Long-term selection for a quantitative character in large replicate populations of Drosophila melanogaster : II. Lethals and visible mutants with large effects

POLYGENIC MUTATION IN DROSOPHILA MELANOGASTER: ESTIMATES FROM DIVERGENCE AMONG INBRED STRAINS

A highly inbred line of Drosophila melanogaster was subdivided into 25 replicate sublines, which were independently maintained for 100 generations with 10 pairs of unselected flies per generation. The polygenic mutation rate (V_M ) for two quantitative traits, abdominal and sternopleural bristle number, was estimated from divergence among sublines at 10 generation intervals from generations 30-100, and from response of each line to divergent selection after more than 65 generations of mutation accumulation. Estimates of V_M averaged over males and females both from divergence among lines and from response to selection within lines were 3.3 × 10^-3 V_E for abdominal bristles and 1.5 × 10^-3 V_E for sternopleural bristles, where V_E is the environmental variance. The actual rate of production of mutations affecting these traits may be considerably higher if the traits are under stabilizing selection, and if mutations affecting bristle number have deleterious effects on fitness. There was a substantial component of variance for sex × mutant effect interaction and the sublines evolved highly significant mutational variation in sex dimorphism of abdominal bristle number. Pleiotropic effects on sex dimorphism may be a general property of mutations at loci determining bristle number.

THE GENETICS OF ADAPTATION: THE GENETIC BASIS OF RESISTANCE TO WASP PARASITISM IN DROSOPHILA MELANOGASTER

There have been very few genetic analyses of "natural" adaptations, that is, those not involving artificial selection or responses to human disturbance. Here we analyze the genetic basis of geographic variation in Drosophila melanogaster's resistance to parasitism by a wasp, Asobara tabida. Our results suggest that population differences in ability to encapsulate parasitoid eggs have a fairly simple genetic basis: 60% of the D. melanogaster genome plays no role in differences between resistant and susceptible populations. Instead, resistance gene(s) are restricted to chromosome two, and may be further restricted to the centromeric region of this chromosome. This finding suggests that natural adaptations-like many responses to artificial selection and human disturbance-sometimes have a simple genetic basis.

Obstruction of adaptation in diploids by recessive, strongly deleterious alleles

Recessive deleterious mutations are common, causing many genetic disorders in humans and producing inbreeding depression in the majority of sexually reproducing diploids. The abundance of recessive deleterious mutations in natural populations suggests they are likely to be present on a chromosome when a new adaptive mutation occurs, yet the dynamics of recessive deleterious hitchhikers and their impact on adaptation remains poorly understood. Here we model how a recessive deleterious mutation impacts the fate of a genetically linked dominant beneficial mutation. The frequency trajectory of the adaptive mutation in this case is dramatically altered and results in what we have termed a "staggered sweep." It is named for its three-phased trajectory: (i) Initially, the two linked mutations have a selective advantage while rare and will increase in frequency together, then (ii), at higher frequencies, the recessive hitchhiker is exposed to selection and can cause a balanced state via heterozygote advantage (the staggered phase), and (iii) finally, if recombination unlinks the two mutations, then the beneficial mutation can complete the sweep to fixation. Using both analytics and simulations, we show that strongly deleterious recessive mutations can substantially decrease the probability of fixation for nearby beneficial mutations, thus creating zones in the genome where adaptation is suppressed. These mutations can also significantly prolong the number of generations a beneficial mutation takes to sweep to fixation, and cause the genomic signature of selection to resemble that of soft or partial sweeps. We show that recessive deleterious variation could impact adaptation in humans and Drosophila.

Polymorphic genes of major effect: consequences for variation, selection and evolution in Arabidopsis thaliana

The importance of genes of major effect for evolutionary trajectories within and among natural populations has long been the subject of intense debate. For example, if allelic variation at a major-effect locus fundamentally alters the structure of quantitative trait variation, then fixation of a single locus can have rapid and profound effects on the rate or direction of subsequent evolutionary change. Using an Arabidopsis thaliana RIL mapping population, we compare G-matrix structure between lines possessing different alleles at ERECTA, a locus known to affect ecologically relevant variation in plant architecture. We find that the allele present at ERECTA significantly alters G-matrix structure-in particular the genetic correlations between branch number and flowering time traits-and may also modulate the strength of natural selection on these traits. Despite these differences, however, when we extend our analysis to determine how evolution might differ depending on the ERECTA allele, we find that predicted responses to selection are similar. To compare responses to selection between allele classes, we developed a resampling strategy that incorporates uncertainty in estimates of selection that can also be used for statistical comparisons of G matrices.

