This paper reports the first direct molecular evidence that X chromosome loss during determination of male aphids (XO) is random. Clonal and sexual females, and males, of the species Sitobion near fragariae were screened using three polymorphic microsatellite loci. Two loci, Sm10 and Sm17, showed the same heterozygous genotypes in all three aphid morphs. The third, Sm11, was heterozygous for the same two alleles in clonal and sexual females, but of the 25 males screened 11 showed the ‘160’ allele and 14 showed the ‘156’ allele. This result indicates X-linkage of locus Sm11, with random loss of the X chromosome during the formation of male embryos. The possible implications of this result are discussed with respect to aphid sex determination, recombination and chromosome evolution.

Genetic instability is associated with aging in many species. One of the initiating factors for genetic instability is the movement of transposable elements (TEs), which occur in all prokaryotic and eukaryotic organisms. The hypothesis that TEs could be involved in the aging process is discussed and the correlation between aging and activity of TEs is analysed in a variety of biological systems.

Until now, with regard to the hobo system of hybrid dysgenesis, natural populations of Drosophila melanogaster have been investigated using only two criteria: at the molecular level, the presence or absence of XhoI fragments 2·6 kb long or smaller; and/or at the genetic level, the ability to induce gonadal dysgenesis sterility in crosses A (females of an E reference strain crossed with males under test) and A* (females under test crossed with males of an H reference strain). Recently, analyses of laboratory strains using these criteria as well as the mobilization of two reporter genes, the male recombination and the number of ‘TPE’ repeats in the S region, revealed a lack of correlation between the different dysgenic parameters themselves, and also between these parameters and the molecular characteristics of the strains. Thirteen current strains derived from world populations were therefore investigated with regard to all these dysgenic traits, to determine discriminating criteria providing a robust method of classifying natural populations and deducing the dynamics of hobo elements in these populations. We show, as in laboratory strains, a lack of correlation between the parameters studied. Therefore, the significance of each of them as well as the nature of hobo hybrid dysgenesis are discussed, to propose an analysis method of the hobo system applicable to natural populations. According to the geographical distribution of hobo activities in world populations and to the variable polymorphism of the number of ‘TPE’ repeats, we propose a new scenario for the invasion of D. melanogaster by hobo elements.

To assess the role of genetic changes in sensitivity to leptin hormone in contributing to responses to long-term selection for fatness, leptin was administered to a long-term fat selected (F) and a control line (C) of mice. These lines differ almost three fold in their percentage of fat (fat%) at about 15 weeks of age. Treated (T) animals received twice-daily intraperitoneal injections of 5 mg/kg leptin from 91 to 105 days of age; untreated (U) animals received equivolume injections of phosphate-buffered saline. Treated compared with untreated animals in both lines had significantly (P<0·05) lower mean body weight, food intake and fatness at the end of test (fat%: CT 3%, CU 7·4%, FT 14·9%, FU21·1%). The differences in response between the lines [(CT−CU)−(FT−FU)] were all non-significant (P>0·05), however. There was a very wide range of fatness (estimated from dry matter content) among FT animals (3–29%), much higher than in FU (15–31%), CT (0·7–6·4%) and CU (2–15%) animals. While sensitivity to leptin remains in the fat line, response appears to vary among animals at the dose level used.

A new hereditary duplication located at the hetechromatin region of the 14th chromosome is described by CBG, CDD and RE banding in a family of Charolais cattle.

RNA viruses consist of populations of extremely high genetic heterogeneity called quasispecies. Based on theoretical considerations, it has been suggested that the unit of selection in such complex genetic populations is not the single viral particle but a set of genetically related particles which form the quasispecies. In the present study we carried out a set of experiments with the vesicular stomatitis virus (VSV) dealing with the evolution of life-history characters under selection acting at two factors either in the same or in opposite directions. The two factors at which selective pressure is applied are the individual and the group. We show evidence that group selection modulates the virulence of VSV populations, in opposition to an unlimited increase in virulence by competitive optimization promoted by individual selection. The results are of relevance for understanding the evolution of parasite virulence.

