Selfish genes and meiotic drive

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If one asks why an organism has a particular feature, such as eyes, it is usually supposed that it is because the trait is ‘good for the individual’. This is another way of saying that although the gene for the feature must have been rare initially, it rose in frequency in the population because it increased the chances of the individual to survive and reproduce. ‘Selfish genes’ are not like this, for they are harmful to the individual. Unlike ordinary deleterious genes, however, selfish genes have some trick that allows them to increase in frequency when rare, despite being harmful.

Meiotic drive genes are one class of selfish gene. The best-described example is Segregation Distorter in the fruitfly Drosophila melanogaster. This is autosomal (that is, not on the X or Y chromosome) and acts in males. Males that have the driver on one chromosome but not the other have half of their sperm killed, the half that do not contain the drive gene. This, of course, is not ‘good for the individual’. But it is ‘good for the gene’, and the drive gene spreads in the population because of the increased numbers of eggs that come to be fertilized by sperm containing it. Males in which both chromosomes have Segregation Distorter are sterile. As the driver spreads, this comes to be an increasingly common occurrence. In consequence, Segregation Distorter does not eliminate the original non-driving chromosome.

Although in no case is the mechanism of drive well understood, in all well-described instances the drive ‘gene’ is actually composed of two tightly linked types of gene. The simplest model proposes that one of these codes for a toxin. Even if only half of the sperm have this gene, all sperm are affected by its toxic product. — unless, that is, they contain the other gene, which codes for the antidote. Unlike the toxin, the antidote's activity is restricted to the sperm that contain the antidote gene.

Initially, most chromosomes will have neither the toxin nor the antidote. When rare, then, the two-gene complex is most likely to be in an individual whose other chromosome is sensitive to the toxin. This male will have half of its sperm killed and the driving gene-complex will spread.

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