Partitioning of variance and estimation of genetic parameters for various bristle number characters ofDrosophila melanogaster

Inheritance of Two Traits With High Plasticity, Developmental Speed, and Body Size, in Tenebrio molitor (Coleoptera: Tenebrionidae)

The study of inheritance of quantitative traits of high plasticity in insects has been limited. The heritability of larval development time and body weight in Tenebrio molitor L. was determined using the method of parent-offspring regression. The parental group of adults obtained from a cohort from one day of oviposition from a stock colony was divided into 28 class groups according to their larval development time and pupal weight. The progeny resulting from these parental classes was grouped in experimental units and allowed to develop to the pupal stage. Means of larval development time and pupal weight of the progeny were compared with their parental class levels using linear regression. The selection of larval development time and pupal weight in the parental classes had a significant impact on the means of larval development time and pupal weight of the progeny. The regression coefficients for larval development time and pupal weight were 0.626 ± 0.02 and 0.408 ± 0.02, respectively. These values represent the proportion of genetic determination of these two traits based on the principles of the method of parent-offspring regression. The apparent independence of larval development time and pupal weight based on their poor linear correlation is discussed.

Regeneration of the variance of metric traits by spontaneous mutation in a Drosophila population

In the C1 population of Drosophila melanogaster of moderate effective size (≈500), which was genetically invariant in its origin, we studied the regeneration by spontaneous mutation of the genetic variance for two metric traits [abdominal (AB) and sternopleural (ST) bristle number] and that of the concealed mutation load for viability, together with their temporal stability, using alternative selection models based on mutational parameters estimated in the C1 genetic background. During generations 381–485 of mutation accumulation (MA), the additive variances of AB and ST approached the levels observed in standing laboratory populations, fluctuating around their expected equilibrium values under neutrality or under relatively weak causal stabilizing selection. This type of selection was required to simultaneously account for the observed additive variance in our population and for those previously reported in natural and laboratory populations, indicating that most mutations affecting bristle traits would only be subjected to weak selective constraints. Although gene action for bristles was essentially additive, transient situations occurred where inbreeding resulted in a depression of the mean and an increase of the additive variance. This was ascribed to the occasional segregation of mutations of large recessive effects. On the other hand, the observed non-lethal inbreeding depression for viability must be explained by the segregation of alleles of considerable and largely recessive deleterious effects, and the corresponding load concealed in the heterozygous condition was found to be temporally stable, as expected from tighter constraints imposed by natural selection.

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.

Genetic differences between populations of Drosophila melanogaster for a quantitative trait: II. Wild and laboratory populations

Crosses were made between four populations of Drosophila melanogaster – three of which were laboratory populations (Kaduna, Pacific and Canberra) and one recently captured (Stellenbosch) – and a line previously selected for low sternopleural bristle number for many generations from a Kaduna/Pacific source. In each of six replicate lines from each cross selection was practised for low sternopleural bristle number, and subsequently these replicates were intercrossed and reselected.

Initially, similar responses were made in each set of lines, but subsequently more variation between replicates was found in Stellenbosch, which was the primary source of lines which responded to a level below that of the original selected line.

It is concluded that this newly captured population contains genetic variability absent from the laboratory populations. Presumably variability has been lost from the latter populations, leaving essentially the same genes segregating in each.

Genetic latitudinal adaptation ofDrosophila melanogaster: new discriminative biometrical traits between European and equatorial African populations

Five biometrical traits (thorax length, wing length and width, sternopleural and abdominal chaetae numbers) were measured on 13 equatorial African strains and 30 French strains. In all cases highly significant differences were observed between the two geographic groups. These results are added to previously known variations concerning adult weight and ovariole number. In each place, the genetic particularities of the wild populations seem to be maintained by the selective pressure from environmental conditions, resulting in a homeostatic focusing of the best fitted average genotype. Analysis within each group showed that variations between strains were in most cases poorly or not correlated, so that partial or total genetic independence between the various traits measured seems likely.

