Ultrastructure of Euspermatozoa of cerithiacean gastropods (Prosobranchia: Mesogastropoda)

Spermiogenesis and Taxonomical Values of Sperm Ultrastructures in Male Crassostrea ariakensis (Fujita & Wakiya, 1929) (Pteroirmorphia: Ostreidae) in the Estuary of the Seomjin River, Korea

Characteristics of the developmental stages of spermatids during spermiogenesis and phylogenetic classicfication of the species using sperm ultrastructures in male Crassostrea ariakensis were investigated by transmission electron microscope observations. The morphology of the spermatozoon of this species has a primitive type and is similar to those of Ostreidae. Ultrastructures of mature sperms are composed of broad, modified cap-shaped acrosomal vesicle and an axial rod in subacrosomal materials on an oval nucleus, four spherical mitochondria in the sperm midpiece, and satellite fibres which appear near the distal centriole. The axoneme of the sperm tail shows a 9+2 structure. Accordingly, the ultrastructural characteristics of mature sperm of C. ariakensis resemble to those of other investigated ostreids in Ostreidae in the subclass Pteriomorphia. In this study, particularly, two transverse bands (stripes) appear at the anterior region of the acrosomal vesicle of this species, unlike two or three transverse bands (stripes) in C. gigas. It is assumed that differences in this acrosomal substructure are associated with the inability of fertilization between the genus Crassostrea and other genus species in Ostreidae. Therefore, we can use sperm ultrastructures and morphologies in the resolution of taxonomic relationships within the Ostreidae in the subclass Pteriomorphia. These spermatozoa, which contain several ultrastructures such as acrosomal vesicle, an axial rod in the sperm head part and four mitochondria and satellite fibres in the sperm midpiece, belong to the family Ostreidae in the subclass Pteriomorphia.

Molecular phylogeny of a circum-global, diverse gastropod superfamily (Cerithioidea: Mollusca: Caenogastropoda): pushing the deepest phylogenetic limits of mitochondrial LSU rDNA sequences

The Cerithioidea is a very diverse group of gastropods with ca. 14 extant families and more than 200 genera occupying, and often dominating, marine, estuarine, and freshwater habitats. While the composition of Cerithioidea is now better understood due to recent anatomical and ultrastructural studies, the phylogenetic relationships among families remain chaotic. Morphology-based studies have provided conflicting views of relationships among families. We generated a phylogeny of cerithioideans based on mitochondrial large subunit rRNA and flanking tRNA gene sequences (total aligned data set 1873 bp). Nucleotide evidence and the presence of a unique pair of tRNA genes (i.e., threonine + glycine) between valine-mtLSU and the mtSSU rRNA gene support conclusions based on ultrastructural data that Vermetidae and Campanilidae are not Cerithioidea, certain anatomical similarities being due to convergent evolution. The molecular phylogeny shows support for the monophyly of the marine families Cerithiidae [corrected], Turritellidae, Batillariidae, Potamididae, and Scaliolidae as currently recognized. The phylogenetic data reveal that freshwater taxa evolved on three separate occasions; however, all three recognized freshwater families (Pleuroceridae, Melanopsidae, and Thiaridae) are polyphyletic. Mitochondrial rDNA sequences provide valuable data for testing the monophyly of cerithioidean [corrected] families and relationships within families, but fail to provide strong evidence for resolving relationships among families. It appears that the deepest phylogenetic limits for resolving caenogastropod relationships is less than about 245--241 mya, based on estimates of divergence derived from the fossil record.

