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Repeated Squash Hybridization. What happens?

Posted by Opuntiagrower none (My Page) on
Mon, May 12, 14 at 0:10

Hi all,

I had a question regarding hybridization of squash cultivars and the phenotypic results that you think would happen.

If you have looked at websites such as baker creek and seedsavers exchange, you have probably seen the many different varieties of heirloom squash they have.

My question is: What would happen if I bought 10 different cultivars of C. maxima, and the first year I crossed 2 of the cultivars, then crossed the resulting hybrid with the third cultivar, then crossed that hybrid with the fourth cultivar all the way to 10. Basically, every year adding a new set of cultivar genes to the gene pool of the "hybrid". Would this be good? I have heard of cases where prolonged outbreeding is actually bad. Can anyone shed some light on what they think might happen?

As an example: Let's say I wanted a squash that looked like the Triamble Squash and I crossed it with an Atlantic Giant. I am assuming the squash would look like a medium to large sized shamrock shaped pumpkin, or something close to that.

What if I then crossed that hybrid with other round pumpkins including Blue Hubbard, Pink Banana, etc. and added a new cultivar each year?

Would the Triamble Squash genes still be present but be buried under other genes so that they would no longer express themselves and the squash would no longer exhibit a phenotypic large shamrock shape ? In that case, it would probably be pointless to cross too many times, right?

Attempt to summarize: Bascially, what happens if you save seeds from a hybrid cross, and each year you cross that hybrid with another cultivar, kind of like a 10 cross hybrid over a period of years. I imagine the result would be extremely unstable, but I wonder if the phenotypes of the very first crosses would no longer show up due to being "buried" under genes from newer, more recent crosses.

ANY HELP IS GREATLY APPRECIATED!


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RE: Repeated Squash Hybridization. What happens?

Hi Opuntiagrower,

First of all, welcome to GardenWeb. I noticed you just recently registered here. As an amateur zinnia breeder, I have confronted the same questions in zinnias that you have with respect to squash cultivars. You have asked some good questions, and it will take more than one message to adequately address them, but I do have a few comments.

"What would happen if I bought 10 different cultivars of C. maxima, and the first year I crossed 2 of the cultivars, then crossed the resulting hybrid with the third cultivar, then crossed that hybrid with the fourth cultivar all the way to 10. Basically, every year adding a new set of cultivar genes to the gene pool of the "hybrid". Would this be good? I have heard of cases where prolonged outbreeding is actually bad. Can anyone shed some light on what they think might happen?"

When you save seeds from an F1`hybrid, the genes tend to randomly "unmix" and recombine in the F2 generation. In other words, the F1 generation will be pretty uniform, given that your cultivars are reasonably well inbred and stabilized (homozygous). But the cellular processes of meiosis involve many random recombinations of genes, such that the F2 generation of a single hybrid can produce a whole spectrum of differently appearing squashes (phenotypes).

So, when you start to add in that third cultivar in the second year of your project, you will be crossing it with potentially thousands of different "virtual" F2 squashes from your first year. I say "virtual" because the pollen grains and egg cells that you will be working with will have been produced by many random recombinations in the meiosis processes.

The breeding possibilities of combining 10 cultivars of squashes involves a potentially astronomical number of different results. Simply because meiosis always produces a very large number of different results when the parent is heterozygous (a hybrid). The pollen grain of a hybrid contains a random recombination of genes. And the egg cell also contains a random recombination of genes. Variations multiplied by variations.

I would suggest you could speed up your process of combining the genes from the 10 different cultivars by not waiting a year to add in each new cultivar. Simply make all possible hybrids of your 10 cultivars the first year. Grow them all and cross them all. How many hybrids could you make that first year from 10 cultivars?

Mathematically, that would be a combination of 10 things taken 2 at a time. Combinatorially, that works out to 45 different hybrids, if we don't distinguish between which cultivar is the female and which cultivar is the male. If you want each cultivar to participate as both a male and a female, that would double to 90 different crosses between your 10 cultivars. You can show this graphically by placing 10 points on a circle and then connecting a line between each of the points with the other points, and put an arrow head on both ends of each line.

It may seem apparent at this time that the planet doesn't have enough land area to grow all the different examples of squashes that could be produced by this project.

In order to cut this down to something that your garden space can accommodate, my suggestion would be make several different crosses the first year, probably less than the 45 different combinations, and to grow as many seeds from those crosses as you have space for the second year. In that second year, carefully evaluate your F1 hybrids, and save seeds from your favorites and also inter-cross some or all of them. In the third year you will get a tremendous variety of different squash phenotypes. So cull them severely, saving only those that appeal most to you. And self and cross your favorites to produce seed for the fourth year. Things will really start to get interesting that fourth year. My zinnias have done some amazing things for me.

You may have some follow-up questions or comments, and I will be glad to respond to them.

Just out of curiosity, what 10 cultivars of squash were you planning to grow?

ZM


This post was edited by zenman on Tue, May 13, 14 at 16:54


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