The eggshell‐matrix protein gene OC‐17 is functionally lost in the viviparous Chinese crocodile lizard

Abstract

Thickness reduction or loss of the calcareous eggshell is one of major phenotypic changes in the transition from oviparity to viviparity. Whether the reduction of eggshells in viviparous squamates is associated with specific gene losses is unknown. Taking advantage of a newly generated high‐quality genome of the viviparous Chinese crocodile lizard (Shinisaurus crocodilurus), we found that ovocleidin‐17 gene (OC‐17), which encodes an eggshell matrix protein that is essential for calcium deposition in eggshells, is not intact in the crocodile lizard genome. Only OC‐17 transcript fragments were found in the oviduct transcriptome, and no OC‐17 peptides were identified in the eggshell proteome of crocodile lizards. In contrast, OC‐17 was present in the eggshells of the oviparous Mongolia racerunner (Eremias argus). Although the loss of OC‐17 is not common in viviparous species, viviparous squamates show fewer intact eggshell‐specific proteins than oviparous squamates. Our study implies that functional loss of eggshell‐matrix protein genes may be involved in the reduction of eggshells during the transition from oviparity to viviparity in the crocodile lizard.

Candidate Genes Associated with Parity Mode Evolution are Under Selection in Oviparous and Viviparous Snakes

“…These results, in conjunction with the literature (Recknagel et al, 2021), may allow researchers to explore further how squamate eggshell formation differs from birds. This will compliment recent advances looking at the presence and absence of avian-eggshell specific genes in squamate genomes (Xie et al, 2022).…”

“…This makes squamates an opportune system for comparative research on the evolution of parity modes, oviparity (egg-laying) and viviparity. A wealth of data on uterine gene expression and functions during gestation and gravidity are now accessible thanks to decades of work—medical and veterinary research on pregnancy, evolutionary research on placental development in mammals, agricultural research on avian egg production, and comparative evolutionary genomics studies on closely related or reproductively bimodal lizards (Brandley et al, 2012; Cornetti et al, 2018; Foster et al, 2020, 2022; Gao et al, 2019; Griffith et al, 2016, 2017; Jonchere et al, 2010; Recknagel et al, 2021; Whittington et al, 2018; Xie et al, 2022).…”

“…Parity mode evolution in snakes may be particularly unique given one study found a 90% probability of viviparity at the MRCA of all snakes (Harrington & Reeder, 2017). Another recent study found viviparous squamates had a significantly reduced number of intact avian eggshell-specific proteins compared to oviparous squamates, a pattern especially apparent in snakes (Xie et al, 2022). This makes snakes an interesting system for studying the evolution of parity modes.…”