Author Topic: Cucurbita moschata × C. pepo  (Read 1077 times)

Klaus Brugger

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Cucurbita moschata × C. pepo
« on: 2019-12-24, 08:25:08 AM »
TL;DR
An op Cucurbita moschata cultivar was pollinated with either of two op Cucurbita pepo cultivars from different subspecies or their hybrid. Eight moschata × pepo F1 plants were grown and pseudo-backcrossed to pepo. Ca. 120 pollinations resulted in ten fruits from four plants. BC1 seeds are poorly developed. Bigger or better filled ones will be sown in 2020. Photos including some pictures of hermaphroditic flowers are below the text.


Background

In 2017, I started a breeding project for a high-quality pepo winter squash by crossing 'Jack Be Little' (JBL) with 'Tonda Padana' (TP). The first one belongs to the acorn cultivar group, subsp. texana, (Gong et al., 2012), while the second one is a pumpkin in the strictest sense, subsp. pepo (compare Paris, 1986).
In 2018, as a side project and out of curiosity, I made quite a few pollinations on the Japanese high-quality moschata 'Kogigu' (K), with pollen from JBL, or TP, or JBL × TP F1. I did not count the pollinations but obtained five mature fruits with some half-filled, viable seeds. The reciprocal crosses with K as the pollenizer did not yield any fruits, even though Wall (1961) stated that crossing is more difficult when moschata is the female parent.

I like good pepo winter squashes because of their starchy flesh, their flavour when roasted, their looks, and their general performance in the field especially in wet and cool years. Including moschata in my pepo breeding project is interesting to me for several reasons. They seem to be somewhat tolerant to the viral diseases relevant here, at least in that they don’t really show symptoms on their fruits. I like the scent and flavour of raw moschata winter squashes. I think eating quality of immature fruits is a valuable trait also in winter squashes and I think moschata can improve pepo cultivars in this respect. In fact, moschata cultivars seem to be the preferred summer squashes in some regions of the world (Decker-Walters & Walters, 2000) and “tromboncino” and “avocado” type squashes seem to have been gaining popularity in the US in recent years. Additionally, moschata is especially heat resistant and moschata fruits often develop a waxy bloom that protects them from the sun and imparts good keeping quality (Andres, 2004). Of course, also carotinoid concentrations and profiles of some moschata cultivars can be interesting, but 'Kogigu' does not have an exceptionally dark or reddish flesh.

Growing the first interspecific hybrid generation

I was able to establish ten healthy plants, two of which I lost later in the season before they had flowers. A few more seeds did germinate, but I discarded all distorted seedlings (there might have been a virus infection). Nine of the ten plants were from the pollinations with the intraspecific pepo hybrid JBL × TP. This might have been because of higher gametic diversity (Wall & York, 1960).
I pseudo-backcrossed the plants to pepo, using pepo as the male parent, in order to overcome possible male sterility or self/sib incompatibility. I used pepo plants (primarily JBL × TP F2) as pollenizers because moschata is already the plasmon donor and I want to prevent the progeny from approaching a moschata species type (after all, my ultimate goal is an improved pepo winter squash).
More than 120 controlled pollinations yielded ten fruits from four plants. For a while, I removed openly pollinated immature fruits to avoid competition for resources. I was especially concerned about possibly better fertilization through the open pollination: I read that zucchini selectively abort fruits on the basis of seed number (Stephenson et al., 1986) … Later in the season, I stopped removing fruits to get more fruits overall (and from more different plants) for phenotypic evaluations (including tastings ;-)). Quite a few fruits from open pollinations did ripen before the first frost.

