Functional evolution of the Ultrabithorax protein

The Hox genes have been implicated as central to the evolution of animal bo dy plan diversity. Regulatory changes both in Hox expression domains and in Hox-regulated gene networks have arisen during the evolution of related ta xa, but there is little knowledge of whether functional changes in Hox prot eins have also contributed to morphological evolution, For example, the evo lution of greater numbers of differentiated segments and body parts in inse cts, compared with the simpler body plans of arthropod ancestors, may have involved an increase in the spectrum of biochemical interactions of individ ual Hox proteins. Here, we compare the in vivo functions of orthologous Ult rabithorax (Ubx) proteins from the insect Drosophila melanogaster and from an onychophoran, a member of a sister phylum with a more primitive and homo nomous body plan. These Ubx proteins, which have been diverging in sequence for over 540 million years, can generate many of the same gain-of-function tissue transformations and can activate and repress many of the same targe t genes when expressed during Drosophila development. However, the onychoph ora Ubx (OUbx) protein does not transform the segmental identity of the emb ryonic ectoderm or repress the Distal-less target gene, This functional div ergence is due to sequence changes outside the conserved homeodomain region , The inability of OUbx to function like Drosophila Ubx (DUbx) in the embry onic ectoderm indicates that the Ubx protein may have acquired new cofactor s or activity modifiers since the divergence of the onychophoran and insect lineages.