Using second- or third-chromosome substitution lines of Drosophila melanogaster, the genetic variation of inducibility and amylase specific activities in three media (starch, normal and glucose) were investigated. Genetic factors on both the second and third chromosomes were responsible for the variation in amylase specific activity and inducibility. In glucose medium, the genetic variance of amylase specific activity estimated for the second-chromosome substitution lines was larger than that for the third-chromosome substitution lines; however, for starch medium and inducibility, the variance was larger for the third-chromosome substitution lines. High correlations for the second-chromosome substitution lines and low correlations for the third-chromosome substitution lines were observed for amylase specific activities in different media. These results suggest that the genetic factor(s) responsible for inducibility or amylase activity variation in an induced medium such as starch should be on the third chromosome and those in the non-induced medium such as glucose should be on the second chromosome. The functional roles of the factors on the second and third chromosomes would be the repression and induction of amylase, respectively.

The historical population genetic processes associated with the divergence of members of the Drosophila virilis species group were examined using DNA sequence variation from two loci. New data on DNA sequence variation from the oskar locus, taken from within and among all five closely related taxa in the virilis phylad of the D. virilis species group, were examined and compared with similar data previously collected from the period locus. Overall, the oskar and period data sets reveal similar patterns of variation. Both loci support the conclusion that the two subspecies of D. americana have had a large historical population size and are exchanging genes in nature. From these data there is little reason to consider them as distinct taxa. In the case of D. novamexicana, from which six lines were sequenced at each locus, there is an intriguing difference in the pattern seen at the two loci. Both loci reveal two distinct groups that are considerably divergent from each other, with very little evidence of gene flow between them. However, the grouping of lines into distinct subgroups based on oskar is different from the grouping based on period. The simplest explanation seems to be that D. novamexicana includes two distinct species, and that the sample of six lines happens to include cases of recent gene exchange. Alternatively, both oskar and period could be linked to sites of strong balancing selection and limited recombination.

Many species of insects bear maternally inherited bacteria which contribute to the physiology and metabolism of their host whereas others bear inherited bacteria which are regarded as parasitic. Parasitic behaviours centre on disruption of the production, survival or fertility of male hosts, through which the microbe is not transmitted. One of the most common of these is early male-killing, where the bacteria kill male embryos they enter. The question arises as to why all inherited symbionts are not male-killers. The male-killing agents identified to date derive from two groups: the Proteobacteria and the Mollicutes. However, bacteria from other groups (Spirochaetes, Flavobacteria) are found to be inherited in insects, but contain no incidence of male-killing. We show here an association of male-killing with a bacterium from one of these groups. The bacterium causing male-killing behaviour in the ladybird Coleomegilla maculata is most closely related to Blattabacterium, a host-beneficial Flavobacteria found in cockroaches and some termites. This result indicates there is little if any taxonomic bar to the evolution of male-killing behaviour. This pattern severely contrasts with the evolution of two other parasitic symbiont behaviours, cytoplasmic incompatibility and the induction of host parthenogenesis, which have been found to be associated exclusively with bacteria from one group: Wolbachia. The result is briefly discussed in the light of the incidence of parasitic and beneficial cytoplasmic elements among insects.

I have studied the consequences of habitat patchiness on the persistence times of deleterious alleles in a random mating population. Results based on computer simulations and supported by analytical approximations suggest that deleterious alleles remain approximately 1/(1−2FST) more generations in the patchy than in a comparable homogeneous population, where 0<FST[les ]0·25 is the fraction of genetic variance due to the sample of families across patches in one generation. In natural populations of Drosophila, therefore, the contribution of deleterious mutants to the genetic variance in fitness might be larger than previously thought. A model of density-dependent viability selection, inspired by the suggestion that deleterious effects can substantially increase when the environment becomes harsher, also gives credence to the analytical results and illustrates that mean persistence times are very sensitive to changes in ecological parameters. If the density dependence model can be taken seriously, there is a clear difficulty in comparing observed and expected levels of genetic variance on the basis of the simplest mutation–selection balance model.

