Comparative Mapping of Quantitative Trait Loci Sculpting the Curd of Brassica oleracea

Copyright 2000 by the Genetics Society of America

Comparative Mapping of Quantitative Trait Loci Sculptingthe Curd of Brassica oleracea

Tien-Hung Lan* and Andrew H. Paterson*,†

*Department of Soil and Crop Sciences, Texas A&M University, College Station, Texas 77843 and †Department of Crop and Soil Science,Department of Botany and Department of Genetics, Applied Genetic Technology Center, University of Georgia, Athens, Georgia 30602

Manuscript received August 18, 1999Accepted for publication April 25, 2000

ABSTRACTThe enlarged inflorescence (curd) of cauliflower and broccoli provide not only a popular vegetable

for human consumption, but also a unique opportunity for scientists who seek to understand the geneticbasis of plant growth and development. By the comparison of quantitative trait loci (QTL) maps constructedfrom three different F2 populations, we identified a total of 86 QTL that control eight curd-related traitsin Brassica oleracea. The 86 QTL may reflect allelic variation in as few as 67 different genetic loci and 54ancestral genes. Although the locations of QTL affecting a trait occasionally corresponded betweendifferent populations or between different homeologous Brassica chromosomes, our data supported othermolecular and morphological data in suggesting that the Brassica genus is rapidly evolving. Comparativedata enabled us to identify a number of candidate genes from Arabidopsis that warrant further investigationto determine if some of them might account for Brassica QTL. The Arabidopsis/Brassica system is animportant example of both the challenges and opportunities associated with extrapolation of genomicinformation from facile models to large-genome taxa including major crops.

THE genus Brassica (tribe Brassicaceae), which in- characteristics to be complex (Kristofferson 1924;Pease 1926).cludes many important crops, is in the taxonomic

B. oleracea originated from the eastern Mediterraneanfamily Cruciferae, as is Arabidopsis thaliana, the firstarea and appears to have been subjected to intensiveflowering plant whose genome will be completely se-selection pressure in the past century (Prakash andquenced. Such a close relationship suggests that cropHinata 1980). Formal genetic studies of B. oleraceaplants of the genus Brassica will be among the earlieststarted in the 1920s (Kristofferson 1924; Detjenbeneficiaries of the Arabidopsis sequence. Among the1926; Pease 1926). The heritability of the curd (inflo-members of the genus Brassica, Brassica oleracea (n 5 9)rescence) size of cauliflower has been studied in particu-has long been of great interest to plant morphologists. B.lar detail (Watts 1964, 1965; Crisp 1977; Freeman andoleracea includes many vegetables with distinct character-Crisp 1979). In these studies, curd weight is closelyistics that are either not known or are less well developedcorrelated with curd diameter, an easily measured, non-in A. thaliana, including the enlarged inflorescence ofdestructive trait that is particularly useful as a selectioncauliflower (B. oleracea subsp. botrytis) and broccoli (B.tool. The genetic basis of curd weight is complex andoleracea subsp. italica); the enlarged stem of kohlrabiconsequently has been described in terms of mathemati-(B. oleracea subsp. gongylodes) and marrow-stem kale (B.cal genetic parameters of multiple loci rather than spe-oleracea subsp. medullosa); the enlarged apical bud ofcific locations and phenotypic effects of individual loci.cabbage (B. oleracea subsp. capitata); and the enlargedCurd diameter showed additive genetic variation andlateral buds of Brussels sprouts (B. oleracea subsp. gemmif-heterosis (Kalloo and Bergh 1993). In related studies,era) (Kalloo and Bergh 1993) as well as morphologi-the genetic factors determining the conformation ofcally simple “rapid-cycling” genotypes with a life cyclecurd was analyzed (Watts 1966). Curd shape was associ-of 6 to 8 wk (Williams and Hill 1986). The fact thatated with acute angles between the curd branches. Moremany members of the species B. oleracea can be inter-recently, DNA markers were used to dissect the inheri-crossed to generate fertile progenies provides an expedi-tance of curd size (Kennard et al. 1994); however, theent means to investigate the genetic basis of enlarge-use of a very small population (95 individuals) con-ment of these organs. Classical quantitative studies showstrained the number of quantitative trait loci (QTL)the inheritance of most of these novel morphologicalthat could be resolved (Lander and Botstein 1989).

This work, based on three B. oleracea F2 populationsand eight curd-related traits, identified a total of 86

Corresponding author: Andrew H. Paterson, Plant Genome Mapping quantitative trait loci, providing a detailed dissection ofLaboratory, University of Georgia, Rm. 162, Riverbend Research Cen-

complex traits in B. oleracea and a comparative analysister, 110 Riverbend Rd., Athens, GA 30602.E-mail: [email protected] of corresponding mutations in Arabidopsis. Its close

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Figure 1.—The parents used in this study. (A) The profound phenotypic difference between B. oleracea var. Cantanese (left)and rapid-cycling B. oleracea (right), 6 wk after planting in the field. The rapid-cycling parent has flowered and set seeds, whilethe flower bud of Cantanese has not even emerged. (B) B. oleracea var. Bugh Kana has small and loose curd. (C) B. oleracea var.Pusa Katki has a large and compact curd. At maturity, the curd of Cantanese will be similar to Bugh Kana.

the row. Brassica seedlings were nursed in the growth chamberrelationship to Arabidopsis provides an efficient way forfor 2 wk before transplanting to the field. Fertilization, irriga-Brassica scientists to utilize the extensive toolboxes oftion, and insecticide application were consistent with commer-

yeast artificial chromosome (YAC)/bacterial artificial cial production conditions.chromosome (BAC) contigs, expressed sequence tags Phenotyping: The following curd-related traits (Figure 2)

were measured:(ESTs), genetic/physical maps, and genomic sequencesavailable in Arabidopsis. Brassica/Arabidopsis compara- 1. Days to budding (dfb): number of days from planting totive maps (Kowalski et al. 1994; Lagercrantz et al. appearance of the first floral bud.

2. Days from budding to flowering (dofdbf): number of days1996; Osborn et al. 1997; Lan et al. 2000) provide afrom budding to the first flower blossom.means to evaluate the relationship between Brassica

3. First-rank branching (rk1): the number of branches withinQTL and mutants, mapped ESTs, and inferred openreading frames in Arabidopsis. Further, the model ofBrassica/Arabidopsis may possibly be extended to moredistantly related plants (Paterson et al. 1996).

