AG Genetics A.G.G.C. © 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.

AG

G

enetics

A.G.G.C. © 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.

IN THE BEGINNING…

To best interpret the genetics behind our beloved Atlantic Giant, we must first understand the basics of plant reproduction. What follows is a very basic tutorial.

It is important at this time to properly classify the Atlantic Giant genetically. It is of the genus Cucurbita, a genus reserved for squash, and of the species maxima. Other members of this species include the Banana, Cinderella, and Burpee’s Prizewinner Hybrid. By definition, it is not a true pumpkin like the Howden’s Field, Connecticut Field, or Sorcerer pumpkins and cannot be crossed with them. These pumpkins are of the species Cucurbita pepo. Furthermore, the difference between today’s Atlantic Giant pumpkin and the Show King giant squash is genetically negligible as these appear to differ only in color. One might look at a true Show King squash as a strain of Atlantic Giant or perhaps even visa-versa.


Family - Cucurbitaceae

Genus – Cucurbita

Species – Cucurbita maxima

Sub-species -

Variety – Atlantic Giant

Strain -


Atlantic Giant and Show King giant squash are both of the species Cucurbita maxima and differ only in color.

Atlantic Giants will not cross with ‘true’ pumpkins.

Points to remember…


MARRYING YOUR COUSIN Plant geneticists broadly group plant reproduction into two categories: inbreeding and outbreeding. Inbreeders are those who reproduce independently of other like plants. Their male and female organs are often contained within the same physical unit and require no third-party means of pollen-to-egg transfer. Tomatoes are a good example of an inbreeding type of plant. Outbreeders on the other hand, are characterized by having two distinctly separate male and female flowers and require an outside means to distribute the pollen between the two. Commonly, bees, wind, or animals provides this means of transfer. All cucurbits are outbreeders. As you might expect, the progeny of inbreeders are mainly produced through self-pollination while those of outbreeders are born through cross-pollination. Both inbreeders and outbreeders technically may be propagated through self and cross-pollination though inbreeders tolerate self-pollination much better. This would seem natural given their reproductive tendencies.


Plants reproduce either through inbreeding or outbreeding.

Cucurbits are outbreeders.



HUH? Within every plant is what’s known as genotypes. These genotypes combine to make up the plant’s appearance, growth, and fruit characteristics. The physical manifestation of genotypes are called phenotypes. Size, shape, and color can all be classified as phenotypes. Constructed of DNA and protein, one or more genes can represent each phenotype and control its state or appearance. Going even deeper, alleles serve as the alternate values for each gene. At least two are present at any given time, one carried over from the mother, one from the father. In any squash, we can have thousands of genes, all coming together to make up the physical characteristics that we see. With this in mind, we can see why an incredible number of variations in progeny are possible. Variance in plants comes from the difference or heterozygosity of the alleles for any given gene. Homozygosity is the sameness of these alleles and manifests itself in the uniformity of the progeny. Self-pollination of plants encourages homozygosity and enhances the predictability of the offspring.


For each generation of self-pollination, the sameness of the allele pairs becomes more frequent lending itself to more uniform offspring. Cross-pollination introduces new gene/allele combinations and therefore gives way to new variations in appearance and habit. These offspring can display characteristics of either or both parents. The probability of these characteristics or phenotypes depends on the dominance and homozygosity of the genes responsible. These genes can be dominant, codominant, incompletely dominant, or recessive in nature. Crossing independent, homozygous varieties creates heterozygosity or hybrids.

Note: When reviewing our hierarchy trees, many plant breeders would consider these pumpkins siblings due to the close relationships many of the pumpkins share.


Genotypes are made up of genes and combine to make up a plants appearance and habit. Phenotypes are the physical manifestation of the genotypes. Alleles represent the possible states for each gene.

Homozygosity is the sameness or purity of the allele pairs.

Heterozygosity is a difference in the allele pairs.

Self-pollination encourages homozygosity. Cross-pollination encourages heterozygosity.

Genes can be dominant, codominant, incompletely dominant, or recessive in nature.

The crossing of two independent, homozygous lines creates hybrids.



ANOTHER DEPRESSION So why is everyone cross-pollinating rather than self-pollinating? If the characteristics you desire are already there, why not just self-pollinate? Why is everyone so eager to further complicate the gene pool? Part of this unwillingness may have some basis in fact. As stated previously, inbreeders naturally tolerate inbreeding or self-pollination. Outbreeders often repel it causing what is known as inbred depression. Poor vigor and even poorer subsequent seed production are the result. Additionally, self-pollination often brings forth the homozygosity of recessive genes. If these genes are negative in nature, the results can be lethal or at least undesirable. Hence, many self- pollinated, outbreeding strains fail to reproduce and carry on the strain.


Here’s the good news. Cucurbits do not seem to be influenced by in-bred depression. Self-pollination appears to have no ill effects and only serves to preserve the genetic qualities of the parent. In fact, tests have been done to try and prove that self-pollination is detrimental to the longevity of these plants and have failed. With this in mind, why not use it to help preserve favorable traits in our favorite seeds?


Inbred depression can sometimes occur as a result of self- pollination but has not been observed in cucurbits.



PRESS “F1” FOR HELP ‘F1 hybrid’. This is a term that is being tossed around quite a bit these days in pumpkin circles but what exactly is it?

