Evolution of viviparity in sceloporine lizards: in utero PO2 as a developmental constraint during egg retention.

Reptilian viviparity evolves through selection for increasingly prolonged egg retention within the oviduct. In the majority of sceloporine lizard species, however, egg retention past the normal time of oviposition results in retarded or arrested embryonic development. In this study, we tested the hypothesis that the amount of embryonic development normally attained in utero is directly related to in utero oxygen partial pressure (PO(2)). The three species of sceloporine lizards we used are characterized by developmental arrest (Urosaurus ornatus), retarded development (Sceloporus virgatus), and normal development (Sceloporus scalaris) when eggs are retained. We incubated eggs of these species for 10 d under conditions that simulated retention in the oviduct at a range of experimental oxygen partial pressures (PO(2)). We estimated in utero PO(2) from a standard curve generated from the stage and dry mass of experimental embryos incubated for 10 d at known PO(2). The standard curve was then used to predict the PO(2) associated with the observed rate of development of embryos retained in utero. The results of this study showed that the degree of embryonic development attained in utero during egg retention was positively associated with in utero PO(2). The results indicate that oxygen availability in utero is associated with interspecific differences in the capacity to support intrauterine development in sceloporine lizards.