MATERIALS AND METHODS

Plant materials: Three B. oleracea F2 populations were usedfor QTL analysis: rapid-cycling B. oleracea (RCB; self-compati-ble) 3 B. oleracea var. Cantanese (CAN; USDA accession no.PI462224), RCB 3 B. oleracea var. Pusa Katki (PK; USDA acces-sion no. PI274783), and RCB 3 B. oleracea var. Bugh Kana(BK; USDA accession no. PI249556), composed of 247, 250,and 246 individuals, respectively (Figure 1). One B. oleraceaF2 population, RCB 3 B. oleracea var. Green Comet (GC; USDAaccession no. G30771) containing 56 individuals, was used tofacilitate the construction of a composite linkage map. Rapid-cycling Brassica was from the Crucifer Genetics Cooperative,Madison, Wisconsin. A single-plant selection derived by selfinga randomly chosen rapid-cycling plant was used. Seed and/or pollen of other B. oleracea varieties were generously providedby Dr. J. McFerson, and Dr. S. Kresovich, USDA/ARS, Geneva,New York. To minimize environmental effect among popula-tions, all the plants used for QTL analysis were grown in thesame experimental field of the Texas Agricultural ExperimentStation in College Station, Texas, in the same period of time(from October 1994 to April 1995). A completely randomizeddesign was used for field planting. Spacing of plants betweenrows and within rows is 1 m. Two parental self-compatible B. Figure 2.—Illustration of curd-related traits studied. Abbre-

viations for the traits are as listed in the materials and meth-oleracea (SCO), one of the other parent and one F1, wereplanted in every row at randomly determined positions within ods section.

1929Comparative QTL Mapping of Brassica

the curd that originated from the main stem (measured dofdbf were highly correlated (r $ 0.73) across the threeon the first flowering day). populations, suggesting that genes that accelerate or

4. Side-branches (sidb): number of lateral branches from thedelay floral meristem initiation have similar effects onmain stem that were outside the curd (measured on thefloral meristem maturation. As expected, traits that werefirst flowering day).

5. Cluster width (cluw): the width of the largest floral cluster related to curd size such as rk1 and headw were alsomeasured on the first flowering day. correlated with one another (r $ 0.53), with the excep-

6. Curd width (headw): the width of the curd measured on tion of cluw (20.27 # r # 0.21). Late flowering timethe first flowering day.

(dfb and dofdbf) was correlated with larger curd size (rk17. Apical shoot length (rrl1st): stem length from the apicaland headw), consistent with previous data (Watts 1964).meristem to where the closest first-rank branch originated

from the main stem (measured 180 days after planting). The strength of correlation between curd size and curd8. First branch length (rrl2nd): length of the branch that is conformation varied in different populations. Specifi-

the nearest to the apical meristem. cally, rrl1st was negatively correlated with rk1 and headwin CAN and PK populations.Genotyping: DNA extraction, electrophoresis, Southern

blotting, and autoradiography were as previously described QTL detected for each trait: Figure 4 illustrates and(Kowalski et al. 1994). The DNA markers and assembly of Tables 3–10 summarize the QTL detected for each trait.the Brassica composite linkage map were described elsewhere

dfb: In the BK population, four QTL were found on(Lan et al. 2000).chromosomes 1, 4, and 7 (2). A full model containingData analysis: Trait means, correlations, and histograms

were calculated by using SAS and Microsoft Excel. Broad sense these QTL explained 64.9% of phenotypic variance,heritabilities were estimated from the variances of F2 and F1 with individual QTL models explaining 20.9–49.7% of(Falconer 1989). Restriction fragment length polymorphism variance. Allele effects were generally consistent with(RFLP) linkage maps were constructed by using MapMaker

the difference between parents, as three of the BK alleles(Lander et al. 1987) and the Kosambi centimorgan function.delayed budding, while one of the SCO alleles delayedQTL likelihood maps, percentage of variance explained, and

gene actions were determined by using interval mapping in budding. BK alleles were dominant or additive to SCOindividual and multiple QTL models by MapMaker/QTL alleles.(Lander and Botstein 1989). The threshold of LOD (loga- In the CAN population, four QTL were found onrithm of odds) 5 2.5 was used, appropriate for the marker

chromosomes 3, 4, 5, and 7. A full model containingdensity and number and length of B. oleracea chromosomesthese QTL explained 90.0% of phenotypic variance,(Lander and Botstein 1989). For each phenotype, the “free

genetics mode” was used to “scan” the whole genome and with individual QTL models explaining 6.5–87.6% oflocate QTL. Then the QTL was “fixed,” and the genome re- variance. Allele effects were generally consistent withscanned to seek QTL that were masked by the largest QTL. the difference between parents, as three of the CANIf a new QTL was found, then the second largest QTL was

alleles delayed budding, while one of the SCO allelesalso fixed, and the genome was scanned again until no furtherdelayed budding. CAN alleles were dominant or additiveQTL were found. For each QTL, LOD 1 and 2 support intervals

were plotted. to SCO alleles.In the PK population, six QTL were found on chromo-

somes 3, 4 (2), 7, 8, and 9. A full model containingRESULTS

these QTL explained 81.9% of phenotypic variance,with individual QTL models explaining 5.9–45.7% ofPhenotypic variation: Figure 3 illustrates the pheno-

typic distribution, parental means, and population variance. Allele effects were all consistent with the differ-ence between parents, as all of the PK alleles delayedmeans for each trait in original units. Log transforma-

tion was used for dfb, dofdbf, rk1, sidb, and headw in all budding. PK alleles were dominant or additive to SCOalleles.populations to improve normality. The distribution of

dfb and dofdbf in the CAN population clearly indicates Across the three Brassica populations, the total of 14significant marker-trait associations may reflect varia-a bimodel distribution, which suggests segregation for

both qualitative and quantitative factors influencing tion at as few as 11 different genetic loci. BK QTL onchromosome 4 correspond closely to CAN and PK QTL,flowering. Further, substantial transgression of parental

phenotypes was observed for sidb (CAN populations), and a BK QTL at the top of chromosome 7 correspondsto a CAN QTL.cluw (all three populations), rrl1st (BK population), and

rrl2nd (BK population). Further, the 11 different genetic loci may be derivedfrom as few as seven ancestral genes. This is based onTable 1 summarizes the broad sense heritabilities for

each trait investigated. Generally, for the same trait, the observation that some pairs of QTL were located inapparently duplicated chromosomal regions, based onbroad sense heritability varied greatly between popula-

tions. Such variations suggest complex inheritance and a detailed map of B. oleracea (Lan et al. 2000). To furtherillustrate the point, we have presented (Tables 3–10)genetic differences among populations. Phenotypes

that showed substantial transgression (list above) had the LOD scores for each QTL and also for putativelyhomeologous (duplicated) locations in the genomes ofbroad sense heritabilities averaging 0.6, markedly lower

than the average of 0.788 for the other phenotypes. the relevant B. oleracea crosses in our study. For example,the chromosome 1 QTL (BK: EST429a17) falls in aCorrelations among traits: From Table 2, dfb and

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Figure 3.—Histograms of curd-related phenotypes measured. The arrows indicate the average values of parents and F2 grown in the same environment. Too few F1

plants were available to obtain meaningful data.

