On the fine structure of spermatozoa of Tachypleus gigas (Xiphosura,Merostomata)

Summary

Among the Arthropoda the Xiphosura are the only group whose spermatozoa resemble the so-called ‘primitive type’, although even here there are some differences. The spermatozoa of Tachypleus consist of a sperm head, a middle piece, and a long flagellum. Though principally quite similar to the spermatozoa of Limulus polyphemus some characteristics are noticeably different: the axonemal pattern (9 × 2 + 0), the shape of the acrosomal vacuole, the different position of the acrosomal filament, and the distribution of mitochondria throughout the cytoplasm. In contrast to what is known from Limulus the nuclear envelope is apparently dissolved over wide areas. Consequences for the interpretation of the acrosomal reaction are discussed. Structural conditions are compared with those in other Chelicerata.     

Sperm Ultrastructure of Two Mollusk Species Geukensia demissa and Modiolus modiolus (Mytilidae, Bivalvia)

Sperm ultrastructure was studied in two Atlantic species of bivalve mollusks Geukensia demissa and Modiolus modiolus (Mytilidae). These spermatozoa have a classic structure common for species with external fertilization: the head of the spermatozoon of G. demissa and M. modiolus consists of an acrosome, including an acrosomal vesicle and periacrosomal material, a nucleus, and a middle part with two mutually perpendicular centrioles, surrounded by a ring of spherical mitochondria. The spermatozoa of G. demissa and M. modiolus differ in the general shape and size of the head (M. modiolus, 5.8 × 4 m; G. demissa, 4.2 × 2.5 m), as well as the structure of the acrosome and the number of mitochondria (M. modiolus, 8–12; G. demissa, 5–6). The spermatozoa of the Atlantic species M. modiolus and M. modiolus (M. kurilensis) from the Sea of Japan are close in ultrastructure, but differ in the number of mitochondria. This could possibly be a reason to consider M. modiolus from the Sea of Japan as a separate subspecies, M. modiolus kurilensis. In ultrastructure, the spermatozoa of G. demissa differ significantly from the spermatozoa of M. modiolus, but are similar to the spermatozoa of species of the genus Brachidontes.     

Ultrastructure of spermatozoa and spermatophores of old world freshwater crabs (Brachyura: Potamoidea: Gecarcinucidae,Potamidae, and Potamonautidae)

We investigated the ultrastructure of spermatozoa and spermatophores of 19 palaeotropical freshwater crab species [12 species of the Gecarcinucidae, 6 of the Potamidae (Potamiscinae), and 1 species of the Potamonautidae (Deckeniinae: Hydrothelphusini)]. The investigated Potamiscinae have densely packed coenospermic spermatophores with the exception of Thaiphusa sirikit and Johora singaporensis that exhibit cleistospermia. In contrast, in the Gecarcinucidae the spermatozoa are loosely embedded in a mucous matrix. The gecarcinucid and potamiscine sperm differ, furthermore, in acrosomal structure and size. The acrosome in the Gecarcinucidae is much smaller and spherical, while the larger acrosome in the Potamiscinae has the tendency to be depressed. In the Potamiscinae, an additional middle acrosomal zone evolved between the acrosome ray zone and the outer acrosomal zone. Within the Gecarcinucidae, a differentiation into two groups (Gecarcinucinae and Parathelphusinae) is not supported by the present spermatological data. The sperm morphology of Hydrothelphusa aff. madagascariensis (Potamonautidae: Deckeniinae) differs from Potamonautes sidneyi (Potamonautidae: Potamonautinae) in acrosomal size and shape, and in the absence of a periopercular rim. A closer relationship of Deckeniinae and Gecarcinucidae cannot be confirmed by spermatology. J. Morphol., 2009. © 2008 Wiley‐Liss, Inc.     

The plasama membrane of Xenopus laevis spermatozoon

Xenopus laevis sperm plasma membrane ultrastructure has been studied by means of freeze-fracture, deep-etching, and lectin-gold binding. Xenopus spermatozoa differ from those of other species in that their plasma membrane does not exhibit topographical domains. In fact, no geometric arrangement or characteristic array of particles is present on fractured plasma membrane. Fractures rarely occur in acrosomal or nuclear membranes. Wheat germ agglutinin receptors are distributed homogeneously, on the plasma membrane of the sperm head and tail.     

