Sperm ultrastructure in Segmentinini (Hygrophila, Planorbidae): Phylogenetic implications and diagnostic utility

The aim of this study was to describe sperm ultrastructure in Segmentinini F.C. Baker, 1945, a tribe of freshwater panpulmonates (Hygrophila), and identify sperm characters that may qualify as candidate synapomorphies of the group. Spermatozoa of seven species representing six genera of Segmentinini were examined by light microscopy and scanning (SEM) and transmission electron microscopy. Sperm ultrastructure is very similar in all species and is generally typical for the Hygrophila. The head consists of a nucleus and conical acrosomal pedestal tipped with an ovoid apical vesicle and is followed by a long midpiece filled with the mitochondrial derivative. The boundary between the midpiece and the next region, the glycogen piece, is marked by a two-partite cylindrical annulus. The glycogen piece is filled with homogeneous mass of glycogen and is followed by a short terminal region, the endpiece. The nucleus has an unusual shape consisting of a barrel-shaped basal region and a narrow apical stem bearing a disc-shaped nuclear keel. The axoneme arises from the centriolar derivative attached to the nucleus with nine wedge-shaped rods. The mitochondrial derivative contains two glycogen-filled helices that spiral apically in opposition to each other, but then become closely apposed and run in parallel. The comparison with other Hygrophila shows that at least three sperm characters (unusual nuclear shape, accessory centriolar rods and arrangement of glycogen helices) can represent potential synapomorphies of the tribe. The shape of the sperm nucleus is readily recognizable by SEM and can be routinely included in taxonomic descriptions of Segmentinini.