Ultrastructural and molecular insights into three populations of Aporrectodea trapezoides (Dugés, 1828) (Oligochaeta, Lumbricidae) with different reproductive modes

Aporrectodea trapezoides is a widely distributed earthworm, consisting of sexual, pseudogamic and parthenogenetic forms. We sampled individuals from five differently reproducing populations: one sexual population (ROB), three parthenogenetic populations (MOL, ALC and SAN), and another population (SHB) in which earthworms had sperm-filled spermathecae (indicating copulation) but seminal vesicles empty of mature sperm (indicating inhibited sperm production). Ultrastructure of spermathecae of ROB, MOL and SHB showed a high number of functional spermatozoa in ROB, no presence of any sperm in MOL, but presence of sperm in SHB, however with morphological evidence for active degradation. Spermiogenesis was complete in ROB, but aberrant in MOL and SHB, which had less germ cells in the seminal vesicles and incomplete maturation of spermatozoa. SHB and MOL also showed a significantly higher amount of lipid droplets than ROB in spermathecae epithelium and seminal vesicles, indicating that the mechanisms involved in nurturing the sperm continues to be functional in SHB and MOL. Molecular analyses of mitochondrial cytochrome c oxidase I and nuclear histone H3 revealed a genetic proximity of the parthenogenetic populations MOL and ALZ to ROB, while SHB and SAN were more divergent and clustered together. Phylogenetic analyses indicated independent origin of uni-parental reproduction.