Maternal effects on offspring fitness in pseudemoia entrecasteauxii : selective advantages and physiological mechanisms

摘要

This thesis focused on why matrotrophy has evolved in viviparous (live-bearing) reptiles. Matrotrophic reproduction is direct supply of nutrients by the mother during gestation (e.g. placental support), and it is rare in viviparous reptiles. Although a large number of studies have investigated the evolution of viviparity in reptiles, we know comparatively little about the evolution of matrotrophic viviparity in reptiles. Matrotrophic reproduction implies complex and increasing maternal-embryonic communications such as nutrient and hormone transfer via the placenta during gestation. These placental nutrient and hormone transfers affect offspring phenotype and, therefore, fitness. Such non-genetic effects on offspring phenotype are so-called maternal effects. However, the importance of maternal effects is still the subject of an ongoing debate in terms of fitness. In addition, there is a very little information on maternal effects and their relation to physiological mechanisms in viviparous reptiles, especially matrotrophic viviparous reptiles.udPseudemoia entrecasteauxii is one of the few known species of matrotrophic viviparous reptiles. The first section of this thesis focused on physiological mechanisms during gestation in P. entrecasteauxii to explore how the timing of placental nutrient and hormone transfer during embryogenesis affects embryonic development and consequently offspring fitness. The second section of this thesis focused on maternal effects and their adaptive significance in P. entrecasteauxii. In matrotrophic viviparous fish, for example, the evolution of matrotrophy may have been related to high maternal energy availability during gestation, which enhances offspring fitness through maternal effects. In reptiles, net energy gain is dependent on the interaction between body temperature, plasma corticosterone concentration and food availability. The effect of maternal net energy gain during gestation on offspring phenotype was examined in two experiments. In the first, the effect of variation in maternal thermal condition and maternal food availability during gestation was investigated. In the second, investigation of the effect of variation in maternal plasma corticosterone concentration and maternal food availability during gestation was conducted. For each experiment, offspring growth rate was examined using reciprocal transplant experiments (i.e. investigation of adaptive significance of maternal effects) because offspring growth rate is usually associated with fitness. In the final experiment, the effects of maternal food quality (i.e. food with β-carotene and food without β-carotene) during gestation on offspring phenotype were investigated by measuring offspring immune capacity in response to antigenic stimulation. I found that the degree of maternal nutrient support during gestation significantly influenced offspring phenotype, and high maternal net energy availability during gestation potentially enhanced offspring fitness. These findings suggest that predictably high maternal energy availability during gestation may have been an important determinant for the evolution of matrotrophic viviparity in P. entrecasteauxii. Furthermore, findings on the effects of maternal corticosterone and maternal β-carotene availability during gestation on offspring phenotype suggest several important considerations for offspring fitness as a consequence of the evolution of matrotrophic reproduction in P. entrecasteauxii. Thus, this thesis contributes significantly to our understanding of the evolution of matrotrophic viviparity in reptiles and also indicates a further direction for research into this topic.