Honeybee Genome May Shed Light on Social Evolution

By David Brown
Washington Post Staff Writer
Monday, October 30, 2006

Bees and people have a lot in common.

We both live in groups and snuggle with others when cold. We both know that staying clean helps prevent disease. We both prepare food for others and leave home to get it even when we aren't hungry. We both can communicate through dancing.

Of course, there are differences.

Bees are an inch long. They copulate while flying. Each winter, the females kick the males out of the house to die.

So to what extent do genes explain our two wildly different evolutionary journeys? Biologists now have a better way of exploring that question, and a whole lot of other ones.

Last week, the Honeybee Genome Sequencing Consortium announced that it had finished copying out the genetic message of Apis mellifera , the world's most important pollinator, maker of nature's best-known sweet food, and object of human fascination and delight for eons. The honeybee becomes the third insect to have its genome fully transcribed, preceded by the fruit fly drosophila and the malaria mosquito anopheles. A flour-eating beetle, an aphid and a wasp are next in line.

The work was done by 150 people in about 20 countries over the past three years. The huge mass of data -- along with that from the other species -- will help sketch a picture of what it means to be an insect, as well as what it means to be a honeybee.

Insects are the most diverse group of animals on Earth, with about 925,000 identified species. The genetic exploration may eventually shed light on the biology of togetherness and cooperation, which bees and people both discovered in the 600 million years since they last shared a common ancestor.

"We can use this genome to go looking for any and every gene that might be involved in the evolution of sociality. But that is down the road," said Hugh M. Robertson, an entomologist at the University of Illinois at Urbana-Champaign.

In a long paper in the journal Nature and several shorter ones in Science, Robertson and his colleagues describe their initial insights on the honeybee genome, which is full of both surprises and confirmed hunches.

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.

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.

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