![]() ![]() With respect to the display hypothesis, the sexual signal function of the scrotal testis is limited to primates. Because, on the one hand, some (such as the hedgehog) have a low core body temperature on the other hand, cetaceans and true seals are able to ease the thermoregulatory threat to a certain extent benefitting from the reproductive countercurrent heat exchangers (CCHEs) and venous system deep within the caudal abdominal cavity, respectively. ![]() For instance, the cooling hypothesis has been argued against on the grounds that the scrotal testis is not obligatory for all mammals. However, each hypothesis has been debated over decades. From another perspective, the ‘galloping hypothesis’ proposes that the scrotal testis originated from galloping, jumping, leaping, and similar movements the externalization of the testes protected the male gonads from fluctuations in intraabdominal pressures. This physiologically hostile environment trained and screened few but high-quality sperms for the ultimate task – fertilization. Additionally, the ‘training hypothesis’ argues that the scrotum exposes the sperm to a physiologically hostile environment due to the finding that poor blood supply was provided for the scrotal testis. According to the ‘display hypothesis’ proposed by Portmann, the externally placed scrotal testis plays a role in sexual signaling and sex recognition. Short extended this hypothesis and suggested that a cooler scrotal testis could be beneficial for maintaining the mutation rate in the male germ line at an acceptable level, without conferring an advantage for spermatogenesis. ![]() Based on the anatomical features that a pair of extracorporeal scrotums provide for thermoregulation (e.g., thin scrotal skin with sweat glands and fine hairs, tunica dartos, pampiniform plexus, cremaster muscle and absence of adipose tissue), the ‘cooling hypothesis’ states that scrotal CDT provide lower temperatures, which are beneficial for spermatogenesis. Specifically, a number of hypotheses have been proposed to explain the variation in testicular position among mammals, although none fully explains the observed variation. For example, the ascrotal testis is accompanied with streamlined body shape in many aquatic taxa (e.g., true seals, cetaceans and manatees). These findings provide general insights into cryptorchidism and have implications for health and infertility both in humans and domestic mammals.įrom an adaptive evolution perspective, the diverse testicular positions in different groups of mammals benefits the adaptation to their unique environments and lifestyles. ![]() Further, we identified cancer resistance and DNA repair as potential protective mechanisms in natural cryptorchids. Our results revealed that alterations in gubernaculum development contributed to the evolution of testicular position in mammals and provided the first support for two hypotheses for variation in testicular position in mammals, the “cooling hypothesis”, which proposes that the scrotum provides a cool environment for acutely heat-sensitive sperm and the “training hypothesis”, which proposes that the scrotum develops the sperm by exposing them to an exterior environment. Genes associated with testicular position were significantly enriched in functions and pathways related to cancer, DNA repair, DNA replication, and autophagy. These genes showed rapid evolution and the signature of positive selection, with specific substitutions in ascrotal mammals. Moreover, genes related to testicular position were significantly associated with spermatogenesis and sperm fertility. In the present study, comparative genomics and evolutionary analyses revealed that genes associated with the extracellular matrix and muscle, contributing to the development of the gubernaculum, were involved in the evolution of testicular position in mammals. However, the evolution of testicular position and the molecular mechanisms underlying the maintenance of health, including reproductive health, in ascrotal mammals are not clear. Although cryptorchidism is hazardous to human health, some mammalian taxa are natural cryptorchids. Mammals have wide variations in testicular position, with scrotal testes in some species and ascrotal testes in others. ![]()
0 Comments
Leave a Reply. |