Lack of nonadditive genetic effects on early fecundity in Drosophila melanogaster

Estimation of genetic purging under competitive conditions

Inbreeding depression for fitness traits is a key issue in evolutionary biology and conservation genetics. The magnitude of inbreeding depression, though, may critically depend on the efficiency of genetic purging, the elimination or recessive deleterious mutations by natural selection after they are exposed by inbreeding. However, the detection and quantification of genetic purging for nonlethal mutations is a rather difficult task. Here, we present two comprehensive sets of experiments with Drosophila aimed at detecting genetic purging in competitive conditions and quantifying its magnitude. We obtain, for the first time in competitive conditions, an estimate for the predictive parameter, the purging coefficient (d), that quantifies the magnitude of genetic purging, either against overall inbreeding depression (d ≈ 0.3), or against the component ascribed to nonlethal alleles (dNL ≈ 0.2). We find that competitive fitness declines at a high rate when inbreeding increases in the absence of purging. However, in moderate size populations under competitive conditions, inbreeding depression need not be too dramatic in the medium to short term, as the efficiency of purging is also very high. Furthermore, we find that purging occurred under competitive conditions also reduced the inbreeding depression that is expressed in the absence of competition.

Social structure and indirect genetic effects: genetics of social behaviour

The social environment modulates gene expression, physiology, behaviour and patterns of inheritance. For more than 50 years, this concept has been investigated using approaches that include partitioning the social component out of behavioural heritability estimates, studying maternal effects on offspring, and analysing dominance hierarchies. Recent advances have formalized this 'social environment effect' by providing a more nuanced approach to the study of social influences on behaviour while recognizing evolutionary implications. Yet, in most of these formulations, the dynamics of social interactions are not accounted for. Also, the reciprocity between individual behaviour and group-level interactions has been largely ignored. Consistent with evolutionary theory, the principles of social interaction are conserved across a broad range of taxa. While noting parallels in diverse organisms, this review uses Drosophila melanogaster as a case study to revisit what is known about social interaction paradigms. We highlight the benefits of integrating the history and pattern of interactions among individuals for dissecting molecular mechanisms that underlie social modulation of behaviour.

Response of body size and developmental time of Tribolium castaneum to constant versus fluctuating thermal conditions

Temperature has profound effects on biological functions at all levels of organization. In ectotherms, body size is usually negatively correlated with ambient temperature during development, a phenomenon known as The Temperature-Size Rule (TSR). However, a growing number of studies have indicated that temperature fluctuations have a large influence on life history traits and the implications of such fluctuations for the TSR are unknown. Our study investigated the effect of different constant and fluctuating temperatures on the body mass and development time of red flour beetles (Tribolium castaneum Herbst, 1797); we also examined whether the sexes differed in their responses to thermal conditions. We exposed the progeny of half-sib families of a T. castaneum laboratory strain to one of four temperature regimes: constant 30°C, constant 25°C, fluctuating with a daily mean of 30°C, or fluctuating with a daily mean of 25°C. Sex-specific development time and body mass at emergence were determined. Beetles developed the fastest and had the greatest body mass upon emergence when they were exposed to a constant temperature of 30°C. This pattern was reversed when beetles experienced a constant temperature of 25°C: slowest development and lowest body mass upon emergence were observed. Fluctuations changed those effects significantly - impact of temperature on development time was smaller, while differences in body mass disappeared completely. Our results do not fit TSR predictions. Furthermore, regardless of the temperature regime, females acquired more mass, while there were no differences between sexes in development time to eclosion. This finding fails to support one of the explanations for smaller male size: that selection favors the early emergence of males. We found no evidence of genotype × environment interactions for selected set of traits.

Sexual conflict and interacting phenotypes: a quantitative genetic analysis of fecundity and copula duration in Drosophila melanogaster

Many reproductive traits that have evolved under sexual conflict may be influenced by both sexes. Investigation of the genetic architecture of such traits can yield important insight into their evolution, but this entails that the heritable component of variation is estimated for males and females-as an interacting phenotype. We address the lack of research in this area through an investigation of egg production and copula duration in the fruit fly, Drosophila melanogaster. Despite egg production rate being determined by both sexes, which may cause sexual conflict, an assessment of this trait as an interacting phenotype is lacking. It is currently unclear whether copula duration is determined by males and/or females. We found significant female, but not male, genetic variance for egg production rate that may indicate reduced potential for ongoing sexually antagonistic coevolution. In contrast, copula duration was determined by significant genetic variance in both sexes. We also identified genetic variation in egg retention among virgin females. Although previously identified in wild populations, it is unclear why this should be present in a laboratory stock. This study provides a novel insight into the shared genetic architecture of reproductive traits that are the subject of sexual conflict.

