As discussed previously in the Annals, interest in squamate development is rapidly accelerating. Our growing community makes this an exciting time to study lizard development, especially in a comparative context. A recent study by Andrews et al. capitalizes on the increasing number of developmental resources for squamates to assess variability in developmental sequences across lizards and snakes. One of our favorite anoles, Anolis sagrei, represents one of the 21 species included in this study. The conclusions of this study speak to several long-standing evolutionary questions and opens up new avenues of investigation that may be of interest the readers of this blog.
The main conclusion of this study is that development evolves – the sequence of developmental events that unfold during morphogenesis to generate a complete organism changes from one clade to the next. In other words, in some clades limb development precedes cranial develop while in others the order is reversed. But morphogenesis is more complex than just limb and cranial development. In total this study charted the course of 20 developmental events to reveal a staggering degree of developmental mosaicism. Not surprisingly, Andrews et al. harness the strength of a molecular phylogeny to perform their evolutionary analyses. But it might also be interesting to consider the topology of the morphological tree when trying to resolve differences in timing among distantly related clades, especially when it comes to differences between the iguanids and gekkonids. Finally, the author’s analyses provide support for the hotly debated “phylotypic stage” – an intermediate stage of development with reduced variation that all chordate embryos progress through.
The conclusions of Andrews et al. have important practical implications for our community. Because developmental sequences evolve, the authors urge caution when trying to apply staging series to distantly related species. Because of the relatively limited sampling of this study – relative to all squamate diversity, not similar studies – it remains unclear where these developmental shifts are occurring in squamate history. Do they occur during significant shifts in morphology or physiology? Perhaps these shifts simply represent the ebb and flow of neutral developmental drift? The anole staging series can readily be applied to the Caribbean radiation of anoles, but what about mainland species and other iguanids? We do not yet know. Additional developmental series are clearly needed to further interpret these patterns and their evolutionary significance.
Ultimately I think that this is interesting study that is worth considering from anyone interested in comparative studies of squamate development. Enjoy!