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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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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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.

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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Evolving spots

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Here’s what seems to be a relatively simple problem in evolution. Within the Drosophila genus (and in diverse insects in general), species have evolved patterned spots on their wings, which seem to be important in species-specific courtship. Gompel et al. have been exploring in depth one particular problem, illustrated below: how did a spot-free ancestral fly species acquire that distinctive dark patch near the front tip of the wing in Drosophila biarmipes? Their answer involves dissecting the molecular regulators of pattern in the fly wing, doing comparative sequence analyses and identifying the specific stretches of DNA involved in turning on the pigment pattern, and testing their models experimentally by expressing novel gene constructs in different species of flies.

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How to make a bat

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The relative length of bat forelimb digits has not changed in 50
million years. (a) Icaronycteris index, which is a 50-million-year-old bat fossil. (b) Extant adult
bat skeleton. The metacarpals (red arrows) of the first fossil bats are already
elongated and closely resemble modern bats. This observation is confirmed by
morphometric analysis of bat forelimb skeletal elements.

or•gan•ic | ôr’ganik | adjective. denoting a relation between elements of something such that they fit together harmoniously as necessary parts of a whole; characterized by continuous or natural development.

One of the wonderful things about how development works is that organisms function as wholes, and changes in one property trivially induce concordant changes in other properties. Tug on one element, changing it’s orientation or size, and during embryogenesis any adjacent elements make compensatory adjustments, so that the resultant form flows, fits, and looks organic. This isn’t that surprising a feature of development, though, unless you have the mistaken idea that the genome encodes a blueprint of morphology. It doesn’t; what it contains is a description of interacting agents that work together in a process to produce a complex result. Changes in genes and regulatory elements can essentially produce changes in rules of development, rather than crudely specifying blocks of morphology.

What does this mean for evolution? It means that subtle changes to the rules of development can be caused by small changes to genes (and especially, to regulatory regions of genes), and that the resulting morphological changes may be dramatic, but are still integrated organically into the form of the organism as a whole. Our understanding of how development works is making it clear that large scale macroevolutionary change may be much easier than we had thought.

Here’s an example where this insight is clarifying the evolution of an organism: the fossil record of bats shows an abrupt appearance of fairly sophisticated creatures with elongated digits, clearly capable of gliding or powered flight, with no known intermediates. We expect there were less fully flight-ready predecessors, but fossil preservation is not kind to small, delicate boned animals. It’s also possible that the transitional period was fairly brief; it looks like turning a paw into a long-fingered membranous wing may be a fairly simple change on a molecular level.

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Bat development

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It always gives a fellow a warm feeling to see an old comrade-in-arms publish a good paper. Chris Cretekos was a graduate student working on the molecular genetics of zebrafish at the University of Utah when I was a post-doc there, and he’s a good guy I remember well…so I was glad to see his paper in Developmental Dynamics. But then I notice it wasn’t on zebrafish—Apostate! Heretic!

Except…it’s on bats. How cool is that? And it’s on the embryonic development of bats. Even cooler! I must graciously forgive his defection from the zebrafish universe since he is working on an organism that is weird and fascinating and important.

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