Evolution of the cichlid mandible

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When we look at the face of another person, we can recognize specific features that have familial resemblances. In my family, for instance, I can recognize a “Myers nose” that my grandmother and my father and some of my siblings and kids have, and it’s different than my wife’s or my mother’s nose. These are subtle differences in shape—a bit of a curve, a knob, a seam—and their inheritance suggests that these differences are specified somehow in the DNA. If you think about it, though…how can whether the profile of a nose is straight or curved be encoded in a linear stretch of nucleotides? The complicated answer is that it isn’t—morphology is a consequence of epigenetic interactions during development—but we know that the alleles present in the genome do contribute in some significant way to three-dimensional form. How?

We don’t know all the details. This is one of those huge research problems that has gaping holes, full of promise and interest, where we don’t understand exactly how all the pieces fit together. However, here’s an important point that is relevant to other, larger issues in evolution: even where we lack full information about mechanisms, we can roughly perceive the shape of the answer, and that helps us rule out many alternative explanations and guides us in the general direction of a more complete understanding.

People’s noses are a difficult subject for research; we don’t get to define human crosses, people tend to be a little snippy about telling them who to breed with and taking their genes apart, and humans are awfully slow to breed. Fish are better experimental animals, much more pliable and faster and more prolific in their breeding. Some fish, such as the African cichlids, also have highly diverse populations and species with easily recognized and often quite dramatic morphological differences—and we can explore how those differences are generated by genetic and molecular differences in development. In particular, we can start to figure out how fish jaws are shaped by developmental processes.

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Axis formation in spider embryos

Blogging on Peer-Reviewed Research

Some of you may have never seen an arthropod embryo (or any embryo, for that matter). You’re missing something: embryos are gorgeous and dynamic and just all around wonderful, so let’s correct that lack. Here are two photographs of an insect and a spider embryo. The one on the left is a grasshopper, Schistocerca nitens at about a third of the way through development; the one on the right is Achaearanea tepidariorum. Both are lying on their backs, or dorsal side, with their legs wiggling up towards you.

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There are differences in the photographic technique — one is an SEM, the other is a DAPI-stained fluorescence photograph — and the spider embryo has had yolk removed and been flattened (it’s usually curled backward to wrap around a ball of yolk), and you can probably see the expected difference in limb number, but the cool thing is that they look so much alike. The affinities in the body plans just leap out at you. (You may also notice that it doesn’t seem to resemble a certain other rendition of spider development).

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Male pregnancy?

Yesterday’s discussion of future biological advances that will piss off the religious right had me thinking about other innovations that I expect will happen within a few decades that might just cause wingnuts to freak out. First thing to come to mind is that it will be something to do with reproduction, of course, and it will scramble gender roles and expectations…so, how about modifying men to bear children? It sounds feasible to me. Zygotes are aggressive little parasites that will implant just about anywhere in the coelom — it’s why ectopic pregnancies are a serious problem — so all we need to do there is culture a bit of highly vascularized tissue in the male abdomen that will serve as a secure home for a few months. We’ll have to play some endocrine games, too, which may effect his love life but will also prepare him to lactate post-partum. There’s the minor anatomical problem that the vagina is a unique tissue, and no, the urethra is not homologous or analogous (fortunately; we wouldn’t want to have to push an 8 pound baby through the penis, even if female hyenas can manage it) — but that’s what c-sections are for. Given money, time, and a few weird volunteers, it could be done.

The next question is, has it been done? Are there any other vertebrates that have males doing the hard work of pregnancy? There were the gastric brooding frogs, which one would think could have made the leap easily — the eggs were just swallowed and developed in the stomach — but only the mothers seemed to have done the job. They’re all extinct, anyway. Male frogs of the genus Rhinoderma brood their young in their mouths, but this is after external fertilization and development, so they’re actually simply holding larvae in a safe place — and they’re also endangered. The precedents aren’t promising.

There is an extremely interesting and successful example, though: the syngnathid fishes, sea horses and pipefish. In all 232 species, the female lays her eggs in a specialized male structure called the brood pouch, where they are fertilized and develop. It’s a true male pregnancy!

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Endless Forms Most Beautiful

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I just finished Sean B. Carroll’s Endless Forms Most Beautiful: The New Science of Evo-Devo the other day, and I must confess: I was initially a bit disappointed. It has a few weaknesses. For one, I didn’t learn anything new from it; I had already read just about everything mentioned in the book in the original papers. It also takes a very conservative view of evolutionary theory, and doesn’t mention any of the more radical ideas that you find bubbling up on just about every page of Mary Jane West-Eberhard’s big book. One chapter, the tenth, really didn’t fit in well with the rest—the whole book is about pattern, and that chapter is suddenly talking about a few details in the evolution and development of the human brain.

So I read the whole thing with a bit of exasperation, waiting for him to get to the good stuff, and he never did. But then after thinking about it for a while, I realized what the real problem was: he didn’t write book for me, the inconsiderate bastard, he wrote it for all those people who maybe haven’t taken a single course or read any other books in the subject of developmental biology. I skimmed through it again without my prior biases, and realized that it’s actually a darned good survey of basic concepts, and that I’m going to find it very useful.

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Cephalopod development and evolution

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People are always arguing about whether primitive apes could have evolved into men, but that one seems obvious to me: of course they did! The resemblances are simply too close, so that questioning it always seems silly. One interesting and more difficult question is how oysters could be related to squid; one’s a flat, sessile blob with a hard shell, and the other is a jet-propelled active predator with eyes and tentacles. Any family resemblance is almost completely lost in their long and divergent evolutionary history (although I do notice some unity of flavor among the various molluscs, which makes me wonder if gustatory sampling hasn’t received its proper due as a biochemical assay in evaluating phylogeny.)

One way to puzzle out anatomical relationships and make phylogenetic inferences is to study the embryology of the animals. Early development is often fairly well conserved, and the various parts and organization are simpler; I would argue that what’s important in the evolution of complex organisms anyway is the process of multicellular assembly, and it’s the rules of construction that we have to determine to identify pathways of change. Now a recent paper by Shigeno et al. traces the development of Nautilus and works out how the body plan is established, and the evolutionary pattern becomes apparent.

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Segmentation genes evolved undesigned

Jason Rosenhouse has dug into the details of the evo-devo chapter of Behe’s The Edge of Evolution and found some clear examples of dishonest quote-mining (so what else is new, you may be thinking—it’s what creationists do). I’ve warned you all before that when you see an ellipsis in a creationist quote, you ought to just assume that there’s been something cut out that completely contradicts the point the creationist is making; Rosenhouse finds that Behe gets around that little red-flag problem by simply leaving out the ellipses.

I just want to expand a little bit on one point Behe mangles and that Jason quotes. It turns out I actually give a lecture in my developmental biology courses on this very issue, the mathematical modeling antecedents to insect segmentation, so it’s simply weird to see Behe twisting a subject around that is so well understood in the evo-devo community, and that was actually well explained in Sean Carroll’s Endless Forms Most Beautiful.

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