三角帆蚌精子的形态及超微结构

运用电子显微镜技术对三角帆蚌精子的形态和超微结构进行研究。结果发现,三角帆蚌精子为原生型,分为头部、中段和尾部,头部呈子弹头形,电子致密且均匀,主要是核所在的区域。核前端由3－4个小的电子致密颗粒组成一个浅弧形的囊泡,为顶体结构,中段具有5个球形线粒体,环绕着两个相互垂直的中心粒。中段末端具有的鞭毛质领结构（flagellar collar)为一电子致密环,与远端中心粒之间由9个分叉的电子致密小片连接。尾部为典型的9＋2结构。     

Ultrastructure of testes and spermatogenesis in the trombiculid mite,Hirsutiella zachvatkini (Schluger)

Summary

Spermatogenesis and sperm ultrastructure of the trombiculid mite Hirsutiella zachvatkini (Schluger 1948) have been investigated using transmission electron microscopy and compared with other arachnids studied. Sperm differentiation takes place in groups of synchronously developed germ cells of the two large sac-like paired testes. Each testis is composed of a secretory epithelium, which occupies their medio-ventral regions, and of a germinative epithelium situated in the latero-dorsal parts of testes together with large somatic cells. The germ cells are represented on sections by spermatogonia, spermatocytes, early, middle and late spermatids, and mature spermatozoa. Spermatocytes and spermatids contain two centrioles, which disappear afterwards, and a small Golgi-like structure forming an acrosomal cistema. Mature spermatozoa, which lie both within the meshes of somatic cells and also free in the lumen of testes, are compact oval aflagellate cells provided with peripheral channels. They also contain an acrosome, flattened between the cell membrane and the round electron-dense chromatin body, an oval body of lesser density lying in close proximity to the chromatin body, and a group of 5–7 mitochondria with spherically arranged cristae situated immediately behind the nuclear bodies. An acrosomal filament may be sometimes seen beneath the acrosome in the middle spermatids and disappears in the mature spermatozoa. These findings show that the mode of differentiation and pattern of organization of the male sex cells in trombiculid mites are of rather primitive type compared with other acarine spermatozoa.     

Fine structure of the spermatozoa of Crassostrea gigas (Mollusca,Bivalvia)

We describe sperm ultrastructure and acrosome differentiation during spermiogenesis in Crassostrea gigas (Mollusca Bivalvia). The sperm cell is a uniflagellated cell of the primitive type. The head region contains a rounded or conical nucleus surmounted by small acrosome. This organelle consists of a membrane-bound acrosomal granule, the contents of which have a homogeneous density, except in the anterior region, which is positive for PTA. The acrosome also surrounds the perforatorium, which includes oriented fibrillar elements: this is the axial body. The middle piece contains four mitochondria encircling two perpendicular centrioles. The distal centriole is provided with a system of mechanical fixation to the plasma membrane, consisting of nine fibers in radial arrangement. The tail flagellum, about 50 m?m long, contains the usual microtubular axoneme. © 1993 Wiley-Liss, Inc.     

The mitochondrial genome structure of Xenoturbella bocki (phylum Xenoturbellida) is ancestral within the deuterostomes

Background  

Mitochondrial genome comparisons contribute in multiple ways when inferring animal relationships. As well as primary sequence data, rare genomic changes such as gene order, shared gene boundaries and genetic code changes, which are unlikely to have arisen through convergent evolution, are useful tools in resolving deep phylogenies. Xenoturbella bocki is a morphologically simple benthic marine worm recently found to belong among the deuterostomes. Here we present analyses comparing the Xenoturbella bocki mitochondrial gene order, genetic code and control region to those of other metazoan groups.     

Comparison of the spermatozoan morphology of Isognomon bicolor and Isognomon alatus (Mollusca, Bivalvia, Isognomonidae)

In this study we used transmission and scanning electron microscopy to examine the spermatozoan structure of Isognomon bicolor and Isognomon alatus. The spermatozoa of both species were of the primitive or ect-aquasperm type. The acrosomal morphologies were essentially similar but the top of the acrosomal vesicle in I. bicolor sperm had a slightly flattened edge whereas the apex of the acrosomal vesicle of I. alatus sperm had a rounded outline. This difference suggested that acrosomal morphology could be an important character for taxonomic differentiation. In the present work, the results demonstrated that the gamete ultrastructure of the two distinct species I. alatus, from Panama, and I. bicolor, from the southeastern region of Brazil, were similar to the other studied species of the superfamily Pterioidea.     

