Of bats, flight and immunity to viruses
Bats are the stealth bombers of the animal kingdom. Equipped with radar-like echolocation, the dark form of the bat allows this creature to stay in the shadows before launching into attack on its unsuspecting prey. Scientists are now increasingly interested in bats for the biological payloads they carry: these include highly pathogenic viruses such as Ebola, rabies, and SARS.
After rodents, bats are the second most numerous mammal species on earth. The increasing interaction between bats and humans might be due to mankind’s need for more land, bringing him closer and closer to the jungle. What allows bats to harbor viruses highly lethal to humans, with no overt signs of illnesses is an important question in public health. Surprisingly, the answer may lie with one of the most characteristic features of the bat: its ability to fly.
There are two major families of bats: the old world fruit bats and the echo-locating (predominantly-) insectivorous bats. In a paper published in Science on January 25th, Guojie Zhang and colleagues at the Beijing Genomics Institute, University of Copenhagen and Australian Animal Health Laboratory, describe a study where they compared the genomes of two wild bats: an Australian fruit bat (Black flying fox) and a Chinese insectivorous bat (David’s Myotis), with the genomes of a number of other mammals, including humans and rhesus macaques. Comparing genomes helps researchers identify how certain genes differ in bats and other mammals. These differences can help identify genes responsible for the evolution of certain features, for instance, flight in bats. Researchers now know that bat flight requires a lot of energy, which led to the selective evolution of genes involved in metabolism. This however raises another problem. Increased metabolism releases free radicals, which can cause DNA damage. To cope with this DNA damage, Zhang’s group found that bat genomes had selectively evolved a number of DNA repair genes.
DNA repair plays an important role in the immune system’s ability to fight pathogens. While looking for genes involved in the bat’s immune system, the researchers found one gene involved in both DNA repair and the immune system.
This discovery led the researchers to hypothesize that the consequence of bat’s evolution of flight may have led to changes in bat immunity.
More interestingly, Zhang and his colleagues identified major differences in genes of Natural Killer cells between bats and the other mammals. Natural Killer (NK) cells play hugely important roles in our immune system’s fight against viruses such as Ebola, SARS and HIV. Both bat genomes in this study lack the same NK cell genes that the other mammals had. This could mean that bat NK cells react to viruses differently. This is important, because most disease we see in humans after viral infections is due to our immune system’s battle against the microbes. Comparing how bat and our NK cells react to viruses could help us understand why bats don’t get sick from these highly dangerous viruses, but we do.
While Zhang’s work is thought provoking, there is still much left to be understood. Is there a causal relationship between bat flight and immunity, or are the links found just correlation? Rather, the differences in NK cells that these researchers found might be more interesting. Studying these differences between bats and humans could provide us with new drug targets that reduce the effects of an overzealous immune response. Tantalizingly, bats enjoy a longer life span than animals of similar size, and this may be due to their unique immunity. Only time will tell if the bat immune system holds the secret to our fountain of youth.
1. Zhang, G. et al. Comparative Analysis of Bat Genomes Provides Insight into the Evolution of Flight and Immunity. Science 339, 456–460 (2013).