The build up of mutation-selection- drift balance in laboratory Drosophila populations

The build up of an equilibrium between mutation, selection, and drift in populations of moderate size is an important evolutionary issue, and can be critical in the conservation of endangered populations. We studied this process in two Drosophila melanogaster populations initially lacking genetic variability (C1 and C2) that were subsequently maintained during 431 or 165 generations with effective population size N(e) approximately 500 (estimated by lethal complementation analysis). Each population originated synchronously to a companion set of full-sib mutation accumulation (MA) lines, C1 and MA1 were derived from an isogenic origin and C2 and MA2 from a single MA1 line at generation 265. The results suggest that both C1 and C2 populations were close to the mutation-selection-drift balance for viability and bristle traits, and are consistent with a 2.5-fold increase of the mutation rate in C2 and MA2. Despite this increase, the average panmictic viability in C2 was only slightly below that of C1, indicating that the expressed loads due to segregating deleterious mutation were small, in agreement with the low deleterious mutation rate (0.015-0.045) previously reported for the MA1 lines. In C1, the nonlethal inbreeding depression rate for viability was 30% of that usually estimated in segregating populations. The genetic variance for bristles regenerated in C1 and C2 was moderately smaller than the average value reported for natural populations, implying that they have accumulated a substantial adaptive potential. In light of neutral and selective predictions, these results suggest that bristle additive variance was predominantly due to segregation of mutations with deleterious effects of the order of 10(-3), and is consistent with relatively weak causal stabilizing selection (V(s) approximately 30).

The quantitative genetics of sexual dimorphism: assessing the importance of sex-linkage

Sexual dimorphism (SD) is a defining feature of gonochorous animals and dioecious plants, but the evolution of SD from an initially monomorphic genome presents a conundrum. Theory predicts that the evolution of SD will be facilitated if genes with sex-specific fitness effects occur on sex chromosomes. We review this theory and show that it generates three testable predictions. For organisms with an XX/XY chromosomal system of sex determination: (1) SD should be associated with X-linked effects; (2) X-linked effects should show strong directional dominance for sexually dimorphic traits favored in males but expressed in both sexes; and (3) SD should be associated with a reduction in the between-sex additive genetic covariance and correlation. A literature review reveals that empirical evaluations of the association between sex-linkage and SD have lagged behind theory. Tests for the presence of sex-linked effects have been plagued by the need to make simplifying assumptions, such as the absence of dominance or maternal effects, that greatly weaken their discriminatory power. Further, most have used comparisons between species or populations, whereas the correct level of analysis is within populations. To overcome these problems, we derive a novel pedigree design that permits separate estimation of X-linked, dominance and maternal effects. We suggest that the data from such a design would be most appropriately analyzed using the animal model. This novel protocol will allow quantitative evaluation of the above predictions, and hence should spur progress in understanding the role of sex-linkage in the evolution of SD.

Naturally occurring variation in bristle number and DNA polymorphisms at the scabrous locus of Drosophila melanogaster

The association between quantitative genetic variation in bristle number and molecular variation at a candidate neurogenic locus, scabrous, was examined in Drosophila melanogaster. Approximately 32 percent of the genetic variation in abdominal bristle number (21 percent for sternopleural bristle number) among 47 second chromosomes from a natural population was correlated with DNA sequence polymorphisms at this locus. Several polymorphic sites associated with large phenotypic effects occurred at intermediate frequency. Quantitative genetic variation in natural populations caused by alleles that have large effects at a few loci and that segregate at intermediate frequencies conflicts with the classical infinitesimal model of the genetic basis of quantitative variation.

Quantitative genetics and fitness: lessons from Drosophila

This paper examines patterns of heritability and genetic covariance between traits in the genus Drosophila. Traits are divided into the categories, morphology, behaviour, physiology and life history. Early theoretical analyses suggested that life history traits should have heritabilities that are lower than those in other categories. Variable pleiotrophy, environmental variation, mutation and niche variation may, however, maintain high heritabilities. In Drosophila the heritabilities of life history traits are lower than morphological or physiological traits but may exceed 20 per cent. The pattern of variation in the heritability of behavioural traits is similar to that of life history traits. Genetic covariance between morphological traits and between morphological and life history traits are all positive but those between life history traits have variable sign. Negative covariance between traits supports the variable pleiotropy hypothesis but other factors such as environmental heterogeneity, or mutation cannot be excluded.

