Output Stream of Consciousness

A commenter at one of the blogs I frequent pointed out in passing this one very cool paper from January 2006 called HOX GENES: Seductive Science, Mysterious Mechanisms in the Ulster Medical Journal.

I love the Hox, or homeobox genes. There aren't too many gene sequences laid out in any sort of order of a body plan, but the Hox genes are. Carroll's book Endless Forms Most Beautiful captures that really well, but this paper has the evolution of the genes in a nutshell.

Homeobox genes are present in the genomes of all animals which have so far been mapped as well as in the genomes of plants and fungi, indicating that the origins are ancient and precede the divergence of these kingdoms. Plants, fungi and unicellular animals do not, however, have clustered homeobox genes. Shortly after the origins of animals the primordial homeobox gene duplicated to form a protohox cluster of two genes which are still present in cnidara such as hydra (Figure 1). Sponges do not have clustered homeobox genes, suggesting that this duplication occurred before the divergence of the parazoa. This is also reflective of the very simple body structure of sponges compared to other multicellular animals.

Figure 1

You can see in their figure 2 the relationship between the Hox gene clusters in the fruit fly and those in humans. Hox genes have been around for a long time. Understanding the evolutionary relationship between Hox genes will help us understand them better. If there is something similar enough that you can model it with fruit flies instead of mice or mice instead of humans, your research costs go down.

One interesting thing that the paper alludes to is the research going on with understanding the relationship between Hox genes and some cancer:

Some investigators have explored the postulate that Hox genes expressed during embryogenesis but down-regulated during adult life are re-expressed in neoplasia-the so called “oncology recapitulates ontology” hypothesis.

Basically, some scientists are postulating that the Hox genes, which help lay out body plan during growth in the first place, but which start to be regulated or turned off in adulthood, sometimes get turned back on and contribute to cancer. It looks like epithelial ovarian cancers may have been discovered to have a link, and they're looking at leukemia.

Pharyngula has a couple of really good posts on Hox genes as well: one for a brief overview and one more recent one what happens when you manipulate Hox genes in mice.