Gene frequency changes following selection may reveal the existence of gene effects on the trait selected. Loci for the selected quantitative trait (SQTL) may thus be detected. Additionally, one can estimate the average effect (α) of a marker allele associated with an SQTL from the allele frequency change (Δq) due to selection of given intensity (i). In a sample of unrelated individuals, it is optimal to select the upper and lower 27% for generating Δq in order to estimate α. For a given number of individuals genotyped, this estimator is 0·25i2 times more efficient than the classical estimator of α, based on the regression of the trait on the genotype at the marker locus. The method is extended to selection criteria using information from relatives, showing that combined selection considerably increases the efficiency of estimation for traits of low heritability. The method has been applied to the detection of SQTL in a selection experiment in which the trait selected was pig litter size averaged over the first four parities, with i=3. Results for four genes are provided, one of which yielded a highly significant effect. The conditions required for valid application of the method are discussed, including selection experiments over several generations. Additional advantages of the method can be anticipated from determining gene frequencies on pooled samples of blood or DNA.

Genetic diversity of natural populations of the two species Naegleria gruberi and N. australiensis collected at the same time in the same area was studied using multilocus enzyme electrophoresis (MLEE analysis). Reference strains and related species were also analysed for comparison. Five loci were found to be polymorphic and allowed identification of 19 different multilocus genotypes among 67 isolates – eight for N. gruberi and 11 for N. australiensis – none being common to the two species. Large departures from Hardy–Weinberg equilibrium were found at all the loci, with significant heterozygote deficiencies in most cases. The two species exhibited strong linkage disequilibrium and the predominance of a few multilocus genotypes within each species was observed. These lines of evidence strongly suggest non-random association of alleles, leading to the conclusion that N. gruberi and N. australiensis have predominantly clonal genetic structures in the wild. Populations consist of a few major clones with some closely related ones – a structure found for several parasitic protozoans and bacteria. This contrasts with the panmictic structure found for N. lovaniensis, another species of this genus, and suggests that closely related species within the genus have evolved to have different reproductive strategies.

Although cryopreservation is routinely used for the storage of a range of biological organisms, few studies have been conducted to determine whether cryopreservation increases the frequency of mutation. A procedure for the cryopreservation of Drosophila melanogaster embryos has recently been developed. Cryopreservation of D. melanogaster is of special interest to geneticists and evolutionary biologists because it would make it possible to assay control and experimental populations simultaneously during long-term studies. Before cryopreserved embryos can be used for such studies, it is first necessary to show that cryopreservation is not mutagenic. We tested for mutagenic effects of cryopreservation in D. melanogaster embryos with an X-linked, recessive lethal assay. The mutation rates of cryopreserved and control flies were not significantly different. We can be 95% certain that cryopreservation does not increase mutation by a factor greater than 2·39. This is the first quantitative estimate of the mutagenic effect of cryopreservation on the germ line of a metazoan. The results are reassuring when considering the genetic impact of cryopreservation on mammalian gametes and embryos.

Since the last glacial period forest trees have expanded to their present range very rapidly, with rates up to 500 m yr−1 for oaks in Europe, which can be explained only by the dispersion of acorns over long distances. We used a stratified dispersal model, including both diffusive and long-distance dispersal of seeds, to simulate the colonization of a 100 km×300 km grid by populations of oak trees. An appropriate rate of spread is obtained with rare dispersal at distances of the order of tens of kilometres. We simulated the effect of stratified versus diffusive dispersal of seeds on the spatial genetic structure at a maternally inherited locus. Founding events associated with stratified dispersal generate a high amount of genetic differentiation among populations, which is likely to persist for a long time after colonization. Using autocorrelation methods, we show that diffusive and stratified dispersals create quite different spatial patterns of variation for the maternally inherited locus. Stratified dispersal creates patchy patterns that are concordant with a previous experimental investigation of chloroplast DNA variation at a regional scale in the oaks Quercus petraea and Quercus robur. For plant populations that have passed through recent episodes of range expansion, long-distance dispersal events are probably the most important factors of spatial genetic structuring of maternally inherited genes at small or medium geographic scales.

In vitro studies using highly purified calf thymus RNA polymerase II and a fragment spanning the first intron of H3.3 as template DNA have demonstrated the existence of a strong transcription termination site consisting of thymidine stretches. In this study, nuclear run-on experiments have been performed to assess the extent to which transcription elongation is blocked in vivo using DNA probes corresponding to regions 5′ and 3′ of the in vitro termination sites. These studies suggest that H3.3 expression is stimulated following the inhibition of DNA synthesis through the elimination of the transcription elongation block. Interestingly, both the in vivo and in vitro experiments have revealed that the transcriptional block/termination sites are positioned immediately downstream of a 73 bp region that has been over 90% conserved between the chicken and human H3.3 genes. The extreme conservation of this intronic region suggests a possible role in maintaining cis-acting function. Electrophoretic mobility shift experiments show that HeLa cell nuclear extracts contain protein factors that bind specifically to the region of transcription elongation block. Furthermore, we demonstrate a correlation between the protein binding activity and the transcriptional block in cells that have been either arrested at the initiation of S phase or were replication-interrupted by hydroxyurea. DNA footprinting experiments indicate that the region of protein binding is at the 3′ end of the conserved region and overlaps with one of the three in-vitro-mapped termination sites.