Non-recombining populations should suffer from four classic population genetic disadvantages: (1) they cannot reverse Muller's Ratchet, the accumulation of deleterious mutations caused by genetic drift and mutation; (2) whenever the fix a favourable mutation they lose all unlinked favourable variants; (3) they tend to lose favourable mutations that are linked to deleterious mutations; and (4) their genetic loads can be quite high when deleterious mutations have synergistic effects. It is commonly assumed that inter-chromosomal recombination (independent assortment) can counter these phenomena, but this has been studied only for the genetic load case. In contrast, many studies have shown that recombination via crossing over can counter these phenomena. Here we first show that segregation alone can strongly decelerate Muller's Ratchet in diploids, i.e. that recombination is not the only way to do so. We then show that inter-chromosomal recombination can indeed deal with phenomena (1) to (3) above very effectively if the genome consists of a moderate number of chromosomes. Therefore, if the above advantages of genetic recombination played a large role in the initial success of eukaryotic sex, the crucial moment in the origin of sex might have been the evolution of inter-chromosomal recombination, i.e. the evolution of genome segmentation, segregation, and syngamy. Crossing over might have become established as a major recombinational device only later, eliminating the disadvantages of extensively segmented genomes.

Resistance to toxicants is a convenient model for investigating whether adaptive changes are associated with pleiotropic fitness costs. Despite the voluminous literature devoted to this subject, intraspecific comparisons among toxicant resistance genes are rare. We report here results on the pleiotropic effect on adult survival of Culex pipiens mutants involved in the same adaptation: the resistance to organophosphorus insecticides. This field study was performed in southern France where four resistance genes sequentially appeared and increased in frequency in response to intense insecticide control. By repeated sampling of overwintering females through winter, we analysed the impact of each of three resistance genes on adult survival. We showed that (i) the most recent gene seems to be of no disadvantage during winter, (ii) the oldest affects survival in some environmental conditions, and (iii) the third induces a constant, severe and dominant survival cost. Such variability is discussed in relation to the physiological changes involved in resistance.

Individual records from the coding of molecular polymorphism (molecular profiles) are particularly useful for the identification of clones or cultivars, in pedigree analysis, in the estimation of genetic distances and relatedness, and as a tool in genome mapping and population genetics. A parametric statistical analysis of molecular profile components can be infeasible because of the huge number of observed markers, the presence of missing values and the high number of parameters required to evaluate the importance of interactions among markers. Moreover, new powerful molecular techniques make possible the analysis of numerous markers at one time; therefore parametric statistical methods could result in troublesome models with more parameters than data. The field of computer-based techniques offers new strategies to cope with the complexity of molecular profiles. We suggest the use of a Genetic Classifier System to evaluate the importance of profile components. The procedure is based on a Genetic Algorithm approach, a numerical technique that simulates some features of the natural selection process to solve problems. A set of isozyme data from a Norway spruce population is analysed in order to assess their ability to predict the individual plant response to the presence of abiotic stresses. The results, obtained by three different computer simulations, show that this computer-based approach is particularly effective for ranking profile components according to their relevance. Genetic Classifier Systems could also be used as a preliminary step to reduce the complexity of molecular data sets.

We studied the dominance of the effects of chromosomes carrying unselected mutations on five life-history traits in Drosophila melanogaster. Mutations were accumulated on the second chromosome for 44 generations in the absence of natural selection. Traits studied were female fecundity early and late in adult life, male mating ability, and male and female longevity. Homozygous effects were estimated for 50 mutant lines, and heterozygous effects were estimated by crossing these lines in a partial diallel scheme. Direct estimates of dominance showed that the effects of mutants are at least partially recessive. Heterozygotes had higher trait means than homozygotes in all five cases, and these differences were significant for late fecundity and female longevity. For all traits, genetic variance was larger among homozygous crosses than among heterozygous crosses. These results are consistent with those of many other studies that suggest that both unselected mutations and those found segregating in natural populations are partially recessive.