Fine structure of scorpion spermatozoa (Buthus occitanus; buthidae,scorpiones)

The mature spermatozoa of Buthus occitanus are threadlike in shape and divided into sperm head, middle piece, and end piece. The sperm head is corkscrew shaped anteriorly and in this region bears an unusual acrosomal complex consisting of a ring-shaped acrosomal vacuole associated with a subacrosomal filament and a perinuclear amorphous component. The subacrosomal filament extends posteriorly into a tube-like invagination of the elongated nucleus. The middle piece is characterized by elongated mitochondria which spiral around the anterior part of the flagellum in an extended collar separated from the flagellum by an extracellular cleft, termed the central flagellar tunnel. In addition to the usual 9 × 2 + 2 axonemal pattern in flagella, 9 × 2 + 1 and 9 × 2 + 3 patterns also were observed. The end piece is represented by the free flagellum. Similarities and diversities of scorpionid spermatozoa are discussed with respect to systematic relationships.     

Cauda epididymal spermatozoa of the rufous hare wallaby, Lagorchestes hirsutus (Metatheria, Mammalia) imaged by electron and confocal microscopy

The ultrastructure of mature Lagorchestes hirsutus spermatozoa is described for the first time, revealing unusual aspects of sperm structure in macropodid species. The sperm head is ovoid rather than cuneiform, lacks a ventral nuclear groove and has an acrosomal distribution over approximately 85–90% of its dorsal surface. Immediately adjacent to the nuclear membrane the peripheral nucleoplasm in most spermatozoa form an irregular series of distinctive evaginations previously not described in the spermatozoa of any other marsupial. The midpiece is extremely thickened and short, containing no helical network or peripheral plasma membrane specializations. Axonemal structure is unspecialized with no connecting lamellae; dense outer fibres are closely adherent to axonemal doublets. The sperm morphology of this species is highly aberrant in comparison to other macropod taxa and supports the retention of Lagorchestes as a distinctive genus. In light of this new information, skeletal and serological data should be re‐evaluated to determine the true taxonomic and phylogenetic position of this species.     

Spermatozoa and spermatogenesis in the northern quahaug <Emphasis Type="Italic">Mercenaria mercenaria</Emphasis> (Mollusca,Bivalvia)

We studied the ultrastructure of spermatogenesis and spermatozoa in the northern quahaug, the clam Mercenaria mercenaria. Spermatogenetic cells gradually elongate. Mitochondria gradually fuse and increase in size and electron density. During spermatid differentiation, proacrosomal vesicles migrate towards the presumptive anterior pole of the nucleus and eventually form the acrosome. The spermatozoon of M. mercenaria is of a primitive type. It is composed of head, mid-piece, and tail. The acrosome shows a subacrosomal space with a short conical contour. The slightly curved nucleus of the spermatozoon contains fine-grained dense chromatin. The middle piece consists of a centriolar complex which is surrounded by four mitochondria. The flagellum has a standard “9 + 2” microtubular structure. The ultrastructure of spermatozoa and spermatogenesis of M. mercenaria shares a number of features with other species of the family Veneridae. M. mercenaria may be a suitable model species for further investigations into the mechanisms of spermatogenesis in the Bivalvia.     

Comparative sperm ultrastructure of<Emphasis Type="Italic"> Neodasys ciritus</Emphasis> and<Emphasis Type="Italic"> Musellifer delamarei</Emphasis>, two species considered to be basal among Chaetonotida (Gastrotricha)

The spermatozoa of two species supposed to be basal to Gastrotricha Chaetonotida, Neodasys ciritus and Musellifer delamarei, were studied in order to supply further elements to the understanding of sperm evolution in Chaetonotida, a group in which a fully parthenogenetic reproduction is dominant. Two considerably different sperm patterns were found: the spermatozoon of N. ciritus has a simple, conical acrosome, a short, condensed nucleus, few conventional mitochondria randomly arranged along the sperm head, and a 9×2+2 flagellum perpendicular to the sperm major axis. The spermatozoon of M. delamarei is a filiform cell with a simple acrosome, a partially condensed nucleus, four mitochondria at the nuclear base, and a flagellum with a 9×2+2 axoneme and large accessory fibers. Some sperm features of M. delamarei are comparable to those of Xenotrichulidae, the only other Chaetonotida producing conventional spermatozoa, whereas the sperm of N. ciritus appears different from all the other patterns known among Gastrotricha, thus knowledge of it does not help in solving the problem of the discussed phylogenetic position of the genus.     