The theory developed by Takahata and colleagues for estimating the effective population size of ancestral species using homologous sequences from closely related extant species was extended to take account of variation of evolutionary rates among loci. Nuclear sequence data related to the evolution of modern humans were reanalysed and computer simulations were performed to examine the effect of rate variation on estimation of ancestral population sizes. It is found that the among-locus rate variation does not have a significant effect on estimation of the current population size when sequences from multiple loci are sampled from the same species, but does have a significant effect on estimation of the ancestral population size using sequences from different species. The effects of ancestral population size, species divergence time and among-locus rate variation are found to be highly correlated, and to achieve reliable estimates of the ancestral population size, effects of the other two factors should be estimated independently.

Homozygous or hemizygous expression of an X-linked wing mutant of Drosophila robusta varies from a rudimentary wing that does not reach the tip of the abdomen (called ‘club’) to forms with full-sized but curled or crumpled wings (called ‘curly’). Homozygous club females crossed to flies from natural populations or laboratory stocks derived from wild flies invariably produce significantly less club male progeny than the 100% expected, most of them exhibiting less severe phenotypes: ‘curly’ forms and wild-type. The male progeny from similar crosses using curly females tend to be predominantly normal. By contrast, the male progeny of outcrossed females homozygous for an X-linked eye colour mutant, vermilion, are all vermilion. The data indicate that natural populations of D. robusta contain suppressors of the wing mutant but not of the eye colour mutant studied. Activity of the suppressors differs by gender: in experiments in which genetic theory expects similar results in the two sexes, males consistently show stronger effects of the suppressors than females.

Based on a deterministic mutation–selection model the concept of error thresholds is critically examined. It has often been argued that genetic information – for instance, an advantageous allele – can be selectively maintained in a population only if the mutation rate is below a certain limit, the error threshold, which is inversely related to the genome size. Here, I will show that such an inverse relationship strongly depends on the fitness model. To produce the error threshold, as given by Eigen (1971), requires that the fitness model is an extreme form of diminishing epistasis. The error threshold, in a strict sense, vanishes as epistasis changes from diminishing to synergistic. In the latter case even the usual definition of error thresholds becomes ambiguous. Initially, a finite sites model has been used to describe error thresholds. However, they can also be defined within the framework of the infinite sites model. I study both models in parallel and compare their properties as far as error thresholds are concerned. It is concluded that error thresholds possibly play a much less important role in molecular evolution than has often been assumed in the past.

A method for finding optimum breeding schemes which maximize genetic gain under index selection with constraints on the rate of inbreeding is derived. The selection index includes information on the candidate and its sibs. Optimization is for the numbers of males and females to be selected and for the index weights when fixed numbers of offspring per generation, heritabilities and time horizons are considered. The expected rate of gain after a number of generations of selection is combined with the expected asymptotic rate of inbreeding (ΔF) in a single objective function which is maximized for finding the optimum solutions. Under restricted inbreeding, optimum designs are very similar for maximizing gains at different time horizons. The optimum number of selected males (for giving maximum gains) increases with the size of the scheme and with the severity in restricting ΔF and decreases with the heritability. Low heritability, less severe restrictions on ΔF and large schemes lead to increases in the relative weights given to performance of relatives in the index. The presence of common environmental effects leads to increases in optimum mating ratio when the heritability is low, to increases in the number of selected males and to more intense selection within families. Gains from index selection are compared with gains from mass selection. Under restricted inbreeding the advantage of optimized index selection over mass selection is only notable when the heritability is low and the scheme is large (in which case indices put more emphasis on family information than mass selection) and when the heritability is high and the scheme is small (in which case indices put less emphasis on family information).

In crosses between inbred lines linear regression can be used to estimate marker effects; these marker effects then allow marker-assisted selection (MAS) for quantitative traits. Weighting of marker and phenotypic information in MAS requires estimation of genetic variance associated with the markers: the usual estimators are biased, resulting in too much weight being placed on marker information relative to phenotypic information. In this paper we develop a cross-validation method to remove this bias, and show by simulation that response to selection using this method is almost as high as that achieved using optimal weighting of marker and phenotypic information.