A tunicate bends the rules

Hox gene clusters are not necessary to build a chordate, a paper published in the September 2 issue of Nature shows. The finding raises questions about established beliefs on the subject of body plan development.

"If you open any textbook, you'll read that the Hox cluster is conserved in all bilaterian animals," Daniel Chourrout at the University of Bergen, Norway, told The Scientist. But his study of the tunicate Oikopleura dioica revealed its Hox genes to be distributed in nine locations around the genome.

Hox gene clusters specify the body plan of most multicellular animals, suggesting that they date back to the common ancestor of worms, arthropods and vertebrates. They are among the most highly conserved of genetic sequences, and display a collinear pattern of expression, whereby their sequence on a chromosome reflects where they are expressed along an animal's anteroposterior axis.

Chourrout said that a degree of Hox fragmentation ...