Genetic diversity of the earthworm Octolasion tyrtaeum (Lumbricidae, Annelida)

• Three genetic lineages were found within O. tyrtaeum.
• Body size does not correlate with lineage in O. tyrtaeum.
• A case on incongruence between nuclear and mitochondrial sequences was found.
• O. tyrtaeum is paraphyletic with respect to the closely related O. cyaneum.

Octolasion tyrtaeum (Savigny, 1826), a cosmopolite earthworm species widespread around the world, is known to consist of two morphologically distinct forms, small (4–8 cm long) and large (10–14 cm) ones that sometimes are found in sympatry. It was demonstrated that these forms belong to significantly divergent mtDNA lineages, which suggests that the differences among these forms are caused by genetic factors. However, these results were in contrast to the allozyme analyses, and nobody has demonstrated that differences among these lineages exist on nuclear level as well.We analyzed mitochondrial cox1 and cox2 and nuclear ITS2 sequences of O. tyrtaeum specimens from various regions of Russia, Belarus, and Kazakhstan. Our sample only included individuals belonging to the small form, however, we found both genetic lineages described by earlier studies, and, in addition, discovered a new genetic lineage. Nuclear ribosomal sequences confirmed that the differences between these lineages are deep. We also found one example of incongruence for several individuals whose mitochondrial sequences belonged to the “small” lineage, but nuclear ITS2 fell into a separate branch on the phylogenetic tree. In addition, O. tyrtaeum was found to be paraphyletic with respect to the closely related species O. cyaneum (Rosa, 1884).