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TABLE 1

Broad sense heritability for curd-related traits in three B. oleracea populations

dfb dofdbf rk1 sidb rrl1st rrl2nd cluw headw

BK 0.89 0.93 0.73 0.69 0.41 0.85 0.30 0.66CAN 0.36 0.31 0.84 0.39 0.89 0.89 0.59 0.72PK 0.81 0.97 0.97 0.27 0.54 0.52 0.47 0.89

Average 0.69 0.74 0.85 0.45 0.61 0.75 0.46 0.76

region that corresponds to the chromosome 4 QTL (PK: flowering. BK alleles were mostly additive to SCO alleles,the one exception being dominant.EST114a11) and chromosome 7 QTL (BK:

EW8C11a12, CAN: WG3D11114); a chromosome 3 In the CAN population, six QTL were found on chro-mosomes 3, 4 (2), 5, 7, and 8. A full model containingQTL (CAN: EW8F03a15) corresponds to the region

of a chromosome 8 QTL (PK: EW8 D10b10); and a these QTL explained 72.3% of phenotypic variance,with individual QTL models explaining 7.3–59.7% ofchromosome 7 QTL (PK: EW6B07a13) corresponds to

the region of a chromosome 9 QTL (PK: EST131a117). variance. Allele effects were generally consistent withthe difference between parents, as five of the CAN allelesFinally, these seven ancestral genes correspond ap-

proximately to the locations of at least 13 known muta- delayed flowering, while one of the SCO alleles delayedflowering. CAN alleles were mostly dominant or additivetions in Arabidopsis that are implicated in flowering

(Table 3). to SCO alleles, the two exceptions being recessive.In the PK population, six QTL were found on chromo-dofdbf: In the BK population, three QTL were found

on chromosomes 4, 7, and 8. A full model containing somes 3, 4, 5, 7, 8, and 9. A full model containing theseQTL explained 91.1% of phenotypic variance, with indi-these QTL explained 48.4% of phenotypic variance,

with individual QTL models explaining 7.8–39.7% of vidual QTL models explaining 7.4–76.6% of variance.Allele effects were all consistent with the difference be-variance. Allele effects were all consistent with the differ-

ence between parents, as all of the BK alleles delayed tween parents, as all of the PK alleles delayed flowering.

TABLE 2

Correlations among curd-related traits in three B. oleracea populations

dfb dofdbf rk1 sidb rrl1st rrl2nd cluw

BK dofdbf 0.82** 1.00rk1 0.56** 0.55** 1.00sidb 0.52** 0.60** 0.33** 1.00rrl1st 0.42** 0.50** 0.15* 0.44** 1.00rrl2nd 0.41** 0.49** 0.09 0.36** 0.81** 1.00cluw 0.03 0.05 0.09 0.03 20.01 20.05 1.00headw 0.53** 0.52** 0.71** 0.36** 0.26** 0.20** 0.21**

CAN dofdbf 0.73** 1.00rk1 0.72** 0.60** 1.00sidb 0.64** 0.55** 0.49** 1.00rrl1st 20.56** 20.41** 20.45** 20.34** 1.00rrl2nd 20.24** 20.17** 20.22** 20.15* 0.39** 1.00cluw 0.16* 0.06 0.02 0.10 0.03 20.02 1.00headw 0.77** 0.69** 0.78** 0.52** 20.45** 20.24** 0.08

PK dofdbf 0.86** 1.00rk1 0.47** 0.49** 1.00sidb 0.73** 0.70** 0.33** 1.00rrl1st 20.02 20.11 20.07 20.11 1.00rrl2nd 20.05 20.12 20.14 20.17* 0.79** 1.00cluw 0.05 0.01 20.23** 0.14* 0.20** 0.17* 1.00headw 0.75** 0.83** 0.53** 0.62** 20.12 20.14 0.03

* a # 0.1; ** a # 0.01.


Figure 4.—QTL maps of curd-related traits, plotted on RFLP linkage maps of B. oleracea RCB 3 CAN, RCB 3 PK, and RCB 3BK F2 populations. Linkage maps of these three crosses used for QTL mapping are also aligned with a more detailed map ofRCB 3 GC, with homeologous locations in Brassica and with homologous locations in Arabidopsis (as described in more detailelsewhere; Lan et al. 2000). The solid circles next to the loci indicate homeologous Brassica loci (chromosomes 1–9, near right)or homologous Arabidopsis loci (chromosomes 1–5, far right) detected by the same probe. All open circles indicate that nopolymorphism was detected for homeologous (Brassica) or homologous (Arabidopsis) loci. The letter “R” next to the probename indicates that the probe hybridizes to a repetitive DNA sequence in Arabidopsis. Chromosome segments that appear tobe homeologous (Brassica) or homologous (Arabidopsis) were connected by shaded columns. Open columns indicate possibletriplicated (Brassica) or duplicated regions (Arabidopsis). The likelihood intervals of QTL associated with different phenotypes(as indicated by fill; see legend) are plotted as bars (1 LOD) and whiskers (2 LOD).

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TABLE 3

Biometrical parameters of QTL associated with days to first bud

Diagnosticmarker and Dominance/ LOD Possible

Popu- Chromo- location on Additive additivity % Variance Mode of LOD at LOD at LOD at LOD at at Arabidopsislation some chromosome effect (d/a) ratio explained LOD gene action 29 site 39 site BK site CAN site PK site mutations

BK C4 EW4D04w16 20.04 1.02 20.9 3.64 D 1.26 1.03 — 4.79 14.96 emf2BK C7 EW8C11a12 20.07 0.00 39.6 22.51 A 1.60 2.08 — 19.81 (0.95) flc,fy,tfl1,emf1,tfl2BKa C1 EST429a17 20.03 0.67 11.0 3.39 DA 2.48 19.51 — 1.04 0.54 flc,fy,tfl1,emf1,tfl2BKa C7 EST151c10 0.02 0.26 1.2 3.06 DA — — — 1.57 (3.22)CAN C3 EW8F03a15 20.05 0.54 9.9 4.31 DA 0.21 0.11 0.28 — 1.43 hy2,vrn1CAN C4 EW9E10112 20.10 1.28 65.3 10.01 D 1.08 0.58 3.08 — 8.35 emf2CAN C5 EST195b13 0.05 0.18 6.5 2.93 DA 0.34 — 0.28 — 0.22 giCAN C7 WG3D11114 20.15 0.50 87.6 37.60 D 5.54 1.52 15.53 — 3.85 co,art1PK C3 EST411c10 20.06 1.07 44.4 4.37 D 0.60 — — — — fhaPK C4 EST114a11 20.07 0.19 44.1 22.95 A 0.65 6.38 2.31 5.54 — tfl2,co,fy,art1,emf1PK C7 EW6B07a13 20.04 0.26 13.9 6.96 A 0.99 6.80 1.22 6.55 — efsPK C8 EW8D10b10 20.02 1.16 5.9 2.80 DA 1.55 0.54 1.16 0.11 — hy2,vrn1PK C9 EST131a117 20.05 0.71 21.9 6.91 D 6.42 0.35 0.67 0.72 — efsPKa C4 EW9B02x11 20.03 0.52 27.9 2.86 DA 0.57 0.97 3.54 4.95 — emf2

a QTL detected after fixing the “largest” QTL. Variance explained by these minor QTL is the portion of residual variance remaining after the major QTL is fixed, so itis not directly comparable to % variance for the QTL detected prior to fixing the largest QTL. Variance explained by the largest QTL is from the original model in whichnone of the QTL are fixed to make variances of each QTL directly comparable to one another. Gene action is indicated as additive (A), dominant (D), recessive (R).Possible modes of gene action that are within LOD 1 of the most likely mode are shown in decreasing order of likelihood, following the system of Paterson et al. (1991).