Response to selection from new mutation and effective size of partially inbred populations. I. Theoretical results

The effects of partial inbreeding on effective population size and rates of fixation of mutant genes are investigated in selected populations. Truncation selection and an infinitesimal model of gene effects for the selected trait are assumed. Predictions of effective size under this model are given for partial selfing and partial full-sib mating and an extension to a more general model is outlined. The joint effect of selection and partial inbreeding causes a large reduction in the effective size relative to the case of random mating. This effect is especially remarkable for small amounts of selected genetic variation. For example, for initial heritability 0·1 and proportion selected 1/6, the ratio of effective size to population size is 0·10 in populations with about 90% selfing while it is 0·85 in random mating populations. The consequence is a reduction in the fixation probability of favourable genes and, therefore, a reduction in the final response to selection. Stochastic simulations are used to investigate the effects of partial inbreeding and selection on fixation and extinction rates of genes of large effect and of recessive lethals with effects on the selected trait. For genes of very large effect, the effective size is not a critical factor and it is expected that partial inbreeding will be efficient in increasing fixation rates of recessive mutants. Lethal recessives are eliminated more frequently and their equilibrium frequency is lower under partial inbreeding, but only when their effects on the heterozygote are not very large.

Long-term selection for a quantitative character in large replicate populations of Drosophila melanogaster: 1. Response to selection

The response to long-term selection for increased abdominal bristle number was studied in six replicate lines of Drosophila melanogaster derived from the sc Canberra outbred strain. Each line was continued for 86–89 generations with 50 pairs of parents selected at an intensity of 20%, and subsequently for 32–35 generations without selection. Response continued for at least 75 generations and average total response was in excess of 36 additive genetic standard deviations of the base population (σA) or 51 times the response in the first generation. The pattern of longterm response was diverse and unpredictable typically with one or more accelerated responses in later generations. At termination of the selection, most of the replicate lines were extremely unstable with high phenotypic variability, and lost much of their genetic gains rapidly upon relaxation of selection.

The variation in response among replicates rose in the early phase of selection to level off at approximately 7·6 around generation 25. As some lines plateaued, it increased further to a level higher than would be accommodated by most genetic models. The replicate variation was even higher after many generations of relaxed selection. The genetic diversity among replicates, as revealed in total response, the individuality of response patterns and variation of the sex-dimorphism ratio, suggests that abdominal bristle number is influenced potentially by a large number of genes, but a smaller subset of them was responsible for selection response in any one line.

Artificial selection with differing population structures

The effect of subdivision of a population on response to artificial directional selection for abdominal bristle number inDrosophila melanogasterwas compared using large, replicated lines. Three different population structures were compared: (i) selection in an Undivided, large population with 50 pairs of parents (treatment U); (ii) selection in each of 10 sublines which were reconstituted every 6th generation by Crossing after Culling the 5 lowest sublines (treatment CC); and (iii) selection in each of 10 sublines which were reconstituted every 6th generation by Crossing after Retaining all 10 sublines (treatment CR). At the end of three cycles of selection and crossing, neither CR nor CC was superior to U; sublining did not increase response to selection. These results agree with the predictions arising from an entirely additive model and provide no evidence for the presence of epistasis.

A comparison of 50-pair lines (U) with several 5-pair lines was made over 31 generations. For the 50-pair lines, there was close agreement between response predicted from the base population (usingih2σp) and observed response throughout all 31 generations of selection. Although the best of the 5-pair lines exceeded the 50-pair lines in the early generations, average response to directional selection in the 5-pair lines soon fell behind that predicted fromih2σp, and soon reached a plateau.

