Fine structure of unusual spermatozoa and spermiogenesis of the mite Megisthanus floridanus Banks, 1904 (Acari: Gamasida: Antennophorina)

The aflagellate spermatozoa of the gamasid mite Megisthanus floridanus are characterized by a large vacuole which contains a cytoplasmic column protruding into the vacuole from the region defined as the posterior part of the cell. The membrane of the column and the inner membrane of the posterior part of the cytoplasmic mantle (outer sheath) surrounding the vacuole bear numerous so-called cellular processes. However, most of the outer sheath is reduced and represented solely by a very thin membrane-like envelope. The posterior part of the cell bears extensive folds. The cell, or, more precisely, the column, shows a deep posterior invagination. This invagination contains extracellular material composed of thin filaments or strands. Peripheral folds emerging from the posterior rim of the cell form a thin-walled tube that contains the same material as the invagination. The elongated nucleus is attached to a peculiar acrosomal complex consisting of a flat acrosomal cisterna that parallels most of the cell membrane, an attachment cone, and a short acrosomal filament which is embedded in a narrow canal within the nucleus. The spermatozoa of M. floridanus represent a peculiar version of the vacuolated type of sperm known to be plesiomorphic within the anactinotrichid Acari. Some details of spermiogenesis are described and consequences for phylogenetic and systematic considerations are discussed.     

The ultrastructure of the peculiar synspermia of some Dysderidae (Araneae, Arachnida)

The present study reports on the ultrastructure features of spermatozoa and spermatogenesis of several species of Dysderidae (Dysdera crocata, Dysdera erythrina, Dysdera ninnii, Harpactea arguta, Harpactea piligera, Dasumia taeniifera). Dysderid spiders are known to possess a peculiar sperm transfer form known as synspermia, characterized by fused spermatozoa surrounded by a secreted sheath. Until now the exact mode of formation of the synspermia is unknown. The present study demonstrates that the spermatids are connected via narrow cell bridges during the entire spermiogenesis as is usual, although in Dysderidae they do not separate at end of the spermiogenesis. Instead, they fuse completely within the testes shortly after the spermatid has coiled to get a spherical shape. The number of fusing sperm cells is different in the different observed species. The species of the genus Harpactea thus have synspermia consisting of two fused spermatozoa; whereas in the species of the genus Dysdera four sperm cells are fused and in D. taeniifera at least three spermatozoa are fused. In contrast with other known families with this peculiar form transfer of sperm, the synspermia in Dysderidae are mainly characterized by a conspicuous vesicular area which extends through the entire synspermium surrounding the cell organelles. Thus, all main cell components (e.g., nucleus, acrosomal vacuole, and axoneme) are covered by the vesicular membrane. The vesicular area seems to be functional and probably it is important during sperm activation in female genital system. Simultaneously to the extension of the vesicular area, the synspermium accumulates large amounts of glycogen. The glycogen is mainly located around the centriolar adjunct and along the axoneme accompanying the postcentriolar elongation of the nucleus. A further peculiar feature is the extremely elongated acrosomal vacuole, which seems to be synapomorphic trait for sperm cells of dysderids. Interestingly, spermatogenesis, including the fusion, exclusively occurs within the testes (in contrast to the formation of coenospermia). In the vas deferens only synspermia were found. The secreted sheath surrounding the spermatozoa is finally synthesized in the parts of the vasa deferentia, which are close to the genital opening where numerous vacuoles and microvilli are seen in the epithelial cells.     