Ultrastructure of spermiogenesis in the gastropod Calliotropis glyptus Watson (Prosobranchia: Trochidae)with special reference to the embedded acrosome

Testicular spermatozoa and sperm development in the archaeogastropod Calliotropis glyptus Watson (Trochoidae: Trochidae) are examined using transmission electron microscopy and formalin-fixed tissues. During spermiogenesis, the acrosome, formed evidently through fusion of Golgi-derived proacrosomal vesicles, becomes deeply embedded in the condensing spermatid nucleus. Two centrioles (proximal and distal), both showing triplet microtubular substructure, are present in spermatids—the distal centriole giving rise to the sperm tail and its associated rootlet. During formation of the basal invagination in the spermatid nucleus, centrioles, and rootlet move towards the nucleus and come to lie totally within the basal invagination. Mitochondria are initially positioned near the base of the nucleus but subsequently become laterally displaced. Morphology of the mature spermatozoon is modified from that of the classic primitive or ect-aquasperm type by having 1) the acrosome embedded in the nucleus (the only known example within the Mollusca), 2) a deep basai invagination in the nucleus containing proximal and distal centrioles and an enveloping matrix (derived from the rootlet), 3) laterally displaced periaxonemal mitochondria, and 4) a tail extending from the basal invagination of the nucleus. Implantation of the acrosomal complex and centrioles within imaginations of the nucleus and lateral displacement of mitochondria effectively minimize the length of the sperm head and midpiece. Such modifications may be associated with motility demands, but this remains to be established. The unusual features of C. glyptus spermatozoa, though easily derivable from ‘typical’ trochoid sperm architecture, may prove useful in delineating the genus Calliotropis or tracing its relationship to other genera within the trochid subfamily Margaritinae.     

Sperm morphology of five species of Colostethus (Anura, Dendrobatidae) from Brazil, with phylogenetic comments

We describe the sperm ultrastructure of five species of Colostethus from Brazil. The general structures of the spermatozoa (acrosomal complex and flagellar apparatus) were similar to species within the Bufonoidea. Colostethus brunneus, Colostethus sp. (aff. trilineatus), Colostethus nidicola and Colostethus sp. had biflagellated spermatozoa, whereas the spermatozoa of Colostethus stepheni had a single flagellum, a characteristic previously unknown for this genus. In addition, the spermatozoa of this species showed some mitochondria within the thick, undulating membrane. These characteristics are apparently common to dendrobatid species that have a single flagellum. Based on these findings, we tentatively consider biflagellarity to be an apomorphic condition in the Dendrobatidae. In addition, the results described here do not support the proposed regrouping of the ‘brunneus’ and ‘alagoanus’ groups in a monophyletic ‘trilineatus’ group.     

Ultrastructure of spermatids and spermatozoa in three polychaetes with modified biology of reproduction: Autolytus sp., Chitinopoma serrula,and Capitella capitata

Unlike the primitive type of spermatozoon found in most polychaetes, the spermatozoon of Autolytus has a bilateral symmetry with elongated nucleus, and the mitochondria surround the posterior part of the nucleus. A rather large disk-shaped acrosome is situated along one side of the anterior part of the nucleus. From the anterior margin of the distal centriole emerge long striated rootlets, which run along the nuclear envelope to the anterior part of the nucleus. The spermatozoon of Chitinopoma serrula has an elongated, slightly bent nucleus, a thimble-like acrosome apically on the anterior surface of the nucleus, and an elongated middle piece containing 4 rod-like mitochondria developed from spherical mitochondria surrounding the basal part of the tail flagellum. In the spermatozoon of Capitella capitata, both nucleus and middle piece are elongated compared to the primitive type. The large and conical acrosome is placed asymmetrically at the nucleus and consists of an acrosomal vesicle and subacrosomal substance. The greater part of the middle piece forms a collar around the initial part of the tail flagellum. The cytoplasm of the collar contains granular material. One or two small mitochondria lie around the 2 centrioles at the base of the nucleus.

These types of spermatozoa represent early steps in the evolution of modified spermatozoa combined with changed biology of reproduction. The modified spermatozoa are larger than the primitive ones.     

