Search

ABSTRACTEusperm and parasperm ultrastructure have been studied for the first time in the neogastropod Plicopurpura patula. Eusperm structure is typical of many Neogastropoda, in which an apical bleb characterizes the conical acrosome with subacrosomal granule; the nucleus is tubular and penetrated to its tip by basal body and axoneme; the mid-piece comprises spiral mitochondria that ensheath the axoneme but terminate at the annulus, defining the junction with the end-piece. In the proximal portion of the end-piece dense tracts of glycogen match the axoneme but these disappear distally. The parasperm is unusual in having no axonemes and no motility was observed. Eusperm do not attach to parasperm, as occurs in the similar nurse cells of Littorinimorpha, thus potential functions may be limited to some form of paternity assurance as occurs in some insects. The nucleus of the parasperm is eliminated during paraspermiogenesis, but there is considerable post-transcriptional secretion activity resulting in the production of a variety of granules, some rich in glycoproteins. A prominent paracrystalline rod accumulates in the centre of the parasperm also from Golgi-derived secretions. Preliminary evidence from proteomics suggests that this parasperm carries the heat shock protein HSP70 that could play a role in fertilization inside the female.

Egg hull formation is described in detail for the first time in Rhyssoplax tulipa from South Africa. Although previous descriptions of hull formation in chitons provide some basis for comparison, the mechanisms we have discovered are totally different. The process begins with the release of clear vacuoles from the oocyte into the intercellular space, which separates it from a layer of follicle cells. Materials contributing to the layers of the hull are formed primarily by micro-apocrine secretion processes, in which extensions of the follicle cells and oocyte are budded off into the intercellular space, rather than by formation and release of vesicles from the Golgi bodies, as previously thought. Later in oogenesis, these secretions become organized into three functional layers, each with a characteristic substructure. Follicle cells mold the shape of the hull spines even down to the petalloid tips that are characteristic of this group. Late in oogenesis the oocyte secretes the vitelline layer beneath the hull and the follicle cells break down and are sloughed off. Differences in the mechanisms of hull formation among chitons are providing a new set of morphological characters that may yield insight into chiton phylogeny. To our knowledge, this is the first description of micro-apocrine secretions during oogenesis in a mollusc, although it has been reported in other phyla.

SummarySpermatogenesis in Serpulorbis squamigerusand S. variabiliswas studied with light and electron microscopy. In spite of their unusual method of accomplishing internal fertilization by the use of spermatophores, the genesis and structure of vermetid eusperm is in most respects typical of Hypsogastropoda (sensu Ponder and Lindberg, 1997). However, vermetid eusperm have evolved a complex acrosome with a number of unique features not previously described in this taxon. Parasperm of vermetids are easily distinguished by their unique shape, but are more similar in their development to parasperm of some Hypsogastropoda than they are to those of Cerithioidea and Campaniloidea. One feature that was thought previously to provide a link with Cerithioidea and related groups, is the presence of the tail brush in mature parasperm. However, in Serpulorbis variabilisthe tailbrush is reduced or lost in some parasperm at maturity and in S. squamigerusit is completely replaced by the terminal bulb, which is synapomorphic with Hypsogastropoda. Also, unlike Cerithioidea, paraspermiogenesis in Vermetoidea always involves the complete elimination of the nucleus and no acrosome is formed. For the first time, nuclear degradation in parasperm of Vermetoidea is shown to progress by an apoptotic mechanism. The many nuclear vesicles that pinch off during this process are broken down by lysosomes secreted by the Golgi body, such that nuclear material is not detected in the mature parasperm. This mechanism of “nuclear apoptosis” is different from that observed in Cerithioidea and Littorinoidea, but is similar to that in some Hypsogastropoda. The Golgi body is involved in producing numerous large glycoprotein granules that fill the body of the parasperm and form a sheath around the axonemal core. This core of axonemes extends anteriorly into a long compound flagellum that produces little motility. Eusperm and parasperm are sealed together inside a spermatophore that dissolves and releases its contents once inside the female reproductive tract. At this point eusperm and parasperm are separate, but in Vermetus aliithere is evidence that, prior to this, they form spermatozeugmata in the vas deferens, which is a derived feature characteristic of some Hypsogastropoda. This and other evidence supports the conclusion of Healy (1988b) that Vermetoidea are not closely related to “lower” caenogastropods, such as Cerithioidea. Rather, they are closer in sperm charactersitics to “lower” Hypsogastropoda, which include superfamilies such as Littorinoidea, Stromboidea, Janthinoidea and Triphoroidea.