Tomato Self Incompatibility and Crossing with Wild Species

I’m going to write a lot. If it’s too much for you to read, my main question is under ‘2’ in this first section.

I’ve been trying to understand something about your SI tomatoes. I understand now (don’t know why it took me so long) that the aim is no merely that they can’t pollinate themselves, but also that they cannot be pollinated by domestic tomatoes. As you’ve said elsewhere about the SI lines you’ve bred, “Pollen flows only from the self-incompatible population to the self-compatible population”.

So I would love some clarity on 2 things, if possible.

1 - I’m guessing that if one adopts an SI system from a wild population into a domesticated line, then if it becomes fully SI, it should still be compatible with the wild species, or at least that wild accession, that the SI system was adopted from. Does that sound right, and is it true in your case?

(Also I’m guessing that, for example, if one adopted a habrochaites SI system into a domestic line, it could still receive pennelli pollen, and if a peruvianum complex SI system, then potentially also could receive pennellii pollen).

2 - For your SI tomatoes, e.g. your ‘elites’, what is the practical result? I understand they can’t pollinate themselves, but what results have you (or others) been getting in practice when you try pollinating them with:
a) pure domestics
b) SC wildings, Q-series, or SC individuals of any of your other lines

That’s what I’ve been wanting to know. Now I’ll just say what has given me renewed appreciation for this project…

I love the idea of increased genetic diversity. And I love the idea of extreme interbreeding for the initiation of a landrace project, so that the population can rapidly adapt to a new region. But in my mind, I had been thinking merely having exserted stigmas would be a good way of having a high enough crossing rate to aim for the traditional landrace method. It seems to me that traditional landraces, of many crops at least, have just enough outcrossing to maintain a relatively consistent phenotype whilst still having the capacity to adapt ‘enough’ over time. This seems appropriate for a population that is already mostly adapted to a specific area, and seems to work great for those traditional landraces, which have been rooted in their land for centuries or millennia.

So it made sense to me that we adapt that method with our ‘modern landrace’ approach by starting with hybrid swarms and a higher rate of crossing, but over time settle into that traditional landrace way with a lower but significant enough rate of crossing, and saving seeds from the group, specifically avoiding the modern ‘purity’ attitude.

But when it comes to tomatoes, I realised that… or at least speculate that, merely having exserted stigma and a diverse starting population, would likely not be sustainable in the medium or long term. (I’m thinking medium term as a few decades.) Because, the higher susceptibility to cross would in most cases mean crossing with neighbours’ tomatoes. And that would mean inheriting all of the problems they have that we have been trying to avoid - inserted stigmas, small flowers, and lack of genetic diversity. So over time ours should also loose their exsertion, and have their genetics diluted back to the general low level of diversity.

This might be saved somewhat by the tendency for modern tomatoes to also include some wild genetics. But our diversity would, in the long run, be limited by the global, or more specifically our neighbourhood’s, level of genetic diversity. As if we brought a fine Scottish single malt whiskey to a barbecue and let it be added to the punch.

And it’s for this reason that I now understand the importance of making an SI population. Not just to make the population diverse and outcrossing, the individuals unable to self pollinate. But rather, to protect them from the world’s domestic tomatoes, by making them unable to be pollinated by them. To me this seems far better than a merely exserted population. It seems like a far far more sustainable way.

One more note…
If this isolation is a key aim, not just the the 100% outcrossing trait, then, I’m thinking that it could also be beneficial to develop multiple domesticated lines using different SI systems. That way, they would not only be isolated from the world’s domestic tomatoes, but also from each other. So gardeners could grow multiple tomato crops, all 100% outcrossing, but still maintain distinct varieties!

For example, imagine if there were 10 varieties using peruvianum’s SI system; 10 using habrochaites; and 10 using chilense. Potentially people could choose 3 varieties from those, one from each group suited to their region and needs, all different, and all easily maintained as distinct lines even if planted right next to each other. And all 100% outcrossers.

And of course no need to stop there. There are at least 7 SI systems available right? And no limit to how many varieties people could develop within each SI system.

Now I know it’s been a huge amount of work just to make your SI tomatoes. But with group effort, I assume that extending this to multiple SI systems is a possibility. Yesterday, I started one attempt - I have a bunch of peruvianum accessions, a few chilense accession, and a couple of corneliomulleri accessions. Here’s the crossing chart I’ve been using for reference, with some of my notes added:

Unfortunately it doesn’t separate the peruvianum complex into the individual species. So, I don’t know which species are compatible. But anyway, I used an SC pennelli accession to pollinate all the peruvianum and chilense accessions I have currently flowering, attempting to using pennellii as a bridge to bring the peruvianum and chilense genetics into a domesticated population. I chose an SC pennellii aiming to avoid clashes with pennellii SI system. [Edit: I just realised chilense is not included in peruvianum complex, I forgot that. But anyway some of my supposed chilense accessions have clearly bent anther cones and I read chilense is meant to always have straight anther cones, so I have doubts that they might in fact be peruvianum, and that’s why I’ve been trying various crosses with them too. I also tried mentor grafts with them but all failed so far).