An F1 hybrid is the resulting cross of two inbred, homozygous lines. The ‘F’ in ‘F1’ stands for filial, ‘1’ represents the first generation. A monohybrid is a plant whose parents were heterozygous for only one trait. A dihybrid is one whose parents were heterozygous for more than one trait. All F1s should produce offspring that are uniform for the trait(s) involved. Burpee’s Prizewinner Hybrid is a very good example of an F1 Cucurbita maxima with the emphasis on shape and color.

Many or you have probably heard the term ‘hybrid vigor’. What does it mean? Do Atlantic Giants experience it?


Inbred lines, when crossed, sometimes experience something called hybrid vigor. Abnormally vigorous plant growth and development may characterize this. This hybrid vigor is only experienced in out-breeding varieties but unfortunately, it has not been observed in the Cucurbita maxima to any appreciable degree.

For those of you hoping to further test this, you would need to cross two stable, inbred lines of Atlantic Giant and then grow the seeds of the progeny. Simply cross pollinating between random Atlantic Giants does not constitute conditions for hybrid vigor. Today’s Atlantic Giant is a very aggressive grower by nature and capable of producing huge fruit. Subsequently, detecting which plants were hybrid vigorous could prove difficult to impossible.


It must be noted as well that hybrid vigor does not always manifest itself in larger fruit. There are many, many hybrid vegetables on the market today with the vast majority of them not being above-average sized cultivars. Hybrid vigor could also mean a more aggressive plant, better disease resistance, or greater seed production. To summarize, hybrid vigor may be difficult to achieve or detect. Even if successful, it does not guarantee a bigger pumpkin.


A hybrid is the result of crossing two homozygous lines.

Hybrids can be either monohybrid or dihybrid.

Hybrid vigor, characterized by abnormally aggressive plant or fruit production, can sometimes occur as a result of hybridization but has not been observed to any appreciable degree in cucurbits.

The random crossing of two Atlantic Giant plants does not constitute conditions for hybrid vigor.

Hybrid vigor does not guarantee a larger fruit.



t tAlleles = T (dominant) and t (recessive)

T

t

Tt Tt

tt tt

50% = T 50% = t


T TExample of an F1 cross

t

t

Tt Tt

Tt Tt

100% = T 0% = t


T tExample of an F2 Cross

T

t

TT Tt

Tt tt

75% = T 25% = t


MAGIC BEANS Many growers seem to be under the impression the Atlantic Giant is some magical variety that somehow transcends the laws of genetics. It is not. In reality, it is a relatively simple, open-pollinated variety that has been bred for size and little more. The laws of nature, genetics, and physics most certainly apply and need to be observed. It is equally important that we strive to find the simplest solutions possible and not overcomplicate what are essentially simple tasks.

So how do we go about properly breeding the squash of our dreams? The method best suited to the task is called genotypic recurrent selection. This method involves the initial selection of seeds from a favorable fruit, the growth and self-pollination of its seeds, and the cross pollination of the next generation of progeny having the traits we desire. The self-pollination between crossed generations is critical as it will help to create sameness amongst the allele pairs. Without it, we are basically nullifying the effects of the cross and further complicating the equation.


¹Selection based on visual criteria alone with intercrossing is called recurrent mass selection. The best improvement will be obtained when parents are carefully chosen for each cycle. Experience has shown that the improvement depends not only on the performance of each plant on its own, but on how well it nicks with other parents in the cross. Certain combinations simply work better than others, regardless of the individual performance of the parents. The strategy used to evaluate nicking ability is progeny testing. Progeny testing requires saving seed of selected plants, and an extra season to evaluate their progeny. If the progeny of a cross all perform well, the cross nicks well and the seed from the original parents is used for the next cycle of recurrent selection. To make the most of recurrent selection, self-pollinate the plants you plan to intercross the next generation. This will double the response to selection by controlling the identity of the male parent.

¹Farmer Cooperative Genome Project - http://www.fcgp.org/


“So what seeds should I be growing? What plant should I use as a pollinator?”

I get questions like this, with no other information provided, emailed to me all the time. No one can answer these questions for you without knowing what your optimum goal is. Just plain big is not a sufficient goal as most of today’s seeds have the firepower to knock out a few 1000+ pounders. Instead, you need to give some thought to which phenotypes you deem desirable. Only then can you map out a strategy in your search for or development of the ultimate seed.

Plant breeding is always done with a specific result in mind. Visualize what you want your pumpkins to look like and start with seeds known to produce similar fruit. Plan your crosses between plants exhibiting the traits you seek. Focus more on breeding with proven, time tested techniques and less on trying to find that magic cross. Random cross-pollination produces random results. Importantly, don’t expect immediate, favorable results. Plant breeding is largely trial and error and strains can take many generations to perfect.


Genotypic recurrent selection, combined with careful self and cross-pollination, is the best way to achieve our breeding goals.

Follow time tested methods and procedures of plant breeding.

Have an end result in mind before you start any cross-pollination and start with lines already proven to produce progeny exhibiting the characteristics you prefer.

Don’t cross-pollinate just for the sake of doing so.

Don’t be disappointed if you don’t immediately get the results you desire.

Keep it simple!



Most importantly…

HavHave e fun!!fun!!Best of luck to all in 2004. May all your pumpkins be big … and just slightly smaller than mine. – Mike Nepereny 02/2004

AG Genetics A.G.G.C. © 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.

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Transcript of AG Genetics A.G.G.C. © 2000-2004 Atlantic Giant Genetics Cooperative. All rights reserved.