Little is known of how gene expression and its plasticity evolves as populations adapt to different environmental regimes. Expression is expected to evolve adaptively in all populations but only ...those populations experiencing environmental heterogeneity are expected to show adaptive evolution of plasticity. We measured the transcriptome in a cadmium-enriched diet and a salt-enriched diet for experimental populations of Drosophila melanogaster that evolved for ~130 generations in one of four selective regimes: two constant regimes maintained in either cadmium or salt diets and two heterogeneous regimes that varied either temporally or spatially between the two diets. For populations evolving in constant regimes, we find a strong signature of counter-gradient evolution; the evolved expression differences between populations adapted to alternative diets is opposite to the plastic response of the ancestral population that is naïve to both diets. Based on expression patterns in the ancestral populations, we identify a set of genes for which we predict selection in heterogeneous regimes to result in increases in plasticity and we find the expected pattern. In contrast, a set of genes where we predicted reduced plasticity did not follow expectation. Nonetheless, both gene sets showed a pattern consistent with adaptive expression evolution in heterogeneous regimes, highlighting the difference between observing "optimal" plasticity and improvements in environment-specific expression. Looking across all genes, there is evidence in all regimes of differences in biased allele expression across environments ("allelic plasticity") and this is more common among genes with plasticity in total expression.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Both theory and experiments have demonstrated that sex can facilitate adaptation, potentially yielding a group-level advantage to sex. However, it is unclear whether this process can help solve the ...more difficult problem of the maintenance of sex within populations. Using experimental populations of the facultatively sexual rotifer Brachionus calyciflorus, we show that rates of sex evolve to higher levels during adaptation but then decline as fitness plateaus. To assess the fitness consequences of genetic mixing, we directly compare the fitnesses of sexually and asexually derived genotypes that naturally occur in our experimental populations. Sexually derived genotypes are more fit than asexually derived genotypes when adaptive pressures are strong, but this pattern reverses as the pace of adaptation slows, matching the pattern of evolutionary change in the rate of sex. These fitness assays test the net effect of sex but cannot be used to disentangle whether selection on sex arises because highly sexual lineages become associated with different allele combinations or with different allele frequencies than less sexual lineages (i.e., "short-" or "long-term" effects, respectively). We infer which of these mechanisms provides an advantage to sex by performing additional manipulations to obtain fitness distributions of sexual and asexual progeny arrays from unbiased parents (rather than from naturally occurring, and thereby evolutionarily biased, parents). We find evidence that sex breaks down adaptive gene combinations, resulting in lower average fitness of sexual progeny (i.e., a short-term disadvantage to sex). As predicted by theory, the advantage to sex arises because sexually derived progeny are more variable in fitness, allowing for faster adaptation. This "long-term advantage" builds over multiple generations, eventually resulting in higher fitness of sexual types.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
Phenotypic sexual dimorphism can be mediated by sex differences in gene expression. We examine two forms of sexual dimorphism in gene expression in Drosophila melanogaster: 1) sex-biased ...gene expression (SBGE) in which the sexes differ in the amount a gene is expressed and 2) sexual dimorphism in isoform usage, that is, sex-specific splicing (SSS). In whole body (but not the head) expression, we find a negative association between SBGE and SSS, possibly suggesting that these are alternate routes to resolving sexual antagonistic selection. Next, we evaluate whether expression dimorphism contributes to the heterogeneity among genes in rmf, the intersexual genetic correlation in body expression that constrains the extent to which a gene's expression can evolve independently between the sexes. We find lower rmf values for genes with than without SSS. We find higher rmf values for male- than female-biased genes (except genes with extreme male bias), even though male-biased genes are known to have greater evolutionary divergence in expression. Finally, we examine population genetic patterns in relation to SBGE and SSS because genes with expression dimorphism have likely experienced a history of sex differences in selection. SSS is associated with reduced values of Tajima's D and elevated direction of selection (DoS) values, suggestive of higher rates of adaptive evolution. Though DoS is highly elevated for genes with extreme male bias, DoS otherwise tends to decline from female-biased to unbiased to male-biased genes. Collectively, the results indicate that SBGE and SSS are differentially distributed across the genome and are associated with different forms of selection.
In dioecious populations, the sexes employ divergent reproductive strategies to maximize fitness and, as a result, genetic variants can affect fitness differently in males and females. Moreover, ...recent studies have highlighted an important role of the mating environment in shaping the strength and direction of sex-specific selection. Here, we measure adult fitness for each sex of 357 lines from the Drosophila Synthetic Population Resource (DSPR) in two different mating environments. We analyze the data using three different approaches to gain insight into the sex-specific genetic architecture for fitness: classical quantitative genetics, genomic associations, and a mutational burden approach. The quantitative genetics analysis finds that, on average segregating genetic variation in this population has concordant fitness effects both across the sexes and across mating environments. We do not find specific genomic regions with strong associations with either sexually antagonistic (SA) or sexually concordant (SC) fitness effects, yet there is modest evidence of an excess of genomic regions with weak associations, both with SA and SC fitness effects. Our examination of mutational burden indicates stronger selection against indels and loss-of-function variants in females than males.
