development
pharyngula | June 8, 2006
The fugu is a famous fish, at least as a Japanese sushi dish containing a potentially lethal neurotoxin that was featured on an episode of The Simpsons. Fugu is a member of the pufferfish group, which have another claim to fame: an extremely small genome, roughly a tenth the size of that of other vertebrates. The genome of several species of pufferfish is being sequenced, and the latest issue of Nature announces the completion of a draft sequence for the green spotted pufferfish, Tetraodon nigroviridis, a small freshwater species.
Tetraodon has about the same number of genes as we do, 20,…
pharyngula | June 8, 2006
Creationists are fond of the "it can't happen" argument: they like to point to things like the complexity of the eye or intricate cell lineages and invent bogus rules like "irreducible complexity" so they can claim evolution is impossible. In particular, it's easy for them to take any single organism in isolation and go oooh, aaah over its elaborate detail, and then segue into the argument from personal incredulity.
Two things, one natural and one artificial, help them do this. Organisms are incredibly complicated, there is no denying it. This should be no solace to the anti-evolutionists,…
pharyngula | June 3, 2006Since Evolgen recognizes the importance of evo-devo, I'll return the favor: bioinformatics is going to be critical to the evo-devo research program, which to date has emphasized the "devo" part with much work on model systems, but is going to put increasing demands on comparative molecular information from genomics and bioinformatics to fulfill the promise of the "evo" part. I'm sitting on a plane flying east, and to pass the time I've been reading a very nice review of the concept of modularity in evo-devo by Paula Mabee (also a fish developmental biologist, and also working in a small…
pharyngula | May 31, 2006Fellow scienceblogger Evolgen has seen the light—evo-devo is wonderful. He's attending a meeting and listening to some of the bigwigs in the field talk about their work, in particular some research on the evolution of gene regulation. While noting that this is clearly important stuff, he also mentions some of the bickering going on about the relative importance of changes in cis regulatory elements (CREs) vs. trans acting elements, transcription factors. I've got a longer write-up of the subject, but if you don't want to read all of that, the issue is about where the cool stuff in the…
pharyngula | May 30, 2006
Call me perverse, but my first thought on seeing this kid was that I desperately want to see an x-ray of the pectoral girdle. It looks to me from this one picture that the lower arm must lack a scapula or a clavicle, or at best have fragments with screwy and probably nonfunctional connections. I don't understand why the doctors are even arguing about which arm could be more functional, if the article is correct. Or why they're even considering it important to lop one off: if there aren't circulatory defects or it isn't impairing the function of the 'best' arm, why take a knife to him?
Poor…
pharyngula | May 26, 2006
One of the most evocative creatures of the Cambrian is Anomalocaris, an arthropod with a pair of prominent, articulated appendages at the front of its head. Those things are called great appendages, and they were thought to be unique to certain groups of arthropods that are now extinct. A while back, I reported on a study of pycnogonids, the sea spiders, that appeared to show that that might not be the case: on the basis of neural organization and innervation, that study showed that the way pycnogonid chelifores (a pair of large, fang-like structures at the front of the head) were innervated…
pharyngula | May 26, 2006
I'm going to introduce you to either a fascinating question or a throbbing headache in evolution, depending on how interested you are in peculiar details of arthropod anatomy (Mrs Tilton may have just perked up, but the rest of you may resume napping). The issue is tagmosis.
The evolutionary foundation for the organization of many animal body plans is segmental—we are made of rings of similar stuff, repeated over and over again along our body length. That's sufficient to make a creature like a tapeworm or a leech (well, almost—leeches have sophisticated specializations), but there are…
pharyngula | May 23, 2006
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…
pharyngula | May 22, 2006
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…
pharyngula | May 19, 2006
Four of my favorite things are development, evolution, and breasts, and now I have an article that ties them all together in one pretty package. It's a speculative story at this point, but the weight of the evidence marshaled in support of the premise is impressive: the mammalian breast first evolved as an immunoprotective gland that produced bacteriocidal secretions to protect the skin and secondarily eggs and infants, and that lactation is a highly derived kind of inflammation response. That mammary glands may have had their origin as inflamed glands suppurating mucus may not be the most…
pharyngula | May 17, 2006I'm going to link to a post on Uncommon Descent. I try to avoid that, because I think it is a vile harbor of malign idiocy, but Dembski has just put up something that I think is merely sincerely ignorant. That's worth correcting. It also highlights the deficiencies of Dembski's understanding of biology.
Dembski makes a strange argument for ID on the basis of a certain class of experiments in developmental biology.
For example, consider how SCNT (somatic cell nuclear transfer) works. You take a mammalian egg (which "just happens to be" a HUGE cell, very easy to experiment on). You take out…
pharyngula | May 11, 2006Crap. Coturnix tagged me with this beautiful bird meme, and I am the wrong person to ask. I don't get out much, preferring to sit in the lab or the library, so my favorite birds are all in pieces and dead. But OK, since he asked…
Bird digits
Bird teeth
Bird brains
Jurassic bird brains
Bird lungs
Oviraptor pelves (does that count?)
Cretaceous bird embryos
Four-winged birds
Waimanu
And Archaeopteryx, of course.
pharyngula | May 8, 2006
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…
pharyngula | May 8, 2006
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…
pharyngula | May 8, 2006
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…
pharyngula | April 30, 2006
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.
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…
pharyngula | April 25, 2006This 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.
pharyngula | April 21, 2006a–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…
pharyngula | April 19, 2006
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…
pharyngula | April 18, 2006The 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…