Devonian Blues

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Lots of people have sent me links to this—thanks, all!—and it’s the perfect thing to lift me out of the finals week blahs, and it’s also just in time for Mother’s Day on Sunday: The Devonian Blues.

Every single girl and every little boy
Was born from the clan of the wayward Dipnoi
Don’t let the preacher man spoil all the fun
Took a lot more than 6 days to get the job done
Amphibians, reptiles, birds, mammals and man
All belong to the fish tribe, doncha’ understand?

Your momma was a lobefinned fish
My momma was a lobefinned fish

Sing along, everyone!

A complex regulatory network in a diploblast

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The Wnt genes produce signalling proteins that play important roles in early development, regulating cell proliferation, differentiation and migration. It’s hugely important, used in everything from early axis specification in the embryo to fine-tuning axon pathfinding in the nervous system. The way they work is that the Wnt proteins are secreted by cells, and they then bind to receptors on other cells (one receptor is named Frizzled, and others are LRP-5 and 6), which then, by a chain of cytoplasmic signalling events, removes β-catenin from a degradation pathway and promotes its import into the nucleus, where it can modify patterns of gene expression. This cascade can also interact with the cytoskeleton and trigger changes in cell migration and cell adhesion. The diagram below illustrates the molecular aspects of its function.

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Hox genesis

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One of the hallmark characters of animals is the presence of a specific cluster of genes that are responsible for staking out the spatial domains of the body plan along the longitudinal axis. These are the Hox genes; they are recognizable by virtue of the presence of a 60 amino acid long DNA binding region called the homeodomain, by similarities in sequence, by their role as regulatory genes expressed early in development, by the restriction of their expression to bands of tissue, by their clustering in the genome to a single location, and by the remarkable collinearity of their organization on the chromosome to their pattern of expression: the order of the gene’s position in the cluster is related to their region of expression along the length of the animal. That order has been retained in most animals (there are interesting exceptions), and has been conserved for about a billion years.

Think about that. While gene sequences have steadily changed, while chromosomes have been fractured and fused repeatedly, while differences accumulated to create forms as different as people and fruit flies and squid and sea urchins, while continents have ping-ponged about the globe and meteors have smashed into the earth and glaciers have advanced and retreated, these properties of this set of genes have remained constant. They are fundamental and crucial to basic elements of our body plan, so basic that we take them completely for granted. They determine that we can have different regions of our bodies with different organs and organization. Where did they come from and what forces constrain them to maintain their specific organization on the chromosome? Are there other genes that are comparably central to our organization?

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Bilateral symmetry in a sea anemone

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There are quite a few genes that are known to be highly conserved in both sequence and function in animals. Among these are the various Hox genes, which are expressed in an ordered pattern along the length of the organism and which define positional information along the anterior-posterior axis; and another is decapentaplegic (dpp) which is one of several conserved genes that define the dorsal-ventral axis. Together, these sets of genes establish the front-back and top-bottom axes of the animal, which in turn establishes bilaterality—this specifically laid out three-dimensional organization is a hallmark of the lineage Bilateria, to which we and 99% of all the other modern animal species belong.

There are some animals that don’t belong to the Bilateria, though: members of the phylum Cnidaria, the jellyfish, hydra, sea anemones, and corals, which are typically radially symmetric. A few cnidarian species exhibit bilateral symmetry, though, and Finnerty et al. (2004) ask a simple question: have those few species secondarily reinvented a mechanism for generating bilateral symmetry (so that this would be an example of convergent evolution), or do they use homologous mechanisms, that is, the combination of Hox genes for A-P patterning and dpp for D-V patterning? The answer is that this is almost certainly an example of homology—the same genes are being used.

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Stromatoveris

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The Cambrian vendobiont S. psygmoglena, gen.sp.nov., composite photo of part and counterpart to show both upper and lower surfaces.

From the pre-Cambrian and early Cambrian, we have a collection of enigmatic fossils: the small shellies appear to be bits and pieces of partially shelled animals; there are trace fossils, the tracks of small, soft-bodied wormlike animals; and there are the very peculiar Edicaran vendobionts, which look like fronds and fans and pleated or quilted sheets. In the Cambrian, of course, we find somewhat more familiar creatures—sure, they’re weird and different, but we can at least tentatively see them as precursors to the modern members of their respective phyla. It’s not surprising, though, that the farther back in time we go, the stranger animals appear, and the more difficult it is to place them in our phylogenies.

So here’s something cool and helpful—it looks like a vendobiont, but it’s been found in the Lower Cambrian fossil beds of Chengjiang. It’s also very well preserved, and has features that suggest affinities to the ctenophores.

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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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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.

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.

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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