Bats are some of the most distinctive mammals around. The only ones capable of powered flight, bat species occupy a variety of ecological niches, from predator to pollinator. The different habits come with some significant specializations, like echolocation and the ability to hibernate through cold weather.
Part of the reason that bats have evolved such extensive specializations is that they've been around for a while—fossil evidence dates back over 50 million years. But now, thanks to our ability to sequence genomes, some researchers have provided a new picture of how bats manage to adapt to such distinctive lifestyles. The DNA sequences suggest that all bats share some adaptations that help them cope with the metabolic demands of flight, while individual species have other adaptations that help them handle echolocation and hibernation.
The work was done by a large international consortium that involved everyone from the BGI (formerly Beijing Genomics Institute) to the Naval Medical Research Center. The team picked two different species of bat: an insectivorous hibernator called Myotis davidii and the black flying fox, Pteropus alecto. The latter feeds on fruit and nectar, and lacks the former's ability to echolocate.
The sequence confirmed that the bats (technically Chiroptera) belong to a large group of mammals called the Pegasoferae that includes the carnivores as well as hoofed mammals like cows and horses. Molecular clock data suggest that bats shared their last common ancestor with horses about 85 million years ago, when the dinosaurs were still around. The split with carnivores was only slightly more recent.
Compared to other mammals, the bat genome is a bit small at 2 billion base pairs (humans, in contrast, have a bit over 3 billion). This may be a product of the high metabolic demands of flight, which make the energy involved in copying unnecessary DNA unfavorable. That doesn't mean that the genome is completely streamlined, though; the flying fox's genome ended up with over 125 copies of a relative of herpesvirus 2 at some point in its past.
A compact genome isn't the only thing that helps bats cope with the energy involved in flight. High metabolic exertion tends to produce oxygen radicals, which damage cellular components, including DNA. So, the authors tested whether the genes that are involved in maintaining DNA integrity showed signs of having undergone evolutionary selection (we've explained how to do that test in the past). Many of the genes involved in repairing DNA damage did show signs of selection, as did genes that help stop cells from dividing if they've picked up too much damage. Both species have also lost a gene that helps cells trigger an inflammatory response when they sense DNA outside of the cell's nucleus.
There were some hints of the two species' specialization in the genome as well. The insect eater has to digest a variety of cellular components, and ended up with four copies of an enzyme that breaks down nucleic acids. In contrast, the flying fox, which gets its energy from sugars in fruit and nectar, had a mutation completely inactivate the gene. The insect eater is also a hibernator, and had acquired extra copies of a gene that's involved in digesting fats. (Gene duplications seem to provide a simple way of ensuring cells produce more of an important protein; they've been found to help bacteria adapt to metabolic demands, too.)
There's also a hint of a change that may contribute to echolocation in the insect-eater. FOXP2 has been implicated in language use in humans and seems to have some distinctive changes in other species that use vocalizations for various purposes. The version of the gene carried by the echolocating species seems to have picked up an unusual number of changes compared to other mammalian species.
The work provides another indication of how flexible genomes can be. Although there are lots of notable differences, the genome as a whole clearly looks a lot like those of other mammals. Most of the obvious adaptations don't involve changing the basic gene content. Rather, they seem to alter the level and function of the proteins the genes code for; other changes have been identified in bats that affect the timing and location of gene expression as well. As we well know, evolution can accomplish a lot by tweaking a common genetic toolkit.