National Geographic has a nice review of the evolution of viviparity in skinks. It summarizes a recent paper by Jim Stewart et al. investigating the morphology and histology of the egg shell and calcium-producing glands in the uterus of the skink, Saiphos equalis.
Saiphos equalis is rare because there exist both oviparous and viviparous populations. Because viviparity must have evolved very recently in this species, S. equalis is therefore a great model system to see this transition of reproductive mode in action.
The story is particularly relevant to me since I have just begun a post-doc at the University of Sydney to study the genetic mechanisms of viviparity in S. equalis with one of the paper’s co-authors, Mike Thompson.
If you think about the transition from laying eggs to giving live birth, it becomes clear that it must have involved some significant morphological and physiological changes including retention of the embryo within the mother until gestation is complete, increasing uterine blood supply to facilitate nutrient and gas exchange with the embryo, potentially suppressing the maternal immune system to prevent rejection of the embryo, and reduction of the calcareous eggshell.
The latter process is particularly interesting because it requires a significant physiological modification of the shell-producing glands. Instead of quickly dumping a bunch of calcium to form an eggshell, the glands in viviparous mothers must instead slowly secrete calcium to nourish the developing embryo during the entire period of gestation.
Stewart et al. found that the thickness of the developing eggshell in the uterus of viviparous mothers is thinner and less developed than those in the oviparous mothers at the same stage of embryonic development. However, they also found that this is not due to the size of the shell-producing glands (i.e., bigger glands = thicker shell), and therefore some other mechanism must be responsible for the physiological changes of shell glands in viviparous mothers.
In other words, we see the hypothesized first steps to viviparity – holding embryos for an extended period of time and reducing the unnecessary eggshell.
What genetic mechanisms underlie this shift in reproductive mode? Check back with me in a year or so.