Comparative sperm ultrastructure of the Planorbidae (Pulmonata: Hygrophila)

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Although the morphology of spermatozoa has been successfully used in the phylogenetic analysis of various animal groups, its utility for inferring phylogenetic relationships within the Planorbidae s. l. remains limited, because the sperm ultrastructure is currently known in only seven species of this family. The purpose of the present study was to study sperm in Planorbis planorbis (Linnaeus, 1758), the type species of the genus on which the family is based, and Segmentina nitida (O.F. Müller, 1774), and to supplement brief earlier descriptions of sperm in Ancylus fluviatilis O.F. Müller, 1774 and Planorbarius corneus (Linnaeus, 1758). Sperm ultrastructure in these four species is typical for the Hygrophila: the sperm cells are subdivided in an anterior-to-posterior sequence into acrosomal complex, nucleus, midpiece, glycogen piece and free flagellum. The spermatozoa of the four species, however, differ significantly in their total length, the shape and size of the head and nucleus, the shape and structure of the acrosomal complex, the internal structure, arrangement and number of ridges and glycogen helices in the midpiece, the structure of the annulus and the length of the glycogen region. The most morphologically distinct sperm, characterized by a unique flask-shaped nucleus, was found in S. nitida. The study presents a set of potentially phylogenetically useful sperm characters and assesses their usefulness for phylogenetic analysis within the Planorbidae and Hygrophila.

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