Extreme heat events are becoming more common as a result of anthropogenic global change. Developmental plasticity in physiological thermal limits could help mitigate the consequences of thermal extremes, but data on the effects of early temperature exposure on thermal limits later in life are rare, especially for vertebrate ectotherms. We conducted an experiment that to our knowledge is the first to isolate the effect of egg (i.e. embryonic) thermal conditions on adult heat tolerance in a reptile. Eggs of the lizard Anolis sagrei were incubated under one of three fluctuating thermal regimes that mimicked natural nest environments and differed in mean and maximum temperatures. After emergence, all hatchlings were raised under common garden conditions until reproductive maturity, at which point heat tolerance was measured. Egg mortality was highest in the warmest treatment, and hatchlings from the warmest treatment tended to have greater mortality than those from the cooler treatments. Despite evidence that incubation temperatures were stressful, we found no evidence that incubation treatment influenced adult heat tolerance. Our results are consistent with a low capacity for organisms to increase their physiological heat tolerance via plasticity, and emphasize the importance of behavioural and evolutionary processes as mechanisms of resilience to extreme heat.
Another SICB, another great presentation from Sylvia Nunez from Thom Sanger’s lab investigating how the interaction between heat and oxygen availability affects development in the brown anole (Anolis sagrei). Last year, Sylvia presented a poster showing that above 33°C, embryonic survival was greatly reduced and many embryos developed craniofacial malformations. With some potential nesting sites for anoles now exceeding 40°C, understanding mechanisms leading to decreased survival is critical.
Low oxygen at sublethal temperatures can recapitulate negative effects on craniofacial development at high temperatures.
As a follow up to this study, Sylvia set out to understand exactly how it is that heat and oxygen can interact to lead to craniofacial deformities. She posited several hypotheses and was able to eloquently test each one, including the neural degeneration hypothesis and the oxygen limitation hypothesis. Specifically, Sylvia noted that disruption in sonic hedgehog has been linked to facial development and that oxygen demand can often exceed oxygen supply at high temperatures.
First, Sylvia tested the oxygen limitation hypothesis by examining whether low oxygen coupled with sublethal (elevated) temperature conditions recapitulate the effects of craniofacial malformation under thermal stress. Previous work in the lab induced craniofacial malformation at 36°C; Sylvia showed you could mimic this effect using 33°C with low oxygen, with a greater rate of malformation than in 27°C (the standard control temperature) with atmospheric oxygen, 27°C with low oxygen, or 33°C with atmospheric oxygen. She then tested whether an increase in oxygen can rescue embryonic survival at high temperatures. To do so, she split eggs among two treatment conditions: 27°C with high oxygen and a hot nest site temperature with high oxygen. She found further support for the oxygen limitation hypothesis – when oxygen availability was increased above atmospheric conditions there were no differences in embryonic survival. Wow! Further, there were no craniofacial malformations in the high temperature treatment when oxygen conditions were high.
To follow up on this finding, she examined if oxidative stress could be the link between temperature and craniofacial malformations using superoxide dismutase (SOD), an enzyme that helps turn the superoxide radical (O2–) into oxygen (O2) or hydrogen peroxide (H2O2), as a marker for oxidative stress in the telencephalon. Indeed, within mere minutes of a temperature increase, SOD becomes upregulated, suggesting that thermal stress contributes to oxidative stress. But Sylvia didn’t stop there. She then treated some embryos with an SOD inhibitor to show that when SOD is absent craniofacial malformations appear.
Overall, Sylvia has very eloquently shown that increased temperature leads to craniofacial malformations via thermal effects on oxidative stress. I cannot wait to see what she presents next year!
Check out this piece in the New Scientist, which picked up on our images of Anolis embryos and Thom’s awesome research!
Embryo of Anolis longitibialis, a trunk-ground anole from the Dominican Republic.
The readers of this blog do not need to be convinced that anoles are an amazing model system in evolutionary biology. New and exciting research often finds its way to the Anole Annals. Here we’ve learned about emerging trends in Anolisgenomics, speciation, and comparative phylogenetics, to list just a few. In recent years, Anolis has also become a model system for developmental biology. For example, a recent study by Dr. Thom Sanger demonstrated that the diversity of limb dimensions among ecomorphs have evolved from similar developmental mechanisms.
This summer I worked a bit with Thom to learn how to stage Anolis embryos using his handy staging series as a guide. The goal of the project was to determine the stage at which female anoles laid eggs under two treatment conditions – a hot treatment (32°C) and a cold treatment (20°C). I had females from three populations of A. cybotes (55, 700, and 1400 meters in elevation), one population of A. shrevei (2450 m), and one population of A. longitibialis (100m). Unfortunately, I was unable to collect very many eggs despite letting the experiment run for six weeks. I did, however, manage to get several beautiful embryos, which I have imaged and staged. Here I’ll provide some pictures and give a few shorthand methods for staging Anolis embryos.
As lineages rapidly diversify, such as in the history of anoles, does their developmental-genetic architecture constrain the rate or direction of evolutionary change? In other words, could the processes controlling the production of variation, the variation that natural selection acts on, affect patterns of phenotypic evolution by generating some phenotypes more readily than others? While theoretical discussions like these have been prevalent for over a century, developmentally-based constraints were not formalized in the context of modern biology until the 1980’s, fueled by an influential paper by Maynard-Smith and colleagues and the re-synthesis of evolutionary and developmental biology. Since then evo-devologists have been testing the plausibility of developmental constraints by examining the developmental bases of traits that have independently evolved multiple times; phenotypes that have repeatedly evolved using the same mechanisms may be indicative of constraint (because the precise interpretation of these patterns and appropriate level of analysis are contentious I will leave further theoretical discussion of constraint to future conversations).
In a recent paper, for which I am the lead author, we set out to examine whether developmental constraints could have affected diversification of anole limb morphology.
