Genome-wide signatures of convergent evolution in Echo locating mammals

by Daniel Considine

 

Most people understand that evolution, for the most part, works in a divergent way. This concept is fairly easy to grasp. When some form of reproductive barrier is introduced to a population, drift and many other factors will take over. When given enough time this will result in the two separated populations diverging to the point where they are now two completely different species, this is the fundamental process of divergence. What is not so intuitive is when two completely distinct species both independently evolve similar features. This is referred to as convergent evolution, and there are some notable examples. The most commonly cited example is how birds and bats both evolved the ability of flight. This is known as an analogous trait, where in two unrelated species face similar environmental challenges and both arrive at the same solution. Analysis of the anatomy of these species makes it clear that their wings are related in no way besides function.

Until recently, researchers were unsure of how convergent evolution worked, if at all, at the molecular level. One major breakthrough was in the analysis of amino acid substitutions at the active site of protease enzymes of different species. A recent paper in Nature (Parker et al, 2013) demonstrated how the nucleic sequence of completely unrelated species show convergent evolution at the site of functionally similar genes. Evidence suggests that echolocation has arisen at three independent times; once in whales and twice in bats. What is truly remarkable about these findings is that in all three instances, there are far more convergent amino acid substitutions to genes involved in hearing than would be expected by chance.

Researchers took 4 distinct bat species, consisting of both echo locating and non-echo locating species, and sequenced their genomes. To align these sequences, they took a further 18 mammal genomes from databases, one of which was the echo locating bottlenose dolphin, to make comparisons. Analysis of this data showed that genes involved with hearing, and in particular those which are important for echolocation, showed similarities with a very high likelihood of convergent evolution. To compound this finding, analysis was also carried out on the genes involved in vision, as bats and dolphins have both undergone adaptation for low-light environments. Again researchers found that the amount of similarities across these sequences strongly supported the hypothesis that convergent evolution was taking place. Finally, analysis of the nature of nucleic sequence substitutions was carried out. This analysis showed that these loci were under strong selection, which adds to the evidence of a convergent theory. One criticism of this methodology however, is their lack of comparison to a non-echolocating species of Cetacean.

So why is this research of importance? For a start it gives biologists new insight into how phenotypic traits can have great similarities, yet still be from completely unrelated species. This research can potentially explain some of the problems evolutionary biologists face. For example when analysing the genome sequences of different species for relatedness, and finding unexpected similarities, which until now would have suggested homology. This research could be taken one step further by measuring tissue-specific gene expression levels, which would not only improve our understanding of how protein-coding genes may converge, but also our overall ability to identify and understand gene regulation at the molecular level.

 

Reference: Parker et al, 2013 Nature

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