ELECTRON MICROSCOPIC OBSERVATIONS OF GUINEA PIG SPERMATOZOA PENETRATING EGGS IN VITRO*

Guinea pig ovarian oocytes matured in vitro were inseminated in vitro with capacitated, acrosome-reacted spermatozoa and sperm penetration through the zona pellucida and into the egg cytoplasm were examined. Sperm heads passing through the zona pellucida had already lost all their acrosomal elements except for the inner acrosomal membrane and the equatorial segment. It was often observed that the texture of the zona material around the sperm head was distorted, giving the impression that the zona pellucida was parted, at least partially, by a shearing force produced by the sperm head advancing through the zona. When eggs were freed from their zonae pellucidae and inseminated, the acrosome-reacted spermatozoa immediately bound to the egg surfaces and began to fuse with the eggs; whereas the spermatozoa with intact acrosomes failed to do so. Fusion began between the egg plasma membrane and the sperm plasma membrane at the central region of the sperm head. The anterior half of the sperm head was engulfed by the egg in a phagocytic fashion, while its posterior half was incorporated into the egg by a fussion between egg and sperm plasma membranes. Incorporation of the sperm tail into the egg was achieved by fusion between the sperm and egg plasma membranes.     

Structure and function of the spermathecal complex in the phlebotomine sandflyPhlebotomus papatasi Scopoli (Diptera: Psychodidae): II. Post-copulatory histophysiological changes during the gonotrophic cycle

The spermathecal complex ofPhlebotomus papatasi Scopoli (Diptera: Psychodidae) undergoes histological and physiological changes during its gonotropic cycle. The present histochemical study revealed a mucopolysaccharide secretory mass in the spermathecae of the newly emerged sandfly. Sperm competition occurs when two or more males compete to fertilize an ovum in the female reproductive tract. In this study, spermatophores of two or more competing males were deposited at the base of the spermathecal ducts, which originate from the female bursa copulatrix. This suggests that females play a role in sperm displacement, which is defined as any situation in which the last male to mate with a female fertilizes maximum number her eggs. A blood meal ingested by the female for ovary development and egg laying stimulates the release of sperm from the spermatophore. The spermatozoa then migrate to the lumen of the spermatheca. The ultrastructure of spermatozoa comprises a head with double-layered acrosomal perforatorium, an elongate nucleus, and the axoneme with a 9 + 9 + 0 flagellar pattern. This axomene differs from the aflagellate axoneme of other Psychodinae. Morphological changes, such as the casting off of the acrosomal membrane, and histological changes in the spermatophore are also described. Mating plugs that have been described previously in sandflies appear to be artefacts. Females ofP. papatasi may be inseminated more than once during each gonotrophic cycle, and additional inseminations may be necessary for each cycle. The relationships between the volumes of the sperm and the spermatheca were calculated to determine sperm utilization and fecundity ofP. papatasi. As the females ofP. papatasi mate polyandrously, the anatomical and physiological complexity of the spermathecal complex may be related to post-copulatory sexual selection.     

Comparative study of sperm ultrastructure of Donax hanleyanus and Donax gemmula (Bivalvia: Donacidae)

The ultrastructure of bivalve spermatozoa can be species‐specific and often provides important taxonomic traits for systematic reviews and phylogenetic reconstructions. Young individuals of the Donacidae species Donax hanleyanus are often identified as samples of Donax gemmula. Hence, the spermatozoa ultrastructure of both species was described in the present work, aiming to identify characters that could be useful for further taxonomic and phylogenetic analyses. D. hanleyanus and D. gemmula spermatozoa were different especially in relation to acrosomal characteristics and chromatin condensation. The spermatozoon produced by D. hanleyanus had a nucleus (exhibiting granular chromatin with a rope‐like appearance) capped by a long and conical acrosomal vesicle, which extended itself outward beyond the anterior nuclear fossa. Otherwise, the nucleus of the sperm cell of D. gemmula showed well‐compacted chromatin, and its acrosome, which was partially inserted into the anterior nuclear fossa, had a bubble‐like tip. In conclusion, the conspicuous ultra‐structural differences found between the spermatozoan morphologies were helpful for the discrimination of the species. In conclusion, our results suggest that analyses of sperm ultrastructure of the bivalves in the family Donacidae can be valuable to investigate their taxonomic relatedness. The present results also contribute to assess the monophyletic status of the family.