NYTimes on Evo-Devo

i-a9329f4fd5321ffc1704eb0634b795d5-finch.jpgThe NYTimes has a excellent summary of the progress in the study of evolution of development (evo-devo). Scientists have been surprised to discover over the years that a relatively small number of closely-related genes control the body plan of animal species, even when those species' body plans differ dramatically. Apparently these same genes are being applied in diverse ways to organize very different body plans.

An example of one of the genes that organizes development is BMP4:

What Dr. Tabin and colleagues found, when looking at the range of beak shapes and sizes across different finch species, was that the thicker and taller and more robust a beak, the more strongly it expressed a gene known as BMP4 early in development. The BMP4 gene (its abbreviation stands for bone morphogenetic protein, No. 4) produces the BMP4 protein, which can signal cells to begin producing bone. But BMP4 is multitalented and can also act to direct early development, laying out a variety of architectural plans including signaling which part of the embryo is to be the backside and which the belly side. To verify that the BMP4 gene itself could indeed trigger the growth of grander, bigger, nut-crushing beaks, researchers artificially cranked up the production of BMP4 in the developing beaks of chicken embryos. The chicks began growing wider, taller, more robust beaks similar to those of a nut-cracking finch.

In the finches with long, probing beaks, researchers found at work a different gene, known as calmodulin. As with BMP4, the more that calmodulin was expressed, the longer the beak became. When scientists artificially increased calmodulin in chicken embryos, the chicks began growing extended beaks, just like a cactus driller.

So, with just these two genes, not tens or hundreds, the scientists found the potential to recreate beaks, massive or stubby or elongated.

"So now one wants to go in a number of directions," Dr. Tabin said. "What happens in a stork? What happens in a hummingbird? A parrot?" For the evolution of beaks, the main tool with which a bird handles its food and makes its living, is central not only to Darwin's finches, but to birds as a whole.

BMP4's reach does not stop at the birds, however.

In lakes in Africa, the fish known as cichlids have evolved so rapidly into such a huge diversity of species that they have become one of the best known evolutionary radiations. The cichlids have evolved in different shapes and sizes, and with a variety of jaw types specialized for eating certain kinds of food. Robust, thick jaws are excellent at crushing snails, while longer jaws work well for sucking up algae. As with the beaks of finches, a range of styles developed.

Now in a new study, Dr. R. Craig Albertson, an evolutionary biologist at Syracuse University, and Dr. Thomas D. Kocher, a geneticist at the University of New Hampshire, have shown that more robust-jawed cichlids express more BMP4 during development than those with more delicate jaws. To test whether BMP4 was indeed responsible for the difference, these scientists artificially increased the expression of BMP4 in the zebrafish, the lab rat of the fish world. And, reprising the beak experiments, researchers found that increased production of BMP4 in the jaws of embryonic zebrafish led to the development of more robust chewing and chomping parts. (Emphasis mine.)

What is the implication of the common use of BMP4 across very different sets of species -- birds and fish?

Used to lay out body plans, build beaks and alter fish jaws, BMP4 illustrates perfectly one of the major recurring themes of evo-devo. New forms can arise via new uses of existing genes, in particular the control genes or what are sometimes called toolkit genes that oversee development. It is a discovery that can explain much that has previously been mysterious, like the observation that without much obvious change to the genome over all, one can get fairly radical changes in form.

"There aren't new genes arising every time a new species arises," said Dr. Brian K. Hall, a developmental biologist at Dalhousie University in Nova Scotia. "Basically you take existing genes and processes and modify them, and that's why humans and chimps can be 99 percent similar at the genome level."

Evo-devo has also begun to shine a light on a phenomenon with which evolutionary biologists have long been familiar, the way in which different species will come up with sometimes jaw-droppingly similar solutions when confronted with the same challenges. (Emphasis mine.)

This has interesting implications when it comes to examples of convergent evolution. In convergent evolution, animals faced with similar circumstances evolve similar solutions -- such as evolving broad flat teeth to chew plants in herbivorous species even when those species are themselves distantly related. If a narrow set of genes is responsible for overall body plan, did the same gene evolve in a similar way to produce that result in both species?

Another example is mimicry. In mimicry, animals looks like one another to steal the benefit of a particular appearance. For example, a moth that is non-poisonous evolves to look like a moth is that is poisonous because the poisonous moth is not eaten as much. The non-poisonous moth gains the advantage of being avoided as food. If a narrow set of genes determines moth coloring, did both species exploit the same genetic solution to result in that type of coloring?

Evo-devo as a field seeks to address the question: how did the immense variety of species develop? The answer has thus far been: by small modifications of a relatively small set of genes that have very large consequences.

For further reading, check out Abzhanov et al. -- which looks at how the expression of calmodulin (CaM) in the embryos of related species of Darwin's finches codes for beak length. It is the paper from which the figure below is taken (click to enlarge). The left depicts various finch species and their skulls. The right depicts staining for CaM in their embryos. Note how the strength of staining increases with beak length.


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