Bats and their viral tenants

Posted on August 8, 2012   by Sruthi Raghavan, Freelance science writer

Bats are an important natural reservoir of gammaretroviruses, a type of retrovirus.    

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We all know that bats roost in trees, caves and even houses. But they seem to be oblivious to, and unaffected by, the many different viruses that dwell within them including emerging human viral pathogens such as Ebola viruses. Bats live in massive populations that are distributed worldwide and have long life spans - attractive characteristics that enable viruses to make bats their allies in their bid for survival.

Scientists from The Pennsylvania State University, in collaboration with other institutes in USA, Australia and China, have determined that bats harbour a distinct and diverse range of gammaretroviruses that share similarity with other mammalian gammaretroviruses. Nine out of 11 bat species host gammaretroviruses. The study, published in the Journal of General Virology, supports previous work that suggests mammalian gammaretroviruses may have evolved from bats.

Gammaretroviral sequences from the brain tissue of a fruit bat Rousettus leschenaultia and insect bat Megaderma lyra, denoted as RIRV (R. leschenaultia retrovirus) and MIRV (M. lyra retrovirus) respectively, were cloned. Genome sequence analysis revealed that RIRV shares 70% similarity with pig retrovirus. MIRV was found to share 72% similarity with pygmy mouse retrovirus and was also very similar to koala retrovirus and gibbon ape leukemia virus. Phylogenetic analyses carried out using amino acid (protein) sequences also supported the above results. These two new gammaretroviruses were shown to be genetically distinct from those previously known.

All the gammaretroviral genomes documented to date in bat species are defective, i.e., they are missing an essential part of their genome due to deletion mutations. Interestingly in this study, both the RIRV and MIRV genomes were found to harbour large deletions at different genomic positions in the pol gene. Products of pol are responsible for synthesizing viral DNA that is integrated into the host genome.  Such retroviral sequences, that may be the product of ancient viral infection, that have become permanently integrated into the genome of an organism are called endogenous retroviruses (ERVs). These are very useful in understanding the genetic diversity of viruses.

When two bat genomes (Myotis lucifugus and Pteropus vampyrus) were analyzed using a novel approach called genomic mining, it revealed that they not only harbour several copies of endogenous gammaretroviruses but also that they fell into three distinct groups. Genomic mining identifies molecular signatures in genome sequences and eliminates the limitations that are otherwise involved in growing micro-organisms in the laboratory.

When 16 ERV genomes from bats were studied using this approach they showed a very high percentage of sequence similarity between them. Genetic divergence (0.5-10%) of these ERVs further indicates a gradual and successive invasion of gammaretroviruses into bats during the course of evolution. Based on this information, researchers estimated the invasion time of these viruses into bat genomes to be between 2.4 and 64.6 million years ago!

Researchers believe that studying the genomes of different bat populations will help us understand the genetic diversity of these gammaretroviruses. Understanding the successful co-existence of bats and these viruses has always been a mystery and studies such this will help us unravel it. This will enable us to better combat these human viral pathogens if and when they decide to make their home in humans.