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

Abstract

SUMMARYFrom 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 comparing X-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 of X-linked genes and genes in haplodiploids. (The theoretical population genetics of X-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 on X-linked genes and the effective population size is smaller. However, deleterious disease genes, maintained by mutation pressure, will give higher disease incidences at X-linked loci and hence rare mutants are easier to detect at X-linked loci. Considering the forces which can maintain balanced polymorphisms, there are much stronger restrictions on the fitness parameters at X-linked loci than at autosomal loci if genetic variability is to be maintained, and thus fewer polymorphic loci are to be expected on the X-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 at X-linked genes and genes in haplodiploids.

Selection at two sex-linked loci

“…which, of course, is the familiar equation for gene frequency change under weak selection (Nagylaki, 1979;Avery, 1984). It also follows that the amount of linkage disequilibrium between the two loci is changing each generation according to the relationship,…”

“…Nevertheless they note that because with weak, directional selection alleles are substituted more rapidly at sex-linked than at autosomal loci (Avery, 1984), linked neutral alleles will also be carried to fixation at the same rate. Thus hitchhiking potentially could be more effective at sex-linked loci as compared to autosomal loci.…”

“…Sex-linked or haplodiploid loci are predicted to have a reduced level of heterozygosity as compared to autosomal loci, whether selection or mutation and random drift are maintaining most of the genetic variability (Lester and Selander, 1979;Pamilo and Crozier, 1981;Avery, 1984). This conclusion is based on single-locus theory only, therefore it is of interest to examine the effect of hitchhiking on genetic variation at sex-linked loci.…”

“…Sex-linked or haplodiploid loci are predicted to have lower levels of genetic variability than autosomal loci whether selection or mutation-drift maintain variability (Avery, 1984). The role of hitchhiking has been discussed (Lester and Selander, 1979;Pamilo and Crozier, 1981) but remains unclear.…”

Heterozygosity at the malate dehydrogenase locus and developmental homeostasis in Apis mellifera

“…(1992) found no significant relationship between degree of inbreeding and FA. Honeybees are already highly homozygous because of haplodiploidy and other factors (Lester & Selander, 1979;Avery, 1984). Therefore it may be difficult to detect differences between inbred and outbred colonies.…”