Differential Expression of <i>Meis2</i>, <i>Mab21l2</i> and <i>Tbx3</i> during Limb Development Associated with Diversification of Limb Morphology in Mammals

<div><p>Bats are the only mammals capable of self-powered flight using wings. Differing from mouse or human limbs, four elongated digits within a broad wing membrane support the bat wing, and the foot of the bat has evolved a long calcar that spread the interfemoral membrane. Our recent mRNA sequencing (mRNA-Seq) study found unique expression patterns for genes at the 5′ end of the <i>Hoxd</i> gene cluster and for <i>Tbx3</i> that are associated with digit elongation and wing membrane growth in bats. In this study, we focused on two additional genes, <i>Meis2</i> and <i>Mab21l2</i>, identified from the mRNA-Seq data. Using whole-mount in situ hybridization (WISH) we validated the mRNA-Seq results for differences in the expression patterns of <i>Meis2</i> and <i>Mab21l2</i> between bat and mouse limbs, and further characterize the timing and location of the expression of these two genes. These analyses suggest that <i>Meis2</i> may function in wing membrane growth and <i>Mab21l2</i> may have a role in AP and DV axial patterning. In addition, we found that <i>Tbx3</i> is uniquely expressed in the unique calcar structure found in the bat hindlimb, suggesting a role for this gene in calcar growth and elongation. Moreover, analysis of the coding sequences for <i>Meis2</i>, <i>Mab21l2</i> and <i>Tbx3</i> showed that <i>Meis2</i> and <i>Mab21l2</i> have high sequence identity, consistent with the functions of genes being conserved, but that <i>Tbx3</i> showed accelerated evolution in bats. However, evidence for positive selection in <i>Tbx3</i> was not found, which would suggest that the function of this gene has not been changed. Together, our findings support the hypothesis that the modulation of the spatiotemporal expression patterns of multiple functional conserved genes control limb morphology and drive morphological change in the diversification of mammalian limbs.</p>