Entomology Today April 30, 2015
Like other social insects, honey bees live in colonies consisting mainly of closely-related members. However, high genetic diversity among the workers is important for the whole colony’s survival because it makes them better equipped to perform the diverse tasks required in the colony, and it means they will likely be less susceptible to disease.
But how can the queen, the colony’s only fertile female, prevent inbreeding and maintain genetic variation?
The queen bee solves the problem in two ways. One is through polyandry — she mates with a score of drones and uses their sperm to fertilize the eggs randomly so that workers often have different fathers. The second is through extremely high rates of recombination.
Recombination, or crossing-over, occurs when sperm and egg cells are formed and segments of each chromosome pair are interchanged. This process plays a crucial role in the maintanance of genetic variation. Matthew Webster and Andreas Wallberg at the Biomedical Centre, Uppsala University, have studied recombination in honey bees. The extreme recombination rates found in this species seem to be crucial for their survival.
By sequencing the entire genome of 30 African honey bees, the research team has been able to study recombination at a level of detail not previously possible. The frequency of recombination in the honey bee is higher than measured in any other animal and is more than 20 times higher than in humans.
Recombination affects how efficiently natural selection can promote favorable genetic variants. In line with this, the researchers have found that genes involved in the new adaptations to the environment in honey bees also undergo more recombination. But recombination is not entirely risk free.
“Recombination is not only beneficial for bees,” Webster said. “When parts of chromosomes [are] broken and exchanged, errors can sometimes occur during their repair due to a process called ‘GC-biased gene conversion.'”
This process leads to gradual fixation of mutations that may be harmful to the honey bee. Although a similar process occurs in humans, it is more than ten times stronger in honey bees. Over time, recombination is expected to lead to a deterioration of the gene pool, a process that seems to have accelerated in bees. The extreme recombination rates — crucial for maintaining genetically diverse honey bee colonies — come with a high price.
“There are no free lunches, not even for a honey bee,” Webster said.