Digital gene expression tag profiling of bat digits provides robust candidates contributing to wing formation

As the only truly flying mammals, bats use their unique wing - consisting of four elongated digits (digits II-V) connected by membranes - to power their flight. In addition to the elongated digits II-V, the forelimb contains one shorter digit (digit I) that is morphologically similar to the hindlimb digits. Here, we capitalized on the morphological variation among the bat forelimb digits to investigate the molecular mechanisms underlying digit elongation and wing formation. Using next generation sequencing technology, we performed digital gene expression tag profiling (DGE-tag profiling) of developing digits in a pooled sample of two Myotis ricketti and validated our sequencing results using real-time quantitative PCR (RT-qPCR) of gene expression in the developing digits of two Hipposideros armiger.

Among hundreds of genes exhibiting significant differences in expression between the short and long digits, we highlight 14 genes most related to digit elongation. These genes include two Tbx genes (Tbx3 and Tbx15), five BMP pathway genes (Bmp3, RGMB, Smad1, Smad4 and Nog), four Homeobox genes (Hoxd8, Hoxd9, Hoxa1 and Satb1), and three other genes (Twist1, Tmeff2 and Enpp2) related to digit malformations or cell proliferation. In addition, our results suggest that Tbx4 and Pitx2 contribute to the morphological similarity and five genes (Acta1, Tnnc2, Atp2a1, Hrc and Myoz1) contribute to the functional similarity between the thumb and hindlimb digits.

Results of this study not only implicate many developmental genes as robust candidates underlying digit elongation and wing formation in bats, but also provide a better understanding of the genes involved in autopodial development in general.