11.05.2006

more on the bee genome
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Here is a follow-up on the bee genome.

The honeybee is the third insect to be sequenced, following the fruit fly and anopheles mosquito.

The honeybee has 10,000 to 15,000 genes arrayed on 16 chromosomes, compared with humans' estimated 24,000 genes and 24 chromosomes (22 regular ones and two sex chromosomes). Comparisons with the fruit fly and mosquito genomes suggest that bees evolved more slowly than either of those other insects. Curiously, some bee genes -- notably the ones responsible for internal "clocks" and circadian rhythms -- are more similar to mammals' genes than flies'.

But the most interesting insights so far come from discoveries of what parts of the bee's genome have been enriched, ignored or discarded by the evolutionary force of natural selection.

Compared with other insects, honeybees have only one-third as many genes involved in recognizing and killing their microbial enemies. This is a surprise for an organism that spends 95 percent of its life in a crowded, moist 94-degree indoor environment hospitable to bacteria and parasites.

But bees are extremely hygienic and prevention-minded. When a developing larva dies, it is removed from its cell in the honeycomb immediately and the carcass is flown a distance from the hive before it is discarded. Nurse bees secrete antimicrobial substances into the food they provide the larvae. Honey, the principal source of food over the winter, does not support microbial growth because of its high-sugar, low-water makeup. Overall, it appears that compared with those of other insects, a bee's genome is less concerned with protecting the individual from disease and more concerned with protecting a larger organism -- the entire colony.

Bees also have fewer genes encoding the proteins that make up their exoskeleton. The researchers speculated that is because they spend their larval stage and much of their early adulthood inside the hive, protected from ultraviolet light and temperature stresses.

But what is lost in the immune system and the skin is gained in the bee equivalent of the nose.

A. mellifera has 170 genes for "odorant receptors," of which 157 are in a gene family so far found only in honeybees. This is far more smelling apparatus than either fruit flies (with 62 receptor genes) or mosquitoes (with 79) possess. It probably reflects the extreme importance of smell in helping bees find flowers and communicate with one another, including with their queen, through pheromones.

There is no genomic smoking gun that explains the species' most remarkable behavior -- the ability of bees to tell one another the location of food sources outside the hive through a ritualized "dance" that uses the sun's position as a point of reference. There is no cluster of brain genes possessed only by bees.

"It's not what you have in your genome but how you use it" that must explain that capacity to learn and communicate, said Jay D. Evans, a scientist at the U.S. Department of Agriculture's Bee Research Laboratory in Beltsville, Md. That is also probably the reason chimpanzees and human beings are so different in cognitive ability despite having 97 percent identical genes, he added.



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