Two legged goats and developmental variation



Variation is common, and often lingers in places where it is unexpected. The drawing to the left is from West-Eberhard’s Developmental Plasticity and Evolution(amzn/b&n/abe/pwll), and illustrates six common variations in the branching pattern of the aortic arch in humans. These are differences that have no known significance to our lives, and aren’t even visible except in the hopefully rare situations in which a surgeon opens our chests.

This is the kind of phenomenon in which I’ve become increasingly interested. I work with a model system, the zebrafish, and supposedly one of the things we model systems people pursue is the ideal of a consistent organism, in which the variables are reduced to a minimum. Variation is noise that interferes with our perception of common underlying mechanisms. I’ve been thinking more and more that variation is actually a significant phenomenon that tells us something about where the real constraints in the system are. It is also, of course, the raw material for evolution.

Unfortunately, variation is also relatively difficult to study.

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I’m redundant-who needs a blogger?


There’s a lovely article in this week’s Nature documenting a transitional stage in tetrapod evolution (you know, those forms the creationists like to say don’t exist), and a) Nature provides a publicly accessible review of the finding, and b) the primary author is already a weblogger! Perhaps there will come a day when I’m obsolete and willl just have to turn my hand to blogging about what I had for lunch.

For an extra super-duper dose of delicious comeuppance, though, take a look at this thread on the Panda’s Thumb. I wrote about Panderichthys, and a creationist (“Ghost of Paley”) comes along to mangle the phylogeny and make wild negative assertions about the validity of interpretations of fossils based on work from the Ahlberg lab…when Martin Brazeau of the Ahlberg lab and author of this new paper shows up to straighten him out.

And for my next trick, let me introduce you to Marshall McLuhan

Brazeau MD, Ahlberg PE (2006) Tetrapod-like middle ear architecture in a Devonian fish. Nature 439:318-321.

Where are the spineless?

Hey! I’m supposed to host the Circus of the Spineless next week (I think on 29 January), and I’ve only received one submission so far! Someone must have written something somewhere about invertebrates, right? There is a set of rules for submissions, but it’s going to be simple: I’ll accept anything about any organisms outside the class Vertebrata.

I’ll spell it out. You can write about the phyla Acanthocephala, Acoelomorpha, Annelida, Arthropoda, Brachiopoda, Bryozoa, Chaetognatha, Cnidaria, Ctenophora, Cycliophora, Echinodermata, Echiura, Entoprocta, Gastrotricha, Gnathostomulida, Hemichordata, Kinorhyncha, Loricifera, Micrognathozoa, Mollusca, Myxozoa, Nematoda, Nematomorpha, Nemertea, Onychophora, Orthonectida, Phoronida, Placozoa, Platyhelminthes, Pogonophora, Porifera, Priapulida, Rhombozoa, Rotifera, Sipuncula, Symplasma, or Tardigrada, and that’s fine. You can even write about the Urochordata, the Cephalochordata, and the Myxini within the phylum Chordata. The overwhelming majority of animal species are fair game, so there is absolutely no excuse if anyone tries to send me a picture of their cat. Understand? Fish, frogs, lizards, birds, mammals, dinosaurs, and your baby pictures are right out.

I’m also going to accept multiple submissions, if you’ve been manic about documenting the breadth of biodiversity. We shall do our best to overcome the bias of the blogosphere for kitties and other furred and feathered and scaled beasties.

Although the tagline for the circus says it is a monthly celebration of “most anything else that wiggles”, I’m also going to break the shackles of metazoan chauvinism, so if you want to send in something about protists, lichens, fungi, plants, whatever, anything but things with a spinal column, I’ll accept them and put them in an honored category of their own.

Orsten fossils

Bredocaris admirabilis

Ooooh, there’s a gorgeous gallery of Orsten fossils online. These are some very pretty SEMs of tiny Cambrian animals, preserved in a kind of rock called Orsten, or stinkstone (apparently, the high sulfur content of the rock makes it smell awful). What are Orsten fossils?