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TABLE 4

Biometrical parameters of QTL associated with days from budding to flowering

Diagnosticmarker and Dominance/ LOD

Popu- Chromo- location on Additive additivity % Variance Mode of LOD at LOD at at LOD at LOD atlation some chromosome effect (d/a) ratio explained LOD gene action 29 site 39 site BK site CAN site PK site

BK C4 EW2B12s14 20.07 20.27 10.0 3.55 A 1.45 0.65 — 5.11 26.2BK C7 EW8C11a11 20.14 20.18 39.7 24.00 A 1.42 2.04 — 19.94 (1.50)BK C8 EW5G04b110 20.04 1.60 7.8 3.49 D 1.40 2.11 — 1.27 3.34CAN C4 EW9E10112 20.12 0.09 15.9 5.49 A 0.51 0.34 1.57 — 26.17CAN C5 EST195b13 0.08 0.78 9.4 3.99 DA 0.46 — 0.24 — 0.75CAN C7 EW8C11a14 20.21 20.37 43.8 21.35 RA 0.81 4.99 23.51 — (1.68)CAN C8 EW8E09c12 20.07 20.80 7.3 2.71 RA 0.71 1.19 (1.87) — (2.15)CANa C3 EW8F03a11 20.06 0.98 4.6 4.04 DA 0.69 1.03 0.11 — 1.20CANa C4 EST453g13 20.12 20.03 11.6 12.63 A 0.68 20.61 3.51 — 26.8PK C3 EST411c10 20.17 1.31 63.6 9.01 D 0.45 — (0.25) (0.17) —PK C4 EST373a116 20.24 0.40 76.6 28.18 A 0.40 (1.68) 1.68 5.00 —PK C7 EW7E01a11 20.09 20.04 11.4 5.72 A 0.18 5.70 0.77 4.79 —PK C8 EW8D10b11 20.07 0.49 7.4 3.35 DA 0.75 1.26 (1.87) 0.81 —PK C9 EST131a114 20.13 0.85 24.6 6.48 D 0.22 5.41 1.13 0.84 —PKa C5 EW8A11a12 20.03 22.09 5.5 2.65 R 0.33 — 0.08 1.61 —

a QTL detected after fixing the largest QTL. Variance explained by these minor QTL is the portion of residual variance remaining after the major QTL is fixed, so it isnot directly comparable to % variance for the QTL detected prior to fixing the largest QTL. Variance explained by the largest QTL is from the original model in whichnone of the QTL are fixed to make variances of each QTL directly comparable to one another. Gene action is indicated as additive (A), dominant (D), recessive (R).Possible modes of gene action that are within LOD 1 of the most likely mode are shown in decreasing order of likelihood, following the system of Paterson et al. (1991).

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TABLE 5

Biometrical parameters of QTL associated with first-rank branching

Diagnosticmarker and Dominance/ LOD LOD LOD

Popu- Chromo- location on Additive additivity % Variance Mode of LOD at LOD at at at atlation some chromosome effect (d/a) ratio explained LOD gene action 29 site 39 site BK site CAN site PK site

BK C7 WG3D11111 20.08 1.24 15.8 4.23 D 0.01 1.27 — 9.52 0.83CAN C1 EST566a12 20.16 0.59 8.2 3.98 DA 0.23 1.30 0.01 — 1.41CAN C3 EW8F03a10 20.15 0.82 7.2 3.72 DA 0.20 0.06 0.90 — 1.28CAN C4 EST453g13 20.23 0.05 14.3 6.59 A 1.22 9.59 1.03 — 15.15CAN C5 EW8F11c10 0.14 0.46 5.9 2.91 DA 0.22 — 0.09 — 0.60CAN C7 WG3D1118 20.33 0.17 27.2 9.64 DA 2.00 6.15 4.18 — 0.82CAN C8 EW8E09c10 20.15 20.54 7.6 3.11 RA 3.98 0.16 (1.45) — (1.82)PK C4 EST122b16 20.27 0.23 34.8 15.97 A 0.46 1.77 1.08 5.70 —PK C5 EW8A11a10 20.06 22.91 7.6 3.35 R 2.40 — 0.22 1.07 —PK C6 WR1D12a10 20.15 20.28 9.2 4.87 RA 1.15 12.1 (0.45) 0.59 —PK C7 EW6B07a13 20.12 0.77 7.9 3.93 DA 1.36 1.91 1.15 2.48 —PKa C1 EST217a13 20.09 20.91 6.2 2.98 RA 1.09 — 0.12 0.52 —PKa C9 EST131a114 20.12 0.50 6.5 3.21 DA 1.32 3.34 1.45 0.64 —


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TABLE 7

Biometrical parameters of QTL associated with cluster width



CAN C1 EW5H0310 1.85 0.73 6.1 3.14 DA 0.23 0.04 1.69 — 0.26CAN C2 EW9F06a10 21.98 20.63 5.9 2.96 RA 2.79 (0.12) (0.12) — 0.04CAN C4 EW9E1010 21.91 21.17 8.0 3.80 RA 2.51 0.45 1.10 — 2.75PK C1 EST12510 1.23 23.17 6.9 2.76 D 1.41 1.70 0.08 0.63 —PK C3 ESST411c10 5.19 1.11 33.4 3.41 D 0.29 — (0.32) (1.54) —PK C4 EW9B02x10 22.30 20.72 5.6 2.75 RA 0.41 1.01 1.12 3.30 —

TABLE 6

Biometrical parameters of QTL associated with side branches



BK C7 EW8C11a11 20.20 20.31 45.9 28.12 A 0.98 1.31 — 12.63 (0.54)CAN C4 EST55b116 20.13 20.36 13.6 5.39 RA 0.83 13.54 2.04 — 0.21CAN C7 EW8C11a15 20.20 20.54 30.4 13.82 RA 0.93 5.07 27.12 — (0.54)CAN C8 EW7C10b17 20.09 20.67 8.1 2.79 RA 0.15 — (0.12) — (0.12)PK C4 EST429e11 20.23 20.25 37.8 17.02 A 0.29 6.03 0.97 4.56 —PK C5 EW8A11a11 20.08 21.83 11.4 4.07 R 0.02 — 0.47 1.04 —PK C7 EW8F0610 20.14 0.40 12.5 6.27 DA 0.10 1.61 5.86 0.63 —PKa C9 EW1G0310 20.09 0.42 3.8 3.03 DA 0.13 5.44 0.06 0.36 —


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TABLE 8

Biometrical parameters of QTL associated with curd width



BK C4 EW4D04w17 20.08 0.52 14.9 3.05 DA 0.20 0.07 — 4.29 18.33BK C7 EW8C11a14 20.07 0.41 10.5 4.54 DA 0.17 1.82 — 8.17 (1.13)CAN C1 EW7C10a10 20.12 0.72 11.3 5.65 DA 5.17 0.23 1.16 — 1.44CAN C4 EST453g14 20.13 20.12 11.7 5.23 A 0.59 8.31 2.31 — 28.78CAN C5 EW7E08a11 0.09 0.80 7.8 3.93 DA — — 0.26 — 0.24CAN C7 WG3D1116 20.19 20.14 22.1 8.54 A 0.67 5.13 4.05 — 1.38CAN C9 EW1G03a11 20.09 20.60 5.1 2.53 RA 4.70 0.50 0.51 — 1.97CANa C3 EST22b14 20.07 1.01 3.7 2.97 DA 4.37 0.84 0.19 — 2.25CANa C8 EW8E09c10 20.08 20.55 6.2 2.60 DRA 4.24 — (1.00) — (1.04)PK C3 EST411c10 20.12 2.67 51.2 2.76 D 0.56 — (0.23) (0.12) —PK C4 EST122b14 20.27 20.12 54.3 29.00 A 0.28 2.43 2.53 4.45 —PK C5 EW8A11a10 20.06 22.21 7.6 2.65 R 0.65 — 0.48 1.43 —PK C7 EW5A12a10 20.10 0.06 6.4 2.86 DA 1.48 1.92 1.45 4.09 —PKa C9 EW8E09b12 20.09 0.24 4.3 4.71 A 1.46 2.43 0.41 2.26 —


1948T

.-H.