I’m lacking space so I have only 1 plant of each accession growing inside, though I have 1 or 2 of several accessions also growing outside, which I will try also once they flower. Next time I should try this on a larger scale but who knows, perhaps this will enable me to start… My hope is that I could either make a pennellii-crossed SI population (or maybe 2 if the chilense and peruvianum each make their own), then cross them to domestics and bring over the SI system, or maybe even cross the F1s directly with domestics and work on getting that double-crossed population SI.

Now, not merely for me but for the community in general, I think it could be really good to have some kind of road map for good methods for making an SI population. A method that increases efficiency of time, reducing the years it would take.

For example, I’m thinking perhaps in the F2 generation, we should

  1. cover some flowers of each individual with little bags, vibrate the flowers, and check if any seedy fruits form? Perhaps this is an efficient way of telling if the plant is SC or not? They pass if no fruits or at least no seeds form.

  2. Perhaps a second check could be to emasculate some flowers of each plant and use domestic pollen. To pass this test they need to not give fruit, or at least none with seeds.

  3. Emasculate some flowers and pollinate with pollen from individuals that pass steps 1 and 2. They pass if seeded fruit forms. (Presumably ideally need to use multiple pollen donors for this to be sure? It would… depend on which part of the SI system the SI donor holds, right? So failing from one donor doesn’t necessarily mean failure?)

  4. Use their pollen to pollinate known fertile individuals - I’m guessing that it would not matter whether the females are SI from the same population or even SC domestics. They pass this pollen fertility test if seeded fruits form.

If they pass all 4 tests, they should be fertile SI individuals.

Step 3 is tricky since one has to know which females are SI, and one would not know this yet in the F2 generation. One could do it in the F3 generation once one knows, or over Winter indoors or just later in the season, once one has worked it out. Or, one could do many tests, keep records, and work it out at the end of the season knowing in retrospect, which would mean more work but cutting down the work by months or even a whole year, depending on ones growing system.

I wonder if this seems a wise approach. And I’m also wondering whether it’s best to work on the SI system and the SI peruvianum/chilense X SC pennellii stage; or at the domestic X [SI peruvianum/chilense X SC pennellii] stage.

If the former, it would have to be worked on again at the latter stage. But… there may be some advantage of working at both stages - it would be good to know whether it would be more efficient to work both or just the latter stage. And I’m guessing if only the latter, that would probably work better the larger the population one is working with. Probably hard to know which would be the best approach without trying, but perhaps you guys who have been working on the SI domesticated lines have insight into this.

That would give rise to a similar question for other systems, for example working on it at:
lycopersicum X SI pennellii stage; or at the lycopersicum X [lycopersicum X SI pennellii] (or [lycopersicum X SI pennellii*] X lycopersicum) stage.

And that would be same for other crosses of SC edibles direct with SI wilds, such as lyc with habrochaites. And just to throw in some fun, we don’t even need lycopersicum involved at all if we don’t want to. We could make a domesticated SI line just out of galapagense, cheesmaniae, or pimpinellifolium; with SI pennellii or habrochaites! Or a mix of sweet SI peruvianum with SC pennellii and exserted accessions of galapagense, cheesmaniae and pimpinellifolium could be quite an exciting mix!

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My interest in self-incompatible (SI) tomatoes focuses on genetic diversity, and survival-of-the-fittest selection. They retain high genetic diversity, and the constant shuffling of the genetics allows for rapid adaptation to changing conditions.

My disinterest in self-compatible (SC) tomatoes stems from their habit of relentlessly becoming more inbred, and loosing genetic diversity rapidly. Every time a tomato self-pollinates, it looses half of it’s remaining genetic diversity.

Some pollen grains from the inter-species hybrids can cross back into the wild lines, carrying some domestic traits with them (I think including the recessive self-compatibility allele).

I only grow one accession of pennellii. It barely ekes out a meager existence in my garden, and grudgingly makes seeds. I attribute that to my super arid climate. Pennellii grows in a lomas environment, getting watered by fog. In my super arid climate I have to mist it every day for it to even survive. Descendants of pennellii might thrive in a moist maritime climate.

If I had it to do over again, I would focus on crosses with habrochaites. Lots and lots of crosses. Over the years, I developed a population of purely wild habrochaites that adapted well to my garden. If starting over, I would first develop a high-population locally-adapted wild variety, and then use that to pollinate whatever SC varieties I want to use. I had a lot of problems associated with making crosses with totally non-locally-adapted wild varieties. Then had to re-select for local-adaptation.