Evidence of a large sex-linked effect accounting for 25% of the divergence between mouse lines selected for body weight has been described previously. A marker-based study was undertaken to determine the number and map positions of the putative X-linked quantitative trait loci (QTLs). An F2 population was generated from a reciprocal F1 between an inbred low line derived from the low selection line and the high selection line. To enable inference of marker-associated QTL effects on the X chromosome, an analytical technique was developed based on the multiple regression method of Haley and Knott. The analysis of data on 10 week weight indicated a single QTL of large effect situated at about 23 cM from the proximal end of the chromosome, with a peak LOD score of 24·4. The likelihood curve showed a single well-defined peak, and gave a 95% confidence interval for the QTL location of 8 cM. The estimates for the additive genotypic effects in males and females (half the differences between hemizygous males and between homozygous females) were 2·6 g in both cases, or 17% and 20% of the 10 week body weight in males and females respectively. Dominance effects in the females were found to be non-significant. No significant X-linked effect on carcass fat percentage was detected, but a single X-linked QTL appears to explain almost the entire X-linked body weight effect.

Eight isofemale lines of Drosophila melanogaster were raised at four temperatures and at four yeast concentrations in their food. Temperature and food show a significant interaction in determining wing length and thorax length, affecting mean size per line and genetic variation between lines. The combination of low temperature and poor food conditions leads to a sharp increase in the genetic variation over lines of both body size characters. The increase in genetic variation in wing length under less favourable conditions is due to an increase in genetic variation of both cell size and cell number. Changes in wing area in response to both temperature and food level follow a common cell size/cell number trajectory. Changes in wing size are obtained by line-specific changes in the cellular composition of the wing, rather than by changes specific for the environmental factor.

Allozyme-associated heterosis has been repeatedly observed in marine bivalves, but its genetic origin remains debatable. A simple explanation is direct overdominance at the enzyme loci scored. An alternative is associative overdominance due to partial inbreeding, affecting the whole genome. The two hypotheses yield different predictions concerning (i) locus-specific effects, (ii) the relationship between heterozygosity and the variance in fitness, and (iii) the expected form of the relationship between the multilocus genotype and mean fitness. The relationship between heterozygosity and growth, a component of fitness, is here analysed in Spisula ovalis (1669 individuals, 9 loci), using statistical models designed to test these predictions. In contrast to most other bivalves, S. ovalis shells display clear annual growth lines allowing accurate quantification of individual age and growth. Our results show (i) that there is no evidence for locus-specific effects, (ii) that the variance in growth decreases significantly when heterozygosity increases, and (iii) that growth is better predicted by a genetic variable optimized for inbreeding than by a variable optimized for overdominance. In addition, the heterozygosity–growth relationship displays a significant variation among annual cohorts, being more pronounced in young cohorts. Although the need to pool alleles and the occurrence of null alleles may limit the efficiency of some of the models used (especially for result (iii)), our results suggest that the heterozygosity–growth relationship is due to inbreeding effects.

In a QTL mapping study with an F2 population of mice, we have shown that one or more sex-linked factors account for a large part of the divergence between mouse lines selected for high and low body weight. Here, we describe a study undertaken to map the putative X-linked quantitative trait loci (QTLs) by backcrossing segments of chromosome from the high line onto an inbred line derived from the low line, thereby removing possible contributions from the autosomes and linked segments of the X chromosome. Sublines containing a regional at the proximal end of the X chromosome were found to be associated with large differences in body weight, and to account for almost all the difference between the lines. A Markov chain Monte Carlo based multipoint linkage analysis incorporating the available marker and phenotypic information from the backcross pedigree was used to map the QTL to a region of about 6 cM. There was no evidence for QTLs elsewhere on the chromosome. The estimated QTL effect is approximately 20% of mean body weight in males and females at 10 weeks. From results obtained from this study and the accompanying F2 analysis, we conclude the presence of a single factor for body weight localizing to about position (±SE) 26·4±1·2 cM on the X chromosome, which increases body weight by approximately 18% at 10 weeks. A strategy to positionally clone the QTL is discussed.

