A brief overview of Hox genes

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In previous articles about fly development, I’d gone from the maternal gradient to genes that are expressed in alternating stripes (pair-rule genes), and mentioned some genes (the segment polarity genes) that are expressed in every segment. The end result is the development of a segmented animal: one made up of a repeated series of morphological modules, all the same.

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Building an animal with repeated elements like that is a wonderfully versatile strategy for making an organism larger without making it too much more complicated, but it’s not the whole story. Just repeating the same bits over and over again is a way to make a generic wormlike thing—a tapeworm, for instance—but even tapeworms may need to specialize certain individual segments for specific functions. At its simplest, it may be necessary to modify one end for feeding, and the opposite end for mating. So now, in addition to staking out the tissues of the embryo as belonging to discrete segments, we also need a mechanism that says “build mouthparts here (and not everywhere)”, and “put genitalia here (not over there)”.

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Naked anaspids

This strange fish is Euphanerops longaevus, which is one of two species of 370 million year old jawless fishes (the other is Endeiolepis aneri, and the paper suggests that they may actually represent differently preserved members of the same species). These are soft-bodied animals that are usually poorly preserved, and are of interest because they seem to have some properties in common with both the lampreys and the gnathostomes, or jawed fishes. Their exact position in the vertebrate family tree is problematic, and the experts go back and forth on it; sometimes they are grouped with the lampreys, sometimes as cousins more closely related to the gnathostomes.

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Euphanerops longaevus, preserved as an imprint. Scale bar, 10 mm.

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ORFans!

Paul Nelson has been twittering about ORFans for some time now—he seems to precede his talks by threatening to make us evolutionists tremble in our boots by bringing them up, but he never seems to follow through. Ian Musgrave got tired of waiting for him to give us a coherent creationist argument about them, and has gone ahead and cut him off at the knees by explaining the place of ORFans in evolution.

In case you’re baffled by the jargon, “ORF” is an Open Reading Frame, or a stretch of DNA bracketed by a start and stop codon; it’s a kind of bare minimum criterion for recognizing an actual gene within a DNA sequence. An ORFan is an orphan ORF sequence, or one that doesn’t have a known function or affinity to other known genes. It is not surprising that genes exist that do not have an easily recognized homology with other genes—novel genes have to arise sometime, and we do not have a complete understanding of all sequences of all organisms.

The short answer is that Nelson is deluded, and ORFans do not conflict with evolution at all…but read Ian’s post for all the details.

Evo-Devo in NYR Books!

This really is an excellent review of three books in the field of evo-devo—

From DNA to Diversity: Molecular Genetics and the Evolution of Animal Design (amzn/b&n/abe/pwll),

Endless Forms Most Beautiful: The New Science of Evo Devo and the Making of the Animal Kingdom (amzn/b&n/abe/pwll), and

The Plausibility of Life:Resolving Darwin’s Dilemma (amzn/b&n/abe/pwll)—all highly recommended by me and the NY Times. The nice thing about this review, too, is that it gives a short summary of the field and its growing importance.

Last chance until the Fall

We’re having our last Café Scientifique Morris of the 2005-2006 school year tonight, at 6:00, at the Common Cup Coffeehouse here in beautiful downtown Morris, Minnesota. Our speaker is Mark Logan of the Mathematics discipline, who’s going to be talking about “Origami in Math and Science”—that wonderful interdisciplinary stuff we liberal arts colleges do so well, tying together math and art.

It’s a good thing we’re doing it tonight so that we don’t suck away Sean Carroll’s audience for the Café Scientifique Chicago tomorrow.

PT-141

Sciencebase has a short article on a potential new aphrodisiac. It’s called PT-141, or bremelanotide, or Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH (“PT-141” is the useful search term if you want to hit up PubMed), and it’s a melanocortin agonist that works directly on the brain. It can be delivered as a nasal spray. It works on men, promoting erections, and it also seems to be effective on women, increasing sexual appetite.

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That question of race

John Wilkins has an excellent linky post on the subject of race. My position on the issue is Richard Lewontin’s (seen here in a RealAudio lecture by Richard Lewontin), and more succinctly stated by Wilkins:

So, do I think there are races in biology as well as culture? No. Nothing I have seen indicates that humans nicely group into distinct populations of less than the 54 found by Feldman’s group (probably a lot more – for instance, Papua New Guinea is not represented in their sample set). And this leads us to the paper by the Human Race and Ethnicity Working Group (rare to see a paper that doesn’t list all the authors). They rightly observe that while there are continental differences in genetics, there is no hard division, and genetic variation doesn’t match up with cultural differences per se. There is a genetic substructure to the human population, but it isn’t racial.

Reimagined humanity

At least someone found my idea of reinventing humanity inspiring: Nemo Ramjet rendered this version of of my hexapodal sapient.

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It’s different than I would have pictured it—the way I juggled about the functionality of the head, I think the face would not have been at all recognizable as human—but the cool thing about imaginations is that ours are all different.

By the way, Nemo says he’s drawn him in the midst of a religious argument, railing against the possibility that humanity could exist in anything other than this divine form, modeled on his God.

Evolving spots, again and again

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a–c, The wing spots on male flies of the Drosophila genus. Drosophila tristis (a) and D. elegans (b) have wing spots that have arisen during convergent evolution. Drosophila gunungcola (c) instead evolved from a spotted ancestor. d, Males wave their wings to display the spots during elaborate courtship dances.

It’s all about style. When you’re out and about looking for mates, what tends to draw the eye first are general signals—health and vigor, symmetry, absence of blemishes or injuries, that sort of thing—but then we also look for that special something, that je ne sais quoi, that dash of character and fashionable uniqueness. In humans, we see the pursuit of that elusive element in shifting fashions: hairstyles, clothing, and makeup change season by season in our efforts to stand out and catch the eye in subtle ways that do not distract from the more important signals of beauty and health.

Flies do the same thing, exhibiting genetic traits that draw the attention of the opposite sex, and while nowhere near as flighty as the foibles of human fashion, they do exhibit considerable variability. Changes in body pigmentation, courtship rituals, and pheromones are all affected by sexual selection, but one odd feature in particular is the presence of spots on the wing. Flies flash and vibrate their wings at prospective mates, so the presence or absence of wing spots can be a distinctive species-specific element in their evolution. One curious thing is that wing spots seem to be easy to lose and gain in a fly lineage, and species independently generate very similar pigment spots. What is it about these patterns that makes them simultaneously labile and frequently re-expressed?

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