Turtle skull development unveils a molecular basis for amniote cranial diversity.

Abstract

Amniote skulls display diverse architectural patterns including remarkable variations in the number of temporal arches surrounding the upper and lower temporal fenestrae. However, the cellular and molecular basis underlying this diversification remains elusive. Turtles are a useful model to understand skull diversity due to the presence of secondarily closed temporal fenestrae and different extents of temporal emarginations (marginal reduction of dermal bones). Here, we analyzed embryos of three turtle species with varying degrees of temporal emargination and identified shared widespread coexpression of upstream osteogenic genes <i>Msx2</i> and <i>Runx2</i> and species-specific expression of more downstream osteogenic genes <i>Sp7</i> and <i>Sparc</i> in the head. Further analysis of representative amniote embryos revealed differential expression patterns of osteogenic genes in the temporal region, suggesting that the spatiotemporal regulation of <i>Msx2</i>, <i>Runx2</i>, and <i>Sp7</i> distinguishes the temporal skull morphology among amniotes. Moreover, the presence of <i>Msx2-</i> and/or <i>Runx2</i>-positive temporal mesenchyme with osteogenic potential may have contributed to their extremely diverse cranial morphology in reptiles.