quote:
I wonder if anyone is familiar with what kind of changes of the hox genes that are responsible for a new family, order or class to evolve? Ive heard that changes in the downstream region of hox genes is caracteristic for species within a family. For different families in the same order, for example dogs, cats or bears, Ive heard that it is mutations within the hox genes themselves. What about bigger morphological differences, such as whales and artiodactyles? What differs there?
There is a great section in the 22 Oct 2004 issue of
Science. The section is called "Genes in Action". It outlines the effect of transcription factors (ie enhancers) of hox genes, and also outlines early embryonic development with respect to gene cascades. One section I found interesting is found on pg's 634-635 discussing the different morphology between starfish and urchins:
Yet subtle variations have had a big impact. For example, there's a five-gene circuit both species share. A key gene in this pathway is
otx, and it sets off the circuit in the sea urchin and the starfish. Hinman has found a tiny change in this enhancer. Between the two species, this enhancer varies by just one binding site, for a transcription factor called t-brain. The starfish has this binding site; the sea urchin does not. In the sea urchin, t-brain works in concert with other regulatory genes and sets off the embryo's skeleton-forming circuitry, a genetic pathway absent in the starfish embryo. But because the
otx enhancer is missing t-brain, the sea urchin must also rely on a different transcription factor to get the
otx gene in the five-gene circuitry to kick in.
Meanwhile the t-brain binding site on the starfish's
otx enhancer keeps
otx focused on genes for the incipient gut. Davidson thinks that ancestral echinoderms had a t-brain site on the enhancer for
otx, one that disappeared from that enhancer in the sea urchin. "This looks like species-specific jury-rigging," he points out. "The evolution of body plans happens by changes in the network architecture". [all typos are mine]
While changes in hox genes may be influential, there is also the enhancement and changes in expression of those hox genes. It is a very dynamic and ever changing phenomena. I just happened to have that issue of Science handy, but it is well worth the read if you happen to be in a library any time soon. It is heavy reading, so you might want to have a genetics text handy as well.