The genome of the honeybee, Apis mellifera, has been sequenced by an international consortium of researchers, and is published in today’s issue of Nature.
Honeybees are of great economic importance, because they pollinate crops, but they also play a vital role in the global ecosystem. They belong to the order Hymenoptera (the social insects), and form large, hierarchical social structures in which there is a division of labour, with different castes performing particular jobs. Yet all the castes making up the bee colony have exactly the same genome.
The nourishment received by a larva determines what caste of bee will develop from that larva. Larvae fed on royal jelly develop into queens. The queen bee remains in the nest, lives much longer than the other castes, and lays 2,000 eggs a day. Drones are male honeybees that develop from unfertilized eggs; their primary role is to fertilize a receptive queen. Worker bees are females which perform duties such as cleaning the hive and attending to the queen. Nurse bees feed larvae and clean the cells in the hive.
In summary, the main findings about the A. mellifera genome are:
- it contains about 10,000 genes;
- it evolved more slowly than the genome of the fruit fly Drosophila melanogaster;
- it contains less genes for immunity, detoxification and taste receptors, but more for olfactory receptors, than the fruit fly;
- in terms of genes for circadian rhythms, RNA methylation and RNA interference, it shows greater similarities to the genomes of vertebrates than it does to that of the fruit fly.
The simplicity of the honeybee’s brain makes it an attractive model for neurobiologists. Despite having a simple nervous system, the honeybee has remarkable cognitive capabilities. For example, honeybees can perform and interpret the waggle dance, a complex mode of visual communication by which they signal to each other the source of food and the location of potential new nest sites.
The honeybee’s brain contains approximately 1 million neurons, and is not too different from that of the fruit fly, yet the two species differ markedly in their social organization and behaviour. The key to understanding these differences may lie in how honeybee genes are regulated. The honeybee genome contains 65 regions encoding microRNAs (miRNAs), short RNA molecules that can bind to DNA sequences and turn genes on or off. These miRNAs could be involved in the social behaviour of the honeybee, as they have been shown to expressed in a caste- and developmental stage-specific manner.
The honeybee brain is stretched laterally in the head capsule between the compound eyes (yellow in the 3D reconstruction on the left). The optic lobes (orange) process visual information received by the compound eyes, and the antennal lobes contain olfactory receptors. In the animation, one of the antennal lobes is coloured blue, and the other is transparent. The mushroom bodies, located in the centre at the top of the brain (one is coloured red in the animation), are known to be involved in complex functions such as social behaviour, learning and memory. Honeybees can learn the odour of their colony, and can also memorize the locations of several food sources which they have been directed to previously by the waggle dance of another bee.
Honeybees have 170 genes encoding odorant receptors, compared to just 60 in the fruit fly genome. This expansion of the odorant receptor family in comparison in honeybees is not surprising, given the importance of olfaction in behaviours such as the recognition of kin and of a diverse set of floral odours. The number of odorant genes closely matches the number of glomeruli in each of the antennal lobes, suggesting that each glomerulus may express just one type of odorant receptor.
The knowledge of gene functions which will follow the completion of the honeybee genome will enable researchers to gain a better understanding of the genetic basis and evolution of social behaviour. There are some parallels between honeybee and human societies, so any findings regarding the genetics of social behaviour in bees could also be applicable to humans.
Update: This month’s issue of the journal Insect Molecular Biology is devoted the honeybee genome. There is free access to all the content.