Lines of mice have been divergently selected on one of two traits: (i) estimated fat content at 14 weeks of age, which has resulted in a 5-fold divergence, and (ii) body weight at 10 weeks of age, which has resulted in a 3-fold divergence. Individuals from each line were castrated or sham operated at 10 days of age and subsequently given either exogenous testosterone or the appropriate control from 14 days of age. Castration increased fat content and decreased lean weight in all lines, an effect which was not reversed by administration of testosterone. Body weight was reduced by around 10% as a result of castration and this effect was at least partially reversed by exogenous testosterone. Analysis of body weight, fat content and lean mass at 10 weeks of age failed to detect any interaction between these treatments and genetic background. It is therefore concluded that testosterone metabolism has not contributed disproportionately to the response to artificial selection in spite of its known effects on growth and body composition.

Mouse populations differing in metabolic rate have been developed through selection for high (MH) and low (ML) heat loss (HLOSS), along with randomly selected controls (MC). Objectives of this study were to (a) compare MH, ML and MC lines for HLOSS and correlated traits of feed intake, body composition and organ weights; (b) compare three widely used inbred mouse lines with MH, ML and MC for the same traits; and (c) investigate potential genotype by diet interaction resulting from feeding diets differing in fat percentage. Heat loss (kcal/day) of MH and ML mice differed by 37% of the mean and remained significant (33%) when HLOSS was expressed on a fat-free mass basis. MH mice consumed more energy than ML with a greater difference in mice fed high-fat compared with standard diets (27% vs 13·9%). Despite greater energy consumption, MH mice were leaner than ML with a difference in total body fat percentage of 40%. The greatest difference in HLOSS between selection and inbred lines was between MH and C57BL/6J (BL), which differed by 26·3%. MH and BL mice also differed in energy intake (15·5%). Body composition of BL mice was similar to MH when fed a standard diet, but similar to ML when fed a high-fat diet. Crosses between MH and ML or between MH and BL would be useful to investigate the genetic regulation of, and identify quantitative trait loci influencing HLOSS, energy intake and body composition. Feeding of a high-fat diet may allow diet-specific loci influencing body composition to be identified in MH and BL lines.

It is known that genetic polymorphisms can be maintained in populations without superiority of the heterozygote subject to constant but non-linear selection through periodic and higher-order behaviour. In this paper we explore evolutionary paths from single equilibria to higher-order attractors and the existence of polymorphisms that do not arise from equilibria. We explore whether there is a continuous range of allelic types that can create such polymorphisms. We use a single-locus genetic model with exponential density-dependent fitness functions and show that there are large parameter ranges in regions of both overdominance and partial dominance where polymorphic attractors exist.

A deterministic analysis is conducted to examine marginal dominance for two linked viability loci influencing inbreeding depression and its graphical inferences. Four estimators of marginal dominance are derived, assuming a biallelic marker locus completely linked to one of the viability loci, and the biases in expected estimates due to the other deleterious locus are discussed. Three conditions under which apparent partial dominance or underdominance could occur are found, i.e. when two multiplicative, partially recessive loci are linked in coupling phase and when two synergistic, highly overdominant loci are linked in coupling or repulsion phases. Expected frequencies of the three marker genotypes in selfed progeny are derived, considering two linkage phases, two types of marker locus position with respect to the viability loci, and the multiplicative and synergistic fitness models. Segregation ratios are generated for the marker locus linked to either two overdominant or partially recessive loci and plotted in gene action graphs to examine the robustness of the graphical inferences of gene action due to the presence of an additional linked viability locus. Under a multiplicative fitness model, the presence of an additional partially recessive or overdominant locus in the vicinity of the marker locus does not greatly affect the graphical inferences of the relative role of partially recessive or overdominant genes in expression of inbreeding depression. A marker linked to two synergistic, highly overdominant loci can behave as though linked to a partially recessive, partially dominant or underdominant locus, even with relatively weak synergism.

