Abstract
BackgroundThe emergence of novel structures during evolution is crucial for creating variation among organisms, but the underlying processes which lead to the emergence of evolutionary novelties are poorly understood. The gnathostome jaw joint is such a novelty, and the incorporation of bapx1 expression into the intermediate first pharyngeal arch may have played a major role in the evolution of this joint. Knockdown experiments revealed that loss of bapx1 function leads to the loss of the jaw joint, because Meckel’s cartilage and the palatoquadrate fuse during development. We used Xenopus laevis and Ambystoma mexicanum to further investigate the function of bapx1 in amphibians. Bapx1 expression levels were upregulated through the use of Ly-294,002 and we investigated the potential consequences of the enhanced bapx1 expression in amphibians to test the hypothesized joint inducing function of bapx1.ResultsWe show that Ly-294,002 upregulates bapx1 expression in vivo. Additionally, ectopic mandibular arch derived cartilages develop after Ly-294,002 treatment. These ectopic cartilages are dorsoventrally oriented rods situated lateral to the palatoquadrate. The development of these additional cartilages did not change the muscular arrangement of mandibular arch-derived muscles.ConclusionsDevelopment of additional mandibular cartilages is not unusual in larval anurans. Therefore, changes in the bapx1 expression during evolution may have been the reason for the development of several additional cartilages in the larval anuran jaw. Furthermore, our observations imply a joint-promoting function of bapx1, which further substantiates its hypothetical role in the evolution of the gnathostome jaw joint.
Highlights
The emergence of novel structures during evolution is crucial for creating variation among organisms, but the underlying processes which lead to the emergence of evolutionary novelties are poorly understood
Effect of Ly-294,002 treatment on tadpole survival Initially embryos of X. laevis were treated with different concentrations of Ly-294,002 at different developmental stages
Embryos treated with 10 μM and 20 μM at the onset of neurulation (NF 13) survived the treatment at moderate rates (27 and 20% respectively) whereas only a minority of embryos treated with 30 μM (2%) and 40 μM (4%) survived
Summary
The emergence of novel structures during evolution is crucial for creating variation among organisms, but the underlying processes which lead to the emergence of evolutionary novelties are poorly understood. The acquisition and incorporation of novel skeletal structures into an existing skeletal environment is a process that can cause morphological diversification. How such evolutionary novelties arose during evolution and under which circumstances they arose is an important question in evo-devo research. The suprarostral cartilage forms the anterior part of the upper jaw, whereas the infrarostral cartilage is part of the lower jaw and movable against Meckel’s cartilage via the intramandibular joint [2] These two additional structures made possible the evolution of numerous different feeding modes in anuran tadpoles, which enabled the decoupling of larval and adult stages. During further evolution several anuran taxa evolved one or more additional
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