A tunicate bends the rules
A chordate without Hox gene clustering challenges textbook thinking
By Stuart Blackman

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 has occurred in some
insects and in the nematode Caenorhabditis elegans, but has not been found
in any vertebrate studied to date. Tunicates are non-vertebrate members of
the chordate phylum, whose affinity with vertebrates is evident in their
tadpole-like larval stage.

Another tunicate, the sea-squirt Ciona intestinalis, also exhibits partial
Hox cluster fragmentation, noted Chourrout, but this is far more pronounced
in Oikopleura. Eight of Oikopleura's nine Hox genes were found to be more
than 250 kilobases from their nearest Hox neighbor on either side. This
extent of fragmentation is matched only in C. elegans, said Chourrout.

Read the rest at The Scientist
http://www.biomedcentral.com/news/20040902/01

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