Blue whales and fin whales have been 'hybridising'

So, here’s one for you. What does Ancient Roman civilisation and a blue whale have in common? Both are remarkably large? Both have seen a catastrophic decline in numbers? Yes to both of those. But, now we can also say that both have a much higher genetic diversity coming in from elsewhere than we previously realised. You see where I’m going with this? A study just out in Conservation Genomics has shown that the north atlantic blue whale, the largest organism on the planet, is getting up to some interesting activities with its smaller counterpart the fin whale, which leads us to ask some questions about what these two species are getting up to in the depths, if you catch my drift. Mark Engstrom is an author on the paper...

Mark - Hybridisation is, in the context that we're discussing it here, integrating between two distinct species.

Will - How common is that in the wild?

Mark - Hybridisation? It depends on which group you're talking about. It's fairly uncommon in mammals, but it certainly occurs on occasion. And it depends on the species, how closely related they are and probably their history of previous contact.

Will - Not everything can hybridise. There's got to be some form of common ground for them to have a successful attempt.

Mark - Yes, you know it's interesting because it used to be that the old definition of a species was two groups of organisms that had their own evolutionary history and could not interbreed. But however, when you look at interbreeding, it's an acquired characteristic. It's something that occurs during evolution to prevent mating between groups that are incompatible already, genetically incompatible. And so it's something that's highly selected against. In groups that haven't been in contact for which it's not so deleterious, it may never evolve. So in the case of the blue whale, one of the things that's really interesting is that blue whales and fin whales are not each other's nearest relatives. The nearest relative of a blue is probably a Sei whale or a Rice's whale, but they do not cross breed with those as far as we know.

Will - And how much of the fin whale's DNA could you detect in the blue whales?

Mark - Well, that's interesting because fin whales and blue whales are again, not that closely related in the sense that they've been separated. The ancestors have been separated for at least probably 8 million years. But we're detecting about a leakage of DNA from fin whale to blue whale of about 3-3.5%. So about 3% of the North Atlantic blue whale's genome is derived from fin whales on average.

Will - When I spoke to an expert in Neanderthal DNA a few episodes ago, he said we've got shared Neanderthal DNA and we may well have got that because we bred with them. Is this a similar situation?

Mark - It's very similar. In fact if you look at the Neanderthals, the percentage of DNA in humans north of Africa is about three to 3.5%. It's very similar to the situation we're seeing between blue and fin whales.

Will - Like with Neandertals, the amount that each one of us alive has today, in terms of Neanderthal DNA, varies. Is it the same with blue and fin whales? Is there sort of a variance across the board?

Mark - There is. So we had some individuals in our historic samples of blue whales. We looked at not only modern blue and fin whales, but we went back to museum specimens and looked at some blue whales from over a hundred years ago. We tried to map them through the process of whaling, which occurred from about 1900 to about 1970 was when the blue whale populations were decimated. We wanted to get a snapshot of their genetic variation prior to whaling, during whaling, and now after whaling. And we had two individuals that did not have any fin whale DNA from our historic samples. And all of our modern samples had at least some, but in our historic sample we had an F1 hybrid, so we had a first generation hybrid between the two. So hybridisation has been going on for quite a while. We don't know how far back in time it goes, but it's been an ongoing phenomenon. And so we get anything from zero to 50% fin whale in our blue whales samples.

Will - As you said, they're not the closest related thing. So I suppose I have to ask a fairly indelicate question here, <laugh>, which is, when a blue whale in a fin whale meet and they decide to hybridise, for lack of a better term. How does that really work? Do they both know what's happening?

Mark - Well, we haven't witnessed a hybridisation event, so we don't know exactly how this works. Blue whales are a huge animal. The reason I work on small mammals is that they're easy to observe and you can get large sample sizes in the small space. If you work on large mammals, especially a blue whale, they occur across a huge area and there are few of them. So there tends to be a lot of facts abating and so on that we don't have a lot of information for because it hasn't been observed very often. However, we have observed and we know how courtship works with blue whales. What happens is the female and the male pair up. Sort of in the fall and into the early winter, you'll get pairs that are travelling together. And then at some point if another male joins so that they'll try to sneak in on the pair, then you wind up getting this wild swimming behaviour where the female just takes off and the males take off with her and the males will fight with one another. You'll have collisions, you'll have breaching and all kinds of things with the males. And then one of the males will typically leave. So the fin whales obviously have managed, at least on some occasions, to either be the primary pair with a blue whale or to win one of these battles with a male blue whale. So when they're mating with female blues. So presumably when you have one male fin whale and one female blue whale, he can successfully mate with her.

Will - It's extraordinary that given the intricacies of the courtship, that increasingly sounds less and less like it's by accident that this is happening.

Mark - Certainly not accidental on the part of the male fin whale.

Will - What a sight to behold that would be...

Mark - <laugh>. Yeah, exactly. We know that when they mate, they do actually do a deep dive. The pair does a deep dive and then they mate on the way back up. So it's not, it's not something that's been observed very often. At least.

Will - One can dream, I suppose.

Mark - <laugh>.