Within-generation mutation variance for litter size in inbred mice

The mutational input of genetic variance per generation (sigma(m)(2)) is the lower limit of the genetic variability in inbred strains of mice, although greater values could be expected due to the accumulation of new mutations in successive generations. A mixed-model analysis using Bayesian methods was applied to estimate sigma(m)(2) and the across-generation accumulated genetic variability on litter size in 46 generations of a C57BL/6J inbred strain. This allowed for a separate inference on sigma(m)(2) and on the additive genetic variance in the base population (sigma(a)(2)). The additive genetic variance in the base generation was 0.151 and quickly decreased to almost null estimates in generation 10. On the other hand, sigma(m)(2) was moderate (0.035) and the within-generation mutational variance increased up to generation 14, then oscillating between 0.102 and 0.234 in remaining generations. This pattern suggested the existence of a continuous uploading of genetic variability for litter size (h(2)=0.045). Relevant genetic drift was not detected in this population. In conclusion, our approach allowed for separate estimation of sigma(a)(2) and sigma(m)(2) within the mixed-model framework, and the heritability obtained highlighted the significant and continuous influence of new genetic variability affecting the genetic stability of inbred strains.

Drosophila bristles and the nature of quantitative genetic variation

Numbers of Drosophila sensory bristles present an ideal model system to elucidate the genetic basis of variation for quantitative traits. Here, we review recent evidence that the genetic architecture of variation for bristle numbers is surprisingly complex. A substantial fraction of the Drosophila genome affects bristle number, indicating pervasive pleiotropy of genes that affect quantitative traits. Further, a large number of loci, often with sex- and environment-specific effects that are also conditional on background genotype, affect natural variation in bristle number. Despite this complexity, an understanding of the molecular basis of natural variation in bristle number is emerging from linkage disequilibrium mapping studies of individual candidate genes that affect the development of sensory bristles. We show that there is naturally segregating genetic variance for environmental plasticity of abdominal and sternopleural bristle number. For abdominal bristle number this variance can be attributed in part to an abnormal abdomen-like phenotype that resembles the phenotype of mutants defective in catecholamine biosynthesis. Dopa decarboxylase (Ddc) encodes the enzyme that catalyses the final step in the synthesis of dopamine, a major Drosophila catecholamine and neurotransmitter. We found that molecular polymorphisms at Ddc are indeed associated with variation in environmental plasticity of abdominal bristle number.

Constrained evolution of a quantitative character by pleiotropic mutation

The long-term response to directional selection and its selection limit are derived for a quantitative character that is controlled by pleiotropic mutations with direct deleterious effect on fitness. Directional selection is assumed to be weaker than the selection acting directly on mutations via deleterious effects (purging selection), which renders all mutations to eventual elimination. The analysis embedding this restrictive assumption indicates that the evolutionary response of the character starting from an equilibrium state, in which mutation and purging selection balance but no directional selection is operating, decreases monotonically with time at an exponential rate. And the fading rate of responses is mostly determined by the direct deleterious effect. Contrary to the expectation by the standard selection limit theory based on fixation of extant genetic variation, the present model predicts that the selection limit depends on the intensity of directional selection, the limit being proportional to the ratio of the directional selection intensity to the direct deleterious effect. A slightly larger genetic variance is maintained at the selection limit than would be without directional selection.

Understanding quantitative genetic variation

Until recently, it was impracticable to identify the genes that are responsible for variation in continuous traits, or to directly observe the effects of their different alleles. Now, the abundance of genetic markers has made it possible to identify quantitative trait loci (QTL)--the regions of a chromosome or, ideally, individual sequence variants that are responsible for trait variation. What kind of QTL do we expect to find and what can our observations of QTL tell us about how organisms evolve? The key to understanding the evolutionary significance of QTL is to understand the nature of inherited variation, not in the immediate mechanistic sense of how genes influence phenotype, but, rather, to know what evolutionary forces maintain genetic variability.

High-resolution mapping of quantitative trait loci for sternopleural bristle number in Drosophila melanogaster

We have mapped quantitative trait loci (QTL) harboring naturally occurring allelic variation for Drosophila bristle number. Lines with high (H) and low (L) sternopleural bristle number were derived by artificial selection from a large base population. Isogenic H and L sublines were extracted from the selection lines, and populations of X and third chromosome H/L recombinant isogenic lines were constructed in the homozygous low line background. The polymorphic cytological locations of roo transposable elements provided a dense molecular marker map with an average intermarker distance of 4.5 cM. Two X chromosome and six chromosome 3 QTL affecting response to selection for sternopleural bristle number and three X chromosome and three chromosome 3 QTL affecting correlated response in abdominal bristle number were detected using a composite interval mapping method. The average effects of bristle number QTL were moderately large, and some had sex-specific effects. Epistasis between QTL affecting sternopleural bristle number was common, and interaction effects were large. Many of the intervals containing bristle number QTL coincided with those mapped in previous studies. However, resolution of bristle number QTL to the level of genetic loci is not trivial, because the genomic regions containing bristle number QTL often did not contain obvious candidate loci, and results of quantitative complementation tests to mutations at candidate loci affecting adult bristle number were ambiguous.