I prefer that the ecosystem does all crossing for me, therefore I don’t pay attention to which crosses with what. I select for huge petals, open anther cones, exposed stigmas. Those traits bring a lot of value for all breeding systems: self-compatible, self-incompatible, and mixed. I also select for plants whose first flower cluster doesn’t set much fruit. That might indicate self-incompatibility.

I tend to lump things together, rather than split them apart, therefore, I would say that tomatoes have only one self-incompatibility system. Different species not being able to cross with each other could happen because of traits other than the self-incompatibility trait.

I believe that no SC variety can successfully pollinate any of the wild species with functioning SI. Pollen only flows from SI → SC.

About 35 s-alleles exist. They control self-incompatibility. An SI population requires at least 3. Preferably many more!!!


I’ve been wondering if that would prove to be the case. Plants are rule breakers and any time we make a rule for them they are liable to break it. I really like that about plants!

With this ~14 species complex of tomatoes we have entire species that are SC, species that are SI, and species that have SC, SI, and mixed populations. This is all natural and varies depending on the advantages the different traits confer in different habitats often driven by natural factors such as local geology. As things change over deep time the traits can change too.

Therefore, it seems fine to me that in my garden I have mixed populations of SC and SI species.

I am currently focusing on Solanum habrochaites, Solanum galapagense, Solanum pimpinillifolium, Solanum cheesemanii, and the multi species glop that is modern domestic tomatoes. I may shift my focus to some other species at some point in the future if there is a compelling reason to do so (or I just get bored).

I am very excited to test Joseph’s next iteration of SI tomatoes that he has mentioned. It should be relatively straightforward to do so. They should cross with pure habrochaites both accepting and receiving pollen. They should not accept pollen from pure domestics. Though they should be able to contribute pollen to domestics. Should be a fun addition to the tomato family and great fun to make crosses with. I can see these experiments with it using a few of my isolation gardens soon and likely for a couple years…

I also think that even a pure SC population is capable of being a landrace style population. If the flowers are exserted they cross at a high rate when grown mixed and very closely together. I have seen the condition where crosses are weaker seedlings and the condition where they are stronger seedlings- in the latter they outcompete the mothers. So when planted densely if any of those stronger seedlings exist they will dominate the population. Though one of the advantages SC species have is that if and when they happen on a really great inbred line it can stick around a long time. Some strains of Solanum pimpinillifolium can be real monsters plant size wise and it is a SC species.

F1 Domestic x Solanum habrochaites crosses tend to be weak seedlings at first and then grow into monster plants. It is an interesting dichotomy- but survivor F1’s put out an enormous amount of highly viable seed. The F2 generation can be weirdly infertile.

With my Solanum habrochaites LA2329 population I have grown out the original packet in entirety so am now growing out seedlings that survived one or multiple generations in my garden already. Backcrossing into them is also a possibility that may have already occurred in any of the years I’ve grown it, but which I have not detected yet. I am curious to try some deliberate back crosses this year with the F1’s and F2’s. Also, the other way around as an 3/4 Solanum habrochaites might be more amenable to back crossing next year.

One note with this: under some conditions the SI gene should be advantageous. If we have a mixed population (and we really should at this point), whichever system is most advantageous should dominate the population eventually given enough generations of natural selection. Though mixed could be the most advantageous under some conditions- just like it seems to be retained in some wild populations.

One of my isolation gardens this year is producing seed that I hope will come back to this group. It is all from the Lofthouse promiscuous project and it wouldn’t surprise me at all if it is mixed SI and SC. It is a mix of XL, polyamarous direct seeded, R18, and seed from several plants I saved seed from over the three years I’ve been growing the project elites. A good test of it might be to save its seed by type, then grow out a row of each type and see how many of those seem to be hybrids with other types from the project. There should be red ones, gold spotted ones, bicolor ones, and perhaps some wild type-ish ones from R18. I also have two semi separate sections of R18 and a 10 foot section of direct seeded The One. In the past one of the things I have noticed is Bicolors from the project producing red offspring at a high rate after being planted near XL the prior year. It is a very dense very competitive planting and the seedlings seem to be growing well. Larger than the other direct seeded projects- maybe better soil in part, but definitely more diverse genetics within the promiscuous project.

In a separate isolation garden, I have The One from a seed mix that includes plants from multiple different gardens last year. All mixed but if it is a good pollen receiver from domestic we should know within a couple months. Providing it doesn’t freeze in the next couple days- don’t really think it will, but it is pretty cold right now for mid-June. A hybrid with The One might have a real competitive advantage direct seeded as it seems a slow variety.

I also hope I manage to make some deliberate hand crosses with R18 and The One this year. Somehow, I didn’t manage either last year or the year before. Though there is still hope of just finding some obvious 2022 natural crosses with The One. Some of the variation in it I noted last year could be from crosses with the original plant and nearby plants also in the promiscuous project.