Spermatozoon ultrastructure of Xenoturbella bocki (Westblad 1949)

Obst, M., Nakano, H., Bourlat, S.J., Thorndyke, M.C., Telford, M.J., Nyengaard, J.R. and Funch, P. 2011. Spermatozoon ultrastructure of Xenoturbella bocki (Westblad 1949). —Acta Zoologica (Stockholm) 92 : 109–115. Here, we report on the sperm ultrastructure of Xenoturbella bocki (Westblad 1949), which we studied for the first time in detail using light, scanning and transmission electron microscopy. The mature spermatozoa are of the bilaterian primitive type, also called aquasperm and develop as uniflagellate sperm consisting of a round head with distinct mitochondria at the base and a 9+2 flagellum of approximately 42 μm in length. The acrosomal complex consists of a small, round electron translucent acrosomal vesicle and a subacrosomal base. There is no separate midpiece, and the mitochondria surround the proximal and distal centriole in the posterior part of the head. The primitive structure of the spermatozoa suggests that these fertilize the egg by free spawning, probably the ancestral mode of fertilization in early bilaterians. When compared to the spermatozoa of other metazoans, we find that the arrangement of organelles in the Xenoturbella sperm shows similarities to a wide range of protostome and deuterostome taxa and does not seem to indicate any particular phylogenetic relationship.     

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.     

Sperm morphology of salamandrids (Amphibia, Urodela): implications for phylogeny and fertilization biology

Mature spermatozoa belonging to four salamander species, Salamandrina terdigitata, Triturus alpestris, Triturus carnifex and Triturus vulgaris, have been investigated by electron microscopy. The sperm ultrastructure of these species was compared with that of previously examined urodeles (36 species and 20 genera) and with that of anurans and caecilians. Many phylogenetic considerations may be inferred as a consequence of comparative spermatology. Urodela appears to be a monophyletic order characterized by three sperm synapomorphies: the acrosomal barb, nuclear ridge and marginal filament. Cryptobranchoidea are confirmed to form a monophyletic suborder having two synapomorphic characters: absence of mitochondria in the tail, and cylindrical shape of the tail axial rod. Within the family Salamandridae, sperm morphology confirms the phylogenetic distance between Salamandrina and Triturus, as already pointed out on the basis of molecular and morphological characters. The very complex ultrastructure of spermatozoa confirms a previous opinion that internal fertilization is the ancestral condition of the Amphibia.     

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.     

Spermatozoa and sperm packages of the European troglophylous scorpion Belisarius xambeui Simon, 1879 (Troglotayosicidae, Scorpiones)

Studies on the sperm morphology in scorpions are rare, but the existing investigations already revealed a remarkable interfamiliar diversity. The present study reports for the first time on the spermatozoa and sperm packages of a representative of the family Troglotayosicidae, the troglophylous species Belisarius xambeui. The spermatozoa are characterized by (1) a thread-like nucleus, which is slightly bent anteriorly; (2) an asymmetrical cap-like acrosomal vacuole, which encloses the anterior tip of the nucleus; an acrosomal filament is absent; (3) an axoneme with a 9 + 0 microtubular pattern; (4) a midpiece consisting of elongated mitochondria coiling around the axoneme; the number can vary between 3 and 6 (mostly 4). At the end of spermiogenesis, the spermatozoa aggregate in order to form oval-shaped sperm packages in which all sperm cells show the same orientation. A single package consists of approximately 150 sperms. A secretion sheath is always absent. The present results might provide new characters for further systematic studies and their phylogenetic implications are briefly discussed.     

Spermatozoal ultrastructure in three Atlantic solenocerid shrimps (Decapoda, Dendrobranchiata)

The spermatozoal ultrastructure in three solenocerid shrimps (Solenocera membranacea, S. africana, and Pleoticus muelleri) from different Atlantic locations was examined with the aim of increasing understanding of the phylogenetic relationships in the Dendrobranchiata. A considerable structural similarity between the sperm of these species and those of penaeid shrimps supports a close affinity between the Penaeidae and Solenoceridae. However, significant differences in the sperm morphology of the previously investigated sicyoniids (namely, a greater complexity of the acrosomal complex) suggest evolutionary separation of the Sicyoniidae from the assemblage Penaeidae-Solenoceridae. Two ultrastructural features distinguish the spermatozoa of the three studied solenocerids from penaeid sperm: 1) separation of the plasma and acrosome membranes at the base of the spike and anterior region of the cap, and 2) asymmetry of the acrosomal cap, which appears to be a synapomorphy of the group. No striking ultrastructural differences were found between the spermatozoa of the closely related species S. membranacea and S. africana, whereas a great number of morphological differences separate the spermatozoa of Pleoticus from those of Solenocera (e.g., shape of the acrosomal cap, structural arrangement of the contents of the whole acrosome vesicle, thickness and distribution of the cytoplasm, and external shape of the spike).     