Lan

and

A.

H.

Paterson

TABLE 10

Biometrical parameters of QTL associated with length of the first branch length



BK C1 EW2E07a115 26.18 0.43 25.5 4.66 DA 1.31 1.57 — 0.09 0.92BK C4 EW4D04w123 24.85 20.56 15.4 2.77 RA 2.55 1.13 — 0.27 0.72BK C7 EW8C11a12 24.91 20.41 15.6 7.02 RA 0.78 1.15 — 0.21 (0.92)BKa C7 EW3A04b123 3.52 0.12 7.5 2.94 DRA — — — 0.09 (0.02)BKa C8 EW5C12f10 3.39 20.82 4.3 2.71 RA 1.99 0.88 — 0.04 0.02


TABLE 9

Biometrical parameters of QTL associated with apical shoot length



BK C1 EW2E07a112 25.20 0.14 22.7 5.27 A 2.71 5.21 — 0.17 1.67BK C4 EW2B12s110 23.65 20.55 11.3 3.05 RA 3.22 4.36 — 4.24 1.36BK C7 EW8C11a11 24.02 20.59 15.2 6.54 R 1.18 2.08 — 0.40 (0.10)BK C7 EW6C11a10 2.89 0.13 7.5 2.69 DA — — — 1.89 (0.02)CAN C3 EW9E05a10 1.69 2.99 9.1 4.04 D 0.15 0.71 0.54 — 0.75CAN C4 EST453g10 3.98 0.48 11.0 4.27 DA 2.04 5.60 2.86 — 1.34CAN C5 EST195b17 23.67 20.36 7.6 2.75 RA — — 0.57 — 1.10CAN C7 EW6B07a10 2.82 0.89 6.8 2.85 D 2.01 4.20 0.10 — 0.04CAN C9 EW1G03a11 4.74 20.54 12.8 5.38 RA 0.32 0.08 0.36 — 0.43PK C1 EW8C11x13 3.31 0.07 11.0 3.18 DRA 1.38 0.16 1.19 1.92 —PKPK C1 WST88c10 20.44 28.95 7.9 2.60 R 0.57 1.27 0.99 0.61 —


PK alleles were mostly dominant or additive to SCO (PK: WR1D12a10) and a chromosome 8 QTL (CAN:alleles, the one exception being recessive. EW8E09c10); a chromosome 4 QTL (CAN: EST

Across the three Brassica populations, the total of 15 453g13, PK: EST122b16) falls in a region that corre-significant marker-trait associations may reflect varia- sponds to a chromosome 7 QTL (BK: WG3D11111,tion at as few as 12 different genetic loci. The BK QTL CAN: WG3D1118); and another chromosome 7 QTLon chromosome 4 corresponds closely to CAN and PK (PK: EW6B07a13) falls in a region that corresponds toQTL, all showing strictly additive gene action. The BK a chromosome 9 QTL (PK: EST131a114).QTL on chromosome 7 corresponds closely to a CAN sidb: In the BK population, one QTL was found onQTL, though the two have somewhat different gene chromosome 7, which explains 45.9% of phenotypicaction (A and RA, respectively). variance. The allele effect is consistent with the differ-

Further, the 12 different genetic loci may be derived ence between parents, as the BK allele increasesfrom as few as nine ancestral genes. The chromosome branching. The BK allele is additive to the SCO allele.3 QTL (CAN: EW8F03a11) corresponded closely to In the CAN population, three QTL were found onchromosome 8 QTL (BK: EW8D10b11, CAN: EW chromosomes 4, 7, and 8. A full model containing these8E09c12); the chromosome 4 QTL (CAN: EST453g13) QTL explained 53.6% of phenotypic variance, with indi-corresponded to a chromosome 7 QTL (BK: EW8C vidual QTL models explaining 8.1–30.4% of variance.11a11, CAN: EW8C11a14); and a second chromosome Allele effects were generally consistent with the differ-7 QTL (PK: EW7E01a11) corresponded to a chromo- ence between parents, as all of the CAN alleles increasesome 9 QTL (PK: EST131a14). QTL in putatively homoe- branching. All of the CAN alleles were recessive or addi-ologous locations often showed different gene action. tive to SCO alleles.

rk1: In the BK population, one QTL was found on In the PK population, four QTL were found on chro-chromosome 7. The model containing this QTL ex- mosomes 4, 5, 7, and 9. A full model containing theseplained 15.8% of phenotypic variance. Its allele effect QTL explained 53.1% of phenotypic variance, with indi-was consistent with the difference between parents, vidual QTL models explaining 11.4–42.2% of variance.where the BK allele increased the first-rank branching. Allele effects were generally consistent with the differ-The BK allele was dominant to the SCO allele. ence between parents, as all of the PK alleles increase

In the CAN population, six QTL were found on chro- branching. PK alleles were mostly dominant or additivemosomes 1, 3, 4, 5, 7, and 8. A full model containing to SCO alleles, the one exception being recessive.these QTL explained 62.9% of phenotypic variance, Across the three Brassica populations, the total ofwith individual QTL models explaining 5.9–27.2% of eight significant marker-trait associations may reflectvariance. Allele effects were generally consistent with

variation at as few as six different genetic loci. The BKthe difference between parents, as five of the CAN alleles

QTL on chromosome 7 corresponds closely to a CANincreased branching, while one of the SCO alleles in-QTL, and a CAN QTL on chromosome 4 correspondscreased branching. CAN alleles were mostly dominantto a PK QTL.or additive to SCO alleles, the one exception being

Further, the six different genetic loci may be derivedrecessive or additive.from as few as four ancestral genes. The chromosomeIn the PK population, six QTL were found on chromo-4 QTL (CAN: EST55b116, PK: EST429e11) fall in asomes 1, 4, 5, 6, 7, and 9. A full model containing theseregion that corresponds to the chromosome 7 QTLQTL explained 55.2% of phenotypic variance, with indi-(BK: EW8C11a11, CAN: EW8C11a15), and a differentvidual QTL models explaining 7.6–41.7% of variance.chromosome 7 QTL (PK: EW8F0610) falls in a regionAllele effects were generally consistent with the differ-that corresponds to the chromosome 9 QTL (PK:ence between parents, as five of the PK alleles increasedEW1G0310).branching, while one of the SCO alleles increased

cluw: No QTL were detected in the BK population.branching. Two PK alleles were mostly dominant to SCOIn the CAN population, three QTL were found on chro-alleles, one PK allele was additive, and three were largelymosomes 1, 2, and 4. A full model containing these QTLrecessive.explained 18.2% of phenotypic variance, with individualAcross the three Brassica populations, the total of 13QTL models explaining 5.9–8.0% of variance. Allelesignificant marker-trait associations may reflect varia-effects were generally consistent with the difference be-tion at as few as 11 different genetic loci. BK QTL ontween parents, as two of the CAN alleles increase clusterchromosome 7 corresponds closely to a CAN QTL, andwidth, while one of the SCO alleles increase clusterCAN QTL on chromosome 4 corresponds to a PK QTL.width. CAN alleles were recessive or additive to SCOFurther, the 11 different genetic loci may be derivedalleles, the one exception being dominant or additive.from as few as six ancestral genes. The chromosome 1

