Ultrastructural studies on the spermatozoa of two primitive crustaceans Hutchinsoniella macracantha and Derocheilocaris typicus

The reproductive system of Derocheilocaris typica (Crustacea, Mystacocarida)

Mystacocarids are dioecious. Their gonopores are on the medial side of the third thoracic limb. The male's paired testes lie in the thorax and abdomen. They develop from paired rows of six small follicles dorsally. In the mature animal they fill most of the abdomen. The spermatophores develop within the follicles from spermatogonia mixed with follicle cells, which support and nourish the spermatocytes and produce the seminal fluid. The short vas deferens runs along the bottom of the testes and then continues forward to the gonopore. The vas deferens has a small group of cells near the gonopore that becomes a closure mechanism. The female has reproductive cells and also support cells that provide nutrition and form the wall of the ovary and oviduct. The unpaired female ovary begins in the third thoracic segment. During maturation, the oocytes are pushed posteriorly. The enormous mature ovum extends into a caudal pocket of the ovary. Starting with its anterior end, this ovum is extruded into the short oviduct, which extends laterally and ventrally to the gonopore. During extrusion, the pocket is reabsorbed from behind. There are no accessory structures connected to the reproductive system, nor any external specializations on the third limb.

ULTRASTRUCTURE OF SPERMIOGENESIS AND THE SPERMATOZOON OF MACRACANTHORHYNCHUS HIRUDINACEUS (PALLAS, 1781) (ACANTHOCEPHALA: ARCHIACANTHOCEPHALA), A PARASITE OF THE WILD BOAR SUS SCROFA

The present paper describes the ultrastructure of spermiogenesis and the spermatozoon of Macracanthorhynchus hirudinaceus, an acanthocephalan parasite of the wild boar Sus scrofa. At the beginning of spermatogenesis, spermatocytes exhibit synaptonemal complexes and 2 centrioles. In the spermatid, only 1 centriole remains, generating a flagellum with a 9+2 pattern. Another ultrastructural feature observed during the spermiogenesis of M. hirudinaceus is the condensation of the chromatin, forming a "honeycomb" structure in the old spermatid and a homogeneous, electron-dense structure in the spermatozoon. The mature spermatozoon of M. hirudinaceus presents a reversed anatomy, as has been described previously in other species of the Acanthocephala. The spermatozoon is divided into 2 parts: an axoneme, and a nucleocytoplasmic derivative. The spermatozoon flagellum exhibits a 9+2 or 9+0 pattern. The process of spermiogenesis and the ultrastructural organization of the spermatozoon of M. hirudinaceus are compared with available data regarding other acanthocephalan species.

How the sperm lost its tail: the evolution of aflagellate sperm

The typical sperm is comprised of a head, midpiece and flagellum. Around this theme there is an enormous diversity of form--giant sperm, multi-flagellate sperm and also sperm that lack flagella entirely. Explaining this diversity in sperm morphology is a challenging question that evolutionary biologists have only recently engaged in. Nonetheless, one of the selective forces identified as being an important factor in the evolution of sperm form is sperm competition, which occurs when the sperm of two or more males compete to fertilize a female's ova. In species with a truly monandrous mating system, the absence of sperm competition means that the selection pressure on males to produce motile sperm may be relaxed. Potentially aflagellate sperm are less costly to produce, both in terms of energy and time. Thus, selection may therefore favour the loss of the sperm flagellum and any other motile mechanisms in monandrous taxa. A review of the literature revealed that 36 taxonomic groups, from red algae to fish, were found independently to have evolved aflagellate sperm. I review what is known about the mating systems of each of these taxa and their nearest sister taxa. A sister-group analysis using this information provided weak evidence suggesting that the evolution of aflagellate sperm could be linked to the removal of selective pressures generated by sperm competition.

Reproductive system of Hutchinsoniella macracantha

The cephalocarid crustacean Hutchinsoniella macracantha is a hermaphroditic species. Ova and sperm develop simultaneously. Ovaries and testes are separate, but the oviducts and vasa deferentia join and exit through a pair of common genital ducts.