SOME types of behaviour are inherited, just like skin and eye colour.
Now, in laboratory experiments on fruit flies, researchers have observed the
natural selection of a behavioural trait and have suggested why the gene that
controls it has evolved in this way.
Fly larvae spend nearly all their time feeding. But larvae carrying the
鈥渟itter鈥� version, or allele, of the foraging gene tend to concentrate
their feeding on one spot, while maggots with the 鈥渞over鈥� allele tend to roam
far and wide as they dine, even if they have plenty of food right under their
feeding hooks. Roving could be considered disadvantageous, as maggots must
expend a great deal of energy dragging themselves along. 鈥淓nergy for a maggot is
very costly,鈥� says biologist Marla Sokolowski of York University in Toronto.
To explain the existence of the rover allele, Sokolowski suggested that
roving might be an advantage under very crowded conditions, in which the maggots
must compete for access to food. To test this idea, she and her colleagues
examined the roaming tendencies of maggots from flies that Larry Mueller, a
biologist at the University of California at Irvine, had kept for other
experiments at both high and low population densities for 286 fly generations
鈥攎ore than a decade.
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Sokolowski evaluated the roving behaviour of isolated larvae by measuring the
distance they crawled in a fixed time. The results, published last week in
Proceedings of the National Academy of Sciences (vol 94, p 7373), show that
the larvae of flies that had been living for generations in crowded
conditions鈥�1000 flies in a 250 millilitre bottle鈥攃rawled on average
6 centimetres in five minutes, which is no mean feat for a 2 millimetre maggot.
On the other hand, larvae from the sparsely populated conditions鈥�50 flies
per bottle鈥攃rawled only about half as far.
In case the difference was due to chance genetic variation between the two
populations, Sokolowski reproduced Mueller鈥檚 crowded and sparsely populated
conditions with flies she collected from an orchard near Toronto. By
crossbreeding, she made half the flies in each environment rovers and the other
half sitters鈥攂ut left the rest of their genetic endowment as it was in the
wild. She maintained the populations at constant density for nearly three years,
and then monitored the mobility of the larvae.
Once again, maggots that lived in crowded conditions typically moved 3
centimetres farther in five minutes than those in sparse conditions. Presumably
the tendency to roam confers an advantage upon flies living in a dense crowd,
says Sokolowski.
After performing various genetic experiments to ensure that the differences
in mobility were due to different alleles of the foraging gene,
Sokolowski concluded that she had mimicked the natural selection of a
single-gene trait, and that population density could explain the existence of
the rover allele. 鈥淲e鈥檝e been looking for a long time for an explanation,鈥� she
says.
Ralph Greenspan of the Neurosciences Institute in San Diego says the paper is
a landmark. 鈥淎ltogether, the story adds up to a tour de force,鈥� he
says. Mueller adds that by testing fly stocks of different origins, Sokolowski
has generated a particularly solid set of data: 鈥淚t鈥檚 really one of the nicest
and most convincing pieces of work in experimental evolution and behaviour ever
诲辞苍别.鈥