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International Congress of Vertebrate Morphology: Beaks and beyond

Posted on behalf of Anna Petherick

Arkhat Abzhanov is a naturalist at Harvard University who is, in some ways, making a career of going over old scientific ground. His recent published work puts a modern spin on the evolution of the finches that Darwin studied in the Galapagos Islands.

For such closely related species, Darwin’s finches have remarkably different beaks. Darwin explained their shapes through natural selection for beaks that enabled finches to consume the various foodstuffs available on the different islands.

During the last plenary session at the 9th International Congress of Vertebrate Morphology in Punta del Este, Uruguay, Abzhanov explained his work with Michael P. Brenner, a Harvard mathematician who works across the street from him in Boston. In February they published a paper showed that the beaks of Darwin’s finches are not as disparate as first meets the eye.

Instead, the species can be grouped by the curve of the upper beak. Within each of these groups, the equation for that curve need only be modified simply, by changing the curve’s length for example, to generate all of the beak shapes of the group’s member species. But between groups a more complicated shape transformation is needed, such as a ‘sheer’ or a ‘rotation’.

Knowing this, Abzhanov then pinned down signaling molecules that regulate where beak cartilage will form in early chick embryos, and showed that the expression of these molecules can be linked with some of the simple within-group shape changes.

But cartilage is only really important in defining the beak shapes of early chick embryos. Thereafter, a bone called the pre-maxillary bone creates the shape of the upper beak in all birds. At the conference, Abzhanov presented unpublished research on Geospiza (a genus of Galapagos finch) in which he identified the signaling molecules that influence how much bone forms, and where it forms on the beak’s cartilage scaffold.

Up- or down-regulating molecules called Dkk3, β-catenin and TGFβRII changed the length and width of the premaxillary bone forming in embryo beaks; another called Bmp4 affected its depth and width later on in development.

Can these same signaling molecules can also explain beak shape in another clade of birds? To answer this question, Abzhanov has turned to Caribbean bullfinches, which are close relatives of Darwin’s finches. He found that the beaks of Caribbean bullfinches can also be grouped according to the equation for the curve of the upper beak — and that a beak shape that he calls “the seed-cracking shape”, which is found in some Darwin’s finches, is also found in three species of Caribbean bullfinch.

Oddly, one of those three species did use the same biomolecular signaling to generate its ‘seed-cracking shape’ beak. It was the Caribbean bullfinch species most closely related Darwin’s finches out of the three. But the two other species employed a different biochemical signal of creating essentially the same morphological result.

Abzhanov has much many more details to grapple with in beak development, but in the long term he would like to expand his research to explain animal morphology in creatures other than birds. The field of work looks fertile because it ties modern molecular biology with another source of old scientific ground: D’Arcy Thompson’s notes on animal forms. This shows that basic mathematical transformations of the body plans of many species produce the forms of other species. Maybe but a few signaling molecules switch a puffer fish body plan into that of a sunfish?

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