Reproductive Modes of Amphibians and Reptiles: Impacts on Conservation and the Development of Assisted Reproductive Technologies

Amphibians and reptiles represent the transition from aquatic to terrestrial life and have evolved an enormous range of reproductive modes and life histories in the process. Male and female reproductive tracts, physiology, and neuroendocrinology reflect this evolutionary transition, but the frequent evolution of viviparity and complex forms of development associated with oviparity have generated an enormous amount of complexity in reproduction. Many species of both taxa are under threat of extinction from a range of threatening processes, with many threats either impacting reproduction directly or being mediated through reproduction. In amphibians, the chytridiomycosis pandemic has reduced survival post-metamorphosis in many species so that females do not achieve sexual maturity and reproduce and disease may cause altered reproductive fitness and terminal investment strategies in reproduction. In reptiles, environmental pollution can cause sex reversal via endocrine disruption and altered sex ratios in temperature-dependent sex determined species that are now at risk from climate change. The complexity of reproductive modes challenges the development of assisted reproductive technologies, a key tool in conserving and reversing loss of genetic diversity in wild and captive populations of threatened species. This is particularly the case where reproduction involves viviparity or extra-reproductive tract brooding of eggs and tadpoles in amphibians such as the Australian hip-pocket frog, or placentation in reptiles. Nevertheless, significant strides have been made in the development of assisted reproductive technologies (ARTs) for conservation purposes, improving the prospects for effective biobanking (cryopreservation) to contribute to the conservation and management of threatened species. ARTs have been applied to an increasing number of threatened amphibian species, and the recent report of the first live young in any reptile species from frozen, thawed sperm is encouraging. More focused research on ARTs and biobanking for threatened species is urgently needed as these are contributing to conservation outcomes; however, this has been hampered by a lack of knowledge on the diverse reproductive modes across amphibians and reptiles. We hope this review helps fill those gaps by recognising current challenges and identifying future opportunities for improved development of ARTs and biobanking technologies.