Variability of sexual organ possession rates and phylogenetic analyses of a parthenogenetic Japanese earthworm, Amynthas vittatus (Oligochaeta: Megascolecidae)

Although earthworms are hermaphroditic animals with biparental sexual reproduction, some parthenogenetic species have been found. Evolutionary trends in parthenogenetic earthworms revealed a reduction in the reproductive organs. To clarify the phylogenetic relationships of parthenogenetic earthworms with different degree of degraded reproductive organs, we conducted a morphological analysis of the reproductive organs and molecular phylogenetic analyses of Amynthas vittatus which usually degraded a part of reproductive organs. Morphological analysis revealed that almost all individuals collected around Mt. Aobayama, Sendai city of northeastern Japan, possessed male pores, while individuals collected from areas located across Hirose River did not. Phylogenetic analysis using mitochondrial DNA sequences of 48 individuals representing 20 populations indicated that almost all individuals collected around Mt. Aobayama belonged to a different lineage from the other populations collected around Sendai, and that almost all individuals collected from across Japan belonged to the latter lineage. We suggest that the difference in the male pore possession rate was caused by histories of each population, but the A. vittatus population found on Mt. Aobayama belongs to a different lineage as compared to the other Japanese populations and not the primitive population. Thus, the parthenogenetic earthworm A. vittatus has undergone at least two morphological evolutionary processes.