Developing New Varieties

Larry Darrah

Research Geneticist and Adjunct ProfessorUSDA-ARS Plant Genetics Research Unit

and Department of Agronomy, UMC

Self- vs. cross-pollinated crops

• Self-pollinated: Uses pollen and egg from the same plant to produce seed. Examples—Wheat and soybeans.

• Cross-pollinated: Uses pollen from one plant to fertilize an egg from another plant. Examples—Corn and squash.

Self- vs. cross-pollinated crops

• Soybean—Selfs easily and crosses with great difficulty; 3-4 seeds per pollination, if you are good! Only about 10% of the crosses set any seed.

• Wheat—Selfs easily and crosses with difficulty; 10-15 seeds per pollination. 95% of the crosses will set seed.

• Corn—crosses and selfs easily; 300-400 kernels per pollination. Anyone can pollinate corn!

• Determines what type of product is available to the farmer (hybrid vs. variety).

Corn anatomy

Tassel - ♂♂• Sheds pollen at maturity

Ear - ♀♀• Each silk is attached to

one ovule.

• Pollen tube grows down silk and fertilizes ovule.

Shoot bagging

Cutting back the ear

Tassel bagging

Pollination

Terminology

• Inbred: A plant that is produced through self-pollination over many generations.

• Hybrid: A plant that is produced by cross-pollinating two inbreds.

Does plant breeding work?

You betcha it does!

Lets look at corn yields over time.

Corn yields 1870-2000(10 tons/ha = 159 bu/a)

1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Year

0

1

2

3

4

5

6

7

8

9

10

Ave

rage g

rain

yie

ld (t/ha)

Varieties; b = -0.00 t/ha/year

How’ed they do that?

Population improvement/basis of gain from selection

Stalk lodging damage

Rind penetrometer use

Rind penetrometer in action

Divergent selection results

Response to selectionin MoSCSSS

0 1 2 3 4 5 6 7 8 9 10 11

Rind penetrometer resistance bins (each 0.25 kg)

0

100

200

300

400

500

Fre

quen

cy

2530 data points for Cycle 0Mean = 4.37 for B73 x Mo17Mean = 4.40 for Cycle 0

2699 data points for B73 x Mo17

B73 x Mo17

Cycle 0

0 1 2 3 4 5 6 7 8 9 10 11

Rind penetrometer resistance bins (each 0.25 kg)

0

100

200

300

400

500

Fre

quen

cy

2446 data points for high2508 data points for low

High sel.

Low sel.

Hig

h m

ea

n =

6.5

4

Lo

w m

ea

n =

3.0

9

Cycle 0 and B73 x Mo17 Cycle 6 low and high

Rind penetrometer resistance

0 1 2 3 4 5 6 7 8 9 10 11 12

Cycle of selection

1

2

3

4

5

6

7

8

9

10

Rin

d pe

netr

omet

er r

esis

tanc

e (k

g pl

ant-1

)

High = 3.60 + 0.17X + 0.20X2

Observed high

Observed low

Low = 3.60 - 0.24X + 0.01X2

Avg = 3.60 - 0.04X + 0.01X2

R 2 = 0.88

R2 = 0.88

Recurrent selection

• Generate families.

• Test family performance.

• Recombine selected families to complete a cycle of selection.

• Extract inbred lines by selfing in selected families and testcrossing at S2

or S3.

Family structure

• Selfed plants (S1 or S2).

• Crossed plants (Half-sib families with various testers.

• Selfed and crossed plants (S1 or S2 testcrosses to various testers.

• Reciprocally crossed plants (two half-sib testcrossing schemes.

Family evaluation (Yield)

• Test 100-300 families.• 2-row plots spaced 30” apart and about 7” between

plants for a total of 60 plants; 26,000-30,000 plants/a.• 4-9 replications at 2-6 locations in one season (3

reps. at 3 locns., 2 reps. at 4 locns., or even 1 rep. at 6 locations might be used.

• Locations are representative of the region of adaptation.

• Combine harvest to obtain grain weight, test weight, and moisture.

Recombination of selected families

• Bulked pollen, 1 male for 2 females.

• Diallel: 1 x 2, 1 x 3, . . . , 9 x 10; in paired rows or by chain crossing where most rows are used as both a male and female where the species allows (as in corn).

Partial Diallel

  6 7 8 9 10

1 1 x 6 1 x 7 1 x 8 1 x 9 1 x 10

2 2 x 6 2 x 7 2 x 8 2 x 9 2 x 10

3 3 x 6 3 x 7 3 x 8 3 x 9 3 x 10

4 4 x 6 4 x 7 4 x 8 4 x 9 4 x 10

5 5 x 6 5 x 7 5 x 8 5 x 9 5 x 10

♂♂♀♀

Germplasm sources for population improvement

• Existing varieties (landraces and improved populations).

• Crosses within heterotic groups of existing elite lines for corn.

• Synthetic populations (contain varieties, lines, other synthetics, etc.). For example, germplasm with resistance to the European corn borer.

Look at many—save a few!

Wheat variety ‘Ernie’

• Came from pedigree selection in a cross of ‘Pike’ and Exp. Mo9965.

• Pike is an old variety with a very mixed pedigree.

Triticale: A “new” crop

• Triticale is a cross of wheat (female) and rye (male).

• Confers traits of high yield and baking quality.• Confers traits of tolerance to acid soils and

salinity, drought tolerance, winter hardiness, rust and mildew resistance, and higher lysine.

• Grown on 7.5 million acres (acid and marginal soils) in the world—primarily Australia, Brazil, France, Germany Poland, and South Africa.

New birdsfoot trefoil with rhizomes

• U.S. trefoil has persistence problems because of root and crown rot.

• U.S. varieties lack rhizomes.• Paul Beuselinck, USDA-ARS, Columbia,

collected a rhizomatous birdsfoot trefoil in Morocco and has bred it into U.S. germplasm resulting in release of ARS-2620.

• Grazing studies show increased persistence in pastures.

No rhizomes Rhizomes

Pedigree selection in self-pollinated crops

Variety A x Variety B

F2

F1

F3F3F3 F3 F3

F4

F5

F6

Can split apart further at F3 or subsequently

Where do most new varieties come from?

• Selfing out of existing varieties and testing.• Crosses among existing lines and varieties

followed by selfing and testing.

Yes, this approach seems to be self-limiting (funnel), but it has worked well thus far in many crops. See the following data from the Kenya Maize Breeding Project:

Genetic variance estimated from ear-to-row selection in Kitale Composite A (E7) from 1965 to 1974

• Cycle 0 355• Cycle 1 287• Cycle 2 242• Cycle 3 470• Cycle 4 316• Cycle 5 283• Cycle 6 586• Cycle 7 213• Cycle 8 1148• Cycle 9 263