Spermatozoa and spermiogenesis of Holocnemus pluchei (Scopoli, 1763) (Pholcidae, Araneae)

Until now, the knowledge on pholcid spermatozoa is based on two species, Pholcus phalangioides and, incompletely, Holocnemus pluchei. To complete this knowledge and to reveal more potential phylogenetic characters, we have investigated sperm ultrastructure and spermiogenesis of H. pluchei. We found that the sperm cells of this species are clearly different from those of P. phalangioides with respect to: (1) the lack of specialization in the cylindrical acrosomal vacuole; (2) a nuclear canal which is located in the periphery and not in the center of the nucleus; (3) a more prominent postcentriolar elongation of the nucleus; (4) the presence of "inner microtubules" in the implantation fossa in early and mid-spermatids; (5) the absence of a helical band of nuclear material; (6) the proximal centriole which is not prolonged; (7) the types of secretion in the seminal fluid (only two types in H. pluchei). Similarities in the spermatozoa of both species concern: (1) a large implantation fossa which contains large amounts of glycogen in mature spermatozoa; (2) absence of a centriolar adjunct; (3) an axonemal basis located in the posterior part of the implantation fossa; (4) the formation of the so-called cleistospermia in the vas deferens. Our results strongly support systematic relationships within Pholcidae placing these two species in different subgroups.     

Fine structure of spermiogenesis, spermatozoa and spermatophore of Saxidromus delamarei (Saxidromidae, Actinotrichida, Acari)

Ultrastructural details of spermiogenesis, spermatozoa and the spermatophore of the early derived actinedid mite Saxidromus delamarei are described. Spermatids and mature sperm cells are provided with up to four acrosomal complexes and nuclei derivatives (chromatin bodies). Due to this reason, the sperm cells may be classified as synspermia, a sperm type found only in some spiders until now. The acrosomal complex is composed of a remarkably complicated vacuole and filament. Other peculiarities of sperm structure correspond to those found in prostigmatic mites, i.e. penetration of the chromatin body by the acrosomal filament and the presence of peripheral invaginations of the plasmalemma. The sperm cells are covered by a thin secretion layer of probably proteinaceous material. Stalked spermatophores are rather large, but simply structured and contain relatively few sperm cells. The results are discussed taking systematical and behavioural aspects into account. In particular, it is suggested that the peculiar mating behaviour of these mites secures both sperm transfer and first male's sperm priority and that this allowed reduction of sperm numbers.     

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.     

The effects of anticalmodulin drugs on the ultrastructure of boar spermatozoa

Trifluoperazine, N-6-aminohexyl-5-chloro-1-naphthalene sulfonamide (W7), and calmidazolium are known to be calmodulin inhibitors and cell membrane soluble substances. In mammalian spermatozoa, calmodulin is present and is retained to mediate several sperm processes, such as sperm activation, sperm-egg fusion, microtubule disassembly, etc. We examined the effects of anticalmodulin drugs on the ultrastructure of freshly ejaculated boar spermatozoa. Whereas all the drugs, at the low concentrations tested, appear to prevent acrosomal alterations, at higher concentrations, they induced these alterations. Unexpectedly, the outer acrosomal membrane appeared to be more sensitive to the drugs than the plasma membrane; vesicles formed within the acrosome from the outer acrosomal membrane even when plasma membrane maintained its structural integrity. These findings were confirmed by the analysis carried out by fluorescent light microscopy by utilizing fluoresceinated Ricinus communis agglutinins to specifically stain the acrosomes.     

