A new study of common vampire bats demonstrates some unique adaptations for its blood-sucking lifestyle, as well as the importance of a holistic approach to understanding the real complexities of evolution.
In 1994 at Cold Spring Harbor Laboratory in New York state, once home of the American eugenics movement, the relentlessly inventive molecular biologist Richard Jefferson introduced the term ‘hologenome’. Subsequently he developed a new perspective on evolution based on a more holistic understanding of the living world.
Jefferson’s approach was to suggest that the “unit of selection” in evolution, the thing that natural selection selects for or against, was not a gene, as Richard Dawkins would have it, or an individual organism alone, but an organism plus all the symbiotic microbes living on and in it. This community of organisms is known as the “holobiont”, and their collective genetic make-up is called the “hologenome”. All the genomes that comprise the hologenome have traits that help the organism survive, thus selection acts across the board.
This holistic approach, Jefferson felt, overcame problems associated with scientific reductionism, which he saw as necessary but insufficient for understanding complex systems.
Marie Zepeda Mendoza of the University of Copenhagen, Denmark, and an international team of researchers have adopted a hologenomic approach to understanding the way the common vampire bat (Desmodus rotundus) has evolutionarily adapted to “sanguivory” – a diet consisting solely of blood.
Sanguivory poses some unique problems. Blood is 78% liquid, high in protein but almost devoid of carbohydrates, low in vitamins and poses health risks due to a plethora of blood-borne pathogens. While D. rotundus has numerous adaptations to this lifestyle, such as specialised teeth and infrared sensing to identify accessible blood vessels, the authors contend that these “alone cannot address all of the challenges posed by this diet”.
Instead they argue that the microbial community, or microbiome, living in the gut of the common vampire bat plays a key role in its adaptation. To investigate exactly how this happens, the team took vampire bat faecal samples. Using the metagenomic technique of shotgun sequencing, a way of taking a genetic snapshot of the microbial community in a mixed environmental sample, Mendoza and colleagues discovered that the microbiome of D. rotundus is radically different from that of other bat species.
Not only did they uncover more than 280 disease-causing bacterial species, as well as a horde of species unique to the vampire bat, but they identified genes associated with various traits, many of which appear to help the bat overcome the problems of drinking blood.
The researchers identified genes that helped the animal exploit the low nutrient value of blood and genes to more efficiently use what fats and sugars are present. They also discovered hologenomic adaptations to the immune system, the better to resist the huge pathogen load. There were also adaptations to assist with the use of iron from blood, and others to help regulate blood pressure and excess nitrogen.
“It is clear from our results that the common vampire bat has adapted to sanguivory through a close relationship between its genome and gut microbiome,” Mendoza and colleagues conclude.
They also note that similar research that does not take into account Jefferson’s idea of combining the genomes of microbes and host might not adequately account for the complexities involved.
Stephen Fleischfresser is a lecturer at the University of Melbourne's Trinity College and holds a PhD in the History and Philosophy of Science.
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