Why the long face?
When most people think of vertebrate sexual dimorphism (differences between the sexes), they think of elephant seals or red deer. Most of us here, of course, think of the pronounced dimorphism in size and shape in many anole species. Indeed, anoles have served as excellent model systems for the study of sexual dimorphism, particularly the evolutionary forces that give rise to it. Although there has been significant progress since Darwin in our understanding of why sexual dimorphism evolves, we have made less progress in the HOW. That is, what mechanisms during development give rise to what are often extreme differences between the sexes when their genomes are so similar?
When we think of vertebrates where males are larger or shaped differently than females, and have weapons or ornaments, we almost immediately think of testosterone as a mechanism underlying the sex differences. Once sexual maturity happens, the testes start cranking out testosterone, thus causing a change in the male’s phenotypic trajectory. While there is certainly evidence for circulating testosterone to have this effect in some lizards, is this always the case, and does it apply to specific body parts and not just overall size? Aside from the circulating hormone, how are receptors involved in the development of dimorphism? In a new paper by Sanger et al., a novel developmental pathway of sexual dimorphism is described for lizards in the carolinensis clade, which are striking in their elongation of male faces relative to females.
Figure 1a. from Sanger et al. (2014), showing the differences in head shape dimorphism among anole clades. Note the long male face in A. maynardi, a member of the carolinensis clade.
Sanger et al. tested whether sex differences in several different pathways led to the observed head shape dimorphism in A. carolinensis compared to two non-carolinensis species (A. cristatellus and A. sagrei) that exhibit shorter male faces. They show, using a combination of developmental and molecular genetic techniques, that the extreme elongation of male heads in carolinensis lizards is not due to an androgen pathway (i.e., testosterone) or the somatropic axis (i.e., insulin-like growth factor). Instead, they found a significant shift in the estrogen pathway. Specifically, at sexual maturity, males decrease expression of estrogen receptors (erβ), which is the beginning of a signaling cascade, ultimately resulting in up-regulation of genes involved in skeletogenesis in the skull of males.
Figure 4 from Sanger et al. (2014), showing the molecular pathway underlying facial elongation in A. carolinensis.
This identification of a novel mechanism for the development of sexual dimorphism will certainly stimulate further evo-devo research in anoles and beyond. For starters, is the same pathway responsible for male facial elongation in other species in the carolinensis clade, or are more ‘traditional’ mechanisms operating there? This important research highlights that investigators need to consider all aspects of signaling systems, including circulating hormones, their receptors, and signal cascades that result from activation of a particular pathway. Clearly this paper by Sanger et al. is an excellent step in the right direction for understanding how developmental pathways lead to adult difference in anoles, and it will also steer other investigators to consider a diversity of developmental mechanisms in their quest to elucidate how adults end up the way they do.
Sanger TJ, Seav SM, Tokita M, Langerhans RB, Ross LM, Losos JB, Abzhanov A. 2014. The oestrogen pathway underlies the evolution of exaggerated male cranial shapes in Anolis lizards. Proceedings of the Royal Society B 281:20140329.
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