Geographic variation was studied at 15 electrophoretic loci (40 alleles) in Italian populations of Ornithogalum montanum Cyr. ex Ten. (Liliaceae). Homogeneity of allele frequencies was assessed by G tests; gene-frequency patterns were described by spatial autocorrelation statistics; matrices of genetic and environmental distance were compared through a series of Mantel's tests, and the zones of highest overall gene-frequency change per unit distance (steep multi-locus clines, or genetic boundaries) were identified. Nineteen allele frequencies appear heterogeneously distributed, but only 3 of them show significant spatial structure. Only 2 allele frequencies are correlated with 1 environmental parameter. Large genetic differences are observed between spatially close populations. These findings support a model of differentiation in which the genetic relationships between isolates do not depend on their spatial distances, but reflect mainly population subdivision and restricted gene flow.

Female interspecific vole hybrids were examined for the expression of the G6PD and GALA genes on the X chromosomes. When one of the parents was a species with a heterochromatin block on the X, and the other parent was M. arvalis, without a heterochromatin block on the X, preferential expression of the genes of the M. arvalis X was consistently observed. When both parental species had heterochromatin on the X, the parental forms of G6PD and GALA were in about equal proportions in the hybrid females. The results of the cytological identification of the active and inactive X on the metaphase spreads in the hybrid females are in agreement with the biochemical results. It is suggested that the observed phenomenon may be due to a nonrandom inactivation of the X chromosome containing a heterochromatin block in crosses involving M. arvalis and by a random inactivation in those with both parents having heterochromatin blocks on the X chromosomes. These results support our previous suggestion that heterochromatin has an effect on X inactivation in female interspecific vole hybrids.

An electrophoretic survey of concentrations of lysozymes M and P was carried out with seven species in the house mouse group (spretus, hortulanus, abbotti, musculus, castaneus, domesticus and molossinus). In most species M is the predominant lysozyme in all tissues tested, except the small intestine, where P predominates if present. In inbred strains of molossinus mice P is more abundant than M in all tissues tested. The phenotypes of high expression of P lysozyme and low expression of M lysozyme in peritoneal and alveolar macrophages were examined genetically. Results of interspecific crosses and backcrosses to domesticus mice support the model that the phenotypes are caused by mutation(s) tightly linked to the lysozyme locus. Alleles at the regulatory loci show additive inheritance.

Two new X-autosome Robertsonian (Rb) translocations, Rb(X.9)6H and Rb(X.12)7H, were found during the course of breeding the Rb(X.2)2Ad rearrangement at Harwell. The influence of these new Rbs on meiotic chromosome segregation was investigated in hemizygous males and heterozygous females and compared to that of Rb(X. 2)2Ad. Screening of metaphase II spermatocytes gave incidences of sex chromosome aneuploidy of 9·2% in Rb(X. 2)6H/Y and 9·6% in Rb(X.9)2Ad/Y males; no metaphase II cells were present in the testes of the Rb(X. 12)7H/Y males examined and no males with this karyotype have so far proved fertile. In breeding tests, 5% of the progeny of Rb(X.2)2Ad/Y males were sex chromosome aneuploids compared to 10% of the Rb(X. 9)6H/Y offspring. The difference was not significant, however. Cytogenetic analyses of metaphase II stage oocytes showed elevated rates of hyperhaploidy (n + l) in Rb heterozygous females over chromosomally normal mice: 4·2% for Rb(X.2)2Ad/+; 2·1% for Rb(X.9)6H/+; 2·2% for Rb(X.12)7H/+ and 1·1% for normal females. There was, however, no statistically significant difference in the rates of hyperhaploidy between the three different Rb types, nor overall between Rb/ + and normal females. Karyotypic analyses of liveborn offspring of Rb heterozygous females revealed low incidences of X0 animals but no other type of sex chromosome aneuploidy. Intercrosses of heterozygous females and hemizygous males yielded 5·5% aneuploidy for Rb(X.2)2Ad and 5·4% for Rb(X.9)6H. In heterozygous females, there was evidence from the metaphase II and breeding test data for all three rearrangements, of preferential segregation of the Rb metacentric to the polar body resulting in a deficiency of cells and progeny carrying a translocation chromosome.

The flux through the de novo fatty acid synthesis pathway was estimated in lines of mice which differed substantially in fat content following 26 generations of selection at 10 weeks of age. Previous estimates of lipogenic enzyme activities had indicated an increase in the capacity for lipogenesis in the Fat compared to the Lean line. Therefore the in vivo flux in lipogenesis was measured in both liver and gonadal fat pad (GFP) tissues of males at 5 and 10 weeks of age, using the rat of incorporation of 3H from 3H2O and 14C from acetate and citra te into total lipids. AT both ages and in both tissues the Fat line had a higher flux, about 20% increase in the liver and up to three-fold increase (range 1·2- to 3·4-fold) in the GFP. We conclude that direct selection for fatness in mice has resulted in metabolic changes in the ratio of de novo fatty acid synthesis, and that the changes are largely detectable before 10 weeks, the age of selection.

