Recombination, or crossing-over, occurs when sperm1 and egg cells are formed and segments of each chromosome2 pair are interchanged. This process plays an crucial role in the maintenance of genetic4 variation. Matthew Webster and Andreas Wallberg at the Biomedical Centre, Uppsala University, have studied recombination in honeybees. The extreme recombination rates found in this species seem to be crucial for their survival. Like other social insects, honeybees live in colonies consisting mainly of closely related members of the worker caste. High genetic diversity among the workers is important for the whole colonys survival. There are several theories as to why: for example, a genetically5 variable workforce6 may be best equipped to perform the diverse tasks required in the colony, and diverse colonies may also be less susceptible7 to disease. But how can the queen, the colonys only fertile female, prevent inbreeding and maintain genetic variationThe queen bee solves the problem in two ways. One is through polyandry. She mates with a score of drones and uses their sperm to fertilize8 the eggs randomly9 so that workers often have different fathers. The second is through extremely high rates of recombination.By sequencing the entire genome of 30 African honeybees, the research team has been able to study recombination at a level of detail not previously10 possible. The frequency of recombination in the honeybee is higher than measured in any other animal and is more than 20 x higher than in humans.Recombination affects how efficiently11 natural selection can promote favorable genetic variants12. In line with this, the researchers have found that genes13 involved in the new adaptations to the environment in honeybees also undergo more recombination. But recombination is not entirely14 risk free.Recombination is not only beneficial for bees. When parts of chromosomes15 broken and exchanged, errors can sometimes occur during their repair due to a process called GC-biased gene3 conversion, says Matthew Webster.This process leads to gradual fixation of mutations that may be harmful to the honeybee. Although a similar process occurs in humans, it is more than ten times stronger in honeybees. Over time, recombination is expected to lead to a deterioration16 of the gene pool, a process that seems to have accelerated in bees. The extreme recombination rates -- crucial for maintaining genetically diverse honeybee colonies -- come with a high price.There are no free lunches. Not even for a honeybee, says Matthew Webster.
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1 sperm n.精子，精液 参考例句： only one sperm fertilises an egg.只有一个精子使卵子受精。 In human reproduction,one female egg is usually fertilized by one sperm.在人体生殖过程中，一个精子使一个卵子受精。
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