Whether the changes brought about by sexual selection are, on the whole, congruent or incongruent with the changes favored by natural selection is a fundamentally important question in evolutionary ...biology. Although a number of theoretical models have assumed that sexual selection reinforces natural selection 1, 2, others assume these forces are in opposition 3–5. Empirical results have been mixed (see reviews in 1, 6–8) and the reasons for the differences among studies are unclear. Variable outcomes are expected if populations differ in their evolutionary histories and therefore harbor different amounts and types of segregating genetic variation. Here, we constructed populations of Drosophila melanogaster that differed in this regard to directly test this hypothesis. In well-adapted populations, sexually successful males sired unfit daughters, indicating sexual and natural selection are in conflict. However, in populations containing an influx of maladaptive alleles, attractive males sired offspring of high fitness, suggesting that sexual selection reinforces natural selection. Taken together, these results emphasize the importance of evolutionary history on the outcome of sexual selection. Consequently, studies based on laboratory populations, cultured for prolonged periods under homogeneous conditions, may provide a skewed perspective on the relationship between sexual and natural selection.
► It is unclear whether sexual selection impedes or facilitates natural selection ► A population's evolutionary history may influence the effect of sexual selection ► The effect of sexually antagonistic alleles is greater in long-adapted populations ► Populations held off their adaptive peak benefit from sexual selection (“good genes”)
The Genomics of Sexual Conflict Rowe, Locke; Chenoweth, Stephen F.; Agrawal, Aneil F.
The American naturalist,
08/2018, Letnik:
192, Številka:
2
Journal Article
Recenzirano
Sexual dimorphism is a substantial contributor to the diversity observed in nature, extending from elaborate traits to the expression level of individual genes. Sexual conflict and sexually ...antagonistic coevolution are thought to be central forces driving the dimorphism of the sexes and its diversity. We have substantial data to support this at the phenotypic level but much less at the genetic level, where distinguishing the role of conflict from other forms of sex-biased selection and from other processes is challenging. Here we discuss the powerful effects sexual conflict may have on genome evolution and critically evaluate the supporting evidence. Although there is much potential for sexual conflict to affect genome evolution, we have relatively little compelling evidence of a genomic signature of sexual conflict. A central obstacle is the mismatch between taxa in which we understand sexually antagonistic selection and those in which we understand genetics.
Variance in fitness is thought to be greater in males than in females in many species. If this is so, there are two potentially contradictory consequences on the efficacy of selection (Nes): greater ...variance in fitness may allow stronger selection (i.e., increased s), but it will also cause stronger genetic drift (i.e., reduced Ne). We develop a simple model to ask how the stronger condition dependency of fitness in males than in females affects selection and fitness variance in each sex to examine the net effect on the efficacy of selection. We measured the phenotypic variance in fitness for each sex in Drosophila melanogaster in different environmental and mating contexts. Variance in fitness was only approximately one and a half to two times higher in males than in females; juvenile mortality likely dampens the difference in variation between the sexes. Combining these results with previous studies of sex-specific selection on mutations, we infer that the increased drift due to males counterbalances the stronger selection on males in this species, leaving Nes similar to what would be expected if both sexes were “female-like” with respect to selection and variance in fitness. Reasons why this could differ in other species are discussed.
Genetically correlated traits do not evolve independently, and the covariances between traits affect the rate at which a population adapts to a specified selection regime. To measure the impact of ...genetic covariances on the rate of adaptation, we compare the rate fitness increases given the observed G matrix to the expected rate if all the covariances in the G matrix are set to zero. Using data from the literature, we estimate the effect of genetic covariances in real populations. We find no net tendency for covariances to constrain the rate of adaptation, though the quality and heterogeneity of the data limit the certainty of this result. There are some examples in which covariances strongly constrain the rate of adaptation but these are balanced by counter examples in which covariances facilitate the rate of adaptation; in many cases, covariances have little or no effect. We also discuss how our metric can be used to identify traits or suites of traits whose genetic covariances to other traits have a particularly large impact on the rate of adaptation.
Data from several thousand knockout mutations in yeast (Saccharomyces cerevisiae) were used to estimate the distribution of dominance coefficients. We propose a new unbiased likelihood approach to ...measuring dominance coefficients. On average, deleterious mutations are partially recessive, with a mean dominance coefficient ~0.2. Alleles with large homozygous effects are more likely to be more recessive than are alleles of weaker effect. Our approach allows us to quantify, for the first time, the substantial variance and skew in the distribution of dominance coefficients. This heterogeneity is so great that many population genetic processes analyses based on the mean dominance coefficient alone will be in substantial error. These results are applied to the debate about various mechanisms for the evolution of dominance, and we conclude that they are most consistent with models that depend on indirect selection on homeostatic gene expression or on the ability to perform well under periods of high demand for a protein.
Many multicellular eukaryotes have reasonably high per-generation mutation rates. Consequently, most populations harbor an abundance of segregating deleterious alleles. These alleles, most of which ...are of small effect individually, collectively can reduce substantially the fitness of individuals relative to what it would be otherwise; this is mutation load. Mutation load can be lessened by any factor that causes more mutations to be removed per selective death, such as inbreeding, synergistic epistasis, population structure, or harsh environments. The ecological effects of load are not clear-cut because some conditions (such as selection early in life, sexual selection, reproductive compensation, and intraspecific competition) reduce the effects of load on population size and persistence, but other conditions (such as interspecific competition and load on resource use efficiency) can cause small amounts of load to have strong effects on the population, even extinction. We suggest a series of studies to improve our understanding of the effects of mutation load.
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