Fixation probabilities and mean times to fixation of new mutant alleles in an isogenic population having an effect on a quantitative trait under truncation selection were computed using stochastic simulation. A variety of population structures and breeding systems were studied in order to find an optimal design for maximizing the fixation probability for recessive genes without impairing that for non-recessives or delaying times to fixation. Circular mating or cycles with repeated generations of close inbreeding alternating with combination of the families proved to be very inefficient. The most successful scheme found, considering fixation probabilities and times to fixation jointly, was to practise individual selection and mate full sibs whenever possible, otherwise mate at random. The benefit was directly proportional to the number of full-sib matings performed, which, in turn, almost exclusively depended on the number of selected individuals with very little effect of selection intensity or magnitude of gene effects. Fixation rates could be well approximated by diffusion methods. When selection was practised in only one sex and, therefore, the proportion of full-sib matings could be varied from zero to one, maximizing the amount of full-sib mating was found to maximize fixation probability, at least for single mutants.

Genetic variance and fixation probabilities are evaluated for a model of a quantitative trait at a balance between mutation, selection and drift in which many alleles can segregate at each locus. If the distribution of effects of new mutant alleles is such that mutations are unconditionally deleterious, as might be the case in natural populations, genetic variance maintained is proportional to the number of mutations occurring in the genome per generation, but is independent of the number of loci at which they appear. If selectively advantageous alleles can occur these tend to interfere to a greater extent with each others' fixation and increasing mutation rate leads to a decrease in the genetic variance as a fraction of the variance maintained in the absence of selection. Fixation probabilities of new mutant alleles approach that for neutral alleles with increasing mutation rate at a locus irrespective of their effects on fitness. The additive genetic variance contributed by the locus may appear to be ‘decoupled’ from the fixation rate of mutant alleles.

An experimental method is proposed for detecting the effects of positive natural selection on DNA polymorphisms. Since beneficial mutations are expected to increase in frequency faster than neutral mutations, variants which have reached high frequencies in a relatively short period could be linked to some beneficial mutation. D. melanogaster has a cosmopolitan polymorphic inversion -In(2L)t - whose age in some local populations has been estimated. Setting the age of In(2L)t as the upper limit for the age of variants, we searched for variants whose frequencies were possibly influenced by positive natural selection. We detected a single candidate whose frequency and distribution met the requirements imposed by our method.

The DNA repair hypothesis for the maintenance of sex states that recombination is necessary for the repair of double-strand DNA damage. In a closed (mitotic) genetic system crossing-over generates homozygosity. This reduces fitness if deleterious recessive alleles become expressed. Thus, outcrossing is required to restore heterozygosity destroyed by recombination. The repair hypothesis is tested by comparing outcrossing sexuality with a hypothetical parthenogenic strategy (the Prudent Reparator) which destroys as little heterozygosity during repair as possible. In the Prudent Reparator, repair of double-strand DNA damage results in a small amount of homozygosity due to gene conversion only, since this process does not render outside markers homozygous. Diploidy, deleterious recessives, multiplicative fitness and linkage equilibrium in mutation-selection balance are assumed. The average fitness of this population increases, and complementation (i.e. masking of recessives in heterozygous form) decreases with the rate of damage per locus. The equilibrium fitness of the Prudent Reparator can be well above that of the sexual population. A lower complementation ability of parthenogens may not be an impenetrable barrier to their successful establishment if the invader's genome is relatively uncontaminated by mutant alleles: there are always such genotypes in the sexual population. Thus, the Prudent Reparator could solve the problem of repairing damage as well as that of invading an existing outcrossing population. As we do not see this strategy widely adopted instead of sexuality, the repair hypothesis is likely to miss some essential feature of the evolution of sex.

This paper describes the relationship between probabilities of identity by descent and the distribution of coalescence times. By using the relationship between coalescence times and identity probabilities, it is possible to extend existing results for inbreeding coefficients in regular systems of mating to find the distribution of coalescence times and the mean coalescence times. It is also possible to express Sewall Wright's FST as the ratio of average coalescence times of different pairs of genes. That simplifies the analysis of models of subdivided populations because the average coalescence time can be found by computing separately the time it takes for two genes to enter a single subpopulation and time it takes for two genes in the same subpopulation to coalesce. The first time depends only on the migration matrix and the second time depends only on the total number of individuals in the population. This approach is used to find FST in the finite island model and in one- and two-dimensional stepping-stone models. It is also used to find the rate of approach of FST to its equilibrium value. These results are discussed in terms of different measures of genetic distance. It is proposed that, for the purposes of describing the amount of gene flow among local populations, the effective migration rate between pairs of local populations, M^, which is the migration rate that would be estimated for those two populations if they were actually in an island model, provides a simple and useful measure of genetic similarity that can be defined for either allozyme or DNA sequence data.