Investigating genes involved in development, scientists now understand how frogs have evolved their unique head structure.
“Nothing in biology makes sense except in the light of evolution” from Theodosius Dobzhansky is one of the most famous quotes regarding evolution. To what extent this is true is a discussion for another time, but what can be said is that evolution can be better understood in the light of development. This is the foundation of the field of Evo-devo (short for Evolutionary Developmental Biology), which aims to understand how different body shapes and patterns evolved. One way scientists study this is to examine how different body structures arise during embryonic development, and how these processes differ between species.
Recently, researchers in the University of Colorado asked the question: how has the development of head skeleton evolved in vertebrates. Taking an Evo-devo approach, the scientists compare the development of various vertebrate head skeletons and examine the underlying mechanisms that contribute to their development and evolution.
Specifically the authors examined genes (genetic material contained in DNA) which are used in the developing embryo of the African clawed frog, Xenopus laevis, and compared their results with existing literature of equivalent genes in mouse, shark and zebrafish embryos. This gave them insight into how skeletal elements in the vertebrate head evolved and what genes may have played a larger role in this process. The authors note that even though the head skeleton has essentially the same function in all vertebrate species, there is remarkable variability in the number, shape and size of its components. This plasticity has likely contributed to the success of vertebrates.
Among all vertebrates, frogs and toads have a unique skull. While they have some structures in common with other vertebrates, they have specialized structures which are hypothesized to be due to their extreme abrupt metamorphosis. During their larval stage most of the skull is made of cartilage, which is then replaced with bone in the adult, unlike other vertebrates where bone is formed in the embryo.
Of all the genes the authors investigated, dlx1/2 and emx2, two genes with opposing effect in balancing cartilage formation, seemed to have a different pattern between Xenopus and other vertebrates, with dlx1/2 reduced and emx2 expanded in the jaw region of the second pharyngeal arch (the structures that resemble gills). The authors suggest that the shift in the pattern of this two genes may explain the reduced amount of cartilage typical of amphibians and evolutionary changes in skull shape and size.
Furthermore the authors also found that two genes involved in the formation of joints, barx1 and gdf5, showed a unique pattern in Xenopus. The authors suggest that unique localization of these genes in the Xenopus is responsible of the formation of new cartilage between bones of the lower jaw, a feature which is not present in most other vertebrates.
Where does this leave us? This work is an impressive screen of several genes in Xenopus and perhaps what makes it more complete is the fact that the scientists did an excellent job compiling the information available in other species to make a model of what pattern could be present in our common ancestor. This study now shows us which genes seem to explain the diversity of the vertebrate skull. What is now left to know is why did the localization of these genes change. This not only can lead to a better understanding of vertebrate evolution by explaining changes in skull shape and size, but also what could be the consequences if these genes are mutated, possibly giving insights into human pathology.