Ex utero culture of viviparous embryos of the lizard, Zootoca vivipara, provides insights into calcium homeostasis during development

A synthetic review: natural history of amniote reproductive modes in light of comparative evolutionary genomics

There is a current lack of consensus on whether the ancestral parity mode was oviparity (egg-laying) or viviparity (live-birth) in amniotes and particularly in squamates (snakes, lizards, and amphisbaenids). How transitions between parity modes occur at the genomic level has primary importance for how science conceptualises the origin of amniotes, and highly variable parity modes in Squamata. Synthesising literature from medicine, poultry science, reproductive biology, and evolutionary biology, I review the genomics and physiology of five broad processes (here termed the 'Main Five') 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 amniotes and squamates. If viviparity did evolve early in the history of lepidosaurs, I offer the nucleation site hypothesis as a proximate explanation. The framework of this hypothesis can be extended to amniotes to infer their ancestral state. I also provide a mechanism and hypothesis on how squamates may transition from viviparity to oviparity and make predictions about the directionality of transitions in three species. After considering evidence for differing perspectives on amniote origins, I offer a framework that unifies (i) the extended embryonic retention model and (ii) the traditional model which describes the amniote egg as an adaptation to the terrestrial environment. Additionally, this review contextualises the origin of amniotes and parity mode evolution within Medawar's paradigm. Medawar posited that pregnancy could be supported by immunosuppression, inertness, evasion, or immunological barriers. I demonstrate that this does not support gestation or gravidity across most amniotes but may be an adequate paradigm to explain how the first amniote tolerated internal fertilization and delayed egg deposition. In this context, the eggshell can be thought of as an immunological barrier. If serving as a barrier underpins the origin of the amniote eggshell, there should be evidence that oviparous gravidity can be met with a lack of immunological responses in utero. Rare examples of two species that differentially express very few genes during gravidity, suggestive of an absent immunological reaction to oviparous gravidity, are two skinks Lampropholis guichenoti and Lerista bougainvillii. These species may serve as good models for the original amniote egg. Overall, this review grounds itself in the historical literature while offering a modern perspective on the origin of amniotes. I encourage the scientific community to utilise this review as a resource in evolutionary and comparative genomics studies, embrace the complexity of the system, and thoughtfully consider the frameworks proposed.

Parallel evolution of placental calcium transfer in the lizard Mabuya and eutherian mammals

An exceptional case of parallel evolution between lizards and eutherian mammals occurs in the evolution of viviparity. In the lizard genus Mabuya, viviparity provided the environment for the evolution of yolk-reduced eggs and obligate placentotrophy. One major event that favored the evolution of placentation was the reduction of the eggshell. As with all oviparous reptiles, lizard embryos obtain calcium from both the eggshell and egg yolk. Therefore, the loss of the eggshell likely imposes a constraint for the conservation of the egg yolk, which can only be obviated by the evolution of alternative mechanisms for the transport of calcium directly from the mother. The molecular and cellular mechanisms employed to solve these constraints, in a lizard with only a rudimentary eggshell such as Mabuya, are poorly understood. Here, we used RT-qPCR on placental and uterine samples during different stages of gestation in Mabuya, and demonstrate that transcripts of the calcium transporters trpv6, cabp28k, cabp9k and pmca are expressed and gradually increase in abundance through pregnancy stages, reaching their maximum expression when bone mineralization occurs. Furthermore, CABP28K/9K proteins were studied by immunofluorescence, demonstrating expression in specific regions of the mature placenta. Our results indicate that the machinery for calcium transportation in the Mabuya placenta was co-opted from other tissues elsewhere in the vertebrate bodyplan. Thus, the calcium transportation machinery in the placenta of Mabuya evolved in parallel with the mammalian placenta by redeploying the expression of similar calcium transporter genes.