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Oh wow I didn’t realise that! Mine are doing fine indoors and with no misting, but with all the plants here, the humidity is probably fairly high. But this is also good news for the UK environment. So that. enthuses me for my pennellii crossing projects!

Yeah, I remember this advice from you and this is what I’m trying to do now with other SI systems. Do you like my idea of using different SI systems for different lines, to keep them separate?

I haven’t grown many habrochaites plants this year, but I do have… hmm, maybe 10~15 or so? And that’s 4 different accessions. So I could try starting a hab cross project too, along those lines. But part of what I’m doing is just trialing these 4 accessions to see which does best here. So maybe I would be best to follow your principle of letting the population adapt to my environment first.

I have one pennellii that I’m not sure if is SC or SI. So I labelled one branch and vibrated the flowers, and will make sure to leave it alone, keep it away from other pollen. Since it’s indoors, if fruit sets then it should be SC.

I suppose I am thinking that if we lump everything together, we get only 1 product. But the world has diverse tomato needs, in terms of climate; taste; size; and a few other things. So I see a potential benefit in developing different lines. For example, cherry tomatoes and large beefsteak tomatoes. Or fresh eating; paste; storage; and self-drying. Or specific lines for Northern areas with low light, cold, and short season, vs. hot bright climates. Or reds, and orange. So I do see benefit of making separate lines. And whilst tomatoes suited to different climates could easily be on the same SI system since they won’t be grown together; if someone wants to grow both cherry tomatoes and large beefsteaks for example, then if they’re 100% outcrossing and grown in the same garden, and compatible with each other, then it won’t be long before they no longer have 2 separate crops, and soon enough (even if it takes several years) they will presumably not only have a mixture, but rather, the population might end up with all medium tomatoes, no?

So do you not see this as a disadvantage of using only 1 SI system? I mean, if the majority of people who want to grow tomatoes, have needs that are not being met by what SI tomatoes are available, then I would assume they will not grow them. But I have the sense that there is potential for this need for somewhat distinct lines to be served by SI tomatoes.

That belief is contradicted by the fact that SC pennellii can pollinate SI peruvianum. Interestingly, SI peruvianum is said to be unable to pollinate SC pennellii. This is a reversal of the common ‘rule’. But yes, I understand that usually, SC species can’t pollinate SI wild species.

But aside from wild species, this does not answer my question about your wild-domestic hybrids. Have you tried manually pollinating them with SC wildings or SC Q-series, for example? If so, what was the result? It would make sense that that would not work, but I was specifically interested in the experiential evidence, if there is any. I would be grateful to hear an answer on that if you’ve tried. Thanks!

I would assume that would be the case with exserted populations of pimp., che. and gal. Though perhaps that same trait is what has been leading Galapagos species to be diluted by pimps and lycs, endangering those species. Well, ‘endangering’ is a subjective term, but anyway, it does perhaps relate to the issue I refer to. And if the resulting hybrids are not exserted, then perhaps that locks them down to the dangers Joseph mentions, directing them to inevitably homologous chromosomes and freezing adaptability.

I will propose an hypothesis. Suppose 2 seedling appear in the population, both equally strong, but one exserted, the other inserted. In later generations, the inserted stays strong, whereas the exserted undergoes further crosses, the offspring of which are less strong. The inserted comes to dominate, and eventually takes over.

Now that would not happen in every case. But I hypothesise that such a mechanism could lead to the eventual dominance of insertion. Which is the danger I have speculated as the one I would like to avoid.

I suggest that insertion may act to enforce that principle.

Out of interest, have you seen that with exserted accessions? Though an affirmative answer will not counter my hypothesis.

Are you saying that some wild populations have multiple SI systems function in the same crossing population?

This highlights my point. Many people like to save seed and would like the same or at least similar tomatoes to what they grew before. With high outcrossing and compatibility, they mix. Do you see what I mean about the potential benefit of having mutually incompatible lines?

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I would suggest that with both exsertion and SI vs. insertion and SC that their evolution and prevalence is environment dependent.

This rule you are trying to create that insertion is a threat to exsertion is probably true in some environments and untrue in others. In environments which enable large population sizes exsertion and SI probably eventually come to dominate. In environments which naturally restrict population size and outcrossing insertion and SC both probably eventually come to dominate. With exceptions in all cases.

So in an environment with a species like pimpinillifolium, domestic, galapagense, and cheesemanii and the only options are exsertion and insertion which trait comes to dominate probably depends on the population size ultimately.

I think the multiple SI systems do have some great potential (if they can be isolated or self isolate) but trying to build such a system is a vast effort. Perhaps not impossible but it has taken Joseph and collaborators many years to get close to developing one such system to close to the point of palatability. Keep in mind that between species crossing may be possible between SI systems from different species and that isolation might be difficult and lead to lots of gloppy results. When we included both Penellii and Habrochaites in Josephs project it may have complicated things a bit. Remember that F2’s with penellii x domestic and habrochaites x domestic and likely other wild x domestic can have considerable reproductive problems.

