Spermiogenesis and spermatozoa ultrastructure in Salminus and Brycon, two primitive genera in Characidae (Teleostei: Ostariophysi: Characiformes)

In Salminus, spermiogenesis is cystic and gives origin to a type I aquasperm. Spermatid differentiation is characterized by chromatin condensed into thick fibres, nuclear rotation, nuclear fossa formation, cytoplasmic channel formation, mitochondrial fusion producing long and ramified mitochondria, and the presence of several membranous concentric rings around the plasma membrane that encircles the cytoplasmic channel. In Salminus and Brycon, spermatozoa are very similar. They exhibit a spherical nucleus and chromatin condensed into fibre clusters, and a deep nuclear fossa. They show a long midpiece with few elongate mitochondria at the initial region and a cytoplasmic channel completely encircled by one or two membranous concentric rings. The flagellar axis is perpendicular to the nucleus and exhibits the classic axoneme (9 + 2). The very strong similarity observed between Salminus and Brycon spermatozoa supports the hypothesis that these subfamilies are likely to have a monophyletic origin.     

Different models of tubificid spermatozeugmata

The structure of the spermatozeugmata of different tubificid species belonging to the subfamilies Tubificinae, Limnodriloidinae and Phallodrilinae was compared to the one of Tubifex tubifex. It was concluded that the spermatozeugmata of Tubificoides and Clitellio (Tubificinae) were very similar to the Tubifex ones in having a parallel arrangement of the central fertilizing sperm surrounded by a cortex made of packed atypical spermatozoa. On the contrary, Kaketio ineri and Marcusaedrilus tuber (Limnodriloidinae) and Bathydrilus formosus (Phallodrilinae) show an organization typical for each species but different from that of Tubifex mainly in lacking a double sperm line. A more comprehensive definition of the spermatozeugmata is proposed and more attention to the morphology of the sperm bundles when describing new oligochaete species is suggested, since it is possible to use this information for taxonomic and phylogenetic purposes.     

Structure of sperm,spermatozeugmata and ‘lateral organs’ in the bivalve Arthritica (Galeommatoidea: Leptonidae)

The position and structure of paired ‘lateral organs’ in the foot of Arthritica semen and Arthritica bifurca might indicate a chemosensory function. In both species part of the organ is also glandular. In A. semen the glandular epithelium is detached piecemeal and, probably by means of the foot, is moved to and grafted upon the gills of the same individual. The transferred epithelia appear as disk‐shaped actively secretory ‘gill bodies’ which, attached to the abfrontal side of the inner demibranch, replace the ordinary unciliated gill epithelium. The secretion is liberated into the suprabranchial chamber, which serves as a marsupium, but its function is uncertain. Arthritica semen is a protandric hermaphrodite and produces very large ova that undergo a direct development that results in a non‐planktonic lecithotrophic crawling juvenile stage. The sperm cells have filiform nuclei that are straight in the euspermatozoa and more or less helicoidal in what is considered to represent paraspermatozoa. By a process of aggregation, spermatozeugmata are formed which consist exclusively either of euspermatozoa or paraspermatozoa. Spermatozoa are stored in the oviduct in A. semen but in paired seminal receptacles in A. bifurca.     

Giant,accordioned sperm acrosomes of the greater bulldog bat,Noctilio leporinus

Sperm of the greater bulldog bat Noctilio leporinus display an architecture that is totally unique among mammalian spermatozoa. The sperm head of Noctilio is extraordinarily large and flat and lies eccentrically with respect to the sperm tail. The major portion of the atypically large acrosome lies anterior to the nucleus and is shaped into a dozen accordionlike folds that run parallel to the long axis of the sperm. The ridge of each fold is shaped into ∼60 minute, evenly spaced rises that extend along the entire length of the fold. We speculate that acrosome ridges may serve to strengthen the sperm head during transport. Mol. Reprod. Dev. 48:90–94, 1997 © 1997 Wiley-Liss, Inc.     

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.     