Comparative spermatology of 11 species of Polyplacophora (Mollusca) from the suborders Lepidopleurina, Chitonina and Acanthochitonina

Transmission electron microscopy of the spermatozoa and spermatogenesis of 11 species (in three suborders Chitonina, Acanthochitonina, Lepidopleurina) of chiton has shown that each species has a sperm with a unique morphology indicating that spermatozoa can be used as a taxonomic character. Although structure is species-specific, similarities between species within suborders and subfamilies can be recognized. The spermatozoa of species from the suborders Chitonina and Acanthochitonina have a head comprising nuclear material only, the anterior portion of which is in the form of a long thin (approximately 80 nm diameter) filament. In many species the centrioles and mitochondria of the mid-piece are lateral in position, the mitochondria often being sited anteriorly alongside the nucleus. By contrast,Leptochiton asellus, a member of the more ancient suborder Lepidopleurina, has a sperm with a head comprising a nucleus and an acrosome. The mid-piece is also more conven­tional in structure with a ring of five or six spherical mitochondria (sited behind the nucleus) that surround the centrioles. The presence of the acrosome inL. asellussuggests that in the more recent chitons the acro­some has been secondarily lost. It is proposed that loss of the acrosome is correlated to a modification in egg-coat thickness. A preliminary examination of the structure of the eggs of three species has shown that those ofL. asellusare surrounded by a very thick chorion (14-30 μm) whereas inAcanthochitona crinitusandDinoplax gigasthere are regions of the chorion that are less than 2 μm thick. The morphological changes that occur during spermatogenesis are very similar in the Chitonina and Acanthochitonina. During spermiogenesis the nucleus elongates to develop a long anterior filament. Chro­matin condensation within the nucleus involves the formation of fibrils that become orientated along its long axis. Closely associated with the elongating nucleus is a manchette. InL. asellus a spherical proacrosomal vesicle appears in the spermatocytes. This vesicle becomes compressed as it matures and simultaneously it migrates to the presumptive anterior end of the spermatid where it invaginates and elongates. Although the pattern of chromatin condensation in the nucleus is similar to that described above, a manchette has not been observed.

Spermatozoon structure of eight species of South African holothurians (Echinodermata)

Transmission electron microscopy of the spermatozoa of eight species (from seven genera) of holothurians has shown that each species has a sperm with unique dimensions, indicating that spermatozoon morphology can be used as a taxonomic character. However, although the structure of the sperm is species-specific, all sperm have a similar appearance which is typical of holothurians. The sperm head is composed of a spheroidal nucleus (about 2 μm diameter) which has a large anterior fossa housing the approximately spherical acrosome. The acrosome is differentiated internally and surrounded by periacrosomal material. The mid-piece consists of a single mitochondrion with lamellar cristae which surrounds the eccentrically positioned centrioles. The conservative form and arrangement of the sperm organelles suggest that sperm morphology cannot be used for phylogenetic investigations within this taxon.

Changes in nuclear structure during eupyrene spermatogenesis in Murex brandaris

Changes in nuclear structure during eupyrene spermatogenesis of Murex brandaris have been studied using light and electron microscopy. In the first phases, spermatogonia show round nuclei, with several electrodense masses of chromatin and a thin layer of heterochromatin associated with the nuclear membrane. Primary spermatocytes possess larger nucleii, with less condensed chromatin, and the synaptonemal complexes are apparent. During spermiogenesis, chromatin becomes lamellar, and the nucleus twists about its principal axis while it elongates. The nuclear twisting is accompanied by a progressive chromatin condensation, which causes a highly electrodense nucleus at the end of the process.

Ultrastructure of the sperm and spermatogenesis in five South African species of the trochid genus Oxystele (Mollusca, Prosobranchia)

Sperm of Oxystele are of the primitive type, which is typical of molluscs using external fertilization. They comprise a head, which is composed of a barrel-shaped nucleus and large conical acrosome; a midpiece of five or six spherical mitochondria; and a tail, with a 9 + 2 arrangement of microtubules. The morphology of the sperm broadly resembles that of the sperm of other trochids as well as that of the sperm of the Pleurotomariidae, Haliotidae, and Fissurellidae but differs considerably from that of the Patellidae. Each of the five species examined was found to have a different sperm morphology. Our results support the suggestion of Heller and Dempster (in preparation) that Oxystele variegata Anton actually consists of two sympatric species. The morphological changes that occur during spermatogenesis are very similar in all species and resemble descriptions given for other molluscs with primitive sperm.