The science of speciation – molecular adaptation in vampire bats

“You can smell the ammonia as soon as you come in.” “See them? Neat, eh?” “Yeah. Those are vampire bats?” “These are vampire bats, yeah.” Nancy Simmons (AMNH): There are 1,300 living species of bats. That is about 20% to 22% of all living mammals. “It’s this one here.” “Oh-ho, there’s a bat.” We think that the great diversity of bats has to do with the fact that they occupy a really unusual niche for a mammal. That is, a nocturnal-flying-mammal niche. “Beautiful.” So, you have some roosting high up in trees. Others close to the ground. “Bugger!” And different bats eating different kinds of things – nectar feeders, fruit feeders, carnivores, insectivores, and even blood-feeding bats, the vampires. “Hi, sweetheart.” I’m an evolutionary biologist, interested in the evolution of diversity in body form of mammals, diets of mammals, species diversity of mammals. And bats are just a wonderful group to work on, because they’re so diverse, so interesting, and they do so many different things. [forest sounds, footsteps] [Lamanai Archeological Reserve - Belize] A typical night of catching bats begins at about five o’clock. We need to give ourselves an hour, hour and a half, to get everything situated before the bats wake up. [music, forest night sounds] “Oh, we got a fourth.” “Nope, it got out. Spoke too soon.” Bats can be told apart with a variety of different traits. The size and shape of the nose leaf varies from one species to another. “What was there?” “The vampire bat.” There are bats that have stripes above and below their eyes. Shape of the ears can vary. “This is a little-nectar feeding bat. It has a long tongue that it can put inside flowers.” So, just looking at the outside of the bat you can tell a great deal about what species it might be. “Here you go. Desmodus rotundus.” “Need help?” “Nope, I’m fine. “Ah, that’s that standard vampire sound.” Toni Piaggio (USDA): There are three species of vampire bats: the hairy-legged, the white-tipped, and the common vampire bat. In this portion of Belize, we only find the common vampire bat. “Don’t want him to flip onto my naked hand.” The three species all feed on blood, but the hairy-legged bat only feeds on bird blood. The white-tipped bat feeds on bird blood, and there have been records of them feeding on mammalian blood. But the common vampire bat specializes on feeding on mammals. “Do not want to be tangled up in there. There we go, now you’re looking photogenic.” “Nice mug.” We don’t entirely understand why there are three vampire bat species, what kind of diversification has happened there. And so, using molecular biology, sequencing DNA, we can really get at this question. [music] David Liberles (Temple University): A long-standing goal in molecular evolution is to understand which are the changes in individual genes that have made each species unique. So, my research group has undertaken a large-scale screen of genes that are candidates for having undergone positive selection. One of the genes that we came across was the gene for plasminogen activator in vampire bats. All organisms have what’s called plasminogen activator, which is active in their tissues and plays an important role in maintaining blood flow throughout the body. In vampire bats, adaptive changes have enabled them to express the activator in their saliva. So, when they bite an animal, it helps to keep blood flowing, allowing the vampire to keep feeding. “So, here we are at the vampire bats and we’ve separated out the bat activator…” When we look at changes in individual genes, we’re ultimately asking how does it affect the fitness of the organism – the probability of surviving, of finding a mate, and of reproducing. And so, this is the biological currency in which evolution works. When we think about things like blood feeding, if you’re able to get a new food source and to reproduce faster, then that’s increasing your fitness and makes you more likely to survive, more likely to reproduce. “So, plasminogen activator is a crucial promoter of breaking up blood clots. Within vampire bats…” When we started this work, the idea that there were changes to this gene in vampire bats was already known to science. However, it was not known how this related to the blood-feeding behavior of different vampire bat species. When we compared the genes of the three species, we found that they were different. While the hairy-legged vampire, which feeds on bird blood, had a normal activator, a modification to the gene in the white-winged and common vampire bat appeared to be related to the ability to feed on the blood of mammals. Mammals have a mechanism that enables them to shut off the activator when bleeding becomes dangerous, allowing blood to clot. The change that we found in the white-winged and common vampire bat enables them to override this clotting mechanism and keep blood flowing in mammals they bite. [music] Once we’ve sequenced a gene, we can build a gene tree reflecting the evolution of that gene. We can then take that gene tree and correlate it with the species tree, adding discoveries of genetic changes within that species. “So, I think this is actually an elegans. See how the knees are naked?” Everything that we observe in how a species interacts with its environment will have some basis, at some level, in genetics. And so, understanding the genetic processes that give rise to how species are different, and how they interact differently with the environment, is a fundamental scientific question that enables us to understand species in general. [Simmons] The age of genomics has just opened up so many new areas for research. We’re able to answer questions that in the past biologists would have liked to know the answers to, but there were no tools. So basically, it’s a whole new toolkit that lets us address all kinds of interesting biological questions that we just couldn’t get at before.