Environmental influences and ecological significance of arrested embryonic development in chelonians
2017-02-06T05:37:06Z (GMT) by
Arrested embryonic development is an important reproductive strategy in the large range of egg laying animals that provide no parental care after oviposition, because it gives their eggs some capacity to respond to changing environmental conditions during development. In this study, I used a comparative approach to identify the external environmental stimuli that cause oviparous reptile embryos to arrest development inside the amniotic egg. This was achieved by investigating how development was directed by maternal (pre-ovipositional development) and nest (post-ovipositional) factors including oxygen tension and temperature, respectively, in eggs of the green sea turtle, Chelonia mydas, and three species of freshwater turtle; the western oblong turtle, Chelodina oblonga, the eastern longneck turtle, Chelodina longicollis, and the Murray River turtle, Emydura macquarii. Initially, I conducted an investigation using an existing long-term data set collected during an ongoing beach monitoring project for the leatherback turtle, Dermochelys coriacea, at Playa Grande, Costa Rica, to establish whether the duration that embryos remain arrested in utero compromises survival after oviposition. This examination revealed that a large proportion of leatherback turtle embryos were failing to resume development after eggs were laid and dying in a state of pre-ovipositional developmental arrest. I then endeavoured to identify the mechanism involved in pre-ovipositional arrest by assessing oviducal O2 availability and the impact of hypoxia on embryonic development in the 4 mentioned species. Eggs of each species were allocated immediately after oviposition to either a normoxic (155 mmHg O2) or a hypoxic (~7.6 mmHg) treatment for 3, 6 or 9 days. Embryonic development of all species progressed normally in normoxia, but in hypoxia development remained arrested and recommenced upon subsequent return to normoxic conditions. I also measured PO2 in oviducts of gravid turtles, with mean oviducal PO2 of 5.9 ± 2.5 mmHg in C. oblonga; 1.6 ± 1.2 mmHg in C. Longicollis; 5.3 ± 2.1 mmHg in E. macquarii; and 2.9 ± 1.4 mmHg in C. mydas. Furthermore, O2 diffusion was measured in green turtle oviducal secretion samples and was significantly lower in oviducal secretion (1.9 ± 0.6 mmHg min-1) than in saline controls (14.2 ± 2.1 mmHg min-1, F = 64.3, P < 0.01). These results suggest that the reduced diffusive ability of oviducal secretion contributes to an extremely hypoxic environment in the oviduct that constrains embryonic development in turtles and maintains pre-ovipositional arrest until after eggs are laid. I also investigated whether incubation temperature influenced arrested embryonic development, by incubating eggs of each species at three different temperatures immediately after oviposition until hatching, and monitoring embryonic development. Results demonstrated that the breaking of pre-ovipositional arrest after eggs are laid occurred independently of temperature in all four species. However, temperature influenced subsequent development beyond this point and at 22ºC, 64% of E. macquarii embryos developed to stage 25 of a 26 stage developmental chronology, but failed to hatch. Similarly, at 24ºC, 45% of C. mydas embryos failed to hatch and died at stage 30 of a 31 stage developmental chronology. Presumably, this was because embryos of both species entered a state of delayed hatching, awaiting a necessary stimulus to trigger pipping that never arose at constant temperature. These findings represent the first evidence that sea turtles possibly retain remnant traces of an ability to delay hatching that they once possessed, or perhaps that they may be pre-adapted to evolve delayed hatching in the future. Finally, I examined the trade-offs between reproductive investment and maternal health in each of the freshwater species included in this study and revealed that females of all three species altered their level of reproductive investment based on health state, manifested by changes in egg mass and size, as well as clutch size. These findings agreed with the physiological constraint hypothesis. Furthermore, no trade-offs existed between clutch and egg size in any of the species under investigation, which refuted the optimum egg size hypothesis and agreed with previous findings in other chelonian species. The findings presented in this thesis contribute to our understanding of how ecology shapes the evolution of developmental processes observed in reptile embryos, in addition to providing evidence of the mechanisms underlying the evolutionary transition between reproductive modes. It sets the foundation for future research, particularly investigations that focus on understanding how maternal effects influence embryonic development both before and after oviposition in reptiles, despite the absence of parental care in many species.