In the PK population, three QTL were found on chro-QTL (PK: EST217a13) falls in a region that corre-mosomes 1, 3, and 4. A full model containing these QTLsponds to the chromosome 5 QTL (PK: EW8A11a10);explained 42.6% of phenotypic variance, with individualanother chromosome 1 QTL (CAN: EST566a12) fallsQTL models explaining 5.6–33.4% of variance. Allelein a region that corresponds to a chromosome 6 QTL


effects were generally consistent with the difference be- variance. Allele effects were generally consistent withthe difference between parents, as three of the BK allelestween parents, as one of the PK alleles increase cluster

width, while two of the SCO alleles increase cluster increase length, while one of the SCO alleles increaseslength. BK alleles were mostly recessive or additive towidth. PK alleles were mostly dominant to SCO alleles,

the one exception being largely recessive. SCO alleles, the one exception being not recessive.In the CAN population, five QTL were found onAcross the three Brassica populations, the total of six

significant marker-trait associations may reflect variation chromosomes 3, 4, 5, 7, and 9. A full model containingthese QTL explained 39.1% of phenotypic variance,at as few as five different genetic loci. The CAN QTL on

chromosome 4 corresponds closely to the PK QTL. with individual QTL model explaining 6.8–12.8% ofvariance. Allele effects were consistent with the differ-Further, the five different genetic loci may be derived

from as few as four ancestral genes. The chromosome ence between parents, as one of the CAN alleles in-creases length, while four of the SCO alleles increase2 QTL (CAN: EW9F06a10) falls in a region that corre-

sponds to the chromosome 4 QTL (CAN: EW9E1010, length. CAN alleles were mostly dominant or additive toSCO alleles, the two exceptions being largely recessive.PK: EW9B02x10).

headw: In the BK population, 2 QTL were found on In the PK population, two QTL were found on chro-mosome 1. A full model containing these QTL ex-chromosomes 4 and 7. A full model containing these

QTL explained 21.4% of phenotypic variance, with indi- plained 18.5% of phenotypic variance, with individualQTL models explaining 7.9–11.0% of variance. Allelevidual QTL models explaining 10.5–14.9% of variance.

Allele effects were generally consistent with the differ- effects were mixed, as one of the PK alleles increaseslength, while one of the SCO alleles increases length.ence between parents, as both of the BK alleles increase

curd width. BK alleles were largely dominant to SCO One PK allele is recessive to the SCO allele and theother shows indications of additive effect, but neitheralleles.

In the CAN population, seven QTL were found on dominant nor recessive effects can be ruled out.Across the three Brassica populations, the total of 11chromosomes 1, 3, 4, 5, 7, 8, and 9. A full model con-

taining these QTL explained 65.5% of phenotypic vari- significant marker-trait associations may reflect varia-tion at as few as 10 different genetic loci. BK QTL onance, with individual QTL models explaining 5.1–28.5%

of variance. Allele effects were generally consistent with chromosome 4 correspond to CAN QTL.Further, the 10 different genetic loci may be derivedthe difference between parents, as six of the CAN alleles

increase curd width, while one of the SCO alleles in- from as few as nine ancestral genes. The chromosome4 QTL (BK: EW2B12s110, CAN: EST453g10) falls increases curd width. CAN alleles were mostly dominant

or additive to SCO alleles, the one exception being a region that corresponds to the chromosome 7 QTL(BK: EW8C11a11).largely recessive.

In the PK population, five QTL were found on chro- rrl2nd: In the BK population, five QTL were found onchromosomes 1, 4, 7(2), and 8. A full model containingmosomes 3, 4, 5, 7, and 9. A full model containing these

QTL explained 66.1% of phenotypic variance, with indi- these QTL explained 47.5% of phenotypic variance,with individual QTL models explaining 15.4–25.5% ofvidual QTL models explaining 6.4-59.9% of variance.

Allele effects were generally consistent with the differ- variance. Allele effects were mixed, as three of the BKalleles increase the branch length, while two of the SCOence between parents, as all of the PK alleles increase

curd width. PK alleles were mostly dominant or additive alleles increase the length. BK alleles were mostly reces-sive or additive to SCO alleles, the one exception beingto SCO alleles, the one exception being recessive.

Across the three Brassica populations, the total of 14 largely dominant. No QTL was found to affect the firstbranch length in CAN and PK populations. The fivesignificant marker-trait associations may reflect varia-

tion at as few as 12 different genetic loci. A BK QTL on different genetic loci may be derived from five ancestralgenes.chromosome 7 corresponds to a CAN QTL, and a CAN

QTL on chromosome 4 corresponds closely to a PK QTL.Further, the 12 different genetic loci may be derived

DISCUSSIONfrom as few as 10 ancestral genes. Several chromosome7 QTL (BK: EW8C11a14, CAN: WG3D1116) fall in a The architecture of the curd (cluster of unopened

flower buds) is an excellent example of the “morpholog-region that corresponds to the chromosome 4 QTL(CAN: EST453g14, PK: EST122b14), and another ical excursions” that appear to be typical of Brassica

evolution. Enlargements of particular organs such aschromosome 7 QTL (PK: EW5A12a10) falls in a regionthat corresponds to the chromosome 9 QTL (PK: the inflorescence of cauliflower (B. oleracea subsp. botry-

tis) and broccoli (B. oleracea subsp. italica), the stem ofEW8E09b12).rrl1st: In the BK population, 4 QTL were found on kohlrabi (B. oleracea subsp. gongylodes), the apical bud

of cabbage (B. oleracea subsp. capitata), and lateral budschromosomes 1, 4, and 7 (2). A full model containingthese QTL explained 48.9% of phenotypic variance, of Brussels sprouts (B. oleracea subsp. gemmifera) (Kal-

loo and Bergh 1993) have led to the widespread usewith individual QTL models explaining 7.5–22.7% of


of Brassica as a major crop. The fact that such morpho- CAN: EW9E10112, PK: EW9B02x11) appear to corre-spond to each other for trait dfb. However, the 86 QTLlogical divergence has occurred among taxa that can

be intercrossed to generate fertile progenies makes we found appeared to represent 67 different geneticloci, illustrating that most QTL differed between popu-Brassica a fascinating subject for genetic analysis.

