Bat wing development

The order Chiroptera, comprising all bats, has evolved the unique mammalian adaptation of flight. Bat wings are modified tetrapod forelimbs. Because bats are mammals, the skeletal structures in their wings are morphologically homologous to the skeletal components found in other tetrapod forelimbs. Through adaptive evolution these structures in bats have undergone dramatic morphological changes. This evolutionary innovation involved many morphological changes such as webbed digits, elongation of the forelimb, and reduction in bone thickness. Recently, there have been comparative studies of mouse and bat forelimb development to understand the genetic basis of morphological evolution. Consequently, the bat wing is a valuable evo-devo model for studying the evolution of vertebrate limb diversity.

Tetrapod limb development involves many signaling molecules such as FGF, BMP, SHH and WNT. The apical ectodermal ridge is a structure found at the distal most tip which becomes a key signaling center for the developing limb. Surprisingly many of the same signaling pathways known to play a role in tetrapod limb development have been found to play a role in bat forelimb development but the timing, intensity, and spatial gene expression of some orthologous genes have changed. Since mice are also mammals, it is convenient to compare morphology and development of forelimbs between mice and bats; these comparisons may elucidate the genetic basis of adaptive bat wing development.

Although many of the molecular mechanisms involved in limb development are conserved between mouse and bat, there are a number of differences primarily seen in gene expression patterns. Surprisingly, the coding regions of many of these genes with different expression domains are highly conserved between mouse and bat. Thus, it is likely that this major morphological transition was a consequence of cis-regulatory changes. Researchers can study the genetic basis of bat wing development by using comparative in situ hybridization to examine gene expression domains and using experimental embryology in mice and bats.

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