Orsten fossils in the strict sense are spectacular minute secondarily phosphatised (apatitic) fossils, among them many Crustacea of different evolutionary levels, but also other arthropods and nemathelminths. The largest fragments we have do not exceed two mm. Orsten-type fossils, on the other hand, have the great advantage in being three-dimensionally preserved with all surface structures in place and thus easier to interpret than any other fossil material. Orsten fossils are preserved virtually as if they were just critical-point dried extant organisms. Details observable range down to less than 1 µm, and include pores, sensilla and minute secondary bristles on filter setae and denticles. Orsten fossils also give an insight of meiofaunal benthic life at small scale, including preservation larval stages, and hence a life zone inhabited by the earliest metazoan elements of the food chain.

It’s a good browse over there. I think it’s useful to remember that the majority of the fauna of the world both extant and half a billion years ago is and was tiny and unfamiliar.

Hemichordate evo-devo

Every biology student gets introduced to the chordates with a list of their distinctive characteristics: they have a notochord, a dorsal hollow nerve cord, gill slits, and a post-anal tail. The embryonic stage in which we express all of these features is called the pharyngula stage—it’s often also the only stage at which we have them. We terrestrial vertebrates seal off those pharyngeal openings as we develop, while sea squirts throw away their brains as an adult.


The chordate phylum has all four of those traits, but there is another extremely interesting phylum that has some of them, the hemichordates. The hemichordates are marine worms that have gill slits and a stub of a tail. They also have a bundle of nerves in the right place to be a dorsal nerve cord, but the latest analyses suggest that it’s not discrete enough to count—they have more of a diffuse nerve net than an actual central nervous system. They don’t really have a notochord, but they do have a stiff array of cells in their proboscis that vaguely resembles one. They really are “half a chordate” in that they only partially express characters that are defining elements of the chordate body plan. Of course, they also have a unique body plan of their own, and are quite lovely animals in their own right. They are a sister phylum to the chordates, and the similarities and differences between us tell us something about our last common ancestor, the ur-deuterostome.

Analyzing morphology is one approach, but this is the age of molecular biology, so digging deeper and comparing genes gives us a sharper picture of relationships. This is also the early days of evo-devo, and an even more revealing way to examine related phyla is to look at patterns of gene regulation—how those genes are turned on and off in space and time during the development of the organism—and see how those relate. Gerhart, Lowe, and Kirschner have done just that in hemichordates, and have results that strengthen the affinities between chordates and hemichordates. (By the way, Gerhart and Kirschner also have a new book out, The Plausibility of Life (amzn/b&n/abe/pwll), which I’ll review as soon as I get the time to finish it.)

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Firefly squid

This is a beautiful little animal with a brief and brilliant life.


Watasenia scintillans is a small (mantle length,~6 cm; wet weight,~9 g), luminescent deep-sea squid, indigenous to northern Japan. Females carrying fertilized eggs come inshore each spring by the hundreds of millions, even a billion, to lay eggs in Toyama Bay (max. depth, 1200 m) and die, thereupon completing a 1-year life cycle.

Watasenia possesses numerous (~800), minute dermal light organs (photophores) on its ventral side. Other organs are scattered over the head, funnel, mantle, and arms, but none is found on its dorsal side. There are five prominent organs beneath the lower margin of each eye. They all emit a bluish light. A cluster of three tiny black-colored organs (<l mm diam) is located at the tip of each of the fourth pair of arms. They emit brilliant flashes of light which are clearly visible to the unaided eye even in a lighted room. Some of the flashes have a cadence resembling that of a terrestrial firefly flashing at night, and thus the squid is known in Japan as the “firefly squid” or “hotaru-ika.”

A billion die every year as a natural part of their lifecycle; all those glittering little creatures dying profligately—Nature is both exuberant and pitiless, it seems.

Tsuji FI (2005) Role of molecular oxygen in the bioluminescence of the firefly squid, Watasenia scintillans. Biochem Biophys Res Commun. 338(1):250-253.