Comparative sperm ultrastructure and spermiogenesis in basal heterobranch gastropods (Valvatoidea, Architectonicoidea, Rissoelloidea, Omalogyroidea, Pyramidelloidea) (Mollusca)

Three distinct groups of basal heterobranch gastropods (='Allogastropoda/Heterostropha') can be distinguished on the basis o f sperm and spermiogenic features: (1) Valvatoidea; (2) Architectonicoidea; (3) Rissoelloidea + Omalogyroidea + Pyramidelloidea. Sperm of pentaganglionate heterobranchs (Opisthobranchia sensu stricto + Pulmonata) conform in all respects to the pattern shown by group (3). Heterobranchia are united, and differentiated from other gastropods, by a shared suite of sperm and spermiogenic features (rounded acrosomal vesicle, nucleus usually helical, highly modified mitochondrial derivative, spermatid acrosome associated with a dense nuclear plaque, formation of the mitochondrial derivative through fusion of numerous small mitochondria along the length of the axoneme). Sperm autapomorphies of the Architectonicoidea and the Valvatoidea suggest that these two superfamilies arc distinct but terminal offshoots from ancestral heterobranchs.