An extraordinary lizard that lays eggs and gives birth – watching evolution in action

A baby Saiphos equalis hatching

The transition from egg-laying (oviparity) to live-bearing (viviparity) represents a key evolutionary innovation that has arisen multiple times across a range of animal groups. The evolution of viviparity is significant because it has enabled several vertebrate groups to become highly successful in exploiting novel environmental conditions. Understanding how viviparity evolves is a question that is linked to the evolution of biological diversity itself and, not surprisingly, has been the focus of considerable research interest.  A new study has now shed light on the possible genetic mechanisms underpinning the evolution of viviparity.

New work by a team of researchers from the University of Sydney has capitalised on the extraordinary reproductive biology of the three-toed skink, a small native lizard that exhibits geographic variation in reproductive mode.

While some skink populations are viviparous, others are oviparous. However, oviparity in three toed skinks is atypical and may represent an intermediate form between ‘true’ oviparity and viviparity. This is because, in contrast to the more conventional oviparity seen in other lizards, oviparous three toed skinks have a long egg retention stage, such that most of the embryonic development has already occurred inside the mother before the eggs are laid.

Using a transcriptomics approach, the researchers found functional similarities between differentially expressed genes between viviparous and oviparous three toed skinks. Intriguingly, genes expressed in oviparous three toed skinks over the reproductive cycle differed from more typical oviparous lizards.

These results suggest that the oviparous mode of reproduction seen in populations of three toed skinks may, indeed, represent an intermediate form between the ancestral egg laying state and live birth.  Importantly, they offer important insights into the genetic changes that may have given rise to viviparity.      

The paper was published in Molecular Ecology.