Examples of embryos with normal and abnormal craniofacial development
Embryos are not just little organisms encapsulated within their eggs. Embryonic development is dynamic; the embryo transitions from one to a few undifferentiated cells to a stage where the various parts like arms, legs, and faces become apparent to a form that resembles the species that will eventually emerge. A panoply of signaling events and rapid rates of cell division are all tightly choreographed to make sure that development proceeds in a predictable, species-specific fashion.
But this dynamism of development also makes the embryo susceptible to environmental perturbation. Heat, chemical exposure, and pathogens can all disrupt normal embryonic development, sending the embryo down paths that may lead to fatality or reduced fitness. In our recently published study, my colleagues, students, and I demonstrate that heat stress, paralleling what will likely be experienced during the 21st century, can induce structural malformations to the brain and face of lizard embryos.
In 2014 and 2015 I was a post-doc in the Cohn Lab at the University of Florida. At the time, I had been dissecting and observing anole embryos for approximately 14 years. Throughout those 14 years I had observed only a handful of malformed embryos, maybe on the order of 10-20 embryos after collecting thousands of embryos from numerous anole species. Yet, in the summers of 2014 and 2015, while working alongside then graduate student Bonnie Kircher, I collected more malformed Anolis sagrei embryos than I had in all of my previous years. I certainly didn’t realize it in the moment as malformed embryos were still relatively rare compared to the total number of embryos we were collecting. But, by the end of my time in Florida the number jumped out at me. Sometimes the rate of development seemed to depart from the normal sequence of development. Other times the embryo was clearly not well and would likely not survive to hatching. Even as other projects accelerated, my interest in these malformed embryos remained piqued. When I began my faculty position Loyola University Chicago, I decided to invest the lab’s time and resources into determining whether this pattern was real or just a chance observation.
In spite of other options–unique genetic mutation running rampant in Gainesville populations of brown anoles seems highly unlikely–I decided to investigate the effects of heat stress on embryonic development. The effects of global warming had been widely discussed as a threat to ecotherm populations and anoles have been at the center of both field and lab observations since the outset. A number of studies have also shown reduced hatching success from lizards incubated at relatively high temperatures. Relating this back to my observations in Florida, it also occurred to me that Bonnie and I used a different collection strategy for our breeding colony those years; we would regularly replenish or add to the colony from field-caught lizards throughout the summer. This raised the possibility that the embryos developing within the gravid females were exposed to the environmental heat stress before being deposited into our waiting hands. Luckily, brown anoles are prolific egg producers, providing my lab with the ability to test whether heat stress induces embryonic malformations under different incubation regimes.
Embryos incubated under conditions reflecting those observed in shady nest sites exhibit malformations in only one to two embryos out of every 100 live embryos. These nest sites tend to be relatively stable in temperature, rarely rising about 30 degrees Celsius. However, embryonic heat stress induces malformations in 10-30% of embryos exposed to incubation conditions that parallel nest sites that would be located in sunny locations. These putative nest sites reach temperatures above 36 degrees Celsius, the critical thermal temperature for the embryos, for up to eight hours per days. The malformations we observed were not evenly spread across the body. Instead, we saw the greatest concentration in the brain and face of the developing lizards. Most malformations included a change in facial proportion, from subtle changes in facial length to pronounced brachycephaly and/or clefting. In one case, the entire face and forebrain were ablated in the embryo! When it comes to the induction of structural malformations, the most sensitive period of development is around oviposition, including the time that the egg is still within the female. Although we do not yet know how many of these embryos would successfully hatch, our experiments do raise concerns about the long-term impacts of global warming on ectotherm development.
Examples of extreme, but rare malformations observed in eggs reared at elevated temperatures.
The consistent pattern of thermal-induced neural and facial anomalies made us think that there may be a common underlying cause of these changes, leading us to create and test a new model of embryonic thermal stress. Based on our understanding of amniote craniofacial development, we predicted that disruption to Hedgehog signaling, one of the earliest signaling pathways needed for facial development, could create the full spectrum of observed malformations. After measuring processes up and downstream of Hedgehog signaling (e.g., cell death and signaling within the presumptive facial cells respectively), it does, in fact, appear that Hedgehog signaling is disrupted in the face of embryos experiencing thermal stress. Depending on the degree of response by a particular embryo, everything from normal to extremely malformed embryo could be induced. At this time, it appears that our model holds for brown anoles and may be applicable in species far beyond anoles and lizards.
In our proposed model, heat stress affects Hedgehog signaling, causing a disruption to normal facial morphogenesis.
There remains much to learn about normal and abnormal facial development in lizards. We do not yet know what other signaling pathways are equally disrupted during thermal stress or whether there are endogenous buffering mechanisms that help to maintain normal development in the face of external stress. Perhaps one of the most important discussions that needs to occur is how we study rare events. These events could be uncommon, but extreme heat events that exceed the “normal” conditions typically observed in the wild. These are increasing in regularity and may have significant impacts on ectotherms later in the 21st century. Alternatively, the rare events could be the emergence of malformed embryos which occur in only a fraction of individuals, even when the average phenotype is not dramatically altered. For species such as the brown anole, this may not be alarming. But for species with relatively few viable hatchings each season, embryonic heat stress could have dramatic impacts on their long-term viability. These developmental perspectives are needed to fully understand the ways that global change will affect the lives and longevity of lizards and other ectotherms.
Embryos are impacted by a range of challenges associated with anthropogenic change. See more in Sanger 2021, Integrative developmental biology in the age of anthropogenic change
- Short Faces, Two Faces, No Faces: Lizards Heads Are Susceptible to Embryonic Thermal Stress - December 15, 2021
- The Super Sticky Super Power of Lizards: a New Outreach Activity for Grade-Schoolers - April 9, 2018
- Updates on the Development of Anolis as a “Model Clade” of Integrative Analyses of Anatomical Evolution - September 4, 2017
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