I think that independent breeders can accomplish great things like what you propose but it requires focus and dogged patience perhaps for decades.


All righty, I apologize that I don’t keep up as well as I could on the tomato thing so this post might be in the wrong thread but, I’m wondering again now about “the one”. Is it an SI or SC? I have three plants growing next to my pimp cross mix that are fruiting right now. Are they mostly likely crossed or selfed?

Is “the one” normally a dwarf? My plants are only about a foot tall with thick stalks, while the pimps next door are closer to four feet.

Two things occur to me:

  1. Don’t SC populations of pimp exist in the native habitats alongside SI species? As for insertion, I don’t know if it’s only exserted pimps that exist alongside SI species, but that would be interesting to know. And, this doesn’t challenge an evolutionary argument. But, I guess it means something.

  2. If the seeds we breed mainly end up with ‘gardeners’, then by your logic that might mean I’m right in assuming they would trend towards insertion over time. And from what I understand, perhaps this is what happened with domestic tomatoes when they spread across the world?

I suppose that still gives the threat that if a population is anything less than 100% SI, the same conditions might trend them towards becoming eventually SC. But I would assume the risk for that may be considerably lower than with insertion, since the merely exserted are directly exposed to dilution from domestic neighbours, whereas the SI population only has internal risks. And whilst that may still pose a minimised risk for small growers saving seed over decades, for seed suppliers the risk should be far less, since they can identify SC plants and not harvest seeds from them, whereas one does not know if the seed saved from an exserted fruit has crossed with an outsider until that seed is grown out. So the suppliers, even if small scale, can far more easily protect the seed supply.

Great, I’m glad to hear that!

Yeah. Well, it seems worth a try!

I feel that Joseph’s experience is very valuable for such continued endeavours. In particular this warming about needing to use multiple SI parents seems quite critical, and I would assume this should make a second attempt considerably faster, potentially anyway, than the first.

Well, it seems SI hab x SI pen can be done. So those seem compatible in one direction. But, so far as I understand the peruvianum and hab SI systems are mutually incompatible in both directions, no? And the peruvianum and pennellii SI systems, so far as I know, are incompatible. Please say so if this is not correct.

I don’t have specific crossing compatibility information about SI accessions of chilense, huaylasense, corneliomulleri, and arcanum, but perhaps there are more mutually incompatible systems available there too?

And even if we had only per. hab. and pen., whilst pen. and hab. would not be 2 way incompatible, if for example there were the following versions in each SI system:

  • Big fruit
  • Small fruit
  • Paste
  • Self-drying

And maybe even strong Sun versions and weak Sun versions, then potentially people in varying climate conditions could choose whichever 2 types they want, in the version for their climate, and make sure they chose one from each SI system, and then grow them in perpetuity! They could even grow a couple of inserted SC lines too, and have 4 lines all protected from each other! And when the climate shifts even more, at least their 2 SI lines will have more chance of continuing to feed them.

I was thinking that mutually incompatible SI populations would protect from this a lot more than exserted lines. Because they ‘self-isolate’. Like growing SI peruvianum in the same garden as SI pennellii or SI habrochaites. I mean, various SI species do grow together in their native habitat also. I guess that’s my main evidence for the success of mutually incompatible SI systems. Unless I’m misunderstanding something here.

I’m assuming that was because both were SI. Perhaps it was a different issue. But this is anyway why I’m attempting to use SC to bridge peruvianum to domestics, and hoping I don’t get that negative effect.

I can’t promise how long I can stay focused. But at least this year I’ve made many cross attempts and many more to come. And I now have 3 sprouted seeds from 2 wilding x arcanum crosses, which are my most exciting fruits harvested so far. Yet to tell if they are genuine crosses and if they will be fertile, but here’s the one that now has poked out of the soil - centre left:

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Oh another question… Joseph’s SI tomatoes, can they receive pollen from SI hab., SI pen., or both? And, any ideas on how easily they can receive pollen from which species?

I choose to avoid technical discussions, and the mind-numbing meticulous record-keeping required. Decades ago, I made the decision to turn away from science and domination, and to live in a more natural cooperative world. If that were the only reason for me to advocate for promiscuous tomatoes, then it would suffice.

I don’t have much else to contribute to this topic, other than restating my belief that self-incompatibility and species-incompatibility represent different things, and that fuzzy biology doesn’t work at all like the either/or mentality that the splitters want to try to impose on living systems. Any domination that the scientists try to impose eventually gets undone by nature.

We grow many phenotypes of out-crossing corn, for hundreds of climates, and maintain thousands of varieties for all sorts of uses. We could do the same with promiscuous tomatoes.


