Insights into the origin of parthenogenesis in oligochaetes: Strong genetic structure in a cosmopolitan earthworm is not related to reproductive mode

Abstract

Little is known about the origin and spatial pattern of the common phenomenon of parthenogenesis in earthworms. Aporrectodea trapezoides is unique in having a global distribution, where asexual forms are broadly known. Here we report two populations in Plasencia (Spain) and Karkra (Algeria) with multiple instances of co-occurrence of sexual and parthenogenetic individuals. We thus studied these populations where the two stages coexist with the aim of understanding the origins of parthenogenesis in earthworms. Two mitochondrial (COII and 16S rRNA) and three nuclear (ITS2, histone H3 and 28S rRNA) genes where evaluated to shed light on the mode, timing and frequency of origin of parthenogenetic forms. A population genetic study showed that in all cases sexual and parthenogenetic earthworms shared mitochondrial and nuclear sequence types, and that there is a strong genetic structure at the geographic level. Genetic divergence in both mitochondrial genes was high (up to 9.61% in COII and 6.12% in 16S rRNA), and ITS2 sequences were identical in individuals from closely related localities within a population. The haplotype networks, AMOVAs, Mantel tests and FST values all support the existence of strong genetic structure at the level of locality, but not due to reproductive mode, therefore providing evidence of multiple recent origins of parthenogenesis in this species.