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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

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Russell, MA (2012) 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. Research Master thesis, University of Tasmania.

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Abstract

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.

Item Type: Thesis (Research Master)
Copyright Information:

Copyright 2012 the Author

Date Deposited: 17 Aug 2012 04:33
Last Modified: 15 Sep 2017 01:06
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