Twenty-five population cages of D. melanogaster were set up, each containing a different wild-type second chromosome and the marker chromosome Cy. In all but one case where contamination apparently occurred, the Cy chromosome persisted in the population at high frequency, showing a selective advantage of Cyj + heterozygotes over wildtype homozygotes. Overall, the results indicate that homozygosity of the entire second chromosome causes a depression in fitness of the order of 85%.
Male.recombination and female sterility, two aspects of hybrid dysgenesis in D. melanogaster, have been studied in crosses between a locally collected wild population and laboratory strains. Dysgenesis occurs in the Fl hybrid of such crosses only if the wild type is used as maie parent and the laboratory strain as female, suggesting an interaction between genotype and cytoplasm. However the results from further crosses are difficult to interpret in terms of a conventional genotype--cytoplasm model, and suggest that for dysgenesis to occur it is necessary-that the wild-type chromosomes be contributed by the male parent. Furthermore, receipt of any of the three major wild-type chromosomes in crosses to laboratory females is sufficient to cause hybrid dysgenesis.A model in terms of spatial organization of chromosomes is put forward to explain these results. It is postulated that (1) normal nuclear functioning requires a definite spatial organiZation of chromosomes, which is presumably achieved by chromosome-membrane associations, (2) chromosomes are inherited from the female parent with spatial ordering preserved, i.e. membranes and associated chromosomes are handed on directly from the female parent, (3) spatial ordering is not necessarily preserved in male gametes, and paternally derived chromosomes carry information enabling them to become correctly organized within the zygote nucleus, and (4) hybrid dysgenesis results when the chromosome(s) from the male of one strain lack the information to become correctly organized in the nucleus of a second strain. The model seems to explain all aspects of the results, and offers the possibility that the present system may yield information on the genetics of membrane development and other aspects of spatial organization in the normal nucleus.
Correlation statistics can be used to measure the amount of linkage disequilibrium (LD) between two loci in subdivided populations. Within populations, the square of the correlation of gene frequencies, r2, is a convenient measure of LD. Between populations, the statistic rirj, for populations i and j, measures the relatedness of LD. Recurrence relationships for these two parameters are derived for the island model of population subdivision, under the assumptions of the linked identity-by-descent (LIBD) model in which correlation measures are equated to probability measures. The recurrence relationships closely predict the build-up of r2 and rirj following population subdivision in computer simulations. The LIBD model predicts that a steady state will be reached with r2 equal to 1/[1+4Nec(1+(k-1)rho)], where k is the number of island populations, Ne is the effective local population (island) size, and rho measures the ratio of migration (m) to recombination (c) and is equal to m/[c(k-1)+m]. For low values of m/c, rho=0, and E(r2) is equal to 1/(1+4Nec). For high values of m/c, rho=1, and E(r2) is equal to 1/(1+4kNec). The value of rirj following separation eventually settles down to a steady state whose expectation, E(rirj), is equal to E(r2) multiplied by rho. Equations predicting the change in rirj values are applied to the separation of African (Yoruba - YRI) and non-African (European - CEU) populations, using data from Hapmap. The primary data lead to an estimate of separation time of less than 1000 generations if there has been no migration, which is around one-third of minimum current estimates. Ancient rather than recent migration can explain the form of the data.
A systematic survey of the sterility of second-generation females produced by backcrossing hybrids of dysgenically interacting strains of D. melanogaster is reported. The effects of Fl rearing temperature and chromosomal constitution on this sterility were explicable in terms of the regulation of a transposable, inserted genetic element called the P factor which is known to be associated with hybrid dysgenesis. No applied radiation or chemical treatment induced dysgenic aberrations where they would not normally be found but, where they were found, EDT A slightly increased and ultraviolet radiation reduced their intensity. The results of previous temperature-shift experiments have implicated different periods during development as having the most effect on female'fertility. The present studies show little effect of interstrain variation on this critical period. Critical periods are identified for male sterility and recombination. Transmission ratio distortion has been shown to occur only in one of the reciprocal-cross hybrids (M x P) of parental strains causing dysgenesis. The degree of distortion in such crosses is shown here to vary between M strains crossed to the same P strain. Few data are available for the second (P x M) cross. The data reported here confirm that whatever distortion occurs in this cross is slight.
It has previously been shown that the ability to induce male recombination may be passed from one chromosome to another, not necessarily homologous, chromosome. This phenomenon is investigated, using strains known to interact in the P-M system of hybrid dysgenesis, and is shown to be analogous to the phenomenon of 'chromosomal contamination' in the I-R system of hybrid dysgenesis, in that the passage of properties from one chromosome to another is correlated with other manifestations of dysgenesis. Unlike the I-R system, contamination can occur in males as well as in females in the P-M system, a result which is consistent with other differences between the two systems. The contaminated chromosome acquires only a portion of the dysgenic properties of the original chromosome, and the inheritance is unstable over more than a few generations.
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