>  Recombination makes new combinations, but does not create new sex alleles, nor allelic variation.

Recombination in and around the sex locus may be selected as a consequence of different mechanisms. Strobeck et al. (1976) proposed that recombination will be favored near genes showing a heterozygous advantage because favorable alleles of genes neighboring such a locus can recombine with different sex alleles. Without recombination, favorable linked genes could not increase in frequency because strong overdominant selection at the linked sex locus would prevent it. A favorable allele could not be more frequent than the sex allele to which it is linked. This could explain the apparent high recombination rate between the sex locus and the centromere.

High recombination may also be expected within the sex locus if detrimental mutations with small effects accumulate faster than they can be eliminated by selection (Kondrashov 1984, 1988; Charlesworth 1990). As detrimental mutations increase, the sex allele loses its complementary function. In complementary sex determination a loss of function of an allele may have severe fitness effects by resulting in the production of a diploid male. Recombination within the gene could rescue the gene function in some gametes and reduce the deleterious effects.

Beye, M., Hunt, G. J., Page, R. E., Fondrk, M. K., Grohmann, L., & Moritz, R. F. A. (1999). Unusually high recombination rate detected in the sex locus region of the honey bee (Apis mellifera). Genetics, 153(4), 1701-1708.

Comment: The main source of genetic diversity is thought to be random mutations. But if these occur on the sex locus, they can cause the sex locus to lose its ability to produce females, instead creating diploid drones which are nonviable. This can produce the shotgun brood pattern we see occasionally. One function of recombination is to rescue allelic diversity which is crucial to sex determination in the honey bee, and other social insects with similar genetic schemes.

PLB

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