The definition of “dwarf” seems arbitrary and capricious to me. However with that said, the domestic ancestor of the Promiscuous Tomatoes project matured with vines about 18" long. Some of the interspecies crosses had vines 30 feet long…

I suspect it is SC. It is a dwarf but perhaps not a rugose dwarf in the dwarf tomato project sense. It picked up its dwarfism through weird interspecies hybridization side effects. It is possible that Payette did the same thing long ago. It might do some crossing if it throws open flowers. The original had very open flowers, but most of the off-spring did not last year. Wouldn’t be surprised if it crossed a bit year before last.

Some accessions of Solanum peruvianum can be pollinated by some accessions of Solanum penellii and possibly by some accessions of Solanum habrochaites.

Self incompatibility and species incompatibility may very well represent different things. I was attempting to cross some domestic x Solanum habrochaites hybrids the “wrong” way the other day and I was struck by the length of their styles. Long style might be too long for the pollen tube of the domestic x pimpinillifolium hybrid I was trying to pollinate it with. That would be a case of physical species incompatibility if it proved true which it may not be. Though it could be a bit mixed in terms of complexity as to how it really works and I haven’t done enough research lately to wrap my head around it.

I suspect that the fourteen species tomato complex acts a bit like a super species at least occasionally leaking genes between species. Probably very advantageous over deep time. Also may prop up the slower evolving SC species over deep time when fancy new genes from the fast evolving SI species sometimes leak into them. Not many crops have such a deep bench of genetics.

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I really like the deep-time concept in regards to tomatoes and super-species. Thanks William for stating it so succinctly. We have another opportunity for that within the cucurbits.

Self-compatible populations tend to form at the extreme edge of a species range. Solanum lycopersicon traveled to the extreme edge of it’s range like 3 times. Once going to Mexico, then going to Europe, then through the heirloom preservation mentality.

It comes back to the fuzzy biology thing… For example in sunroots, the self-incompatibility trait works only 99.5 percent of the time. (I didn’t check the exact number, but it’s in that vicinity). Therefore, the goldfinches could expect to find about one seed in every 40 flowers. My field produces around 13,000 sunroot flowers per year. That creates lots of opportunities for new varieties to express themselves, even in one self-incompatible clone.

Yes, I really like that way of looking at it. I’m starting to think that, with wild plants, we frequently call things “species” that we would actually call “varieties of the same species” in a domesticated plant population.

If they can all cross, and especially if they can cross easily, it’s probably helpful to think of them more like phenotypically distinct varieties than like separate species.

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You’re probably more of an expert than I am, but I wonder . . . what if most crops do have such a deep bench of genetics? I keep finding more examples of crops that have numerous wild relatives they are sometimes compatible with. And an awful lot of our favorite crops (for instance, wheat and strawberries and sweet potatoes) began as interspecies hybrids.

As for the long style, Joseph mentioned somewhere that a broken-off style is still susceptible to pollination at the spot where it broke. If you want to make a cross in that direction (for instance, to preserve the wild cytoplasm), maybe you could try cutting off half the style and then pollinating it?


I have seen electron-microscope photos of pollen growing in styles.

In the self-incompatible tomatoes, the pollen grows for some distance before the plant’s natural defenses against self-pollination stop the growth. Shorter styles may not stop the growth before it reaches the ovary.

Alan Kapular taught me about the importance of growing lots of species close together, for the sake of encouraging the occasional inter-species hybrid. If I see a weed in my garden, and it’s the only one like it, I allow it to continue growing, for the sake of deep genetic diversity.

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Ah that’s a good point! Do regular gardeners have an easy time saving seed from multiple heirloom varieties keeping them differentiated? If so, perhaps my worries are exaggerated.

Indeed. There’s a paper on using SC pennellii LA0716 to cross with peruvianum, and by coincidence before I read that paper I tried using the same accession to cross with a bunch of peruvianum accessions. I’ll update here on the forum if I get successful crosses.

Yes I read that … if I remember correctly, there are SC and SI populations within the same species that are mutually incompatible. Makes you wonder about the definition of ‘species’! It is after all a conceptual box, made up by speculating creatures.

I read about SI mechanisms starting up at a specific stage in flower development, such that it may be possible to cross domestic to some SI species/accessions if you use a flower 5 days before it opens. I tried this only a couple of times, one I saw didn’t work, the other must be hidden somewhere in my ‘jungle’ and I guess I’ll see sooner or later if it worked. Tough to work on such small flowers, I should make more attempts I guess but so many things to do!

Also here’s a chart on how far pollen went in various tests:


Due to that consideration I tried many arcanum and peruvianum cross attempts with some pimps, in case the shorter style helped. Just 1 peruvianum accession, and 2 (and likely their cross) accessions of arcanum. Yet to harvest their fruits though turns out crossing those with domestics and wildings gave fruit in several cases, but… is it the endosperm… not formed in almost all cases. So for those it might not be a style length issue to worry about. I’m now growing 3 wilding x arcanum seedlings (or what is hopefully so anyway), from 2 different wildings. I didn’t do tests myself, I used mix pollen for selfing, but it seems those arcanum are said to be (likely?) facultative-SC.

