Each self-incompatible plant has two (different) s alleles. Each pollen carries one s allele.
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I would love to learn more about the inheritance of SI. Does the above (I added highlight) mean that if we cross an SC with an SI, the F1 will be all SI, due to the SI allele being dominant? And then 75% of F2 being SI, 25% of which would have 2 SI alleles?
If not, then what percentages of SI would we be expecting to find in the F1 and F2?
Thanks!
The self-compatibility (SC) of domestic tomatoes arises from (at least) two broken genes. HT-A and HT-B that may work together in some sort of additive (co-dominant) state. The F1 between SC and SI contains working copies of both genes. https://onlinelibrary.wiley.com/doi/10.1111/tpj.12424
The math gets really fuzzy really quickly, cause to return to a functional self-incompatibility (SI) system, we also need to account for petal size, style length, peduncle length, petal color, etc… A plant self-eliminates if it has a working self-incompatibility system and pale, tiny, closed flowers.
I select for huge flower petals, exerted stigmas, bright colors, and bold flower displays, because I can see them with my own eyes. I don’t need a DNA lab. Those traits provide value in both a SC and SI system.
I love this image which shows the fuzzy nature of tomato genetics… Papers by reductionist splitters show this sort of data as only a black/white line with none of the nuances that actually exist.
Source:
Slice of PLOS: 57 Varieties of Tomato
March 29, 2016 Roli Roberts
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Landraces don’t have to be crossed with wild populations. They do that with tomatoes because they are so inbred.
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I am interested to know how people maintain their seed stock of the self-incompatible tomatoes. How many plants of a given species are needed to keep them viable? Do you rely on hand pollination? I currently have one each of a few self-incompatible species growing and I am concerned with being able to maintain my seed supply for tomato crossing projects in coming years.
I appreciate the input!
Self-incompatible tomatoes cannot pollinate themselves. Therefore, if you grow only one plant, then they probably won’t produce seeds. Except that the various species might cross.
That’s the same situation I observe with tomatillos. People whined to me at farmer’s market that their tomatillo grew great, and flowered profusely, but didn’t make fruit. Someone ignorantly sold them only one plant. Whenever I sold tomatillos, I put 2 or three plants in each pot, so that they have a pollinator.
The minimum population size is 2 plants that aren’t closely related… If they are closely related 2 plants might be insufficient.
I aim for around 20-30 plants in my garden, even if they grow together in a clump.
To really maintain the diversity, population sizes of around 70 plants work well.
I rely on insect pollination. The s-alleles that are most rare have the highest chance of successful pollination, so things tend to balance out, and retain rare genetics.
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I got a packet of LA2329 a SI strain of Solanum habrochaites I think now it was perhaps 2020?. I grew out ten plants the first year and one of the 10 produced seed. So in 2021 I grew out both the saved seed and the remainder of the packet to do a proper grow out. I saved some seed again in 2022 from a smaller growout. I also successfully crossed it with some plants from Joseph’s promiscuous project in 2021 and again in 2022. In 2022 I also crossed it with Joseph’s Big Hill HX-9 and with a cross I made with Big Hill. In 2023 If any LA2329 seed got produced I accidentally mixed it with Promiscuous x LA2329 F2 and F3 seed. I have five clumps of seedlings germinated for 2024 I’ll keep the clumps together and make a row of five clumps. I really hope a bit of back crossing may have occurred by this point but I’ve had no proof of it yet. If its happening eventually some phenotypic variation will show up. I might in a few more years do another large growout of the LA2329 to try to refresh the seed again, but I also have a few other Solanum habrochaites accessions and might switch to working with one of them if I get to a point where I think I’ve done what I can with LA2329 which has some documented potential for arthropod resistance which is really interesting. Tomato seed tends to be long lived so it could be set aside for some years and then come back to. I haven’t gotten a very great seed crop from this particular accession yet but was able to grow enough for my own use for some time. If I were a few zones warmer than 6A I think Solanum habrochaites would be quite a bit easier to work with and produce larger seed crops for me. It seems to always be on the verge of a great seed crop if there were just a few more weeks of growing season. I now have so much F2 and F3 seed of hybrids between it and promiscuous that I’m thinking about attempting to direct seed some. Having some crosses made also takes some pressure off of trying to maintain the original accession.
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When crossing Solanum lycopersicum (female) with Solanum habrochaites (male) or another wild type, I would like to know the appearance of the leaves in the F1 generation. Do the leaves of the wild type tend to be dominant or recessive? Is it apparent early after sprouting the F1 generation that a cross has occurred or is it necessary to wait for the F2 generation for traits to show themselves? Thanks ahead of time for those that can share their experiences.
