Shaping animal body plans in development and evolution by modulation of Hox expression patterns.

Most animals exhibit distinctive and diverse morphological features on their anterior-posterior body axis. However, underneath the variation in design and developmental strategies lies a shared ancient structural blueprint that is based on the expression patterns of Hox genes. Both the establishment and maintenance of the spatial and temporal distribution of Hox transcripts play an important role in determining axial pattern. The study of many animal systems, both vertebrate and invertebrate, suggests that the mechanisms used to establish Hox transcription are nearly as diverse as the body plans they specify. The strategies for maintenance of Hox expression pattern seem more conserved among different phyla, and rely on the action of Pc and trx group genes as well as auto- and cross-regulation among Hox genes. In mice, the sharing of regulatory elements coupled with auto- and cross-regulation could explain the conservation of the clustered arrangement of Hox genes. In contrast, fly Hox genes seem to have evolved insulators or boundary elements to avoid sharing regulatory regions. Differences in Hox transcription patterns can be correlated with morphological modifications in different species, and it seems likely that evolutionary variation of Hox cis-regulatory elements has played a major role in the emergence of novel body plans in different taxa of the animal kingdom.