I also just opened up attempts to cross those with Columbianum, which is a Columbian landrace of domestics supposed to be able to act as a bridge to peruvianum. No proper seeds for the peruvianum attempt, but full load of seeds for the arcanum. Which actually makes me suspicious… but I guess I’ll see when I grow some out.

It may also be possible to cut styles to make them shorter. Some suggest using walnut oil on the end of the cut to hold the pollen - water would make the pollen explode. I don’t know if anyone has actually had success with this method though. I made a few attempts - there are so many hanging labels in my ‘jungle’ that I only check them when fruits are ready to harvest, so no update yet as to whether any of those worked. Though I did not make enough attempts to give the method a fair trial.

I like dendrograms. Kind of allows us to see where on that lake shore of Joseph’s each accession lies (well, to some degree anyway) rather than just having the species concepts as boxes not knowing the relations between them:

These charts are interesting too, using mitochondrial DNA and other methods to see relationships and evolutionary trajectory:


Here’s a chart that I made for my own reference, which makes it easy to see which species are more closely related, and whether they’re SI or SC:

In case anyone’s interested, here’s that book’s take on the species concept:


Our goal for this study is to apply a phylogenetic concept and classification to sections and series, and a more pragmatic, practical concept incorporating a wide variety of data to species. Sections Lycopersicon, Lycopersicoides, and Juglandifolia, and the informal “Lycopersicon” species group are unambiguously monophyletic. The “Arcanum,” “Eriopersicon,” and “Neolycopersicon” species groups may be monophyletic, but there are ambiguities in various data sets regarding these.

Ultimately, all large-scale monographs rely on morphological characters to provide identifications for the many specimens needing determinations, but species concepts may also be influenced by molecular, ecological, and crossing relationships, despite inherent potential conflicts between biological and phylogenetic concepts. Our decisions relied on clear morphological discontinuities to define the easily distinguished species S. habrochaites, S. lycopersicoides, S. pennellii, and S. sitiens. The following closely related species are generally easy to distinguish but sometimes intergrade: 1) S. lycopersicum, S. pimpinellifolium, 2) S. cheesmaniae, S. galapagense (sometimes also with introduced S. pimpinellifolium), 3) S. arcanum, S. chmielewskii, S. neorickii, 4) S. corneliomulleri, S. peruvianum, 5) S. chilense, S. huaylasense. Specific characters used for recognition are detailed with each species description and in the keys. Potential reasons for variability and intergradation are recent divergence and hybridization. Despite the variability in tomato species, our decision to recognize the four segregants of S. peruvianum s.l. (Peralta et al. 2005) is based on a pragmatic combination of phylogeny and morphology, and reflects evolving, recognizable entities within the complex.

We do not recognize taxa below the species level, most notably the small-fruited tomatoes known to many as “var. cerasiforme.” The name “cerasiforme” has been used to refer to putatively wild forms of S. lycopersicum that have been regarded as progenitors of the cultivated tomato (although see Frary et al., 2000, and Nesbitt & Tanksley, 2002). It is impossible to distinguish wild from cultivated forms using herbarium specimens, and we regard many specimens labeled as “var. cerasiforme” to be possible revertants from cultivation (i.e., feral plants) or possible hybrids of wild and weedy taxa. Many cultivar names have been proposed (often not validly published, see Appendix 1) as formal taxa following the principles laid out in the International Code of Botanical Nomenclature (McNeill et al. 2006, and earlier editions), but cultivars would be more usefully named using The International Code of Nomenclature of Cultivated Plants (Spooner et al. 2003; Brickell et al. 2004). In addition to species groups, we distinguish four weakly defined morphotypes within S. arcanum that show discrete geographic ranges but exhibit so much overlap of character states, especially in leaf morphology, that consistent assignment of any given specimen to a morphotype can be difficult in the absence of geographical data.

And here’s a section some of you might find useful:


Mating systems in wild tomato species vary from allogamous self-incompatible to facultative allogamous and self-compatible, to autogamous and self-compatible (Rick 1963, 1979, 1982b, 1986b; Table 1). The self-incompatibility system in tomatoes is gametophytic and controlled by a single, multiallelic S locus (Tanksley & Loaiza-Figueroa 1985).