The enlarged curd of cauliflower (B. oleracea subsp. lations. Further, the comparison of ostensibly homeolo-gous locations of duplicated Brassica chromosomal re-botrytis) is under complex genetic control. By the com-

parison of QTL maps constructed from three different gions also showed mixed results. The 67 putativelydifferent genetic loci accounting for the QTL foundF2 populations, we have identified a total of 86 QTL

that control eight curd-related traits in B. oleracea. The could be accounted for by 54 ancestral genes—i.e.,about one-third of the QTL had possible homoeologs.three crosses shared one common parent, a rapid-

cycling Brassica strain that does not generate a curd On the basis of a genome of nine chromosomes withaverage length of 100 cM and QTL likelihood intervalsbut instead quickly flowers. Among the three different

parents, B. oleracea var. BK, which generates small curd averaging 25 cM in length, these levels of correspon-dence are somewhat higher than would be expected tocompared to varieties CAN and PK, is believed to be

the progenitor of broccoli and cauliflower (Song and occur by chance, but are not nearly so striking as thelevels of correspondence found between much moreOsborn 1992). CAN and PK, from Italy and India, re-

spectively, represent curd-forming types of diverse mor- distantly related members of the grass family (Lin et al.1995; Paterson et al. 1995). The level of diversity ofphology and geographical origin. The possibility that

BK may represent a progenitor of types such as CAN QTL among these three populations was also reflectedin comparisons to another population in which similarand PK was supported by the observation that the BK

population showed simpler genetic control of most traits have been mapped. Kennard et al. (1994) foundthree QTL associated with days to first bud that mappedtraits; across the eight curd-related traits studied, only

20 QTL were found in BK, vs. 34 in CAN and 32 in PK. to the same general regions of three of our QTL, buta fourth was on a linkage group where we found noAmong the 86 QTL detected, 18 displayed domi-

nance/additivity (d/a) absolute values larger than 1 (up QTL for this trait. A single QTL for bud cluster widthdid not correspond to any of the five QTL that we reportto 8.95), suggesting overdominance. Except traits rrl2nd

and cluw all the traits detected one to seven loci showing for this trait. Each of two QTL for leaf lamina lengthdid correspond to two of the five QTL that we foundoverdominance across three populations. Closer exami-

nation of the 18 QTL reveals that two genetic loci ac- for this trait. One QTL each for leaf lamina width andpetiole length corresponded to one of the QTL wecount for many of the cases of overdominance in differ-

ent traits. One, on chromosome 3 near marker EST411c, found (for each trait), while a second QTL for eachtrait did not correspond to any of the three additionalhas positive d/a values; the other, on chromosome 5

near marker EW8A11a, has negative d/a values. Both QTL we found for each trait. Two QTL associated withinternode length corresponded in the two differentloci primarily appeared in the PK population. It has

been observed that crosses made between parents of studies, but a third QTL for this trait found by Kennardet al. (1994) did not correspond to any of the fourdifferent origins generally exhibit greater heterosis than

crosses between parents of similar origins in Brassica additional QTL that we found for this trait.The discovery that the joint effects of two mutations(Grand and Beversdorf 1985; Lefort-Buson et al.

1987; Brandle and McVetty 1990; Ali et al. 1995). in Arabidopsis, CAULIFLOWER and APETALA1, couldproduce a plant with a curd-like inflorescence (KempinFor most traits, we found more overdominant QTL in

CAN and PK populations than in BK populations, fur- et al. 1995) is in contrast to the complex genetic controlwe report for Brassica. Other investigations of possiblether supporting the notion that BK may represent a

progenitor of types such as CAN and PK (Song and interactions between these genes in Brassica (Carr andIrish 1997) show that each of the two duplicated copiesOsborn 1992).

By the comparison of three diverse B. oleracea geno- of APETALA1 in B. oleracea contain insertions that leadto premature translation termination (Lowman andtypes, we sought to investigate the extent to which curd-

related traits were under common genetic control. Of Purugganan 1999). One CAULIFLOWER homologthat has been studied in Brassica does not contain aparticular interest was the notion that we might be able

to discern particular genes that might have been funda- loss-of-function mutation such as would be predicted(Kempin et al. 1995), but does contain other changesmental to the evolution of the curd-like inflorescence,

based on finding QTL at corresponding locations in all that may have phenotypic consequences. Both the CAU-LIFLOWER and APET ALA1 mutations map to Arabi-three populations. The discovery of a mutation that

results in a “curd-like” phenotype in Arabidopsis (Kem- dopsis chromosome 1. Due to the lack of polymorphism,we were unable to map the Arabidopsis genes in Brassicapin et al. 1995) has often been considered to support

this notion. In a few cases, the locations of QTL did directly. However, from our comparative map, we pre-dict they will be mapped to Brassica chromosomes 1appear to correspond in all three populations. For ex-

ample, the chromosome 4 QTL (BK: EW4D04w16, and 5, in locations that approximately coincide with the


location of QTL that influence curd size, including rk1 dominant). In addition, homologous QTL on chromo-somes 7 (PK: EW6B07a13) and 9 (PK: EST131a117)QTL, EST566a12 (CAN), and EW8F11c10 (CAN). It

should be noted that the “curd-like” mutation that oc- possess different gene action, too, which might alsosuggest two different loci. The allelic variation of a ho-curred in Arabidopsis primarily referred to the pheno-

type of “semisterile” flowers and does not appear to be mologous locus may reflect the different allelic effectsamong different crosses or may be the result of mutationaccompanied by the “enlarged inflorescence” observed

in cauliflower or broccoli. (either gain-of-function mutation or loss-of-functionmutation) at a particular locus. The latter, includingOur data (both herein and previously published;

Kowalski et al. 1994) are consistent with the notion homology-dependent gene silencing, i.e., cosuppres-sion, caused by DNA methylation has been a commonthat the Brassica genome may be relatively rapidly evolv-

ing. The diversity of QTL we found among genotypes feature in many plant species including Arabidopsis(Napoli et al. 1990; Furner et al. 1998).and generally low correspondence of QTL in ostensibly

homeologous locations may reflect relatively rapid for- Naturally occurring variants at QTL loci in Brassicamay prove useful for new investigations of plant develop-mation of new alleles in Brassica. The high level of

duplication, perhaps triplication, in the Brassica ge- ment. For example, the curd is essentially a cluster ofbranches near and surrounding the apical meristem.nome would of course facilitate this.

