Spermiogenesis and Sperm Structure in Relation to Early Events of Fertilization in the Limpet Tectura testudinalis (Muller, 1776)

Abstract

Spermiogenesis and fertilization in the limpet Tectura testudinalis (Mollusca: Archaeogastropoda: Patelloidea) were examined using scanning and transmission electron microscopy, as well as light microscopy. Spermiogenesis was similar to that described for other Lottiidae (Patelloidea), but some key differences were noted. We observed that the acrosomal vesicle forms on the plasma membrane from several Golgi-derived proacrosomal vesicles during the early spermatid. Evidence from anti-actin antibody staining of a related species, T. scutum, indicates that the acrosome probably contains G-actin in three different locations, but there are no preformed actin filaments. The flagellum of mature sperm tapers to a thin, filamentous end-piece. The mature egg of T. testudinalis is enclosed by a thin vitelline layer and a thick jelly coat bound by follicle cells. Where a mature sperm encounters the intact jelly coat, its acrosome tip may elongate, up to twice its original length. The acrosome reaction is not induced, but by this mechanism the acrosome bridges the jelly coat and extends down far enough to contact the vitelline layer. During the elongation of the acrosome, microfilaments are formed inside the tip. As well, new membrane required for this process is supplied by elimination of any slack in the plasma membrane and also perhaps by spontaneous formation of vesicles inside the acrosome. Once the tip of the sperm contacts and binds with the vitelline layer it undergoes the acrosome reaction, in which the plasma and acrosomal membranes fuse and roll back in a manner typical of scaphopods and some polychaetes. Sperm also may bind directly with the vitelline layer when the jelly coat is absent. Furthermore, the acrosome tip may undergo spontaneous fusion with a naked egg microvillus that formerly connected with a follicle cell process. Thus, the sperm either individually or collectively are equipped to deal with any one of several potential routes to successful fertilization of the egg. Current theories of gastropod phylogeny place the limpets as basal to the Gastropoda, but new evidence presented here supports the idea that they are a divergent group, with some unique innovations in sperm design.