Binucleate and biflagellate spermatozoa in Tricholepidion gertschi Wygodzynsky (Insecta, Zygentoma)

The phenomenon of sperm pairing is known from some species of the apterygotan insect order Zygentoma, and has been described as the close apposition of two sperm cells. When released from the testes, they are single cells; pairing taking place in the deferent ducts. In a study of the relic species Tricholepidion gertschi, Zygentoma, sperm pairing was found to be due to a true fusion of two partners along their entire sperm head regions. The spermatozoon thus formed has two acrosomes, two nuclei and two separate sperm tails. The biflagellate spermatozoon swims with coordinated movements of its two flagella only when the two flagella lie close together but is totally uncoordinated when separate. The spermatozoon is about 50 microm long, thus much shorter than those of related apterygotan species. The mechanism of sperm cell fusion is unclear, although it appears that a 55-nm wide layer of electron dense substance, here termed the peripheral lamina, may play a role in delimiting the extent of sperm fusion.     

Morphology of the male reproductive system and sperm ultrastructure of the egg parasitoid Gryon pennsylvanicum (Ashmead) (Hymenoptera,Platygastridae)

Gryon pennsylvanicum is a platygastrid hymenopteran that has lately received increasing attention in Europe due to its possible use in biological control of the conifer seed bug pest Leptoglossus occidentalis. Here the male reproductive system and the spermatogenesis of this species, along with those of Gryon muscaeformis, are examined ultrastructurally for the first time. The male genital system is formed by a pair of testes, each containing only one follicle, a pair of accessory glands and deferent ducts connected to a single ejaculatory duct. All the stages of spermatogenesis are described in detail. Characteristic features of the Gryon spp. sperm, which are 100 μm long, are the presence of a polygonal nucleus, only one mitochondrial derivative, the occurrence of the centriole adjunct and a typical insect 9 + 9 + 2 flagellar axoneme. The single derivative, however, results from a process in which one of the two mitochondria is lost during spermiogenesis. Unlike in other insects, two centrioles occur in spermatids as a consequence of the ameiotic parthenogenesis. These characteristics stand as a valuable tool for phylogenetic inferences. Furthermore this study suggests a useful strategy for laboratory mass rearing.     

Ultrastructural studies on the reproductive system of gyrocotylidea and amphilinidea (Cestoda): Spermatogenesis, spermatozoa, testes and vas deferens of Gyrocotyle

The ultrastructure of the vas deferens, testes, spermatogenesis and spermatozoa of Gyrocotyle urna and G. parvispinosa is described. The vas deferens is ciliated and syncytial. Within the testes primary spermatocytes arise from the primary spermatogonia by incomplete mitotic divisions; the primary spermatocytes undergo two meiotic divisions leading to spermatids. In early spermatids microtubules are formed at the cell periphery. Later the spermatozoal cytoplasm (the ‘middle-piece’) grows out and the two spermatozoal flagella with their typical 9 + ‘1’ axonemes are formed. During ciliogenesis the flagella are at an angle of about 60° to the axis of the middle-piece. The flagella are inserted into basal bodies terminating in striated rootlets. Subsequently, the nucleus and isolated mitochondria migrate into the central axis. The angle between the flagella and the axis decreases; the flagella are incorporated to form the spermatozoon. In mature spermatozoa no basal body or rootlet elements were found. The phylogeny of parasitic Platyhelminthes is discussed with respect to the evolution of spermatozoa. The reduction of the acrosinoid granules which are found in spermatozoa of free-living Platyhelminthes and the incorporation of the spermatozoal flagella into the sperm body constitute autapomorphies of the Neodermata (the parasitic Platyhelminthes). Included in the Cestoda because of several common derived characters, Amphilinidea and Gyrocotylidea are the only cestodes with spermatozoa containing mitochondria. Their absence in Cestoidea—all taxa with a six-hooked larva and other characteristics—is an autapomorphy of this group.     

An ultrastructural analysis of mature sperm from the crayfish Pacifastacus leniusculus,Dana

Sperm from the crayfish, Pacifastacus leniusculus, resemble other reptantian sperm in that they are composed of an acrosome, subacrosomal region, nucleus, membrane lamellar complex, and spikes which radiate from the nuclear compartment. The acrosome (PAS positive vesicle) can be subdivided into three regions: the apical cap, crystalline inner acrosomal material, and outer acrosomal material which is homogeneous except for a peripheral electron dense band. The nucleus contains uncondensed chromatin and bundles of microtubules which project into the spikes. The orientation of the microtubule bundles relative to the nuclear envelope near the base of the subacrosomal region suggests that the nuclear envelope may function in the organization of the spike microtubules.