The homeotic (Hox) genes establish the body plan in early embryonic life, and are arranged along the chromosome in the order of their spatial and temporal expression. The Hox locus is regarded as the jewel of evolutionary biology, as cross-species conservation of the clustered arrangement and expression patterns of its constituent genes is strong evidence for our humble evolutionary beginnings.

But now, a small marine organism has broken the rules, as Daniel Chourrout and colleagues showed in a study in Nature. Unlike all other animals that have bilateral body symmetry, the Hox genes of Oikopleura dioica are located far apart with no indication of clustering — a finding that, so far, is restricted to Caenorhabditis elegans. With regard to the precise position of the Hox genes in O. dioica, Nipam Patel, from the University of California at Berkeley, USA, speculated that “for all we know they'll be on different chromosomes.” (The Scientist, 2 September 2004).

So, if O. dioica can develop normally despite the separation of its Hox genes, what is the driving force behind the conservation of Hox clustering?

Similar to C. elegans, the chromosomal position of the Hox genes still correlates with spatial expression in O. dioica, but the coordination of temporal expression is lost. As Patel points out, the rapid embryonic development of these small organisms might allow them to “employ mechanisms that don't require strict anterior and posterior timing.” The need for a coordinated timing mechanism would, however, favour the maintenance of Hox clusters in higher organisms.