It’s really obvious early on, especially if the domestic mother was potato leaved.
The shape of leaves of hybrids tend to be mid-way between the leaves of the parents. This counts the number of leaflets on pure wild, F1 hybrid, and pure domestic.
Here’s what leaves of pure wild look like.
These are all leaves of interspecies hybrids with wild as pollen donors.
This is an F1 hybrid. The smaller leaf is pure wild.
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Interesting. I made four crosses domestic x habro with four different domestic tomatoes, all with regular leaf. The F1s all strongly resembled the habro father in leaf shape, plant habit and flower structure (all had strongly exserted stigmas) and fruits. The F2 also had the habro leaf shape though other features were more attenuated. For example, only a few of the F2s had exserted stigmas and the fruits were more varied. One even tasted pretty good! This is to be expected I guess in the F2.
I was surprised at the dominance of the habro traits in the F1. Given that Joseph saw more of a blend of traits I’m guessing the results vary depending on the particular strain of habro used. I don’t know where mine originated. They were given to me by a long time seed collector I knew. Where she got them from I don’t know. She has since died so I have no way of finding out.
I made the experiment of direct seeding the large amount of F2 and F3 Promiscuous x LA2329 it was an interesting experiment but a total failure in terms of seed production. I got zero seeds back. Most of the experiment failed to set berries. The few that did set did not have time to ripen. Would have been much more successful in a longer growing season. However, I also suspect potentially some backcrossing to LA2329. The entire growout had wild type leaves and poor pollen production. I also suspect it was largely SI though I also suspect that it may have missed a critical window of bumblebee activity. It may be very important in my climate to have the 50% Solanum habrochaites plants quite a bit older than the domestics. That is started inside earlier. It might be interesting to repeat the experiment with even earlier domestic genetics in the cross- such as say a cross between Solanum habrochaites and Sweet Cherriette a very early domestic at 35 DTM from transplant. Though 50% crosses seem to greatly favor Solanum habrochaites in my experience even in the F2 and subsequent generations, though that could be partly due to back crossing, so who knows?
Any suggestions which wild tomato relatives you all would expect to be promising for the north of The UK? That would be damp, relatively cool summers with not too many sun hours.
I caught the suggestion of Penellii for damp(er) maritime climes. Any others you might suggest?
I’m looking to get a wild population to self select in my garden as a first step to a Scotland proof tomato, and it seems wise to start with those that have the best chance out here.
Many thanks for any suggestions and the interesting discussion!
If breeding tomatoes for damp maritime climates, I’d recommend the two species from the Galapagose Islands:
Solanum galapagense
Solanum cheesmanae (Galapagose tomato)
And also the more widespread sister species:
Solanum pimpinelifolium (current tomato)
These are inbreeding, or facultative out-crossing species, therefore genetic diversity is much lower than in the self-incompatible out-crossing only species like Solanum pennellii or Solanum habrochaites. They bear red or orange fruits, so you don’t have to eliminate the green-fruited traits.
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Thank you!
Oeh, really interesting that they’re wild but inbreeding. I assumed all the wild varieties would be sort-of geared toward self-incompatibility. An Island thing? That’s bound to be a bottleneck. Thank you for pointing that out and saving me a lot of time.
As you may have guessed, I want to spend as little time on the nitty gritty of genetics as possible, and basically grow as much diversity as I can and select candidates by eye and let them breed themselves into local adaptability.
But if they’re not promiscuous, it may need more targeted intervention at stages.
Very helpful, thanks! 
If the only thing you did was incorporate one, or a few of the red-fruited wild species into your population, and then inter-planted with domestics, and then selected for open flower types, you would move the cross-pollination rate from like 3% to around 10%. That would help with local adaptation.
If you started with domestic tomatoes known to have open flowers, and inter-planted with strains of the wild tomatoes known for open flowers, then they would do the crossing themselves. Domestic tomatoes with open flowers include the beefsteaks, and some cherry tomatoes. Potato-leaved are rumored to have more open flowers, but I haven’t grown enough of them myself to verify.
The wild, red-fruited tomatoes bear pea-sized fruits, therefore crossing with domestic tomatoes can yield larger fruits.
Maintaining a rich pollinator habitat really helps with cross pollination, especially if they include buzz-pollinating species.
That sounds very doable in my garden, thank you. The search for open-flowered domestic tomatoes is on!
This summer I took a cutting from the most vigorous volunteer tomato I’ve ever seen. The cutting set new fruit (unripe, but there in all its promising-ness) and is still growing happily despite the cold.
Carol Deppe’s breed your own vegetables kept me up and reading into the small hours. So by spring I’ll be ready to try some plant magic.
Thanks!