It seems like a lot of phenomena are getting lumped in the definition of outbreeding depression: traits that we don’t want getting in our populations in general, genetic incompatibilities, or niche incompatibilities. The way I’ve understood outbreeding depression is when you simply get weaker or lethal seedlings instead of more vigorous ones, such as the study Malcolm posted in beans. Another term I’ve seen for this is hybrid dysgenesis. In these cases there are severe genetic incompatibilities which is self limiting to the spread of these combinations, although there are tricks like grafting the lethal or dysgenic seedlings can save them sometimes. But when you look at the definition of outbreeding depression, by many accounts its unhelpfully broad, such as “crossing plants separated by millions of years of evolution or from different climates.” I think a results oriented definition is more helpful, such as weak genetically incompatible hybrids. If you combine this with traits that we as breeders simply do not want, or say with an altered flower structure that a pollinator from a species original climate will not fit in, that strikes me as a unnecessarily broad which if nothing else is confusing for newbies. For example low fruit set in a hybrid is considered outbreeding depression in the broad sense, but for me I wouldn’t use the term when the seedlings are strong and often more adapted than at least one of the parents.
Let’s say I have a set of F1s of two species and 20% are are chlorotic and look weak. Is that outbreeding depression, of course. But the other 80% are super strong and just as outbred. In this case the result is just a bad combination rather than the outbreeding per se.
This is why it is difficult to determine what is or is not actual outbreeding depression.
Is the pollination problem do to flower structure, or is it that the plant is genetically sterile?
I would say it is likely outbreeding depression and just the luck of the genetic recombination for which plants ended up with the favorable vs the unfavorable gene combinations.
Yes, I think that strictly speaking outbreeding depression should refer to genetic incompatibilities, rather than unwanted traits or a loss of local fitness.
While some cases can be obvious (seedlings that fail to thrive, infertility) I’m more interested in milder cases that might not be noticed; presumably, there is a gradual curve from “totally normal” to “inviable”. The more distant the populations involved, genetically speaking, the more likelihood of problems. A lot of our landracing projects might fall into a sort of middle zone; for instance, crossing red and green cabbages, or cabbages and broccoli. These are not interspecific hybrids, but presumably their last common ancestor could be hundreds of years in the past, since breeders would likely use varieties within the respective pools as starting points for new breeding work. Or crosses between the different pools of C. pepo squash, or the bean example above. Mid-range problems could hamper a landrace project more than the extreme problems, which would be quickly weeded out.
I wonder if over time, such things would work themselves out of a landrace population? Would it involve one of the genotypes going extinct? Or would the two populations (each of which could contain several original variants) continue to exist side by side, each year generating a certain percentage of hybrids that are culled due to their lower vigor?
In the case of the beans above, the problem is related to different phaseolin types; would a landrace eventually become homogenous for one type?
@MarkReed Very interesting about the sweet potato “vampires”! That does sound like an inbreeding depression sort of problem. I wonder if different compatibility classes have something to do with the fact that different sweet potato varieties vary so much in their ability to produce true seeds? Maybe some seemingly infertile varieties simply belong to a rarer class, and there are no compatible pollen sources nearby? Or maybe such plants represent a cross between classes; able to grow, but a genetic dead end, rather like the runner bean/common bean crosses you mentioned above? “Irish” potatoes also have a complicated genetic ancestry, and can have similar issues with true seed production; I wonder.
I think they do work themselves out and both can happen in nature. Sometimes both species continue along with a new subspecies. Other times one or both parent populations become extinct.
It’s a little easier to see within animal populations. The hyper-evolution of the Galapagos and other island environments contain many examples.
In North America we have modern crossed populations between wolves and coyotes known as “coy-wolves”.
Even in humans we mixed with Neanderthals, they went ‘extinct’ but we still carry many genes from them and other archaic humans.