Forging links between population and quantitative genetics

An initially rare allele with a large effect on a quantitative character is expected to exhibit the following behaviour in artificial selection lines: 1. It should change in frequency, or be lost by chance, at rates predictable from the effects of the allele on the quantitative character and the selection regime imposed. 2. At the phenotypic level the behaviour of the allele should cause (a) asymmetrical responses to bidirectional selection, (b) variation among replicate lines in response to selection corresponding to the behaviour of the allele in individual lines, (c) changes in heritability in lines in which the allele increases in frequency and (d) selection response compatible with the effects of the allele. This paper reports an experimental evaluation of these predictions utilizing a rare allele of large effect (sm (lab) ) detected in a sample of the Canberra outbred population of Drosophila melanogaster at a frequency of 1/120. Homosygosity for this allele reduced abdominal bristle number by more than 50%, altered the abdominal bristle pattern and reversed the sexual dimorphism for abdominal bristle number. Experiments were done to characterise sm (lab) and all evidence indicates that it is a single allele with a very large effect.Bidirectional selection for abdominal bristle number was carried out in three high and three low lines from this sample of the Canberra population. The sm (lab) allele rose in frequency and went to fixation in two of the low lines (in 10 generations) but was lost from the third. These times to fixation were slower than the expectations derived from computer simulations of the behaviour of such an allele but this can be attributed to the lower fitness of sm (lab) homozygotes. The proportions of lines with the allele fixed or lost were compatible with expectations. At the phenotypic level, the behavior of sm (lab) had the expected consequences, namely, (i) asymmetrical responses to bidirectional selection, (ii) variation in response among replicate low lines corresponding to the behaviour of sm (lab) , (iii) changes in heritabilities in the lines in which sm (lab) went to fixation, and (iv) selection responses compatible with the effects of the allele.A test for rare alleles of large effect was proposed, based on the expected pattern of change in heritability under artificial selection. This test was applied to the high selection lines but no evidence was found for important effects due to rare alleles of large effect increasing abdominal bristle number, a conclusion consistent with other independent evidence.This work provides experimental corroboration of the links between population genetics and quantitative genetics.

Selection for increased abdominal bristle number in Drosophila melanogaster with concurrent irradiation : II. Populations derived from an outbred cage population

Replicate lines, each initially with one hundred pairs of parents selected at 50% intensity, were derived from the Canberra strain. In later generations population size was reduced and selection intensity increased. Three lines were selected without irradiation and five with 1000 r X-rays per generation for thirty generations. Selection was continued until generation 66. Long-term responses were similar in unirradiated and irradiated lines, and there was evidence that genes with large effects influenced response patterns in both groups of lines.

The effects of population size and selection intensity in selection for a quantitative character in Drosophila. 3. Analyses of the lines

1. In order to determine the nature of the genetic variation causing the response to selection in our lines (Joneset al.1968), various analyses were performed.

2. There was no consistent change in heritability, estimated from half-sib correlation or from the phenotypic correlation between the bristle numbers of two abdominal segments, after 10 to 20 generations of selection.

3. Realized heritabilities over the 10 generations subsequent to the heritability estimations were less than in the early generations but bore little relationship to the estimated values.

4. Six lines contained recessive lethals with appreciable effects on bristle number as indicated by high variances, large regression on relaxation and large response to reverse selection.

5. Reverse selection lines taken from the main lines at generation 40 indicated that genetic variation was still present in almost all of the lines. Only one line failed to respond to further forward or to reverse selection.

6. The three highest lines were crossed in pairs and reselected. Two of the three possible crosses gave further response, exceeding the higher parent after one and three generations, but the other cross failed to pass the highest parent line.

7. A combination of large gene effects, linkage, and gene interaction effects have been suggested as the cause of irregularities in the response of the lines. It has not been possible to determine the relative importance of these effects.

The effects of population size and selection intensity in selection for a quantitative character in Drosophila. I. Short-term response to selection

1. The response to selection for increased number of bristles on one abdominal segment was studied over 12 generations using a factorial design of three population sizes (10, 20 and 40 pairs of parents) and five selection intensities (10, 20, 40, 80% and controls).

2. The responses on the average agreed well with those expected from the estimated base population heritability, but individual replicates diverged considerably.

3. Larger populations tended to give greater response to selection, due mainly to larger realized heritabilities.

4. There was no consistent effect of selection intensity on realized heritability.

5. For populations with the same number of individuals scored, less intense selection gave greater realized heritabilities.