When two closely related species mate and form so-called hybrids, over time, why doesn't this lead to the disappearance of the two parent species? Princeton's Molly Schumer has been comparing the genetic profiles of two closely related fish. What she's finding is that some combinations of genes contributed by the two parent species don't work well together, owing to the presence of some species-specific adaptations. And although individually, their contributions are quite small, at the level of an entire genome, they exert a powerful selective force, as she explains to Chris Smith...
Molly - In the past 10 years, we've begun to realise that maybe in-between different species or hybridisation is really common. That raises a little bit of a problem in understanding how species or different groups of organisms stay distinct in the face of this genetic flow between them. We study two species of freshwater fish, Xiphophorus birchmanni and malinche that hybridise quite often. But we don't see a lot of evidence that these species are really breaking down. And so, we wanted to understand what genetic barriers might be existing between them.
Chris - Is this because Molly, when you get a hybrid that the hybrid itself then cannot continue to interbreed with members of the individual species and that stops the further evolution of that hybrid?
Molly - So, in a lot of species, hybrids are inviable, are infertile, but that isn't really the case in these species which is part of the reason they're very interesting.
Chris - So, perhaps the hybrids are more vulnerable to being caught by a certain predator and this keeps their numbers down.
Molly - Or they're less effective at foraging or finding mates, or any of these sort of things that are important in the wild, but not apparent to us in the lab.
Chris - So, how did you explore this genetically to try to find out what was going on?
Molly - Basically, the idea behind what might be breaking down in hybrids is that you have these two genomes in the parental species that are evolving independently. They accumulate mutations and then when you reintroduce these genomes to each other in hybrids, they can interact poorly. And to give sort of a specific example of what that would look like if you have two proteins that work together in the same pathway and in the two species, they've accumulated difference and they can't work together anymore. Any hybrid that has the combination of those two proteins will be selected out of the population. So then when you go and sample hybrids from the population you'll see an absence of that combination, just sort of the footprint of selection on that population. And so, that's basically what we did is, we collected hundreds of hybrids from two populations and looked for that signature of the missing genotypes.
Chris - What did you find? Were there areas of the DNA which clearly, if mix them together in a hybrid were a toxic combination? They just didn't crop up when you went looking for them.
Molly - Yeah. So surprisingly, we found 207 pairs of regions that were significantly missing in both hybrid populations. This was a lot more than we expected. These two species have some clear differences in terms of their ecology. For example, malinche has adapted very cold environments and birchmanni is adapted to warm environments. So, we expected that we might see a few regions associated with ecological differences, but we did not expect to find so many.
Chris - What does this tell us more generally about how hybrids are or aren't maintained in populations?
Molly - So, I think the bottom line here is that when you look at hybrids in the lab, you're missing a lot of the selection that's going on. And one of the things that I didn't mention earlier is that even though we find so many pairs, when we estimate selection on these interactions, the selection is actually quite weak. So, around 2% or 3% disadvantage for that hybrid genotype. What that means is that in this particular case, many selection effects that are weak are contributing to reproductive isolation between the two species as opposed to the more classic cases where you have strong selection effects on just a few sites.
Chris - Because you've said there are lots of little contributions and so, a big number times a small number still makes a fairly big number which is why you see this effect, do you think that the same sorts of genes are going to crop up in different species though or do you think that the kinds of genes that exert this fairly toxic effect, they're going to end up being species-specific?
Molly - We actually did not see a strong pattern in terms of the kinds of genes that were involved. So, there are sorts of different analysis that you can do to see if there's over representation of certain kinds of genes. We did not actually see that any particular category of genes was over represented which suggests this is kind of a broad effect that many genes are involved in contributing to isolation in hybrids. I think that'll be the case in other species as well. Though in some species, they have found that genes on the X chromosome for example are significantly more important in contributing to reproductive isolation, which is not something that we found...