Hox and ParaHox Genes in Evolution, Development and Genomics

The discovery of the homeobox motif and its presence in each gene of the Hox clusters revolutionized the fields of developmental biology and evolutionary developmental biology (1, 2), providing a rapid entrance into investigating the mechanisms of development of almost any animal taxon as well as dramatically altering conceptions on the extent of genetic conservation across the animal kingdom. The wide-reaching implications stemming from Hox biology and the rapid developments in the field due to technological advances, such as improved sequencing techniques and RNAi, make this special issue of Genomics, Proteomics & Bioinformatics a timely marker of progress. Despite the intensive research on Hox genes over the last few decades, much about the biology of these genes remains mysterious. Focusing investigations on powerful genetic model species, such as fruit flies and mice, have contributed much to our understanding. However, the comparative approach, involving studying the genes in non-traditional model systems that are primarily selected for their position within the animal phylogeny rather than their immediate tractability in the lab, has been vital to the progress of Hox biology, even from its earliest days. A prime example of this was the discovery that the ParaHox genes probably evolved as an evolutionary sister cluster to the Hox cluster, deep in animal ancestry (3). Consequently, the review articles in this special issue are written with this comparative emphasis to the fore.