Traits of the interspecific hybrids

Since one of the parents was an intraspecific hybrid, the first interspecific hybrid generation was quite heterogenous. The following notes are just some general observations.
The leaves of the hybrid plants looked somewhat intermediate between the parental species. Silver leaf mottling started late and was not very pronounced.
The stigmata were quite orange, more resembling moschata (Křístková et al., 2004). The male flowers produced lots of pollen – something that I did not really expect – and were heavily visited by honeybees. I cannot say anything about pollen viability though – I only tried one self-pollination, which failed. One plant made peculiar hermaphroditic flowers instead of male ones with stamina and carpellate structures but not too well-developed ovaries. No fruits formed from these flowers.
I had feared that the fruits of these hybrids might be bitter, because there is a paper describing fruits of a hybrid between non-bitter cultivars of C. mixta/C. argyrosperma (a close C. moschata relative) and C. pepo being bitter (Borchers & Taylor, 1988). However, both immature and mature fruits tasted fine.
Immature fruits reminded me more of moschata summer squashes. Mature fruits had a “melony” moschata scent when raw but a more pepo-like potato and chestnut flavour when baked. The most important thing to me, however, was the obviously high starch content inherited from the pepo parent. The baked fruits had a great dry and non-stringy texture.
The exterior of many fruits looked almost like pure moschata, but some fruits clearly did show characteristics of both parental species (e.g. waxy bloom and a distinct noncontiguous pepo striping pattern).

Outlook

Most harvested seeds were rudimentary or very badly filled. I hope that some of the better developed ones (zero to three per fruit) will be able to germinate and give rise to a BC1 generation. Fruits from open pollinations later in the season did not necessarily have more filled seeds per fruit, but some seeds are really well-filled. I probably will not grow these, however, since many different varieties, including some from a neighbouring field, are possible fathers. In case that I can get some BC1 seedlings established, I am planning to allow them to inter-pollenize freely, if I can find a location isolated enough.



Sources

Andres, T. C. (2004). Diversity in tropical pumpkin (Cucurbita moschata): cultivar origin and history. In A. Lebeda & H. S. Paris (Eds.), Progress in Cucurbit Genetics and Breeding Research. Proceedings of Cucurbitaceae 2004, the 8th EUCARPIA Meeting on Cucurbit Genetics and Breeding. July 12––17, 2004. (pp. 113–118). Retrieved from http://cuke.hort.ncsu.edu/cgc/conferences/cuc2004proceedings.pdf#page=113

Borchers, E. A., & Taylor, R. T. (1988). Inheritance of Fruit Bitterness in a Cross of Cucurbita mixta x C. pepo. HortScience, 23(3), 603–604.

Decker-Walters, D. S., & Walters, T. W. (2000). Squash. In K. F. Kiple & K. Coneč Ornelas (Eds.), The Cambridge World History of Food. Volume One (pp. 335–351). Cambridge, United Kingdom: Cambridge University Press.

Gong, L., Paris, H. S., Nee, M. H., Stift, G., Pachner, M., Vollmann, J., & Lelley, T. (2012). Genetic relationships and evolution in Cucurbita pepo (pumpkin, squash, gourd) as revealed by simple sequence repeat polymorphisms. Theoretical and Applied Genetics, 124(5), 875–891. https://doi.org/10.1007/s00122-011-1752-z

Křístková, E., Křístková, A., & Vinter, V. (2004). Morphological variation of cultivated Cucurbita species. In A. Lebeda & H. S. Paris (Eds.), Progress in Cucurbit Genetics and Breeding Research. Proceedings of Cucurbitaceae 2004, the 8th EUCARPIA Meeting on Cucurbit Genetics and Breeding. July 12–17, 2004. (pp. 119–128). http://cuke.hort.ncsu.edu/cgc/conferences/cuc2004proceedings.pdf#page=119

Paris, H. S. (1986). A proposed subspecific classification of Cucurbita pepo. Phytologia, 61(3), 133–138. https://biodiversitylibrary.org/page/13046661

Stephenson, A. G., Winsor, J. A., & Davis, L. E. (1986). Effects of Pollen Load Size on Fruit Maturation and Sporophyte Quality in Zucchini. In D. L. Mulcahy, G. B. Mulcahy, & E. Ottaviano (Eds.), Biotechnology and Ecology of Pollen (pp. 429–434). https://doi.org/10.1007/978-1-4613-8622-3_69

Wall, J. R. (1961). Recombination in the genus Cucurbita. Genetics, 46(12), 1677–1685. https://www.genetics.org/content/genetics/46/12/1677.full.pdf

