High mitochondrial DNA sequence diversity in the parthenogenetic earthworm Dendrobaena octaedra

Apomictic parthenogens are clonal organisms with limited genetic opportunity for increasing diversity beyond mutation. However, such species can be successful and have been shown to harbor more genetic diversity than might be expected. Here we surveyed diversity of the cytochrome oxidase subunit I gene from the mitochondrial genome of the earthworm Dendrobaena octaedra, an apomictic parthenogen. Diversity estimates made previously from allozyme markers for this species were high, but could have been affected by a detection bias, namely variable expression of alleles in the polyploid genome. We found similarly high mtDNA diversity over three localities in Finland, each represented by two sites; either with or without anthropogenic soil metal contamination. A single haplotype was most common over all sites, and over two thirds of all haplotypes sampled were similar to it in sequence (only varying by 1-3 substitutions). However, more divergent rare haplotypes were also found in one locality, Imatra, in southeast Finland close to the Karelian Isthmus. Metal contamination in the soil did not have a significant effect on genetic diversity, although metal exposure is known to be detrimental to the worms. There was no evidence for cryptic species within D. octaedra and phylogenetic analyses showed some structure of lineages that may have diverged in historical glacial refugia. Other mechanisms, such as rare genetic exchange with closely related species or high mutation and dispersal rates may explain high genetic diversity in D. octaedra.