The population genetics of haplo-diploids and X-linked genes

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Abstract
SUMMARY: From the available electrophoretic data, it is clear that haplodiploid insects have a much lower level of genetic variability than diploid insects, a difference that is only partially explained by the social structure of some haplodiploid species. The data comparingX-linked genes and autosomal genes in the same species is much more sparse and little can be inferred from it. This data is compared with theoretical analyses ofX-linked genes and genes in haplodiploids. (The theoretical population genetics ofX-linked genes and genes in haplodiploids are identical.)X-linked genes under directional selection will be lost or fixed more quickly than autosomal genes as selection acts more directly onX-linked genes and the effective population size is smaller. However, deleterious disease genes, maintained by mutation pressure, will give higher disease incidences atX-linked loci and hence rare mutants are easier to detect atX-linked loci. Considering the forces which can maintain balanced polymorphisms, there are much stronger restrictions on the fitness parameters atX-linked loci than at autosomal loci if genetic variability is to be maintained, and thus fewer polymorphic loci are to be expected on theX-chromosome and in haplodiploids. However, the mutation-random drift hypothesis also leads to the expectation of lower heterozygosity due to the decrease in effective population size. Thus the theoretical results fit in with the data but it is still subject to argument whether selection or mutation-random drift are maintaining most of the genetic variability atX-linked genes and genes in haplodiploids.