Monday, March 16, 2009

The unity of life


Sometimes an illustration in Nature or Science stirs a more spontaneous response than the text it accompanies, such as this delightful rendering of the nematode worm Caenorhabditis elegans and the fruitfly Drosophila melanogaster, from the March 12 issue of Nature.

Except for the bacterium E. Coli, no two creatures have been more relentlessly studied by scientists than C. elegans and Drosophila. In the case of the worm, every one of its precisely 959 cells has been catalogued, including 302 neurons. Who would have guessed you could do so much with so little. Fruit flies have been faithful servants of science since they were adopted by T. H. Morgan in his important studies in genetics that began at Columbia University in the early years of the last century. They are ideal research animals, small enough to breed in the lab in large numbers, but large enough to examine with only modest magnification. And they have a short life cycle, which means they can be bred through many generations during a typical graduate student's time of study.

But enough, I have sung their praises before.

Why here, why now? Nature reports a study that shows the worm and the fruit fly produce similar relative amounts of analogous proteins, even though levels of the messenger RNAs that code for these proteins vary widely between the species, "illustrating that regulating protein abundance is more important than maintaining gene-expression levels." These relative abundances have been maintained across 600 million years of separate evolution.

And if that were not enough, two research articles in the same issue of Nature explore the sensitivity of Drosophila's hearing organ (in the fly's antennae) to gravity and moving air. The organ is not only atuned to courtship songs, it also lets the fly know which way is up and when to hunker down out of the wind. According to Nature:
In the ear, hearing receptor cells also show clear adaptations for fast responses relative to gravity-sensitive receptor cells. This similarity extends the list of recently discovered parallels between fruit fly and vertebrate ears, including related genes for specifying ear development, the involvement of particular classes of ion channel, and evidence for mechanisms that amplify the mechanical input to the sensor. Such similarities excite the speculation that hearing organs in fruitflies and vertebrates arose from a sensory structure present in a common ancestor, rather than independently as was long thought.
On the face of it, you and I and the nematode and the fruit fly are about as different as one might imagine. At the level of genes and their expression as chemical strategies for survival, the ties that bind go deep into geological time. The unity of life that Darwin proposed 150 years ago is written in the DNA.