Candidate quantitative trait loci and naturally occurring phenotypic variation for bristle number in Drosophila melanogaster: the Delta-Hairless gene region

Delta (Dl) and Hairless (H) are two chromosome 3 candidate neurogenic loci that might contribute to naturally occurring quantitative variation for sensory bristle number. To evaluate this hypothesis, we assessed quantitative genetic variation in abdominal and sternopleural bristle numbers among homozygous isogenic third chromosomes sampled from nature and substituted into the Samarkand (Sam) inbred chromosome 1 and 2 background; among homozygous lines in which the wild-derived Dl-H gene region was introgressed into the Sam chromosome 3 background; and among Dl-H region introgression lines as heterozygotes against the Sam wild-type strain and derivatives of Sam into which mutant Dl and H alleles had been introgressed. Variation among the Dl-H region introgression lines accounted for 36% (8.3%) of the total chromosome 3 among line variance in abdominal (sternopleural) bristle number and for 53% of the chromosome 3 sex x line variance in abdominal bristle number. Naturally occurring alleles in the Dl-H region failed to complement a Dl mutant allele for female abdominal bristle number and sternopleural bristle number in both sexes, and an H mutant allele for both bristle traits in males and females. These results are consistent with the hypothesis that naturally occurring alleles at Dl and H contribute to quantitative genetic variation in sensory bristle number.

Population and quantitative genetics of many linked loci in finite populations

Theoretical studies on the effects of linkage on variability of quantitative traits and response to directional selection in finite populations are reviewed. Emphasis is given to predictions that can be based on observable parameters, such as population size, chromosome lengths and the increment in variance from new mutations. Although truncation selection produces negative linkage disequilibrium in infinite populations, simulation results show that the effects of linkage on response are more pronounced in finite populations. Substantial linkage disequilibrium at the DNA sequence level is being found in population surveys. Some of the results and their interpretation are discussed.

The nature of quantitative genetic variation revisited: lessons from Drosophila bristles

Most characters that distinguish one individual from another, like height or weight, vary continuously in populations. Continuous variation of these 'quantitative' traits is due to the simultaneous segregation of multiple quantitative trait loci (QTLs) as well as environmental influences. A major challenge in human medicine, animal and plant breeding and evolutionary genetics is to identify QTLs and determine their genetic properties. Studies of the classic quantitative traits, abdominal and sternopleural bristle numbers of Drosophila, have shown that: (1) many loci have small effects on bristle number, but a few have large effects and cause most of the genetic variation; (2) 'candidate' loci involved in bristle development often have large quantitative effects on bristle number; and (3) alleles at QTLs affecting bristle number have variable degrees of dominance, interact with each other, and affect other quantitative traits, including fitness. Lessons learned from this model system will be applicable to studies of the genetic basis of quantitative variation in other species.

Response to selection from new mutation and effective size of partially inbred populations. II. Experiments with Drosophila melanogaster

Divergent artificial selection for abdominal bristle number in Drosophila melanogaster has been carried out starting from a genetically homogeneous base population. Lines with two different systems of mating, random (P lines) or between full sibs whenever possible (about 50%), random otherwise (I lines) were compared. Responses after 40 generations of selection were mostly due to one or two mutations of large effect (0.2 to 2 phenotypic standard deviations) per line. Ten mutations affecting the selected trait were individually studied (five lethal and five non-lethal, these being predominantly additive). These mutations satisfactorily explain the response attained, although some minor mutations may also be involved. No evidence of epistasis for bristle number was found. The average final divergence was 57% larger in the P lines, but it was mostly due to lethals or highly deleterious mutations. Thus, after relaxation of selection, the ranking reversed and the mean divergence became significantly larger in the I lines (14%). Analysis of inbreeding showed that the very small amount of variation created by spontaneous mutations (a heritability for the selected trait of about 3%) was responsible for a reduction in the effective size of about 50% in the I lines (relative to the case with random selection), but only about 10% in the P lines. Mutational heritabilities estimated from the response to selection (0.05-0.18%) were within the range usually found for this trait in previous experiments. REML estimates account for correlations between relatives, and were much larger in those lines where the response was due to lethal mutations, as these do not contribute to response after reaching maximum frequency.

