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AG Hybrid Vigor By Joe Ailts - Dec 2000

B.S. Biotechnology, University of Wisconsin-River Falls

When it comes to seed selection for next spring, all growers want the "best" genetics. But what does that mean? And how do you go about deciding which seeds are the best? Often selections are made based on what a particular seed has produced in the past. Growers want "proven" seed. Seeds that "consistently" produce big fruit. But why do some seeds seemingly produce larger fruit than others? There has to be an explanation, and I propose one theory in the following text.

Hybrid vigor occurs when two genetically different sources of DNA combine to form progeny that exhibit non-typical results. A horse and donkey cross produces a mule. It has the endurance of the horse, and strength of the Donkey, but cannot reproduce. The same type of results is found in the plant world. Typical characteristics of hybrid vigor as it may apply to pumpkins include: consistent, marked increase in size of fruit and plant mass, increased growth rate, and lack of seeds or sterile seeds.

Looking back through the archives, there appear to be a few distinct groups of seeds that appear in most backgrounds. Some of these seed lines differ enough genetically that I believe some degree of hybrid vigor is expressed. As an example I will use the following seed lines- those based on "Lloyd" genetics, and those based on "holland" genetics of the early 90's. When a "Lloyd-Holland" cross is made, pure Lloyd genetics are combined with pure Holland genetics. The resulting seeds typically produce above average vigorous pumpkins. The 723, 846, and 815 are good examples of what I have termed a "vigor cross". Many of the 1000+ lb pumpkins grown to date are the result of a vigor cross.

Lets take the 815 Checkon as an example. This fruit was grown on the 977 Andersen plant, and was pollinated by the 935 Lloyd. The 977 was grown on the 1006 Greer, which has a significant amount of Holland genetics in its background. This sets the stage for a vigor cross. The 977 seed is unique, because it is the result of a "selfed" cross. When you self-pollinate a flower, this serves to "purify" its genetics by not allowing "foreign" genetics enter the cross. By performing the self-pollination, this cut a possible 8 different grandparents on the female side to four, which, like I said, "purified" the Holland genetics. The 935 is also very unique situation where its parents both came from the same selfed pumpkin. This cut a possible 8 grandparents down to 2, producing very "pure" genetics. So what you are left with is the Neilly-Craven cross on the Lloyd side, and the 792 Holland, 722 Holland, 500 Dill, and 502 Ciliberto on the Andersen (holland) side. The result of all this is an extremely vigorous seed, the 815 Checkon.

Here are some statistics that I believe to be both interesting and significant. Check it out:
Based on data obtained from the AGGC, the average size pumpkin grown and officially weighed between 1999 and 2000 is 601lbs. The average size pumpkin as the result of a "vigor cross" in the same time period is 634lbs. That’s a 5% difference. Not necessarily an eye opener, but something to think about none-the-less. Here’s the kicker though- the average size of a pumpkin grown where the FEMALE is the 935 and the male is some other genetically different seed is 692lbs. That is a 13% difference. I consider that to be quite significant.

Another interesting statistic- As of this writing, 9 of the 15 largest fruit ever grown are what I consider vigor crosses.

I believe that the genetics in many of the Lloyd fruit (935, 876.5) differ enough from others to produce some degree of hybrid vigor. Keep this in mind though- hybrid vigor is not a day and night situation. There are varying degrees. The interesting part is I believe that we as growers can manipulate the genetics of our seeds to maximize the degree of vigor introduced into a seed. The more "purified" a seed’s background is, the better chance it has for vigor. It makes sense- if a seed has many of the same ancestors at least 4 generations back, this significantly reduces the possibility of that seed sharing common genes with another seed of a different background. The 815 Checkon is a perfect example. Decrease the number of different ancestors, increase the possibility of hybrid vigor.

Of course I must say the evidence provided here is by no means conclusive. This is just another theory like many others out there. However, I feel that this theory holds a significant amount of merit, which can be backed up scientifically. Here is a statement found in a chapter on squash breeding, from a book titled Breeding Vegetable Crops: "…investigators have found support for the idea that inbreeding in Cucurbita does not decrease vigor... absence of inbreeding depression does not signify that hybrid vigor in Cucurbita is lacking. A number of investigators have found significant evidence for hybrid vigor." Atlantic Giants were not mentioned specifically in the book, the preceding text was in reference to all species of Cucurbita, which certainly includes the big pumpkins.

Ever-improving grower knowledge and growing practices are likely the major cause of recent advances in giant fruit size. But…having an education in the biology field exposed me to the importance of genetics in an organism. My idea is that by adopting theories from both aspects of growing, i.e., environment and genetics, you will be well on your way to maximizing the potential of fruit in your garden.

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