Ultrastructural observations of spermatozoa of several tetragnathid spiders with phylogenetic implications (Araneae, Tetragnathidae)

The present study reports on the ultrastructure of spermatozoa and spermatogenesis of 12 tetragnathid spiders (10 Tetragnatha species [T. boydi, T. dearmata, T. extensa, T. montana, T. nigrita, T. obtusa, T. pinicola, T. reimoseri, T. shoshone, T. striata]; Pachygnatha listeri and Metellina segmentata). All species develop typical cleistospermia with a coiled nucleus in the center and a coiled axoneme in the periphery of the cell. Remarkable differences in the sperm ultrastructure of the investigated species comprise the shape of the main sperm cell components (nucleus, acrosomal complex, implantation fossa, and centriolar complex). Within the observed Tetragnatha species, three types of sperms were characterized: T. montana-type, T. boydi-type, and T. striata-type. The highly derivative T. montana-type is characterized by the following remarkable features: an extremely elongated nucleus, shaped like a corkscrew and twisted around the axoneme (before coiling); a deep implantation fossa; a corkscrew-shaped acrosomal vacuole; after the coiling process, the nucleus is coiled five to six times in the center of the spermatozoon and the axoneme is coiled five to six times peripheral to the nucleus. The T. boydi-type hardly differs from the T. montana-type, but is remarkable due to the triangular-shaped nucleus (in cross section). The T. striata-type differs especially by a peculiar acrosomal vacuole with a long, slightly curved process and a short appendix, as well as a nucleus that describes only three loose coils around the axoneme (before coiling). The spermatozoa of Pachygnatha listeri and especially Metellina segmentata differ strikingly from the described Tetragnatha-types and are similar to more primitive araneomorph spermatozoa, such as Hypochilus pococki. The described Tetragnatha-types completely correspond with Okuma's (1988a,b, J Fac Agr Kyushu U 32:165-181, 32:183-213) classification of Tetragnatha species. Furthermore, our results suggest an early derivative systematic position of Pachygnatha within Tetragnathinae and the position of Metellina within the Tetragnathidae.     

Ultrastructural observations of spermatozoa and spermiogenesis in Wandella orana Gray, 1994 (Araneae: Filistatidae) with notes on their phylogenetic implications

Spermatozoa and spermiogenesis of the prithine filistatid spider Wandella orana are described. The spider produces coenospermia, i.e. sperm aggregations that include several single sperm cells commonly surrounded by a secretion sheath. One sectioned coenospermium in W. orana contains at least five spermatozoa. During copulation many coenospermia are transferred into the female. Coenospermia are regarded as a peculiar transfer form of sperm which occurs in early derivative spiders such as Liphistiomorphae and Mygalomorphae. The only exception which was found in Araneomorphae until now was the filistatine spider Filistata insidiatrix. Our observation is the second case and supports the view that Filistatidae represent an early derivative taxon. Furthermore, the individual sperm cells show characteristics which also may be regarded as being plesiomorphic. There is a cone-shaped acrosomal vacuole, a very long acrosomal filament, a rather stout nucleus and a small implantation fossa. The axoneme shows the 9x2+3 pattern of microtubules which is synapomorphic in Megoperculata (Uropygi, Amblypygi and Araneae). The finding of coenospermia in two distant taxa of Filistatidae may have consequences for phylogenetic and systematic considerations.     

A dendrobranchiate, Peisos petrunkevitchi (Decapoda, Sergestidae), with reptant-like sperm: a spermiocladistic assessment

The spermatozoon of Peisos petrunkevitchi differs significantly from those of any of the investigated dendrobranchiates in the anterior fusiform acrosome, lacking a spike, and embedded in (instead of capping) the nuclear region. In contrast, the position of the acrosome and the internal arrangement of its contents, as well as the apomorphic presence of a pair of centrioles (absent in all known dendrobranchiate spermatozoa) at the base of the acrosomal perforatorium, indicate a close affinity between this sperm plan and that found in reptants, especially anomurans and brachyurans. Based on the present and previous observations on decapod spermatozoal ultrastructure, we review the phylogeny of dendrobranchiate spermatozoa in the perspective of most recent phylogenetic analyses of malacostracan crustaceans.     

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.