|The question of eusociality in bees remains a genomic mystery|
The Honeybee Genome Sequencing Consortium announced that it had finished sequencing the genome of Apis mellifera. The honeybee is the third insect to have its genome fully sequenced, preceded by the fruitfly, Drosophila melanogaster and the malaria mosquito, Anopheles gambiae. A flour beetle, an aphid and a wasp are next in line. The work was done by 150 scientists from about 20 countries over the past three years. Dr. Dan Graur and graduate student Eran Elhaik from the Department of Biology and Biochemistry at the University of Houston took part in the compositional analyses of this uniquely AT-rich genome.
The honeybee has 10,000 to 15,000 genes arrayed on 16 chromosomes, compared with humans' estimated 24,000 genes and 24 chromosomes. Comparisons with the fruitfly 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 mammalian genes than dipteran genes.
Compared with other insects, honeybees have only one-third as many genes involved in recognizing and killing bacterial pathogens. This is a surprise for an organism that spends 95 percent of its life in a crowded, moist, and warm indoor environment hospitable to bacteria and parasites. This may be due to bees being extremely hygienic and practicing preventive medicine. For example, when a 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 other insects, a bee's genome is less concerned with protecting the individual and more concerned with protecting the larger organism -- the colony. Bees also have fewer genes encoding exoskeletal proteins. It is thought that this paucity may be due to the fact that beesspend their larval stage and much of their early adulthood inside the hive, protected from ultraviolet light and other stresses.
But what is lost in the immune system and the skin is gained in the bee equivalent of the nose. The bee genome contains 170 olfactory-receptor genes, of which 157 are in a gene family so far found only in honeybees. In comparison, fruitflies and mosquitoes have 62 and 79 genes, respectively. Smell seems to be of extreme importance 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 fruitfly's 68 and the mosquito's 76.
But there is no genomic smoking gun that explains the species' most remarkable behavior- its eusociality. Nor can we yet identify the genes responsible for communication, most famously for the ritualized "dance" that is used to communicate food location among bees. There is no cluster of brain genes possessed only by bees.