Octopus kaurna
Figure from Cephalopods: A World Guide (amzn/b&n/abe/pwll), by Mark Norman.
Pharyngula
Evolution, development, and random biological ejaculations from a godless liberal
Read and discuss:
Or talk about anything you want. The pope’s presence annihilates ice cream and tampons. Bill Frist really needs to take a shower before working in the Senate. What kind of penalties would be appropriate for Kenny Boy? I’m sure you can all think of something to talk about—I’m buckling down for a few hours to finish reviewing a paper.
I just learned (via John Lynch) about a paper on cetacean limbs that combines developmental biology and paleontology, and makes a lovely argument about the mechanisms behind the evolution of whale morphology. It is an analysis of the molecular determinants of limb formation in modern dolphins, coupled to a comparison of fossil whale limbs, and a reasonable inference about the pattern of change that was responsible for their evolution.
One important point I’d like to make is that even though what we see in the morphology is a pattern of loss—whale hindlimbs show a historical progression over tens of millions of years of steady loss, followed by a near-complete disappearance—the molecular story is very different. The main players in limb formation, the genes Sonic hedgehog (Shh), the Fgfs, and the transcription factor Hand2, are all still present and fully functional in these animals. What has happened, though, is that there have been novel changes to their regulation. Even loss of structures is a consequence of changes and additions to regulatory pathways.
I’m going to briefly summarize an interesting new article on cnidarian Hox genes…unfortunately, it requires a bit of background to put it in context, so bear with me for a moment.
First you need to understand what Hox genes are. They are transcription factors that use a particular DNA binding motif (called a homeobox), and they are found in clusters and expressed colinearly. What that means is that you find the Hox genes that are essential for specifying positional information along the length of the body in a group on a chromosome, and they are organized in order on the chromosome in the same order that they are turned on from front to back along the body axis. Hox genes are not the only genes that are important in this process, of course; animals also use another class of regulatory genes, the Wnt genes, to regulate development, for instance.
A gene can only be called a Hox gene sensu stricto if it has a homeobox sequence, is homologous to other known Hox genes, and is organized in a colinear cluster. If such a gene is not in a cluster, it is demoted and called simply a Hox-like gene.
Hox genes originated early in animal evolution. Genes containing a homeobox are older still, and are found in plants and animals, but the particular genes of the Hox system are unique to multicellular animals, and that key organization arrangement of the set of Hox genes in a cluster is more unique still. The question is exactly when the clusters arose, shortly after or sometime before the diversification of animals.
If you take a look at animal phylogeny, an important group are the diploblastic phyla, the cnidarians and ctenophores. They branched off early from the metazoan lineage, and they possess some sophisticated patterns of differentiation along the body axis. We know they have homeobox containing genes that are related to the ones used in patterning the bodies of us vertebrates, but are they organized in the same way? Did the cnidaria have Hox clusters, suggesting that the clustered Hox genes were a very early event in evolution, or do they lack them and therefore evolved an independent set of mechanisms for specifying positional information along the body axis?
Rabbits are the most freaky of all mammals (right, Chris?) They’re thriving, though—they’re all over my lawn, and if this summer is anything like last year, I’ll have to go out about once a week and scrape one off the road running by my house.
I did not know that interesting fact about their scrotum, though.
Whoa, it’s been a while since I’ve said anything about my infatuation with cephalopods (since, like, the last post…). Let’s correct that with a nifty paper I found on octopus suckers.
Here’s a typical view of a tangle of octopus arms, all covered with circular suckers. The octopus can cling to things, grasp prey and other objects with those nifty little discs, and just generally populate people’s nightmares with the idea of all those grappling, clutching, leech-like appendages.
Hmmm…this video of an octopus attacking a man looks as phony as the battle between Bela Lugosi and the rubber octopus in Bride of the Monster to me. It makes for an entertaining break in the grading slog I’m in right now, but it would have been much improved if the octopus had won.
(via Phil)
