Why do certain species of fruit flies prefer some fruits over others? Two biologists have looked at the genetic basis behind the evolution of fruit preference, in a paper in this month's issue of Genetics (an incredible issue which happens to contain another amazing, pioneering, paradigm-shifting, ground-breaking paper).
Many insects specialize in feeding on just one or a few types of plants. This fact isn't that surprising, since plants have all sorts of defenses for warding off insects, including the production of toxic chemical compounds, and insects that feed on plants have typically evolved ways to get around the defenses of one type of plant, but not another. This phenomenon is dramatically played out in the hundreds of species of fruit flies around the world, many of which specialize in just one or a few types of fruit. Researchers at Michigan State and UNC Chapel Hill have looked at the genetic changes that enabled one species of fruit fly to specialize in the fruit of the Morinda plant, also known as cheese fruit, a fruit that is toxic to other flies.
The fruit fly species D. simulans and D. sechellia split off down separate evolutionary branches less than 500,000 years ago, and during that time, D. sechellia has managed to adapt to eating cheese fruit, which D. simulans despises. What genetic changes in D. sechellia produced this gastronomical shift?
To begin to answer this question, the researchers used two strategies to compare these two closely related but behaviorally different species: 1) They looked at which genes were switched on or off when the flies were exposed to cheese fruit, and 2) They compared the D. simulans and D. sechellia genomes to see what genes D. sechellia had gained or lost.
The results of the first comparison, looking at which genes are switched on and off in the presence of Morinda fruit, are only a first step towards understanding what evolutionary changes occurred in D. sechellia. But the gene expression changes did show one interesting feature: D. sechellia had a tendency to shut down many genes in the presence of Morinda fruit.
How does shutting down genes make D. sechellia not turn up its nose at Morinda fruit? The researchers found a concrete example of how this works. Cheese fruit smells foul to D. simulans (which is probably a strategy by Morinda to keep pesky flies away), but because of a broken gene, the smell doesn't bother D. sechellia. The gene Obp56e has been irreparably damaged in the D. sechellia genome, and apparently this gene is necessary to smell whatever nasty odorant is secreted by Morinda.
The biologists tested this idea by knocking out Obp56e in another fruit fly species that dislikes Morinda fruit. Flies which hated Morinda fruit could be transformed into flies that would eat Morinda fruit simply by breaking their Obp56e gene. It's possible that a broken Obp56e gene in an ancient D. sechellia individual paved the way for this species to specialize in getting around Morinda's chemical defenses.
Join me tomorrow, here at Adaptive Complexity, for day 22 of 30 Days of Evolution Blogging. Evolution as a science is alive and well. Each day I will blog about a paper related to evolution published in 2009.
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