The favorable vs unfavorable genetic combinations is the point right ? Hybridizers talk about “combinability” often, which is selecting the ones that have great combinations in the early generations to use as parents. You frequently get the whole range in a population of hybrids from great vigorous combinations, mediocre, to weak lethal ones. I just wanted people to get that you can and many times do get both hybrid vigor and hybrid dysgenesis in the same sibling group of seedlings, with the same parents. Of course its a double edged sword. It’s not the genetic distance that’s always the problem per se, it just depends. The same way in nature you can get hybrids that can take over a niche, or ones that select themselves out.
The difference is that in nature, the very distant crosses will not usually survive due to fertility issues.
But for us, in a situation like the flower structure not being suitable for pollinators, we can hand pollinate and back-breed or cross-breed and select for improved flower structure to the point that the plant is again self-sustainable.
You can actually get hybrid vigor and outbreeding depression in the same individual offspring.
In any offspring there will be a range of vigor and traits but the more genetic distance between the parents the higher the % of offspring likely to have a more negative genetic interaction.
@Ascentropic Good point about the range in hybrid populations, particularly in later generations as things shake out.
@JinTX Yes, I suppose speciation is how this is solved in the wild.
I guess what I’m concerned about is the following: let’s say that in a landrace population, there are two populations with two genes, call them A and B. Both are “good” genes in their own way. But a plant that has them both will be yield 50% less than either parent.
I imagine that, on relevant timescales (a few decades) this landrace would simply produce 50% defective plants in each generation. (And let’s imagine that this defectiveness is something that only shows up later, so that it isn’t just a matter of overplanting and culling seedlings.) The AAs and the BBs would be fine; each set, of course, would have many other variations, as other genes combined in different ways. But any crosses between the pools (which on average would be about 50% of the seeds each year) would be defective. Or am I not thinking this through correctly?
If I am correct, would it be worthwhile to source the starting material for landraces from just one major pool; for instance, only including Mesoamerican beans, or only including one of the two C. pepo pools, or whatever? As opposed to my understanding so far, which is that including as widely diverse of a selection as possible would produce the greatest adaption over time.
Yes I think in that situation one or both of those genes would become less common over time with enough genetic diversity.
The exception would be if both individual genes provided a strong advantage when separated. Think of it as a more complex version of the gene that causes sickle cell anemia in humans being beneficial (preventing malaria) with only one copy.
I asked chatGPT if there is a difference between hybrid dysgenesis and outbreeding depression, and according to it they are similar, but it gave me some ways to distinguish them in practice. I will post it in case anyone is interested. Caveats of course being an LLM, check the results.
"Timing and Generational Effects:
Hybrid Dysgenesis: Usually manifests strongly in the first generation of hybrids, with rapid sterility or abnormalities linked to transposon activity.
Outbreeding Depression: Often takes multiple generations to fully manifest, as recombination breaks down the advantageous combinations of genes that had evolved in the parental populations. If negative effects increase over time, it suggests outbreeding depression.
Experimental Crosses and Controls:
Hybrid Dysgenesis: Controlled crosses between populations that differ in transposon types (e.g., crosses where only one parent carries active transposons) can induce dysgenesis. If crosses between two strains lead to severe effects that do not occur in reciprocal or intra-population crosses, it points to hybrid dysgenesis.
Outbreeding Depression: If similar crosses show a gradual decline in fitness or are environment-specific (i.e., hybrids perform poorly in certain environments compared to parents), it suggests outbreeding depression rather than dysgenesis.
…Hybrid dysgenesis is more chaotic, transposon-driven, and germline-focused, while outbreeding depression unfolds over time through the disruption of coadapted gene complexes or chromosomal mismatches."
This suggests that sometimes what people are calling outbreeding depression is actually hybrid dysgenesis.
Is hybrid dysgenesis not a specific form of outbreeding depression?
It sounds like maybe the difference between the two, according to these definitions, is that hybrid dysgenesis means fatal or steril in the first generation. And outbreeding depression means there are able to be future generations.
