Maggs X 

Across amniotes, squamates represent the only clade with highly variable parity modes, oviparity (egg-laying) and viviparity (live-birth). Despite this, relatively little is known about how oviparity and viviparity evolve at the genomic and physiological levels in squamates. Within the context of interdisciplinary medical, poultry science, and reproductive biology literature, I review the genomics and physiology of reproduction across five broad processes expected to change during transitions between parity modes—eggshell formation, embryonic retention, placentation, calcium transport, and maternal-fetal immune dynamics. Throughout, I offer alternative perspectives and testable hypotheses regarding proximate causes of parity mode evolution in squamates. This review is the first time that the maternal-fetal immune dynamics of viviparous squamates is considered in the context of the modern medical understanding that embryos are not analogous to allografts (e.g., organ transplants). In the discussion, I present two new pathways through which early Lepidosaurs may have transitioned rapidly between oviparity and viviparity with no intermediate stages. Rather than emphasizing the feasibility of transitions in either direction, I posit that oviparity and viviparity are relatively minor variations of a shared process. I encourage the scientific community to embrace the complex physiology and evolutionary history of reproduction in squamates.