Friday, February 09, 2007

Having it out

Drosophila melanogaster, the "black-bellied dew lover," is better know to you and me as the fruit fly that swarms annoyingly about our food trash. Never mind. Sing its praises. Perhaps no other creature has contributed so much to our understanding of life.

The fruit fly was adopted by T. H. Morgan in his important studies in genetics that began at Columbia University in the early years of this century. These studies led to the classic textbook of Morgan, Sturtevant, Muller, and Bridges, "Mechanisms of Genetic Inheritance," which in 1915 established the link between genes and chromosomes. Since that time, much of what we know about mutation, speciation, and other genetic phenomena has been discovered with populations of fruit flies in nature and in the lab. Drosophila is an ideal research animal. It is small enough to breed in the lab in large numbers, but large enough to examine with only modest magnification. And it has a short life cycle, which means it can be bred through many generations during a typical graduate student's time of study.

Lately, Drosophila has been teaching scientists about the genetic basis of aggression.

Try this experiment (ala geneticists Ralph Greenspan and Herman Dierick of the Neurosciences Institute in San Diego). Put 120 male fruit flies and 60 virgin females into an enclosure with 11 dispersed cups filled with fly food. Care to guess what the males do first? Yep, the first thing they think about is sex. Then they settle down to sup.

As they sup, they establish and defend their sugar-cup territories. And that's where the scientists come in. Greenspan and Dierick selectively bred the most aggressive flies for 21 generations, creating a superaggressive line of flies that were meaner and scrappier than their normal cousins. Then the scientists used DNA microarrays to look for changes in gene expression, and identified 42 genes that had significantly increased or decreased their activity. One gene in particular, Cyp6a20, stood out among the aggressive flies as less active than normal, an enzyme-producing gene whose reduced activity may make the flies hypersensitive to pheromones.

This is just one of many recent experiments looking for the genetic basis of aggression, summarized in a fine article by Greg Miller in the January 12 issue of Science. It's a bit early to say what these experiments tell us about vertebrate aggression, but I dare say if you put 120 human males, 60 virgin females, and 11 scattered caches of beer and pizzas, in a sealed space, the observed behaviors would not be all that dissimilar from those of drosophila.

As I believe I mentioned here before, female fruit flies can be aggressive too, although with a lot more pushing and shoving and less slugging it out.