The self-incompatibility system has shown strong relationships with the degree of outcrossing, allelic diversity, floral display, and degree of stigma exsertion in wild tomatoes. Rick (1982b) investigated the genetic bases of self-compatibility, self-incompatibility, and flower characters by studying interspecific hybrids between the self-compatible (SC) S. pimpinellifolium, used as recurrent parent, and the two self-incompatible (SI) species S. habrochaites and S. pennellii. He postulated that three independent genetic phases, most probably regulated by different unlinked genes or gene complexes, are essential for successful functioning of the self-incompatibility system. These genes are operating on: 1) prevention of self-fertilization, 2) changes in the flower structures to ensure cross-pollination, and 3) development of secondary flower characters to attract pollinators. He concluded that the evolution of the mating system in wild tomatoes proceeded from self-incompatibility, as the ancestral condition, to self-compatibility, and probably never reversed to self-incompatibility. Changes from self-incompatibility to selfcompatibility are expected to arise frequently and independently (Rick 1982b). This trend has been found in S. habrochaites and S. pennellii; both species have self-incompatible and self-compatible populations. The self-incompatible populations occupy the center of their species geographic distributions, and have higher genetic variation, larger flower parts, and exserted stigmas. Self-compatible populations occur toward the northern and southern edges of the ranges of S. habrochaites and S. pennellii, have less genetic variation, smaller flower parts, and little or no stigma exsertion (Rick et al. 1979; Rick & Tanksley 1981). The change from self-incompatibly to self-compatibility has been reported in only one population of S. peruvianum (Rick 1986b).

In the self-compatible species, the extent of outcrossing and genetic variation is also related to floral display and degree of stigma exsertion. Within S. pimpinellifolium, the most northern and southern populations at the margins of the species range are highly autogamous with little or no genetic variation, have small flower parts, and little or no stigma exsertion, while the centrally located facultative allogamous populations have higher genetic variation, larger corollas, and marked stigma exsertion (Rick et al. 1977). A comparison of different genotypes of S. pimpinellifolium in experimental plots in Peru showed that different outcrossing rates could be largely attributed to differences in floral characters, especially the level of stigma exsertion, rather than to differences in numbers and types of pollinators (Rick et al. 1978). Smaller flower size in selfing forms of S. pimpinellifolium is due largely to variations in the growing time of individual flowers, with the larger outcrossed flowers growing (i.e., remaining open) for longer time periods than the smaller, selfing flowers (Georgiady & Lord 2002). Four QTLs (total anther length, anther sterile apical appendage length, style length, and flowers per inflorescence) cause major phenotypic variance (Georgiady et al. 2002). Early floral stages showed no significant differences; thus, the difference in size in these flower size transitions can be attributed to a simple heterochronic change in growth (Georgiady & Lord 2002). Chen and Tanksley (2004) have suggested that a locus on chromosome 2 is largely responsible for stigma length, and that the tightly linked genes in this compound locus represent a co-adapted gene complex controlling mating behavior.

Two self-compatible sister species, S. chmielewskii and S. neorickii, illustrate another example of changes in flower characters associated with outcrossing and genetic variation. Solanum neorickii is exclusively autogamous, has low intra-populational genetic variation, small flowers, and stigmas included in the anther tube. In contrast, the facultative allogamous S. chmielewskii exhibits higher levels of heterozygosity, larger flower parts, and exserted stigmas. Rick et al. (1976) postulated that S. neorickii evolved from S. chmielewskii. All populations of S. chilense are self-incompatible. The species in the outgroup sections S. lycopersicoides, S. sitiens, S. ochranthum, and S. juglandifolium, are exclusively self-incompatible.

The Endosperm Balance Number (EBN) crossability phenomenon was analyzed for Rick’s two wild tomato complexes by Ehlenfeldt and Hanneman (1992). The EBN hypothesis (Johnston et al. 1980; Ortiz & Ehlenfeldt 1992; Hanneman 1994) postulates that in the absence of stylar barriers, the success or failure of a cross is determined primarily by a 2:1 maternal to paternal balance in the endosperm, independent of ploidy. The EBN data supported the hypothesis of two intra-fertile groups as proposed by Rick (1979). Rick’s “Esculentum” complex showed uniformity of EBN values, which can be compared to the 2×(1EBN) species in potato. On the other hand, the “Peruvianum” complex showed variable values for EBN, but most comparable to 2×(2EBN) potato species (Ehlenfeldt & Hanneman 1992). These authors hypothesized that Rick’s “Esculentum” and “Peruvianum” complexes are separated by a system analogous to the 2×(1EBN) S. commersonii Dunal and 2×(2EBN) S. chacoense Bitter crossability groups. This putative isolating mechanism may restrict or suppress gene flow among sympatric populations (Ehlenfeldt & Hanneman 1992), and may play a role in the reproductive isolation in tomatoes, such as the S. arcanum assemblages in northern Peru.

In the paper I mentioned above, ‘TESTING THE SI × SC RULE- POLLEN–PISTIL INTERACTIONS…’, there are some nice images. For example:


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Quick question - for the SI alleles, bearing in mind we need a sufficient number of them… since tomatoes are diploid, would one parent have 2 of those alleles potentially different from each other? If so, presumably the cross would only get 1, but if there are multiple seeds in the crossed fruit, we might get both among them?