Wall, J. R., & York, T. L. (1960). Gametic diversity as an aid to interspecific hybridization in Phaseolus and in Cucurbita. Proceedings of the American Society for Horticultural Science, 75, 419–428.
« Last Edit: 2019-12-24, 08:28:59 AM by Klaus Brugger »

esoteric_agriculture

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Re: Cucurbita moschata × C. pepo
« Reply #1 on: 2020-01-18, 01:35:10 PM »
Fantastic! Thanks for sharing this!
Concise, well written paper.
Lovely squashes.
Very deep mildly acidic clay loam with abundant sandstone and quartzite gravel and stones. Very high water table, Border of Koppen climate Oceanic and Humid Subtropical, USDA Zone 6b, very windy frost pocket valley at the foot of a lonely mountain, historic dairy and orchard county.

Klaus Brugger

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Re: Cucurbita moschata × C. pepo
« Reply #2 on: 2020-01-20, 12:01:37 PM »
Fantastic! Thanks for sharing this!
Concise, well written paper.
Lovely squashes.

Thank you so much, highly appreciated!

Here are two more pictures of fruits cut open.

Ferdzy

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Re: Cucurbita moschata × C. pepo
« Reply #3 on: 2020-01-20, 01:15:07 PM »
Interesting!

I see you commented about a paper stating that their mixta/argyrosperma x pepo was bitter. I grew I plant this summer that turned out to be such a cross, and while it was frankly rather dull it was in no way bitter. I'm sure it can happen, but I think maybe he just got unlucky.

Klaus Brugger

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Re: Cucurbita moschata × C. pepo
« Reply #4 on: 2020-01-25, 09:05:56 AM »
Interesting!

I see you commented about a paper stating that their mixta/argyrosperma x pepo was bitter. I grew I plant this summer that turned out to be such a cross, and while it was frankly rather dull it was in no way bitter. I'm sure it can happen, but I think maybe he just got unlucky.

Thank you, that's some really valuable information!
I guess that means that in at least one of the two species there's more than one genetic configuration conditioning non-bitterness.

Nicollas

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Re: Cucurbita moschata × C. pepo
« Reply #5 on: 2020-03-09, 01:07:08 PM »
That is a good article ! Fantastic work

lieven

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Re: Cucurbita moschata × C. pepo
« Reply #6 on: 2020-04-12, 09:10:59 AM »
I'm looking forward to the next generations. Combining the best of both species!

Klaus Brugger

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Re: Cucurbita moschata × C. pepo
« Reply #7 on: 2020-04-12, 03:28:41 PM »
Thank you, Nicollas and lieven!

Time to give you a short update. Most of the bigger, more promising BC1 seeds were still empty or had only very little embryos: Picture 1 shows a dry embryo, picture 2 shows a hydrated, excised one. I didn't manage to establish plants from embryos of this size, although it might be possible even without hormones.
No seed was perfectly filled, but five did germinate more or less normally (picture 3). None of the plants are what I'd call vigorous (unlike the F1!) and they all seem more or less weird. See for example the one in picture 4 that chose to grow a quite long epicotyl. I've never seen this before in Cucurbita.

Edit: An interesting detail is that all five plants grew from seeds of the same mother.
« Last Edit: 2020-04-12, 03:45:15 PM by Klaus Brugger »

William S.

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Re: Cucurbita moschata × C. pepo
« Reply #8 on: 2020-04-12, 10:14:43 PM »
If I excised the embryo in photo two after first surface sterilizing the seed coat and put it into sterile culture I would expect to successfully grow a fine plant or plants from it. Assuming of course I had time (I don't school/work/parenting) and my plant tissue culture kit up and running and some fresh media ordered and any needed hormones, though I wouldn't be surprised if that just grew. Pretty cool. 
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Lauren

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Re: Cucurbita moschata × C. pepo
« Reply #9 on: 2020-04-13, 09:54:20 AM »
I took four that looked similar to that (seeds about half full), pulled them out of their shells and just tucked them in damp sphegnum moss. Two of the four grew, although the seed leaves looked odd. They didn't like the hydroponics, so they died later.