Mutation tlx was induced in pea by gamma-irradiation in the homeotic gene Tl, responsible for transformation of the upper leaflets into tendrils. Heterozygotes tlx/Tl+ possess ‘flat tendrils’ having narrow rudimentary leaf blades. By 15 generations of selfing the line Delta was established, in which closely linked loci Tl and His1 were maintained heterozygous, His1 encoding the most abundant histone H1 subtype. The heterozygous chromosome region was flanked by several recessive markers. The genotype of line Delta with respect to loci His1 and Tl was: tlxHis12/Tl+His11. There were derived two sublines, Delta-s and Delta-f, homozygous for each of the His1 alleles but heterozygous for the Tl and flanking markers. The three lines were compared for a number of quantitative traits, including parameters of flat tendrils reflecting the expression of the gene Tl in heterozygote, and for recombination rate within the heterozygous segment. The line Delta-s exceeded Delta-f in terms of the level of the first inflorescence, length of the internode 8–9, number of pods on the main stem and mean seed mass, but was inferior regarding the mean number of seeds per pod. The flat tendrils were wider in the Delta-s as compared with the Delta-f line ; the difference was highly significant and exceeded 15% at node 17. Recombination rate between the flanking markers also showed significant differences, being highest in the Delta-f and lowest in the Delta line. We do not exclude the possibility that the observed differences in the quantitative traits are caused by the substitution of alleles of the histone H1 gene. The effects of such substitution appeared to be especially strong for expression of the homeotic gene Tl.

Allozyme markers were used to estimate mating system parameters in nine fragmented populations of the grassland daisy Rutidosis leptorrhynchoides that differed in size and spatial isolation. Multilocus estimates of outcrossing rate did not differ significantly among populations, all indicating a high level of outcrossing (tm=0·84–1·0). Small populations showed greater divergence than large populations between the allele frequencies in the population and those in the pollen pool, indicating paternal bottlenecks. Isolated populations of fewer than 200 individuals also exhibited higher correlations of outcrossed paternity (rp) than larger populations, indicating the production of more full-sibs within families. The combination of paternal bottlenecks and correlated paternity increases the genetic identity of progeny across families and predisposes populations to biparental inbreeding in subsequent generations. As over half the remaining populations of R. leptorrhynchoides contain fewer than 200 plants, such second-order inbreeding may threaten the viability of the species if it is associated with significant inbreeding depression.

Theories on the evolution of recombination in regard to its ability to increase mean fitness require a consistent source of negative linkage disequilibrium among loci affecting fitness to show an advantage to recombination. Here we present evidence that, at least theoretically, genetic variation for recombination can spread in very large populations under a strictly multiplicative-fitness, deleterious-allele model. The model uses only Mendelian genetics in a multi-locus context to show that a dominant gene for recombination can spread when rare and resist invasion when common. In non-equilibrium populations driven by Muller's ratchet, the gene increases its probability of fixation by increasing the probability of being associated with the best individuals. This occurs at an optimal level of recombination. Its action results in both an immediate and a long-term advantage to recombination amongst the proto-meiotic organisms modelled. The genetic mechanism lends itself naturally to a model for the evolution of meiosis, where modern-day gametes are seen as derivative of ancient unicellular organisms.

Previous work has shown that genetic diversity at a neutral locus is affected by background selection due to recurrent deleterious mutations as though the effective population size Ne is reduced by a factor that is calculable from genetic parameters such as mutation rates, selection coefficients, and the rates of recombination between sites subject to selection and the neutral locus. Given that silent changes at third coding positions are often subject to weak selection pressures, it is important to develop similar quantitative predictions of the effects of background selection on variation and evolution at weakly selected sites. A diffusion approximation is derived that describes the effects of the presence of a single locus subject to mutation and strongly deleterious selection on variation and evolution at a partially linked, weakly selected locus. The results are validated by computer simulations using the Ito pseudo-sampling method. We show that both nucleotide site diversity and rates of molecular evolution at a weakly selected locus are affected by background selection as though Ne is reduced in the same way as for a neutral locus. Heuristic arguments are presented as to why the change in Ne for the neutral case also applies with weak selection. As in the case of a neutral locus, the number of segregating sites in the population is poorly predicted from the change in Ne. The potential significance of the results in relation to the effects of recombinational environment on molecular variation and evolution is discussed.

Characterizing deleterious genomic mutations is important. Most of the few current estimates come from the mutation–accumulation (M-A) approach, which has been extremely time- and labour-consuming. There is a resurgent interest in implementing this approach. However, its estimation properties under different experimental designs are poorly understood. By simulations we investigate these issues in detail. We found that many of the previous M-A experiments could have been more efficiently implemented with much less time and expense while still achieving the same estimation accuracy. If more than 100 lines are employed in M-A and if each line is replicated at least 10 times during each assay, an experiment of 10 M-A generations with two assays (at the beginning and at the end of M-A) may achieve at least the same estimation quality as a typical M-A experiment. The number of replicates per M-A line necessary for each assay largely depends on the magnitude of environmental variance. While 10 replicates are reasonable for assaying most fitness traits, many more are needed for viability, which has an exceptionally large environmental variance. The investigation is mainly carried out using Bateman–Mukai's method of moments for estimation. Estimation using Keightley's maximum likelihood is also investigated and discussed. These results should not only be useful for planning efficient M-A experiments, but also may help empiricists in deciding to adopt the M-A approach with manageable labour, time and resources.

An increasing amount of evidence indicates that different forms of environmental stress influence the expression of genetic variance in quantitative traits and, consequently, their evolvability. We investigated the causal components of phenotypic variance and natural selection on the body condition index (a trait often related to fitness in wild bird populations) of blue tit (Parus caeruleus) nestlings under contrasting environmental conditions. In three different study years, nestlings grown under a poor feeding regime attained lower body condition than their full-sibs grown under a good feeding regime. Genetic influences on condition were large and significant in both feeding regimes, and in all three study years. However, although estimates of additive genetic variance were consistently higher in the poor than in the good environment, heritability estimates for body condition index were very similar in both environments due to higher levels of environmental variance in the poor environment. Evidence for weak genotype×environment interactions was obtained, but these contributed little to variance in nestling condition. Directional natural selection on fledging condition of nestlings was detected, and there were no indications of year or environmental effects on the form and intensity of selection observed, in a sample of 3659 nestlings over four years. However, selection on fledging condition was very weak (standardized selection gradient, β=0·027±0·016 SE), suggesting that, in the current population, the large additive genetic component to fledging condition is not particularly surprising. The results of these analyses are contrasted with those obtained for other populations and species with similar life-histories.

A simple mathematical model of genic directional selection is developed to study frequency changes of genetic marker alleles that are partially linked to a quantitative trait locus (QTL) under artificial selection. The effects of population size, number of generations of artificial selection, recombination between marker locus and QTL, and the strength of selection on the change in allele frequency are analysed by the diffusion equation approach and by stimulation. Using these results, we investigate the power of statistical tests for the detection of QTLs based on the observation of significant marker allele frequency changes in selection experiments. The probability of inferring the correct location of a QTL is also obtained.