Inversion polymorphism and accumulation of lethals in selected lines of Drosophila melanogaster

Five long-term selected lines of Drosophila melanogaster were monitored for the presence of lethals and inversions on the third chromosome for a period of nearly 100 generations after the cessation of selection. The results provide an example of the action of inversions as a trap for genes with homozygous detrimental effects in small populations. Lines polymorphic for cosmopolitan inversions In(3R)C and In(3L)P maintain linked lethal and detrimental genes in a near-balanced system. Consideration of the high selection response attained despite inversion polymorphism leads to the conclusion that extreme responses to selection can be obtained without exploiting the potential from large genomic regions.

The Segregation Distorter (SD) complex and the accumulation of deleterious genes in laboratory strains of Drosophila melanogaster

Segregation Distorter (SD) associated with the second chromosome of D. melanogaster is found in nature at equilibrium frequencies lower than 5%. We report extremely high frequencies of SD (30-50%) in two selected strains, established in 1976, and show it to be responsible for the accumulation of deleterious genes in chromosome II. Samples of chromosomes extracted over a 4-year period were characterized with respect to distortion, sensitivity, lethality, sterility, and inversions. SD chromosomes were inversion-free as they have been shown to be in the Mediterranean area. The cosmopolitan inversion In(2L)t was found associated with SD (+) chromosomes. Lines polymorphic for SD have accumulated linked lethal and female-sterile genes approaching a near balanced system. It is proposed that deleterious genes linked in coupling to SD were accumulated by the balancing effect of distortion, while drift and restricted recombination account for the accumulation of deleterious genes linked in repulsion by a mechanism similar to Muller's ratchet. Our results should not be viewed as a particular case as SD chromosomes associated with detrimental genes and inversions are present in almost all populations around the world. The system could evolve in the way we describe whenever equilibrium conditions are broken down in small populations and lead to an increase in SD frequency.

Spontaneous mutation for a quantitative trait in Drosophila melanogaster. I. Response to artificial selection

Divergent selection for abdominal bristle number was carried out for 47 generations, starting from a completely homozygous population of Drosophila melanogaster. All lines were selected with the same proportion (20%) but at two different numbers of selected parents of each sex (5 or 25). A significant response to selection was obtained in 25 lines (out of 40). In most cases, it could be wholly attributed to a single mutation of relatively large effect (> 0.3 phenotypic standard deviations). A total number of 30 mutations were detected. In agreement with theory, larger responses in each direction were achieved by those lines selected at greater effective population sizes. A large fraction of mutations were lethals (10/30). Thus, the observed divergence between lines of the same effective size selected in opposite directions was smaller than expected under neutrality. The ratio of new mutational variance to environmental variance was estimated to be (0.52 +/- 0.09) x 10(-3).

Association of genetic defects with yield and type traits: the weaver locus effect on yield

The association of recessive genetic disorders with yield and type traits was investigated. The frequency of a defective gene could be increased by selection if it is positively associated with selected traits, despite efforts to reduce it. Genetic defects considered were weaver in Brown Swiss and rectovaginal constriction and limber leg in Jerseys. Data sets for linkage analysis consisted of 245 sons of 9 carrier sires, 1036 sons of 16 carrier sires, and 557 sons of 10 carrier sires, respectively. Weaver carrier sons had higher producing daughters than noncarrier sons within all 9 sire families. Weaver carrier cows have an advantage of 673.6 kg milk and 26.0 kg fat and a disadvantage in rear legs score, indicating that the condition may not be completely recessive. Carriers of the other defect genes have no advantage for milk production, are scored lower for pelvic angle, and limber leg carriers have more desirable udders. Estimates of defect gene frequencies in 264,000 Jersey cows show a decrease over time for rectovaginal constriction and limber leg; in 97,723 Brown Swiss cows, frequency of the weaver gene increased over time. Gene frequencies in daughters of the youngest sires were 5.48, 2.13, and 8.89%, respectively. Consistently higher yield evaluations of weaver carrier sons within each sire family, large advantage in production of weaver carrier cows, and increasing gene frequency over time indicate that a chromosome segment with major effect on yield is tightly linked to weaver in Brown Swiss.

