Sperm Cells of a Primitive Strepsipteran

The ultrastructure of the spermatheca of Mordellistena brevicauda (Coleoptera, Tenebrionoidea) and the associated bacterial cells

The ultrastructural study on the female reproductive system of the beetle M. brevicauda (Mordellidae) confirmed the positive correlation between the length of the sperm and the size of the female seminal receptacle (Spermatheca). The spermatheca of the species is characterized by an apical bulb-like structure where the spermathecal duct forms numerous folds filled with sperm. At this level many bacterial cells are present intermingled with the duct folds. Some are organized in large structures, such as bacteriomes, while other are single bacteriocytes. The latter are often found near the basal lamina of duct epithelium. In addition, some bacteria are visible in the cytoplasm of the duct epithelial cells. Interestingly, bacterial cells have never been observed in the duct lumen. The possible function of the bacterial cells is discussed.

The sperm structure of the Scraptiidae (Coleoptera; Tenebrionoidea)

The sperm ultrastructure of two members of the Scraptiidae Anaspis pulicaria and A. lurida was studied. The results confirm the general organization of the sperm in the superfamily Tenebrionoidea. The sperm bundles at the end of the spermiogenesis show the same peculiar antiparallel distribution at the two opposite poles of the germ cyst, observed in other Tenebrionoidea. The sperm have a bi-layered acrosome, a long cylindrical nucleus with two infoldings at its basal region, two elliptical equal mitochondrial derivatives and two triangular accessory bodies. The flagellar axoneme has the common 9 + 9 + 2 microtubular pattern that at the tail end results disorganized. All these sperm characteristics are quite similar to those found in Pythidae, a closely related family, according to molecular data.

New Findings on the Sperm Structure of Tenebrionoidea (Insecta, Coleoptera)

Simple Summary

Tenebrionoidea, with more than 30,000 described species and 30 currently recognized families, is a superfamily of difficult taxonomy. The aim of this work is to support the basal position of the Mordellidae among the beetle tenebrionoids. They have a low number of sperm cells per cysts, contrary to the more derived families of the group; moreover, their sperm are not distributed in two bundles at the opposite poles of the cysts, as occurs in the higher taxa, but their sperm flagella form a loop in the median region so that sperm nuclei are positioned close to the tail end. The sperm structure of two members of higher families, Oedemeridae and Tenebrionidae, are investigated to confirm the data mentioned above. The sperm looping, which also occurs in the closely related Ripiphoridae, could be the consequence of the growth asynchrony between the cyst size and the sperm length. The Mordellidae sperm are characterized, not by small mitochondrial derivatives and accessory bodies, but by a peculiar stiff and immotile thin flagellar posterior region provided with only accessory tubules.

Abstract

The sperm ultrastructure of a few representative species of Tenebrionoidea was studied. Two species belong to the Mordellidae (Mordellistena brevicauda and Hoshihananomia sp.), one species to Oedemeridae (Oedemera nobilis), and one species to Tenebrionidae (Accanthopus velikensis). It is confirmed that Mordellidae are characterized by the lowest number of spermatozoa per cyst (up to 64), a number shared with Ripiphoridae. In contrast, in the two other families, up to 512 spermatozoa per cyst are observed, the same number present, for example, in Tenebrionidae. Also, as in the other more derived families of tenebrionoids studied so far, during spermatogenesis in O. nobilis and A. velikensis, sperm nuclei are regularly distributed in two sets at opposite poles of the cysts. On the contrary, the Mordellidae species do not exhibit this peculiar process. However, during spermiogenesis, the bundles of sperm bend to form a loop in their median region, quite evident in the Hoshihananomia sp., characterized by long sperm. This process, which also occurs in Ripiphoridae, probably enables individuals to produce long sperm without an increase in testicular volume. The sperm looping could be a consequence of the asynchronous growth between cyst size and sperm length. The sperm ultrastructure of the Mordellidae species reveals that they can be differentiated from other Tenebrionoidea based on the shape and size of some sperm components, such as the accessory bodies and the mitochondrial derivatives. They also show an uncommon stiff and immotile posterior flagellar region provided with only accessory tubules. These results contribute to a better knowledge of the phylogenetic relationship of the basal families of the large group of Tenebrionoidea.

