Ultrastructure of spermiogenesis in the gastropodHeliacus variegatus (Architectonicidae), with description of a banded periaxonemal helix

Abstract

Spermiogenesis and spermatozoa of the marine gastropod Heliacus variegatus Gmelin were examined using transmission electron microscopy (TEM). Mature spermatozoa are composed of an acrosomal vesicle, helical nucleus, elongate midpicce, annulus, glycogen piece and short end piece. The midpiece consists of a 9 + 2 axoneme, nine coarse fibres, and a banded helix, all enclosed by a continuous mitochondrial sheath (with multiple, helically coiled grooves). Anterior extensions of the mitochondrial sheath and banded helix form a double sheath around the basal half of the nucleus -an arrangement possibly unique in the Mollusca. During spermiogenesis, dense plaques delineating the anterior and posterior poles of the spermatid nucleus become attachment sites for the acrosomal vesicle and the axial complex (respectively). As the nucleus condenses and elongates, midpiece formation involves fusion of numerous, oblong mitochondria along the length of the axoneme. The coarse fibres and banded helix of the midpiece probably are derived through centriolar activity. Results of the study support inclusion of the Architectonicidae within the Heterobranchia, but in view of midpiece specializations, do not clarify the precise relationship of the family within this subclass.

Comparative sperm ultrastructure and spermiogenesis in basal heterobranch gastropods (Valvatoidea, Architectonicoidea, Rissoelloidea, Omalogyroidea, Pyramidelloidea) (Mollusca)

“…Groups classically assigned to the Prosobranchia but now considered to be primitive heterobranchs such as the Valvatoidea, Pyramidelloidea and Architectonicoidea, have become the focus of detailed anatomical and phylogenetic investigations (Haszprunar 1988;Rath 1988;Ponder 1990aPonder , b , 1991. Significantly, the taxonomic implications of sperm and osphradial ultrastructural research on these groups have largely been in accord with results suggested by comparative anatomy (Thompson 1973;Haszprunar 1985a, b;Healy l982,1988aHealy l982, , b , 1990aHealy l982, , 1991Healy & Jamieson 1991). In this account I survey sperm ultrastructure and development within the basal Heterobranchia and relate these data to the classification and phylogeny of the families and superfamilies presently assigned there.…”

“…The following summary and supporting micrographs ( Fig. 4A-L Medina et al 1985Medina et al ,1986Medina et al ,1988Healy l982,1988bHealy l982, ,1990aHealy l982, , 1991Healy & Jamieson 1991).…”

“…Investigated species of the Architectonicidae (from the genera Architectonica, Philippia, Heliacus and Granosolariurn) exhibit most of the sperm features typical of the Heterobranchia and likewise, features of spermiogenesis clearly support placement in this subclass (Healy 1982(Healy , 1988a(Healy , 1991Healy & Jamieson 1991). Certain features of spermiogenesis, such as the structure of the spermatid acrosome and nucleus, suggest that the Architectonicoidea lie closer to the Omalogyroidea/Rissoelloidea/ Pyramidelloidea/Pentaganglionata assemblage than do the Valvatoidea.…”

Evolutionary relationships of euthyneuran gastropods (Mollusca): a cladistic re-evaluation of morphological characters

“…Plesiomorphic spermatozoa are found in externally fertilizing gastropods, whereas modified spermatozoa are found in internally fertilizing gastropods such as neritimorphs, the vetigastropod genus Zalipais (Healy, 1990a) and Apogastropoda. The characters used here have been taken from the studies of Thompson (1973b), Healy (1983a, b, 1986a, b, c, 1987, 1988, 1990a, b, 1991, 1993), Healy & Willan (1984, 1991), Koike (1985), Healy & Jamieson, 1989, 1991) and Ponder & Lindberg (1997).…”