Is this assuming we are crossing F1’s with F1’s and not accounting for back-breeding or crossing with another line?
One of the papers I looked at seemed to be referring to what chat GPT is calling outbreeding depression; it said that in some cases the F1 is more vigorous and productive than either parent (because it has a full complement of genes from both) but that in subsequent generations the offspring can become worse than either parent, because recombination fragments these combinations.
I’m guessing that what chat GPT is calling Hybrid Dysgenesis would be more likely in wide-crosses between species, and what it is calling outbreeding depression would be more likely between varieties in the same species, though I’m just guessing.
That sounds like what happened with the runner x common beans. They certainly were vigorous growers, going to the top a twelve-foot trellis, flopping down and growing back up it again, multiple times. But that giant mass produced only those few seeds.
Can outbreeding depression affect grafted plants? I grafted two sweet potatoes together using the mentor graft. Both of the sweet potatoes were the same species, but they were from different populations. The root was from a Korean population, and the graft was from an American population. I attempted this graft twice, and both times it resulted in very stunted plant growth. Is it possible these sweet potatoes are from populations that are so distant that the graft resulted in outbreeding depression? If this is not an example of outbreeding depression, then does anyone know what is causing the stunted growth? Thank you for your help.
@MarkReed Do you no longer want your hybrid runner beans? If so, how would you feel about sending me a few seeds? They sound like they would be great for biomass production. I understand if you are unable or don’t desire to send me seeds. Thank you…
It would not be outbreeding depression because the plants are grafted and not the result of breeding.
It could be some effect of incompatibility relating to the grafting of the two varieties. And if so, there is probably a correct term for it but I do not know what that would be.
Hopefully someone with a lot of knowledge on grafting will be able to help.
There was never more than a few of those runner x common beans and I don’t think I still have them.
I don’t know much about grafting. The only time I ever tried it was with the sweet potatoes and wild sweet potatoes and was surprised that it worked. Not so much that it could be done with those two species but that I was able to do it.
Thank you for your reply about the runner x common beans. It was very nice of you to consider me.
I know you have been doing work with sweet potatoes, but have you done any work with the different leaf colors? This website shows all sorts of different sweet potato leaf colors including some variegated types. I personally am very interested in the potential leaf colors that might be discovered. Leaf shape is also quite interesting.
I suppose @Ascentropic might know the answer to my grafting problem. He knows a lot about grafting. His Substack is what inspired me to try grafting in the first place.
Those vampires sound fascinating. Have you ever gotten one to make seeds? Not being able to make chlorophyll is obviously not a desirable trait, but if they can survive despite that . . . I dunno, that’s just interesting!
Since sweet potato leaves are edible, their leaves might have a different flavor than usual. I know people sometimes blanch asparagus and sea kale shoots because they like the flavor better. Would chlorophyll-less sweet potato leaves have a blanched flavor?
I also wonder if there could be an advantage to a plant that can only be grown in a dim space. Maybe as a novelty houseplant? If they look cool? Especially if they can manage to flower, since sweet potato flowers are pretty?
Obviously growing chlorophyll-less vampires that grow extremely slowly and fry upon contact with direct sunlight is not the goal, but . . . (laugh). It’s fun to try to imagine valuable uses for weird things that sound useless at first.
They are a curiosity but no, I have never bothered to pamper one in hopes of getting seeds. I kept one on the bottom shelf of my outside work bench one time, and it bloomed a little, but I doubt it actually set any seeds, I didn’t pay it a lot of attention except to throw a little water on it now and then. I’d guess it grew at about 1/10 the rate of a normal plant, if that.
Your novelty houseplant idea is interesting. If another one shows up, I may investigate that a bit. I have terrible problems with spider mites on sweet potatoes as house plants, along with low temps, low humidity and low light, only more robust plants survive. The vampires are anything but robust, but I guess they wouldn’t mind the low light.
There are about a hundred other things that I have observed once or twice with sweet potatoes but to know for sure about any one of them would be a major undertaking.