The sperm ultrastructure of members of basal Tenebrionoidea (Coleoptera)

The sperm ultrastructure of some beetles of Tenebrionoidea was studied with particular attention to those of the Ripiphoridae, Mordellidae, and Meloidae. These three groups are often thought to form a clade, which is the sister group of the remaining Tenebrionoidea. The testes of the two former families have thinner but longer spermatic cysts containing fewer and longer sperm. Within each cyst all sperm cells have the same orientation, but cross sections showed that the orientation of the axonemes alternate between adjacent cysts, possibly due to the cysts bending on themselves. In both families the sperm has a bilayered acrosome and the flagellum, which shows mitochondrial derivatives starting laterally to the nuclear base, has a typical 9 + 9+2 axoneme with accessory tubules provided with 16 protofilaments in their wall, and well-structured triangular shaped accessory bodies. In Mordellistena sp (Mordellidae) sperm, both mitochondrial derivatives and accessory bodies are somewhat asymmetrical. Moreover, the flagellum shows a very thin and long tail end provided with only accessory tubules. Meloidae species have testes with thicker sperm cysts containing numerous shorter sperm. Within the individual cysts the sperm flagella exhibit an alternating orientation of their axonemes as consequence of a peculiar spermatogenetic process. The flagellar structure is similar to that of the above-mentioned species, but the accessory bodies are not well defined and constituted by fuzzy material. In Mylabris hieracii (Meloidae) sperm, the acrosome is flat with a conspicuous perforatorium and its nucleus has a peculiar quadrangular section. Berberomeloe majalis sperm has a large acrosome with an unusual pentagonal perforatorium. The centriolar structure of Mylabris variabilis shows a complex of dense radial links connecting the microtubular structures to the plasma membrane. These results suggest that Ripiphoridae have a closer relationship with Mordellidae than with Meloidae. These findings are in agreement with results obtained with molecular data.

The reproductive system of Ceracis cornifer (Mellié) and first description of sperm structure in a minute tree-fungus beetle (Tenebrionoidea: Ciidae)

Studies on the spermatogenesis of Tenebrionidae beetles (Tenebrionoidea) have shown an unusual organization of spermatozoa, in which they are arranged antiparallelly within the testicular cysts. Despite such works, many taxa of Tenebrionoidea remain to be studied, including the minute tree-fungus beetles (Ciidae). Among the challenges in the study of the internal morphology of ciids is their small body size (about 2 mm or less), which makes dissections and comparisons extremely arduous. Here, we provide, for the first time, an anatomical and histological study of the reproductive system (female and male) and a description of sperm structure of Ceracis cornifer Mellié (Ciidae), under light and transmission electron microscopes. We pointed out the growth of a single oocyte at a time in females, aside of evidences toward a constant copulatory activity that can provide a continuous supply of sperm in their small spermatheca. In males, sperm have an antiparallel organization within the cysts, a condition observed so far only in members of Tenebrionoidea. Furthermore, we observed sperm dimorphism in C. cornifer. We described, for the first time, this characteristic in a Tenebrionoidea taxon, and discussed the possible relationship between sperm dimorphism and antiparallel organization of sperm.

The sperm ultrastructure of Pytho depressus (Linnaeus, 1767) (Coleoptera, Pythidae)

The sperm ultrastructure of Pytho depressus (Pythidae) is described in this study. The sperm are short cells, about 85-90 μm long, with an acrosome consisting of three layers, a cylindrical nucleus, which at its base has the initial region of two mitochondrial derivatives. The flagellum has two well-developed triangular accessory bodies, and a 9 + 9+2 axonemal pattern with accessory tubules provided with 16 protofilaments in their wall. The structure and shape of the accessory bodies are diagnostic characters within the superfamily. The sperm morphology of P. depressus can be easily distinguished from those of Ripiphoridae, Meloidae and Tenebrionidae. The P. depressus sperm are organized in cysts as in other species of the group but the sperm are not well aligned and show an antiparallel orientation, a feature also observed in other tenebrionids. The phylogenetic implications of the observed sperm features are discussed in the context of comparative sperm ultrastructure of other insect species.

Spermiogenesis of the hangingfly Terrobittacus implicatus (Huang and Hua) (Mecoptera: Bittacidae)

The structure of spermatozoa is able to provide valuable characters in resolving phylogenic relationships in Metazoa, especially in insects. Such data, however, are greatly deficient in Mecoptera. Here, we studied the spermiogenesis and ultrastructure of sperm in the hangingfly Terrobittacus implicatus (Huang and Hua) using transmission electron microscopy. The results show that the spermatogenesis of T. implicatus occurs within sperm cysts, following a pattern commonly found in insects. The microtubular doublets of spermatid axoneme exhibit a hooklike projection from the B-subtubule in the early period, but the projection disappears in the mature stage. The mature spermatozoon of T. implicatus is a filiform cell that is pronouncedly elongated and has a bi-layered acrosome, a nucleus with two lateral longitudinal grooves, a neck region with the centriole adjunct, a flagellum with a simple 9 + 2 axoneme, two extra-axonemal accessory structures, two accessory bodies, and two mitochondrial derivatives of unequal size, and a prominent glycocalyx. The basic structure of spermatozoa of T. implicatus is similar to that of other Mecoptera studied. However, this species shows characteristics unique in Bittacidae, such as the reniform appearance of the centriole adjunct, two triangular accessory bodies with granular materials, and two asymmetric mitochondrial derivatives with a circular profile in cross-section. The potential utilization of the sperm ultrastructure for understanding the phylogeny of Bittacidae is briefly discussed.