Comparative data help to highlight how tools from What makes it interesting is that the occurrence of thosebranches near the apical meristem violates the rule ofArabidopsis might be used to quickly explore for genes

that may directly account for Brassica QTL. In these apical dominance, which states that the germination ofaxillary buds should start from those buds that are dis-studies we have identified 14 dfb and 15 dofdbf QTL that

associate with earliness in B. oleracea. Ten of these QTL tant from the apical meristem, i.e., near the ground. Incontrast, there also exists a “spreading type” Brassicafall in a homologous region involving the tops of B.

oleracea chromosomes 1, 4, and 7, which correspond to (vs. heading type, which makes curd), which generatesmany axillary branches, but does not make a curd. Thus,a segment of Arabidopsis chromosome 5 that contains

seven flowering mutations (tfl1, flc, tfl2, co, fy, art1, emf1); the trait of sidb was a measurement of Brassica conforma-tion (spreading vs. heading) as well as a test of the apicala homologous region in the middle of chromosome 7

and the lower half of chromosome 9, which corresponds dominance theory. QTL controlling sidb all fall in thesame location as QTL controlling trait rk1, suggestingto a region of Arabidopsis chromosome 1 containing

the mutation efs ; a homologous region involving the that QTL controlling the size of a curd in a heading-type Brassica may have the same effect in spreading-typemiddle of chromosomes 3 and 8, which corresponds

to a region of Arabidopsis chromosome 3 containing Brassica. Variations in apical dominance may largelyexplain the difference between these phenotypes. Fi-mutations hy2 and vrn1; a region of Brassica chromo-

some 3, which corresponds to a region of Arabidopsis nally, Brussels sprouts (B. oleracea subsp. gemmifera),which produce many axillary buds that are not trans-chromosome 1 containing the mutation fha; a region

in the middle of chromosome 5, which corresponds to formed to axillary branches, appear to have a phenotypeof “neutral” apical dominance, lending further supporta region of Arabidopsis chromosome 1 containing the

mutation gi; and a region on the bottom of chromosome to our theory.On the basis of the progeny of crosses with B. oleracea7 in which comparative mapping data were insufficient

to determine if there were corresponding mutations var. Romanesco, Watts (1966) suggested that acuteangle of a curd was genetically controlled. Similar phe-in Arabidopsis. As the Arabidopsis genome sequence

unfolds, these and other candidates will provide a foun- notypes were found in our Brassica populations. As illus-trated in Figure 5, C–E, the relative length of apicaldation for the use of “resequencing” methods to explore

levels and patterns of allelic variation in Brassica that shoot and first branch will influence the acute angleof a curd, and therefore is a good indication of curdmay help to implicate some of these in the genetic

control of complex traits in well-defined Brassica gene conformation. Noticeably, from the histogram, we cansee that CAN and PK, both with large curds, have shortpools (Wang et al. 1999).

Compared to two previous reports (Osborn et al. rrl1st and rrl2nd with respect to SCO, while BK hasroughly equal rrl1st and rrl2nd to SCO, which implies1997; Bohuon et al. 1998), we have identified at least

six additional Brassica QTL corresponding to known that there might be an interaction between curd confor-mation and curd size. Further crosses of SCO to otherArabidopsis flower-timing mutations. The two dfb QTL

EW8C11a12 (BK) and WG3D11114 (CAN) located on Brassica varieties should reveal more details.Head width (5 curd diameter), generally consideredthe top of Brassica chromosome 7, although very close

to each other and both with very high LOD score (.20), to be the best nondestructive proxy for the mass of thecurd (Watts 1964, 1965; Crisp 1977; Freeman andappear to correspond to different loci. Evidence sup-

porting this finding derives from the nonoverlapping Crisp 1979), was closely related to both branching pat-tern and flowering time. In the BK and CAN popula-LOD-support interval and the different gene action of

two QTL (EW8C11a12 is additive and WG3D11114 is tions, the closest correlate of head width was the number


density. From the above, one can see the power of QTLmapping. By comparing associated phenotypes, one caneither confirm an identified QTL or detect new allelesthat differentiate the phenotype, so that a complex traitcan be analyzed in detail.

The Brassica genus, and B. oleracea in particular, illus-trates the “specialized” features that distinguish majorcrops from A. thaliana, a botanical model for moleculargenetic studies. The prospect of a complete sequencefor Arabidopsis in the near future will open new doorsinto many aspects of plant biology by using comparativeapproaches, with special importance for its close rela-tives such as Brassica. With the morphological diversityFigure 5.—The bars indicate the first-rank branches (rk1)of Brassica and the abundance of Arabidopsis resources,of a curd. As illustrated, A and B show that a curd with thewe believe that establishing the Cruciferae as a modelsame amount of first-rank branches could have different curd

size (headw); C, D, and E show three different conformations for dissecting crop development will bear fruit soon.of a curd, which can be documented by the different length

We thank Scott Davis, Mark Hussey, and Terry Thomas for criticalbetween apical shoot and first branch. The angle between thesuggestion, Jim McFerson, Stephen Kresovich, Kenneth Feldmann,thin lines is the acute angle of a curd, where curd C has anand JoVan Currie for technical help, and the Pioneer HiBred Produc-angle .1808, D 5 1808 and E ,1808.tion Ltd. for providing a subset of the DNA probes used. We thankthe Texas Higher Education Coordinating Board, United States De-partment of Agriculture Plant Genome Program, and Texas Agricul-of first-rank branches. In the PK population, the closesttural Experimental Station for funding.

correlate of head width was the number of days betweenthe first bud and first flower. Earliness has long been apriority for crop breeders because reducing the time in

LITERATURE CITEDthe field also reduces the cost and risk of crop cultiva-tion. Several QTL controlling earliness (dfb, dofdbf) fall Ali, M., L. O. Copeland and S. G. Elias, 1995 Relationship between

genetic distance and heterosis for yield and morphological traitsinto the same chromosomal regions as QTL that controlin winter canola (Brassica napus L.). Theor. Appl. Genet. 91:the size of the curd (rk1, headw). Such an association118–121.

could be caused by pleiotropic effects or by tightly Bohuon, E. J. R., L. D. Ramsay, J. A. Craft, A. E. Arthur, D. F.Marshall et al., 1998 The association of flowering time quanti-linked genes. From an applied standpoint, the breedingtative trait loci with duplicated regions and candidate loci inof early varieties that also produce high yield shouldBrassica oleracea. Genetics 150: 393–401.

utilize varieties carrying QTL for curd size that are not Brandle, J. E., and P. E. B. McVetty, 1990 Geographical diversity,parental selection and heterosis in oilseed rape. Can. J. Plant Sci.associated with late-flowering QTL. For example, PK70: 935–940.carries rk1 QTL (EST21713) on chromosome 1, where

Carr, S. M., and V. F. Irish, 1997 Floral homoeotic gene expressionno late-flowering QTL were detected. defines developmental arrest stages in Brassica oleracea L. vars.While the number of first-rank branches, rk1, appears botrytis and italica. Planta 201: 179–188.

Crisp, P., 1977 Genotype x environment interactions in early winterto be a major component of curd width, curd densitycauliflowers in south-west Britain. J. Hort. Sci. 52: 357–366.also needs to be taken into account. As illustrated in Detjen, L. R., 1926 A preliminary report on cabbage breeding. Proc.

Figure 5, A and B, both curds possess five first-rank Am. Soc. Hort. Sci. 23: 325–332.Falconer, D. S., 1989 Introduction to Quantitative Genetics. Longmanbranches; however, they have different width, due to

Scientific & Technical Press, New York.the growth angle of the branch relative to the stem.Freeman, G. H., and P. Crisp, 1979 The use of related variables

Thus, measuring only the curd width to represent the in explaining genotype-environment interactions. Heredity 42:1–11.mass of a curd could be misleading. Therefore we mea-

Furner, I. J., A. Sheikh and C. E. Collett, 1998 Gene silencingsured rk1, cluw, and headw at the same time to betterand homology-dependent gene silencing in Arabidopsis: genetic

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Comparative Mapping of Quantitative Trait Loci Sculpting the Curd of Brassica oleracea

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