Analysis of lethals in selected lines of Drosophila melanogaster

Five lines of Drosophila melanogaster that reached an extreme phenotype after long-term selection for increased dorsocentral bristle number, were analysed for the presence of lethals. Seven chromosome II and three chromosome III lethal types were detected in four of the lines, at frequencies ranging from between 6% and 36%. No lethal had any demonstrable effect over the selected trait. In one line, where almost every chromosome II was a lethal carrier, it was shown that the main lethal (at a frequency of 36%) was associated with the transmission ratio distortion in males. The processes which could lead to the accumulation of this lethal and others linked in disequilibrium to it is discussed. Some results suggest similar mechanisms for the accumulation of lethals in the other lines. These findings show that causes other than the direct effect of artificial selection must be taken into account when trying to explain the accumulation of lethals in selected lines.

Models of long-term artificial selection in finite population with recurrent mutation

The effects of mutation on mean and variance of response to selection for quantitative traits are investigated. The mutants are assumed to be unlinked, to be additive, and to have their effects symmetrically distributed about zero, with absolute values of effects having a gamma distribution. It is shown that the ratio of expected cumulative response to generation t from mutants, , and expected response over one generation from one generation of mutants, , is a function of t/N, where t is generations and N is effective population size. Similarly, , is a function of t/N, where is the increment in genetic variance from one generation of mutants. The mean and standard deviation of response from mutations relative to that from initial variation in the population, in the first generation, are functions of . Evaluation of these formulae for a range of parameters quantifies the important role that population size can play in response to long-term selection.

Accumulation of lethals in highly selected lines of Drosophila melanogaster

Four synthetic lines of D. melanogaster selected for low sternopleural bristle number for 50 generations were screened for lethals on chromosome III when their mean score equalled 2.5. Each line originated from a cross between line M (previously selected for the same trait during 130 generations) and a different unselected cage population. Line M was already known to carry a recessive lethal on chromosome III affecting the selected trait, such that the bristle score of the lethal heterozygote was lower than that of the viable homozygote. Tests revealed 18 lethals, 15 of these present in at least two lines. Each line carried from 10 to 16 lethals. All lines carried groups of lethals present on the same chromosome, and at least six lethals in each line were included in such an association with a frequency of 0.18 or higher. It appears that the lethal affecting bristle score in line M has protected a segment of chromosome III from natural selection and that the remaining 14 lethals have accumulated later in that line.

Long-term selection for a quantitative character in large replicate populations of Drosophila melanogaster : Part 4: Relaxed and reverse selection

Reverse and relaxed selection were carried out in sublines which were derived from six replicate lines of Drosophila during 86-89 generations of selection for increased abdominal bristle number, and the reverse selection sublines were reciprocally crossed with selection lines of their origin.The results of serial relaxed selection initiated at different generations of selection confirm that the accelerated responses observed in the selection lines were largely due to deleterious genes, particularly lethals, with large effects on the selected character. The decline in mean bristle number under relaxed selection was not much different between crowded and uncrowded relaxed sublines.Reverse selection initiated at generation 57 was very effective, though it failed to bring the mean back to the base population level, and the genetic differences between replicate sublines (two from each of the six lines) indicate that low bristle number genes were probably rare in the selection lines. The genes which were still segregating after 57 generations of selection, on the average, did not show any directional dominance. The contribution of the X-chromosome to selection response was proportional to its chromosome length.

Long-term selection for a quantitative character in large replicate populations of Drosophila melanogaster : Part 3: The nature of residual genetic variability

Six replicate lines of Drosophila melanogaster, which had been selected for increased abdominal bristle number for more than 85 generations, were assayed by hierarchical analysis of variance and offspring on parent regression immediately after selection ceased, and by single-generation realised heritability after more than 25 generations of subsequent relaxed selection.Half-sib estimates of heritability in 5 lines were as high as in the base population and much higher than observed genetic gains would suggest, excluding lack of sufficient additive genetic variance as a cause of ineffective selection in these lines. Also, there was considerable diversity among the six lines in composition of phenotypic variability: in addition to differences in the additive genetic component, one or more of the components due to dominance, epistasis, sex-linkage or genotype-environment interaction appeared to be important in different lines.Even after relaxed selection, single-generation realised heritabilities in four lines were as high as in the base population. As a large proportion of total genetic gain must have been made by fixation of favourable alleles, the compensatory increase of genetic variability has been sought in a genetic model involving genes at low initial frequencies, enhancement of gene effects during selection and/or new mutations.