Ultrastructure of spermatozoa in Orthalicidae (Mollusca, Gastropoda, Stylommatophora) and its systematic implications

Abstract

Sperm morphology of orthalicid gastropods Clessinia pagoda, Spixia tucumanensis, Plagiodontes daedaleus (Odontostominae) and Drymaeus hygrohylaeus, D. poecilus, Bostryx stelzneri (Bulimulinae) are examined and described for the first time using transmission electron microscopy. Spermatozoa show the general characteristic of Pulmonata: an acrosomal vesicle, sperm nucleus helical, mitochondrial derivative forming a continuous sheath with paracrystalline material and coarse fibers associated with axonemal doublets. Features in the acrosomal complex and shape of the nucleus distinguish orthalicid sperms from other stylommatophoran. The acrosomal pedestal is traversed by fine striations in all species examined except in S. tucumanensis. The structure and thickness of the perinuclear sheath with a single or double layer of electron-dense material ensheathing the nuclear apex is characteristic of the group. The presence of a subnuclear ring in Drymaeus, Bostryx and Clessinia species is also reported. A data matrix of eleven species per 34 characters (16 sperm plus 18 anatomical and shell characters) from orthalicids plus other stylommatophoran and systellommatophoran representative species was constructed. Three cladistic analyses (sperm-based, anatomical-based and a combined sperm + anatomical-based) were performed to test the phylogenetic potential of sperm ultrastructure in orthalicid systematics and understand how sperm characters affect the topology and resolution of the obtained trees. Stylommatophora resulted in a monophyletic clade in the sperm-based and in the combined-character analysis. Orthalicidae is monophyletic only in the combined-character cladogram. Within Orthalicidae, Odontostominae is recovered as a monophyletic clade in all analyses, while Bulimulinae is paraphyletic in all trees except in the combined phylogeny. The present study and cladistic analyses performed support the hypothesis that characters on sperm ultrastructure are informative for stylommatophoran systematic and phylogenetic approaches, providing synapomorphies at familiar, subfamiliar and generic level.