La gestation et la ponte (Gestation and Oviposition)

Today's post is from Antonio Cordero of Universität Tübingen.

Recently there has been a lot interest in understanding flexibility of developmental processes in response to environmental variability, which is referred to as developmental plasticity. In fact, a recent special issue of the Journal of Experimental Zoology A highlights many recent advances in this area. For example, embryos can control how they utilize energy (metabolism) by reducing oxygen consumption via physiological adjustments, as we previously showed in wall lizard embryos that developed in a high elevation environment.

Processing of wall lizard embryos in the lab.

Wall lizard embryos are interesting to study because they begin to develop in the “womb” of their mother (gestation), as is the case in many lizards. However, when they reach a certain stage of development they are released by their moms, while encased in calcified egg, to the external environment. This provides an excellent opportunity to examine interactions between embryonic physiology and the maternal versus external environments.

In 2018, team PODARCIS set out to determine whether gravid wall lizard mothers and their embryos displayed physiological adjustments while exposed to high elevation conditions (see post about the beginning of this experiment). A key prediction of this experiment was that developing embryos would better adjust to low oxygen availability if they had already undergone gestation in such conditions. In fact, females of many high-elevation lizard species retain their eggs for longer while undergoing gestation. As a result, the time that embryos are exposed to the external environment is reduced and thus their likelihood of surviving to adulthood is enhanced. To address this, I was in charge of determining the stage of development in preserved wall lizard eggs that had been recently laid by their mothers.

Wall lizard embryos are translucent and measure 5-8 mm by the time they are laid by their mothers for incubation in a nest. I used the standardized staging table of Dufaure & Hubert (1961) to classify embryos into stages of development based on external features. Wall lizard embryos are essentially indistinguishable from that of any lizard species at this point: limbs are but tiny tissue buds, the tail is hardly discernible, the face and jaw are not yet put together, the eye is far from fully formed, and many other organs are not yet set in place. Moreover, paired somites can be counted to estimate the size and stage of embryos. Somites are transient mesodermal structures that subsequently give rise to bone and muscle tissue associated with, for example, the rib cage and limb system.

Thus far, we found that wall lizard embryos in our experiment were laid between stages 25-28 of Dufaure & Hubert. Furthermore, whether eggs were from low- or high-elevation populations or were laid at low- or high-elevation or not did not appear to affect the average stage at oviposition. These results are informative because they generally support the prediction that stage at oviposition, and thus egg retention, is not a highly plastic trait. One intriguing hypothesis to explain these results is that maybe mothers employ physiological adjustment to ensure that their eggs are laid on schedule and that embryos will be left to their own devices to compensate for high-elevation conditions, as we previously showed in Cordero et al. (2017). To test this hypothesis, we will have to figure out how to measure physiological parameters directly related to egg production in gravid mothers while altering both temperature and oxygen. Still, we succeeded in demonstrating that reduced oxygen availability in the environment of the mothers did not affect the rate at which their offspring develop while in the uterus.

Our treatment groups did not differ with respect to stage at oviposition (left panel).
On average, wall lizard embryos were at stage 27 (right panel).