Neutrality tests on mtDNA: unusual results from nematodes

McDonald-Kreitman tests of neutrality on mitochondrial DNA (mtDNA) of butterflies, Drosophila, and a variety of vertebrates usually show excess (over the neutral expectation) intraspecific polymorphism at nonsilent sites. These results are of great interest because they are the opposite of what is usually found for nuclear genes, in which the neutral pattern or evidence of adaptive divergence between species is usually observed. However, only vertebrates and insects have been tested so far, so it is not clear whether this intriguing pattern is typical for mtDNA in all taxa. Here I tested three pairs of nematode species and found that they all show a deficit of replacement polymorphism. Taken at face value, this result suggests that adaptive evolution proceeds more efficiently in nematode mtDNA than in mtDNA of vertebrates or insects. An alternate explanation is that the nematode pattern is an artifact of silent-site saturation that results from the rapid and composition-biased way in which nematode mtDNA evolves. Further studies are needed to distinguish between these two hypotheses.