A comparison of genetic variability at X-linked and autosomal loci in kangaroos, man and Drosophila

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

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 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.

Selected features of marsupial genetics

As a consequence of the ancient separation of the marsupial and eutherian lineages, comparative genetical studies of these two mammalian taxa can be particularly informative. The potential for marsupial genetical research has been enhanced by the development of laboratory colonies of three 'model' species--Macropus eugenii, Monodelphis domestica and Sminthopsis crassicaudata. In this paper two selected aspects of marsupial genetics are reviewed, one involving cytogenetics and the other linkage. Marsupials provide a spectacular example of karyotypic conservation. The so-called 'basic karyotype' (2n = 14) is probably ancestral in all extant marsupials. Karyotypes that do not conform to this basic arrangement are thought to have been derived from it. A notable feature of the basic karyotype is that it has been retained, possibly for as long as 150 million years, in morphologically, behaviourally and ecologically diverse species from at least five Australian and two American families; this suggests that selective forces, presently unknown, have acted to conserve the basic chromosome form and number in these species. With respect to genetic linkage, family studies in S. crassicaudata and more recently M. domestica have indicated extreme differences between the sexes with the recombination frequencies for linked loci being very much greater in males than in females, a situation that is strikingly different from that in eutherian mammals. These differences in linkage values are paralleled by differences in the number and distribution of chiasmata during male and female meiosis. Prospects for further research in marsupials, particularly research that builds upon the observations of karyotypic conservation and genetic linkage, are noted.

Insertion-deletion variation at the yellow-achaete-scute region in two natural populations of Drosophila melanogaster

We have surveyed the region of the X chromosome of Drosophila melanogaster which encodes the yellow, achaete and scute genes for restriction map variation. Two natural populations, one from North Carolina, U.S.A. and the other from southern Spain were screened for variation at about 70 restriction sites and for variation due to DNA insertion or deletion events in 120 kilobases of DNA. Mean heterozygosity per nucleotide was estimated to be 0.0024 and 15 large insertions were found in the 49 chromosomes screened. Extensive disequilibrium between polymorphic sites were found across much of the region in the North Carolina population. The frequency of large insertions, which usually correspond to transposable genetic elements, is significantly lower than has been observed in autosomal regions of the genome. This is predicted for X-linked loci by certain models of transposable element evolution, where copy number is restricted by virtue of the recessive deleterious effects of the insertions. Our results appear to support such models. The deficiency of insertions may in this case be enhanced by hitch-hiking effects arising from the high level of disequilibrium.