Linkage analysis and map construction using molecular markers is far more complicated in full-sib families of outbreeding plant species than in progenies derived from homozygous parents. Markers may vary in the number of segregating alleles. One or both parents may be heterozygous, markers may be dominant or codominant and usually the linkage phases of marker pairs are unknown. Because of these differences, marker pairs provide different amounts of information for the estimation of recombination frequencies and the linkage phases of the markers in the two parents, and usually these have to be estimated simultaneously. In this paper we present a complete overview of all possible configurations of marker pairs segregating in full-sib families. Maximum likelihood estimators for the recombination frequency and LOD score formulas are presented for all cases. Statistical properties of the estimators are studied analytically and by simulation. Specific problems of dominant markers, in particular with respect to the probability of detecting linkage, the probability of obtaining zero estimates, and the ability to distinguish linkage phase combinations, and consequences for mapping studies in outbred progenies are discussed.

We consider using microsatellites for paternity checking and parent identification in different population structures, and allowing for possible typing errors or mutations. Statistical rules derived from the Bayesian and the sampling approaches are discussed in the case involving the choice of the true father–mother pair among a finite set of possible parental pairs. General situations are investigated by means of random simulations, in order to characterize the joint influences of the number and polymorphism of typed loci, the population structure and size, and error rates. Approximate expressions are provided that give the efficiency of a set of markers for identifying the parents in various mating schemes. The importance of a non-zero value for the typing error rate in the likelihood is highlighted.

The variance effective size (Ne) was formulated for populations of monoecious plant species that are partly asexually propagating with discrete or overlapping generations. It was shown that partly asexually reproducing populations have larger or smaller effective sizes (ratios to the census size N) than fully sexually reproducing populations, according to whether the term Vc/c¯ is smaller or larger than the term (Vk/k¯ +1−β)/2, where c¯ and Vc are the mean and variance of the number of progeny asexually produced per plant per year, respectively, k¯ and Vk are the mean and variance of the number of gametes contributed per plant per year, respectively, and β is the selfing rate of each plant. Asexual reproduction has no effect on Ne when the two terms are equal, as is true when the numbers of both sexually and asexually produced progeny per plant per year are Poisson-distributed (Vc/c¯=1 and Vk/k¯=1+β). Populations with a larger generation length (L) tend to have a smaller effective size: for a population model of age-independent survival and fecundity with an annual rate δ of asexual reproduction, Ne declines asymptotically to N(2−β)/{3−β +Vk/k¯ +(2Vc/c¯ −Vk/k¯ −1+β).δ} as L gets large, which simplifies to N(2−β)/4 under a Poisson-distributed reproductive contribution. The trade-off relation of Ne and L, however, does not always hold: for stage-structured populations, increase in the survival rate of juveniles may act to increase both Ne and L.

Theoretical arguments are presented which suggest that each advance of Muller's ratchet in a haploid asexual population causes the fixation of a deleterious mutation at a single locus. A similar process operates in a diploid, fully asexual population under a wide range of parameter values, with respect to fixation within one of the two haploid genomes. Fixations of deleterious mutations in asexual species can thus be greatly accelerated in comparison with a freely recombining genome, if the ratchet is operating. In a diploid with segregation of a single chromosome, but no crossing over within the chromosome, the advance of the ratchet can be decoupled from fixation if mutations are sufficiently close to recessivity. A new analytical approximation for the rate of advance of the ratchet is proposed. Simulation results are presented that validate the assertions about fixation. The simulations show that none of the analytical approximations for the rate of advance of the ratchet are satisfactory when population size is large. The relevance of these results for evolutionary processes such as Y chromosome degeneration is discussed.