Reduced inbreeding depression due to historical inbreeding in Drosophila melanogaster: evidence for purging

An important issue in conservation biology and the study of evolution is the extent to which inbreeding depression can be reduced or reversed by natural selection. If the deleterious recessive alleles causing inbreeding depression can be 'purged' by natural selection, outbred populations that have a history of inbreeding are expected to be less susceptible to inbreeding depression. This expectation, however, has not been realized in previous laboratory experiments. In the present study, we used Drosophila melanogaster as a model system to test for an association between inbreeding history and inbreeding depression. We created six 'purged' populations from experimental lineages that had been maintained at a population size of 10 male-female pairs for 19 generations. We then measured the inbreeding depression that resulted from one generation of full-sib mating in the purged populations and in the original base population. The magnitude of inbreeding depression in the purged populations was approximately one-third of that observed in the original base population. In contrast to previous laboratory experiments, therefore, we found that inbreeding depression was reduced in populations that have a history of inbreeding. The large purging effects observed in this study may be attributable to the rate of historical inbreeding examined, which was slower than that considered in previous experiments.

Relaxation of selection with equalization of parental contributions in conservation programs: an experimental test with Drosophila melanogaster

Equalization of parental contributions is one of the most simple and widely recognized methods to maintain genetic diversity in conservation programs, as it halves the rate of increase in inbreeding and genetic drift. It has, however, the negative side effect of implying a reduced intensity of natural selection so that deleterious genes are less efficiently removed from the population with possible negative consequences on the reproductive capacity of the individuals. Theoretical results suggest that the lower fitness resulting from equalization of family sizes relative to that for free contribution schemes is expected to be substantial only for relatively large population sizes and after many generations. We present a long-term experiment with Drosophila melanogaster, comparing the fitness performance of lines maintained with equalization of contributions (EC) and others maintained with no management (NM), allowing for free matings and contributions from parents. Two (five) replicates of size N = 100 (20) individuals of each type of line were maintained for 38 generations. As expected, EC lines retained higher gene diversity and allelic richness for four microsatellite markers and a higher heritability for sternopleural bristle number. Measures of life-history traits, such as egg-to-adult viability, mating success, and global fitness declined with generations, but no significant differences were observed between EC and NM lines. Our results, therefore, provide no evidence to suggest that equalization of family sizes entails a disadvantage on the reproductive capacity of conserved populations in comparison with no management procedures, even after long periods of captivity.

A measure of the within-chromosome synergistic epistasis for Drosophila viability

In order to detect possible synergistic epistasis for viability in Drosophila melanogaster we assayed the relative viability of chromosomes II in: (i) panmixia, (ii) forced total homozygosity, and (iii) homozygosity for, on the average, half of their loci. As these genotypes were constructed using exactly the same set of chromosomes in the three cases, the design allows us to estimate the inbreeding depression rate at two different inbreeding levels in the absence of purging natural selection. Overall, no consistent synergistic epistasis was found. However, there was a small fraction of chromosomes whose severely deleterious effect when homozygous was almost significantly larger than expected from their viability when homozygous for half of their loci. This suggests occasional but important synergistic epistasis, which might confer evolutionary advantage to recombination in tightly linked genomes. Nevertheless, such epistasis is unlikely to be an evolutionary advantage driving the evolution of sexual anisogamous reproduction, as its contribution to overall viability is small when compared with the two-fold cost of anisogamy.

Mutation-selection balance accounting for genetic variation for viability inDrosophila melanogasteras deduced from an inbreeding and artificial selection experiment

We carried out an experiment of inbreeding and upward artificial selection for egg-to-adult viability in a recently captured population of Drosophila melanogaster, as well as computer simulations of the experimental design, in order to obtain information on the nature of genetic variation for this important fitness component. The inbreeding depression was linear with a rate of 0.70 +/- 0.11% of the initial mean per 1% increase in inbreeding coefficient, and the realized heritability was 0.06 +/- 0.07. We compared the empirical observations of inbreeding depression and selection response with computer simulations assuming a balance between the occurrence of partially recessive deleterious mutations and their elimination by selection. Our results suggest that a model assuming mutation-selection balance with realistic mutational parameters can explain the genetic variation for viability in the natural population studied. Several mutational models are incompatible with some observations and can be discarded. Mutational models assuming a low rate of mutations of large average effect and highly recessive gene action, and others assuming a high rate of mutations of small average effect and close to additive gene action, are compatible with all the observations.