Recknagel, H., Carruthers, M., Yurchenko, A.A. et al. “The functional genetic architecture of egg-laying and live-bearing reproduction in common lizards.” Nat Ecol Evol 5, 1546–1556 (2021). https://doi.org/10.1038/s41559-021-01555-4
Convergent evolution is a phenomenon where different species evolve similar characteristics though they come from very different lineages. For example, birds, bats and insects have all evolved the power of flight through very different evolutionary pathways. In this context, it is an impressive statistic that live-bearing behaviour or viviparity (as opposed to laying eggs to beget young ones or oviparity) has evolved independently in vertebrates at least 150 times. In reptiles alone, it has evolved independently some 121 times, and to specify further, in squamates (a group including lizards, snakes and worm lizards or amphisbaenians) it has evolved independently 115 times. It is perhaps not surprising then, considering the many routes taken by evolution in forging this difference, that within this group there should be seen this rare incidence of both behaviours (oviparity and viviparity) within the same species: the Eurasian common lizard ( Zootoca vivipara ).
- In the Eurasian common lizard, one finds the rare incidence of both behaviours — oviparity and viviparity.
- Researchers have done a comparative study within the same species to understand this phenomenon. They studied mainly the genetic basis for gestation period and eggshell traits and found that more genetic variants were involved in gestation time than with eggshell traits. This indicates that the genetic basis of the retention of the embryo is more complex than that of eggshell traits.
- The study helps in understanding the genetic basis of the evolution of pregnancy.
The subject of study
In the paper entitled, “The functional genetic architecture of egg-laying and live-bearing reproduction in common lizards,” published in Nature Evolution and Ecology , Hans Recknagel et al study a rare development: different geographically separated populations of the same species Z. vivipara , the Eurasian common lizard, show on the one hand egg-laying behaviour and on the other, live-bearing behaviour. The two populations have diverged as recently as nearly 4 million years ago, and so this may even be seen as a transition stage in moving from one parity to another. The researchers, who are from Universities of Glasgow and Bristol, U.K.; Ljubljana, Slovenia and Paris Saclay, France, have studied the genomic comparison between the two populations in an attempt to characterise the genetic basis of viviparity and oviparity and to better understand how the evolution took place. In the process, they have also seen hybrid members, in areas where the two populations overlap.
Viviparity versus oviparity
The significance of this study is attributed to the following aspect: Usually, comparative studies of viviparity and oviparity are done between different species – one of which is egg-laying and the other is live-bearing. The problem with conducting a comparative study like this is the complexity of the differences between the species which can obscure understanding. Therefore, seeing the two kinds of behaviour within the same species is a powerful tool. The researchers have analysed both the gene expression and the genomics data of the two groups to identify the genes that play out in the reproductive mode. It is also very useful that hybridisation between the two parities occurs both naturally and in the lab, perhaps because the two populations diverged only about 4 million years ago.
The development of viviparity is accompanied by changes in the mother and the development of the embryo. For instance, active physiological exchange of water, gas and calcium from mother to embryo and an adjusted immune response for maternal-foetal communication.
In advanced stages of viviparity, more complex placentation is seen with enhanced nutrition transfer from mother to baby. Also, the mother’s immune response may be modified to prevent abortion and support foetal development. In the Eurasian common lizard, viviparity is at a relatively early stage.
In order to characterise the two types of behaviour, the researchers compared the eggshell thickness, gestational time and developmental stage of the young one at birth. Offspring of oviparous females were seen to be covered in a thick shell with calcium crystals. On the other hand, neonates of females in viviparous clutches were born surrounded by a thin uncalcified membrane. This characterisation helped them understand and mark out hybrid individuals. The researchers sampled a contact zone where two groups overlapped and found that 14% of adults inherited at least 10% of their genes from the alternative parity mode, and at least 6.1% of adults were first generation hybrids. First-generation hybrid females showed intermediate properties with respect to eggshell thickness, gestation time and developmental stage at the time of expulsion of the egg from the oviduct to the environment.
The study was able to pinpoint the genes that differed between the two groups. It studied mainly the genetic basis for gestation period and eggshell traits and found that more genetic variants were involved in gestation time than with eggshell traits, indicating that the genetic basis of the retention of the embryo is more complex than that of eggshell traits. They found 210 single nucleotide polymorphisms and 439 genes associated with the gestation period and only 17 single nucleotide polymorphisms and 38 genes associated with eggshell trait variation. (Single nucleotide polymorphisms, abbreviated as SNPs and pronounced ‘snips’, are variations in the gene produced by the change of a single nucleotide, for example, when cytosine is replaced by thymine in a particular stretch of DNA.)
The study helps in understanding the genetic basis of the evolution of pregnancy, and the fact of seeing both parities in the same species makes this study unique and insightful.