Author Topic: Bitterness in interspecific squash hybrids  (Read 316 times)

Klaus Brugger

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Bitterness in interspecific squash hybrids
« on: 2018-12-21, 08:38:32 AM »
Hello,

I'm very much interested in crosses between domesticated Cucurbita species, mostly in order to transfer quality traits among the species. Have you ever encountered bitter fruits in hybrids between edible cultivars of different species, particularly in crosses with C. pepo? I already have a few seeds C. moschata × C. pepo F1 that I will trial in 2019, but I'd like to hear some experiences or thoughts for my season planning. There seems to be one particular paper* about C. argyrosperma × C. pepo and since C. argyrosperma and C. moschata are quite close, I fear that I might have to deal with cucurbitacins in my progeny.

Thank you,
Klaus

*Borchers, E.A. and Taylor, R.T. (1988): Inheritance of fruit bitterness in a cross of Cucurbita mixta x C. pepo. HortScience, Volume 23, pp. 603–604.
« Last Edit: 2018-12-21, 09:18:49 AM by Joseph Lofthouse »

Joseph Lofthouse

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Re: Bitterness in interspecific squash hybrids
« Reply #1 on: 2018-12-21, 09:19:34 AM »

Of the squash I have worked with, only the pepo species is still wild enough to be bitter, and then only in the  wild species, or in populations that have been contaminated with bitterness from the wild species. Pepos have not naturalized in the wild here, and my neighbors don't grow decorative gourds, so I don't have to worry about my inter-species crosses becoming bitter.

Klaus Brugger

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Re: Bitterness in interspecific squash hybrids
« Reply #2 on: 2018-12-22, 08:26:23 AM »
Thank you!
I can't get hold of the paper but it seems like there might be some epistasis going on in certain interspecific Cucurbita hybrids, making fruits of two non-bitter populations bitter.
Like red lettuce from a green × green cross (well, that's within one species, of course).

Joseph Lofthouse

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Re: Bitterness in interspecific squash hybrids
« Reply #3 on: 2018-12-22, 11:18:28 AM »
Yikes! If the bitterness in that particular cross is controlled by 3 dominant epistatic genes, then that complicates selection. Complicates the math too, but here is the most simple analysis, presuming merely that all three alleles need to be present.

If simple Mendelian inheritance is working on unlinked genes, then among the offspring:

27 of 64 will carry at least one copy of each bitter gene, and thus 42% will be bitter.
36 of 64 will carry some of the bitterness genes waiting to manifest in future generations.
1 in 64 will have no bitterness genes.

Interspecies hybrids tend to set seed poorly, and to germinate poorly, and to have fertility problems, so there might not be enough seed available in early generations to do much selection. The F1 generation in interspecies crosses is often crossed with one of the parent species, so there may be an opportunity during back-crossing to devise a scheme to select against the bitterness genes.

Hmm. What would my protocol be to select against bitterness?

Taste the cotyledons to see if bitterness manifests in them like it does in cucumbers. If differences are noted, cull the plants with bitter leaves. Otherwise, let the plants set fruit. Taste the fruits from each plant. Cull any plants with bitter fruits. Cull all the fruits in the patch, and allow the non-bitter plants to promiscuously pollinate. Then I would do recurrent mass selection. Mass selection against multiple dominant epistatic alleles is very quick. (After only one generation of selection, the rate of bitterness drops to 1 fruit in 5 or less). Recurrent mass selection doesn't ever eliminate the bitterness genes completely, but it would be a fine population for a home gardener that tastes every fruit in every generation before saving seeds from it.


« Last Edit: 2018-12-22, 01:05:04 PM by Joseph Lofthouse »

Klaus Brugger

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Re: Bitterness in interspecific squash hybrids
« Reply #4 on: 2018-12-23, 01:40:32 PM »
Thank you very much for your detailed answer! From what I’ve read now* it might be difficult to stabilize intermediate types in a C. pepo × C. moschata cross. I will probably go for several rounds of backcrossing with the male parent of the initial cross as the recurrent parent. Maybe the cytoplasm of the other species will make future interspecific crosses easier. Establishing bridge lines among the three most important Cucurbita species is definitely possible**.

*compare
 Wall, J. R. (1961): RECOMBINATION IN THE GENUS CUCURBITA. Genetics, vol. 46, no. 12, pp. 1677–1685. http://www.genetics.org/content/46/12/1677

**compare
Zhang, Q., Yu, E. und Medina, A. (2012): Development of Advanced Interspecific-bridge Lines among Cucurbita pepo, C. maxima, and C. moschata. HortScience, vol. 47, no. 4, S. 452–458. http://hortsci.ashspublications.org/content/47/4/452.full.pdf+html

Joseph Lofthouse

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Re: Bitterness in interspecific squash hybrids
« Reply #5 on: 2018-12-23, 01:54:30 PM »
Establishing bridge lines among the three most important Cucurbita species is definitely possible**.

Bwah ha ha! I keep worrying that species barriers are going to break down in my garden, if I keep playing with interspecies hybrids. If/when that happens, I'll have to call it a feature, rather than an unfortunate mingling of species...


Klaus Brugger

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Re: Bitterness in interspecific squash hybrids
« Reply #6 on: 2018-12-23, 02:28:06 PM »
Haha, yes, there's two sides to this medal  ;D.