Honeybee Genome May Shed Light on Social Evolution

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.

At the same time, honeybees have a paucity of taste receptors -- 10, compared with the fruit fly's 68 and the mosquito's 76.

Over the eons, taste has evolved primarily as a mechanism of defense. Plants evolve toxins to protect themselves from being eaten; animals evolve receptors to detect the toxins. (This is part of the reason human beings have far more bitter than sweet receptors, bitter being the taste of poisonous plant alkaloids.) Bees, however, have a far more congenial relationship with the plant world. They pollinate the plants and the plants feed them.

"Honeybees for a long time have not needed a lot of gustatory receptors because they have not been in a chemical arms race with plants," Robertson said. He added that an unanticipated, but entirely logical, discovery in the bee genome supports this theory. Compared with other insects, A. mellifera has only half the number of genes encoding the enzymes that detoxify dangerous compounds.

But 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. That is also probably the reason chimpanzees and human beings are so different in cognitive ability despite having 97 percent identical genes, he added.

One line of research the completed genome will help advance is which genes constituted the raw material that through natural selection allowed cooperative behavior to emerge in honeybees.

Work already done by Gene E. Robinson, a neurobiologist at Urbana-Champaign and a co-leader of the genome consortium, and by Gro Amdam and Robert E. Page Jr. at Arizona State University, suggests that reproduction and nutrition genes evolved greatly in honeybees.

"Food and sex -- it is not unreasonable to hypothesize they were important in the development of social behavior," Robinson said.