Should I mate or should I wait? : The morphology of sperm storage, and its consequences for sperm viability and mating strategies in a temperate skink species, Niveoscincus ocellatus
Creators: name: Russell, MA
Publisher: name: University Of Tasmania
Year: 2023

description: Sperm storage is a central phenomenon to the reproductive cycle of a wide range of vertebrate species. Although the morphological mechanisms of sperm storage and its consequences on sperm viability have been described separately in many taxa, there is a paucity of studies which have combined detailed descriptions of morphological variation with accurate estimates of sperm viability and its links to male and female mating strategies in the wild. This is particularly true in reptiles, despite the relatively high occurrence of sperm storage within this group. This thesis addresses these shortcomings by investigating seasonal morphological changes in sperm storage organs, along with temporal variation in sperm viability in epididymal sperm and its links to mating patterns in Niveoscincus ocellatus, a temperate-climate reptile with both male and female sperm storage. I show that N. ocellatus sperm viability varied throughout the mating period, being most viable early in the mating season before decreasing in viability later. In accordance with these changes, mating patterns also varied across the mating season. Specifically, copulations were more abundant early in the season (when sperm were most viable and abundant) compared to late in the season when copulation frequency was significantly lower. In contrast, I found no evidence for variation in mate choice across the mating season; males preferred large, unmated females to small, promiscuous ones throughout. In addition, I observed structural changes in sperm production and storage organs (e.g., the germinal epithelium and seminiferous tubules of the testes, the epithelium and lumen of the epididymal ducts, and the musculature, ciliation, lumen, and secretory activity of oviductal segments) across the mating season. In most cases these changes are closely linked to the variation observed in reproductive cycles and mating patterns reported here and elsewhere. Specifically, the epididymides varied in size in accordance with sperm abundance and usage and closely corresponded with the observed mating patterns. The cauda epididymis swelled in size as sperm were transferred into this storage location, and decreased in size as the mating season progressed and sperm abundances decreased. Additionally, the female oviduct underwent significant morphological changes in conjunction with vitellogenesis, and secretory activity was heightened during periods of mating activity and sperm storage, increasing in concentration at the start of the mating season. This work also generated several additional observations. For example, I found that oviductal sperm storage occurs in different locations to those previously described for skinks (i.e. the anterior and posterior vagina, as opposed to the infundibulum), supporting recent suggestions that morphological mechanisms of sperm storage vary between species much more than once believed. Furthermore, I found that spermatogenesis begins in late autumn, but a long refractory period halts sperm production until late summer, when spermatogenesis resumes in preparation for the beginning of the mating season in early autumn. This expands on previous work which implied that spermatogenesis began in late summer. Finally, I found a rise in live sperm proportions in conjunction with a significant decrease in sperm abundance in spring, suggesting that, dead or defective sperm are either evacuated, or leak out of the epididymides to increase mating success from copulations occurring in their second period of mating activity. In summary, my results confirm previous descriptions of the reproductive cycle of male and female N. ocellatus and provide further information as to the progression of spermatogenesis, and the location of sperm storage in the female reproductive tract. Many of these changes correlate with changes in sperm production, usage and viability, and ultimately variation in mating patterns. This thesis thus provides an insight into the consequences of sperm storage in temperate-climate reptiles and the morphological features underlying it. In doing so, I provide an opportunity for further research to investigate the consequences of mating strategies with respect to sperm storage, for fertilization success, offspring phenotype and ultimately reproductive success.