Investigating the evolution of long egg retention and viviparity in Saiphos equalis

The transition from an ancestral oviparous (egg-laying) mode of reproduction to a derived viviparous (live-bearing) state has occurred more than 150 independent times in vertebrates. However, little is known of the genetic changes underpinning the large physiological changes that are necessary for transitions between parity modes. Here, we use the bimodally reproductive skink Saiphos equalis as a model to investigate the genetics underpinning the evolution of viviparity. Saiphos equalis is ideal for this purpose since some populations exhibit oviparous long egg retention, and others are viviparous. We used RNA-seq data to generate a transcriptome for S. equalis, and used differential expression analysis to investigate changes in gene expression throughout gravidity in each parity mode. We found that many genes related to uterine remodelling, embryonic gas and water exchange, calcium transport, and immune regulation were differentially expressed throughout gravidity in both oviparous and viviparous S. equalis. Collectively, we conclude that differences between oviparity and viviparity can evolve through changes in gene expression, but many physiological changes throughout gravidity are similar in both oviparous long egg retention and viviparity.