Sex-specific fitness landscapes in the evolution of egg-laying vs live-birth

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The evolution of live-birth (viviparity) from egg-laying (oviparity) is regarded as one of biology's great and repeated transitions. Making and laying eggs, which develop and hatch externally is very different from giving birth to fully developed young. Not only the machinery of reproduction is involved, but also an assortment of ecological and life history strategies differ between egg-laying and live-bearing species. Even though it is a very complex evolutionary step, live-bearing has evolved many times in amniotes: once early in mammals and more than one hundred times in reptiles.While entire lineages fix in one state or the other, obviously only the reproductive adult females express oviparity or viviparity, i.e. lay eggs or give birth. However according to evolutionary theory, to some extent we expect all individuals in a population experience the consequences of being at intermediate phases that lie between two alternative and distinct adaptive states, such as egg-laying and live-bearing. We can predict - but have never tested - that sexes might respond differently to the life-history costs and benefits of being oviparous or viviparous, with potentially dramatic consequences for evolution. Case studies to finely disentangle the evolutionary and organismal biology of live-bearing, including intermediate phases, are vital if we are ever to infer what ecological conditions drive changes in reproductive mode. Our proposed project is highly interdisciplinary as it spans reproductive biology, physiology, genomics, and evolutionary theory. It is "blue-skies" discovery science, which is driven by deep curiosity about the animal diversity of the natural world and how it arose. Outcomes from this study will open new doors in our understanding of reproductive mode evolution and propel future research across biological systems. While mammals might seem far removed from lizards, in fact we have shown that there are deep shared genetic toolkits of live-bearing from mammals to reptiles. Consequently the genes and evolutionary forces we study in lizards also echo across many animal species, including humans.