The mating system of a natural population of Bulinus globosus from the Chiweshe area, Zimbabwe, was studied with mother—offspring data using isozyme genetic markers. The study was done in response to work on the genetic structure of this population which suggested a limited extent of cross-fertilization. Of the 24 adults whose progenies were analysed, at least 15 showed evidence of outcrossing and 9 had results consistent with selfing. These results show that the two modes of reproduction are important under natural conditions and the mating system of this population is considered to be ‘partially-selfing’.

In Drosophila melanogaster and closely related species, polymorphism has been shown to be reduced at loci located in regions of low recombination on the X chromosome and on the fourth chromosome, which does not normally recombine. Thispositive correlation between nucleotide polymorphism level and recombination rate is not predicted by standard neutral theory and therefore must result from natural selection and genetic hitchhiking along the chromosomes. We report here the near-complete absence of variation at concertina (cta), a locus located in the β-heterochromatic base ofchromosome 2L, a region of strongly reduced recombination. A 1.2 kilobase region containing coding regions and introns was sequenced from each of nine lines of D. melanogaster and nine lines of D. simulans representingworldwide collections. Variation is significantly reduced in cta in both species compared with other available loci on the same chromosome. Two analyses of background selection demonstrate that the reduction in variation at cta, considered in combination with other loci on chromosome 2L or alone, is consistent with the background selection model.

This study extends previous observations that chromosome loss in somaticcells of juveniles of the pacific oyster Crassostrea gigas is associated with reduced growth rate. All four studies designed to examine this association (two usingrandom population samples and two using full sibs) produced the same result. This consistent effect appears to be unrelated with the commonly, but not consistently, observed correlation between degree of allozyme heterozygosity and growth. We propose thatthe inverse relationship between aneuploidy and growth is due to the unmasking of deleterious recessive genes caused by ‘progressive haploidization’ of somatic cells. Because unmasking of deleterious recessive genes by random chromosome lossisunlikely in polyploid cells, this hypothesis may also provide an explanation for theobservation that artificially produced polyploid shellfish usually grow at faster rates than normal diploid ones.

We identify a fixed species difference in the relative concentrations of the cuticular hydrocarbons 2-methyl hexacosane and 5,9-pentacosadiene in Drosophilapseudoobscura and D. persimilis, and determine its genetic basis. In backcross males, this difference is due to genes on both the X and second chromosomes, while the other two major chromosomes have no effect. In backcross females, only the second chromosome has a significant effect on hydrocarbon phenotype, but dominant genes on the X chromosome could also be involved. These results differ in two respects from previous studies of Drosophila cuticular hydrocarbons: strong epistasis is observed between the chromosomes that producethe hydrocarbon difference in males, and the difference is apparently unrelated to the strong sexual isolation observed between these species.

An HSR in chromosome 1 which is found in many feral populations of Mus musculus domesticus was shown in previous studies to consist of a high-copy long-range repeat cluster. One such cluster, MUT, showed distorted transmission ratios when introduced by female parents. MUT/ + offspring were preferentially recovered at the expense of + / + embryos in the progeny of ♀ MUT/+ x♂ +/+ but were found at the expected 1:1 ratio in reciprocal crosses. Preferential recovery of maternal MUT was due to lethality of postimplantation + /+ embryos. There was no distortion of the recovery ratio in MUT/+ x MUT/MUT progeny: maternal MUT and + clusters were present among live implants at a 1:1 ratio. Maternal and zygotic effects therefore contribute to the phenomenon. The mechanism of their interaction is unknown.

Theoretical models of the effects of selection against deleterious mutations on variation at linked neutral sites (background selection) are used to predict the relations between chromosomal location and genetic variability at the DNA level, in Drosophila melanogaster. The sensitivity of the predictions to variation in the mutation, selection and recombination parameters on which they are based is examined. It is shown that many features of the observed relations between chromosomal location and level of genetic diversity in D. melanogaster can be explained by background selection, especially if the weak selective forces acting on transposable elements are taken into account. In particular, the gradient in diversity in the distal portion of the X chromosome, and the lack of diversity on chromosome 4 and at the bases of the major chromosomes, can be fully accounted for. There are, however, discrepancies between predicted and observed values for some loci in D. melanogaster, which may reflect the effects of forces other than background selection.

Despite the increasing number of studies on the magnitude of Ne/N ratios, much remains unknown about the effects of demographic and environmental variables on Ne/N. We determined Ne/N for seven population size treatments, ranging from N = 2 to N = 960, in the red flour beetle Tribolium castaneum. Ne/N decreased with increasing N, as evidenced by a significant negative relationship between log N and Ne/N. Our results are consistent with other published data on the relationship between Ne/N and N. Effective population sizes in large populations may be much smaller than previously recognized. These results have important implications for conservation and evolutionary biology.

We have studied variability maintained in a quantitative trait by the balance between symmetric mutation and direct stabilizing selection with a fluctuating optimum. Using a simulational computer model, we have found that wide fluctuations, such that the range of the optimum changes exceeds the width of the fitness curve, increase the trait variance, often by two or three orders of magnitude, over its value under constant selection. This happens because such fluctuations cause frequent allele substitutions at the loci that control the trait. At any particular moment the variance is increased mostly due to one or several loci where more than one allele is currently common. The data on fluctuating selection in nature are reviewed

Both stabilizing and directional selection acting on one or many quantitative traits usually reduce the genetic variance in a polymorphic population. Amphimixis and recombination restore the variance, pushing it closer to its value under linkage equilibrium. They thus increase the response of the population to fluctuating selection and decrease the genetic load when the mean phenotype is far from optimum. Amphimixis can have a short-term advantage over apomixis if selection fluctuates frequently and widely, so that every genotype often has a low fitness. Such selection causes high genetic variance due to frequent allele substitutions, and a high load even with amphimixis. Recombination in an amphimictic population is maintained only if selection is usually strong and effectively directional. A modifier allele causing free recombination can have a significant advantage only if fluctuations of selection are such that the load is substantial. With smaller fluctuations, an intermediate recombination rate can be established, either due to fixation of alleles that cause such a rate or due to the stable coexistence of alleles causing high and low recombination. If many traits simultaneously are under fluctuating selection, amphimixis and recombination can be maintained when selection associated with individual traits is weaker and the changes in their mean values are smaller than with a single trait. Still, the range of changes of the fitness optima in each trait must be at least of the order of the trait's standard deviation, and the total load must be high.

In plant species, typical gene mapping strategies use populations initiated from crosses between two inbred lines. However, schemes including more than two parents could be used. In this paper, a new approach is introduced which uses a four-way cross population derived from four inbred lines. The four-way cross design for mapping quantitative trait loci (QTLs) provides tests for QTL segregation in four lines simultaneously in one experiment. Therefore, it is a more economical strategy than oneusing line crosses between only two lines. The new strategy also increases the probability of detecting QTLs if they segregate in one line cross but not in the other. A multiple linear regression analysis is used for QTL detection. It is proven that the expected residual variance from the regression analysis differs from the pure environmental variance. Correction for the bias is proposed and verified by computer simulations.