Co-evolution of gametes of the Greater Bandicoot Rat,Bandicota indica- a murine rodent from South-East Asia

Ultra-imaging in applied animal andrology: The power and the beauty

Ultrastructural studies of the male gamete provide relevant complementary data of value for the clinical assessment of semen quality and assist in determining phylogenetic and structural/functional relationships. This is illustrated using semen samples and testicular material from vulnerable wild animals (cheetah and rhinoceros), commercially exploited exotic birds (ratites and tinamou) and poultry (chicken and duck). Transmission electron microscopy (TEM) was employed to record sperm and spermatid ultrastructural detail on a comparative basis. The power of the technique was demonstrated using normal and abnormal (the knobbed acrosome defect) formation of the acrosome in the cheetah and rhinoceros. The structural similarities of the defect across species was apparent. The determination of phylogenetic associations was illustrated by comparing structural characteristics between ratites (ostrich, emu and rhea), the tinamou and poultry (chicken and duck), highlighting the morphological peculiarities evident in the midpiece and proximal principal piece of the sperm tail. A clear distinction was obvious between the ratites and tinamou on the one hand and the Galliform and Anseriform birds on the other. The potential power of using molecular techniques in conjunction with ultrastructural studies to explain structural/functional relationships was demonstrated by describing a transient elaboration of the perinuclear theca that occurs during a specific stage of spermiogenesis in ratites, and which can only be imaged using TEM. The inherent aesthetic appeal of the structurally complex normal and defective male gamete was also emphasised.

Testicular Structure and Spermatogenesis in the Naked Mole-Rat Is Unique (Degenerate) and Atypical Compared to Other Mammals

The naked mole-rat (NMR) queen controls reproduction in her eusocial colony by usually selecting one male for reproduction and suppressing gametogenesis in all other males and females. Simplified, polymorphic and slow-swimming spermatozoa in the NMR seem to have been shaped by a low risk of sperm competition. We hypothesize that this unique mammalian social organization has had a dramatic influence on testicular features and spermatogenesis in the NMR. The testicular structure as well as spermatogenic cell types and its organization in breeding, subordinate and disperser males were studied using light microscopy and transmission electron microscopy. Even though the basic testicular design in NMRs is similar to most Afrotheria as well as some rodents with intra-abdominal testes, the Sertoli and spermatogenic cells have many atypical mammalian features. Seminiferous tubules are distended and contain large volumes of fluid while interstitial tissue cover about 50% of the testicular surface area and is mainly composed of Leydig cells. The Sertoli cell cytoplasm contains an extensive network of membranes and a variety of fluid-containing vesicles. Furthermore, Sertoli cells form numerous phagosomes that often appear as extensive accumulations of myelin. Another unusual feature of mature NMR Sertoli cells is mitotic division. While the main types of spermatogonia and spermatocytes are clearly identifiable, these cells are poorly organized and many spermatids without typical intercellular bridges are present. Spermatid heads appear to be malformed with disorganized chromatin, nuclear fragmentation and an ill-defined acrosome formed from star-like Golgi bodies. Rudimentary manchette development corresponds with the occurrence of abnormal sperm morphology. We also hypothesize that NMR testicular organization and spermiation are modified to produce spermatozoa on demand in a short period of time and subsequently use a Sertoli cell "pump" to flush the spermatozoa into short tubuli recti and simplified rete testis. Despite the difficulty in finding cellular associations during spermatogenesis, six spermatogenic stages could be described in the NMR. These numerous atypical and often simplified features of the NMR further supports the notion of degenerative orthogenesis that was selected for due to the absence of sperm competition.

Sperm morphology of the Rattini – are the interspecific differences due to variation in intensity of intermale sperm competition?

It is widely accepted that in mammals a causal relationship exists between postcopulatory sexual selection and relative testes mass of the species concerned, but how much it determines sperm size and shape is debatable. Here we detailed for the largest murine rodent tribe, the Rattini, the interspecific differences in relative testes mass and sperm form. We found that residual testes mass correlates with sperm head apical hook length as well as its angle, together with tail length, and that within several lineages a few species have evolved highly divergent sperm morphology with a reduced or absent apical hook and shorter tail. Although most species have a relative testes mass of 1–4%, these derived sperm traits invariably co-occur in species with much smaller relative testes mass. We therefore suggest that high levels of intermale sperm competition maintain a sperm head with a long apical hook and long tail, whereas low levels of intermale sperm competition generally result in divergent sperm heads with a short or non-existent apical hook and shorter tail. We thus conclude that sexual selection is a major selective force in driving sperm head form and tail length in this large tribe of murine rodents.