Ultrastructure alterations induced by gamma irradiation in spermiogenesis of the ground beetle, Blaps sulcata: reference to environmental radiation protection

Ultrastructure alterations in spermiogenesis of the ground beetle, Blaps sulcata (Coleoptera: Tenebrionidae) were studied in normal adult males and in two male groups irradiated by gamma rays of 3 and 16 μSv/h dose rate. Ultrastructure examination of irradiated sperms revealed many alterations both in the head and in the flagellum regions of mature sperm. Alterations of the head region included nuclei with uncondensed chromatin materials and irregular nuclear envelope. Abnormal flagella contained malformed mitochondrial derivatives and damaged axonemes in addition to the absence of accessory bodies. Bi- and multi-flagellate sperms (with two, three, and four axonemes) were the most common alterations. Sperm cells with residual bodies were also obtained. Agglutinated sperms and sperms with enlarged and/or vacuolated cytoplasm were common. Sperm abnormalities were more pronounced in males irradiated by 16 μSv/h dose rate than those irradiated by 3 μSv/h. Spermiogenesis alterations induced by irradiation of B. sulcata may be used as a monitoring model for predicting the effects of environmental radioactivity.

Structure and Evolution of Insect Sperm: New Interpretations in the Age of Phylogenomics

This comprehensive review of the structure of sperm in all orders of insects evaluates phylogenetic implications, with the background of a phylogeny based on transcriptomes. Sperm characters strongly support several major branches of the phylogeny of insects-for instance, Cercophora, Dicondylia, and Psocodea-and also different infraordinal groups. Some closely related taxa, such as Trichoptera and Lepidoptera (Amphiesmenoptera), differ greatly in sperm structure. Sperm characters are very conservative in some groups (Heteroptera, Odonata) but highly variable in others, including Zoraptera, a small and morphologically uniform group with a tremendously accelerated rate of sperm evolution. Unusual patterns such as sperm dimorphism, the formation of bundles, or aflagellate and immotile sperm have evolved independently in several groups.

The sperm ultrastructure and spermiogenesis of Tribolium castaneum (Coleoptera: Tenebrionidae) with evidence of cyst degeneration

Previous studies on the spermatogenesis of tenebrionid beetles showed the unusual formation of two antiparallel sperm bundles per cyst. In this work we reported this feature also in Tribolium castaneum using light and transmission electron microscopy. The sperm structure of T. castaneum, similar to other tenebrionids, consists of a three-layered acrosome, an elongated nucleus and a flagellum with a 9+9+2 axoneme, two accessory bodies and two asymmetric mitochondrial derivatives. The presence of two antiparallel sperm bundles per cyst also in Meloidae and Rhipiphoridae suggests that it is a strong trait synapomorphic for Tenebrionoidea. The huge degeneration of whole sperm cells in several cysts of testes during spermiogenesis is also described.

The sperm ultrastructure of Stictoleptura cordigera (Füssli, 1775) (Insecta, Coleoptera, Cerambycidae)

The spermatozoa of the longhorn beetles Stictoleptura cordigera were ultrastructurally described in this paper. They have an apical bilayered acrosome, an elongated nucleus, a centriole with star-shape links, two asymmetric mitochondrial derivatives partially crystallized and a 9+9+2 flagellar axoneme with accessory tubules provided with 16 protofilaments in their wall. A centriole adjunct is present and gives rise to two thick laminae as accessory bodies, also asymmetrical, to which two relatively small puff-like structures of different size are connected. These features were previously found in the sperm of the cerambycid Morimus asper. The strict similarity of the cerambycid sperm characters with those of curculionoids indicates a clear phylogenetic relationship between Chrysomeloidea and Curculionoidea.

Overview on spermatogenesis and sperm structure of Hexapoda

The main characteristics of the sperm structure of Hexapoda are reported in the review. Data are dealing with the process of spermatogenesis, including the aberrant models giving rise to a reduced number of sperm cells. The sperm heteromorphism and the giant sperm exceeding the usual sperm size for length and width are considered. The characteristics of several components of a typical insect sperm are described: the plasma membrane and its glycocalyx, the nucleus, the centriole region and the centriole adjunct, the accessory bodies, the mitochondrial derivatives and the flagellar axoneme. Finally, a detailed description of the main sperm features of each hexapodan group is given with emphasis on the flagellar components considered to have great importance in phylogenetic considerations. This study may be also useful to those requiring an introduction to hexapod reproduction.