Douglas B. Menke

How is vertebrate morphology encoded in the genome, and how does vertebrate morphology evolve? Vertebrate limbs are ideal structures in which to study these questions because they are both highly patterned and show remarkable changes in size and form in animals adapted to running, swimming, hopping, digging, or flying. Although many of the genes that control limb growth and patterning have been identified, we know relatively little about how the expression of these genes is regulated and to what degree changes in the regulation of these limb genes has contributed to the evolution of divergent limb morphologies. My lab uses a combination of comparative genomics and functional assays in knockout and transgenic mice to identify key cis-regulatory elements that control the expression of developmentally important limb genes. Current work is focused on the Tbx4 gene, which encodes a T-box transcription factor critical for the formation of the hindlimb. We have demonstrated that hindlimb expression of Tbx4 is controlled by at least two distinct enhancer elements, deletion of one of which reduces expression of Tbx4 in the hindlimb during embryogenesis and produces mice with characteristic decreases in the sizes of bones in the hindlimb. Comparative sequencing of vertebrate species that have evolved partial or complete hindlimb reduction indicates that reduction in the hindlimb size of some species has been accompanied by sequence changes in key hindlimb control regions of Tbx4. Future work will include the use of mouse transgenics and knock-in strategies to test the functional significance of Tbx4 hindlimb control elements during normal mouse development and the biological effects of sequence changes seen in other species.