My previous contributions to the basic concepts in science collection were on gastrulation and neurulation, so let’s add the next stage, and the one I named the blog after: the pharyngula.
Way back in the early 19th century, Geoffroy St. Hilaire argued for a radical idea, that vertebrates and most invertebrates were inverted copies of each other. Vertebrates have a dorsal nerve cord and ventral heart, while an insect has a ventral nerve cord and dorsal heart. Could it be that there was a common plan, and that one difference is simply that one is upside down relative to the other? It was an interesting idea, but it didn’t hold up at the time; critics could just enumerate the multitude of differences observable between arthropods and vertebrates and drown out an apparent similarity in a flood of documented differences. Picking out a few superficial similarities and proposing that something just looks like it ought to be so is not a persuasive argument in science.
Something has changed in the almost 200 years since Geoffroy made his suggestion, though: there has been a new flood of molecular data that shows that Geoffroy was right. We’re finding that all animals seem to use the same early molecular signals to define the orientation of the body axis, and that the dorsal-ventral axis is defined by a molecule in the Bmp (Bone Morphogenetic Protein) family. In vertebrates, Bmp is high in concentration along the ventral side of the embryo, opposite the developing nervous system. In arthropods, Bmp (the homolog in insects is called decapentaplegic, or dpp) is high on the dorsal side, which is still opposite the nervous system. At this point, the question of whether the dorsal-ventral axis of the vertebrate and invertebrate body plans have a common origin and whether one is inverted relative to the other has been settled, and the answer is yes.
I’ve been looking forward to seeing these little jewels in print since I saw Kuratani talk about them at the SICB meetings in January. Hagfish are wonderfully slimy jawless chordates that have been difficult to raise in the lab—although if you poke a whale corpse rotting in the cold deeps you’ll find them swarming everywhere. The Kuratani lab has managed to keep animals of the species Eptatretus burgeri alive and healthy in a lab aquarium maintained at cold temperatures (16°C), and has even had success in breeding them. That object to the right is a single hagfish egg, brown and leathery-shelled and surprisingly big—it’s an inch and a half long!
They collected 92 eggs, and then another limitation emerged: it took 5-7 months for embryos to develop in a small number of the eggs. Hagfish aren’t going to be your typical fast-developing model system, I’m afraid, but they are extraordinarily cool animals, and it’s good to see work beginning on them.
This evening, I am watching an episode of that marvelous and profane Western, Deadwood, as I type this; it is a most excellently compensatory distraction, allowing me to sublimate my urge to express myself in uncompromisingly vulgar terms on Pharyngula. This is an essential coping mechanism.
I have been reading Jonathan Wells again.
If you’re familiar with Wells and with Deadwood, you know what I mean. You’ll just have to imagine that I am Al Swearingen, the brutal bar-owner who uses obscenities as if they were lyric poetry, while Wells is E.B. Farnum, the unctuous rodent who earns the contempt of every man who meets him. That imagination will have to hold you, because I’m going to restrain myself a bit; I’m afraid Wells would earn every earthy sobriquet I could imagine, but I’ll confine myself to the facts. They’re enough. The man completely misrepresents the results of a paper and a whole discipline, and does it baldly on the web, as if he doesn’t care that his dishonesty and ignorance leave a greasy, reeking trail behind him.
Let’s start with Wells’ own words.
I must disagree with Larry Moran, who accuses the field of evo-devo of animal chauvinism — not that it isn’t more or less true that we do tend to focus on metazoans, but I disagree with an implication that this is a bad thing or that it is a barrier to respectability. Larry says we need to cover the other four kingdoms of life in greater breadth, which I agree is a fine idea. I would like to have a complete description of the genome of every species on earth, a thorough catalog of every epistatic interaction between those genes during development, a hundred labs working on each species, and a massive collection of papers for each one documenting every step and every protein and every variation in their development. I would like it tomorrow.
I think we all agree that that would be impractical. The question is how we will focus our research to maximize our use of limited resources, and get us useful answers that will lead us in productive directions. Larry is advocating maximizing our phyletic breadth by following organisms representative of the greatest amount of diversity. He is proposing this in opposition to the proposal from Jenner and Wills, who suggest a different strategy — and I find myself agreeing more with Jenner and Wills than with Moran.
When we had last seen our basic embryo, it had gone through gastrulation — a process in which cells of a two-layered sheet had moved inward, setting up the three germ layers (endoderm, mesoderm, and ectoderm) of the early embryo. In particular, cells at the organizer, a tissue that induces or organizes migrating cells, had rolled inwards to set up specific axial mesoderm structures: the prechordal plate, which will underlie cranial structures, and the notochord, which resides under the future hindbrain and spinal cord. At this point, the embryo has an outer layer of ectoderm, and lying under part of it, a band of prechordal mesoderm and notochord. In addition, the ectoderm is loaded with a molecule called BMP-4, a member of the TGF-β family of signaling molecules, and under its influence will go on to make skin, not nervous system. So what next?
My latest column for Seed, Variant Genes-in-Waiting, is now online. If you subscribed, you would have already read it earlier this week.
By the way, my mom subscribes, too, and she gives it a thumbs up. I’ll have to find out what she thinks about my next column, which is all about beetle testes (and that’s all you get to know about it—you’ll just have to wait).
The latest Nature reveals a new primitive mammal fossil collected in the Mesozoic strata of the Yan mountains of China. It’s small and unprepossessing, but it has at least two noteworthy novelties, and first among them is that it represents another step in the transition from the reptilian to the mammalian jaw and ear.
Here is a spectacularly pretty and weird animal: stalk-eyed flies of the family Diopsidae. There are about 160 species in this group that exhibit this extreme morphology, with the eyes and the antennae displaced laterally on stalks. They often (but not always) are sexually dimorphic, with males having more exaggerated stalks—the longer stalks also make them clumsy in flight, so this is a pattern with considerable cost, and is thought to be the product of sexual selection. The Sphyrocephala to the right is not even an extreme example. Read on to see some genuinely bizarre flies and a little bit about the development of this structure.
Stevie C sent along this article on
An unusual presentation of supernumerary breast tissue (just what were you googling for, Stevie?), in which a woman reports an annoying growth on her foot, and when examined, is discovered to have a breast growing there, complete with nipple and fatty tissue (but in this case, no glandular tissue).
It’s in the Dermatology Online Journal, not the Onion.
I hadn’t heard of this before myself, but it’s fascinating. These supernumerary breasts can pop up all over the place, including the face, back, and thigh (and foot, obviously). They can be functionally complete, and can even lactate. The authors report some weak and sometimes contradicted associations with other oddities, but no causal mechanism is known. These cases of autonomous self-organization and recruitment of organs are extremely interesting—it suggests that a breast would be a fairly easy tissue to grow in a dish. I’d love to know what the molecular signal for initiating differentiation—I suspect it’s something simple and common.
