Transcriptomics of differences in thermal plasticity associated with selection for an exaggerated male sexual trait.

The information about the magnitude of differences in thermal plasticity both between and within populations, as well as identification of the underlying molecular mechanisms are key to understanding the evolution of thermal plasticity. In particular, genes underlying variation in the physiological response to temperature can provide raw material for selection acting on plastic traits. Using RNAseq, we investigate the transcriptional response to temperature in males and females from bulb mite populations selected for the increased frequency of one of two discrete male morphs (fighter- and scrambler-selected populations) that differ in relative fitness depending on temperature. We show that different mechanisms underlie the divergence in thermal response between fighter- and scrambler-selected populations at decreased vs. increased temperature. Temperature decrease to 18 °C was associated with higher transcriptomic plasticity of males with more elaborate armaments, as indicated by a significant selection-by-temperature interaction effect on the expression of 40 genes, 38 of which were upregulated in fighter-selected populations in response to temperature decrease. In response to 28 °C, no selection-by-temperature interaction in gene expression was detected. Hence, differences in phenotypic response to temperature increase likely depended on genes associated with their distinct morph-specific thermal tolerance. Selection of males also drove gene expression patterns in females. These patterns could be associated with temperature-dependent fitness differences between females from fighter- vs. scrambler-selected populations reported in previous studies. Our study shows that selection for divergent male sexually selected morphologies and behaviors has a potential to drive divergence in metabolic pathways underlying plastic response to temperature in both sexes.

6Pgdh polymorphism in wild bulb mite populations: prevalence, environmental correlates and life history trade-offs.

Genetic polymorphism in key metabolic genes plays a pivotal role in shaping phenotypes and adapting to varying environments. Polymorphism in the metabolic gene 6-phosphogluconate dehydrogenase (6Pgdh) in bulb mites, Rhizoglyphus robini is characterized by two alleles, S and F, that differ by a single amino acid substitution and correlate with male reproductive fitness. The S-bearing males demonstrate a reproductive advantage. Although the S allele rapidly fixes in laboratory settings, the persistence of polymorphic populations in the wild is noteworthy. This study examines the prevalence and stability of 6Pgdh polymorphism in natural populations across Poland, investigating potential environmental influences and seasonal variations. We found widespread 6Pgdh polymorphism in natural populations, with allele frequencies varying across locations and sampling dates but without clear geographical or seasonal clines. This widespread polymorphism and spatio-temporal variability may be attributed to population demography and gene flow between local populations. We found some correlation between soil properties, particularly cation content (Na, K, Ca, and Mg) and 6Pgdh allele frequencies, showcasing the connection between mite physiology and soil characteristics and highlighting the presence of environment-dependent balancing selection. We conducted experimental fitness assays to determine whether the allele providing the advantage in male-male competition has antagonistic effects on life-history traits and if these effects are temperature-dependent. We found that temperature does not differentially influence development time or juvenile survival in different 6Pgdh genotypes. This study reveals the relationship between genetic variation, environmental factors, and reproductive fitness in natural bulb mite populations, shedding light on the dynamic mechanisms governing 6Pgdh polymorphism.

Sexual and ecological selection on a sexual conflict gene.

Sexual selection and conflict can act on genes with important metabolic functions, potentially shaping standing genetic variance in such genes and thus evolutionary potential of populations. Here, using experimental evolution, we show how reproductive competition intensity and thermal environment affect selection on phosphogluconate dehydrogenase (6Pgdh)-a metabolic gene involved in sexual selection and conflict in the bulb mite. The S allele of 6Pgdh increases male success in reproductive competition, but is detrimental to S-bearing males' partners. We found that the rate of the S allele spread increased with the proportion of males in the experimental populations, illustrating that harm to females is more easily compensated for males under more intense sexual competition. Furthermore, we found that under equal sex ratio, the S allele spreads faster at higher temperature. While the direction of selection on 6Pgdh was not reversed in any of the conditions we tested, which would be required for environmental heterogeneity to maintain polymorphism at this locus, our study highlights that ecological and sexual selection can jointly affect selection on important metabolic enzymes.

Does operational sex ratio influence relative strength of purging selection in males versus females?

According to theory, sexual selection in males may efficiently purge mutation load of sexual populations, reducing or fully compensating 'the cost of males'. For this to occur, mutations not only need to be deleterious to both sexes, they also must affect males more than females. A frequently overlooked problem is that relative strength of selection on males versus females may vary between environments, with social conditions being particularly likely to affect selection in males and females differently. Here, we induced mutations in red flour beetles (Tribolium castaneum) and tested their effect in both sexes under three different operational sex ratios (1:2, 1:1 and 2:1). Induced mutations decreased fitness of both males and females, but their effect was not stronger in males. Surprisingly, operational sex ratio did not affect selection against deleterious mutations nor its relative strength in the sexes. Thus, our results show no support for the role of sexual selection in the evolutionary maintenance of sex.

Relative costs and benefits of alternative reproductive phenotypes at different temperatures - genotype-by-environment interactions in a sexually selected trait.

BACKGROUND: The maintenance of considerable genetic variation in sexually selected traits (SSTs) is puzzling given directional selection expected to act on these traits. A possible explanation is the existence of a genotype-by-environment (GxE) interaction for fitness, by which elaborate SSTs are favored in some environments but selected against in others. In the current study, we look for such interactions for fitness-related traits in the bulb mite, a male-dimorphic species with discontinuous expression of a heritable SST in the form of enlarged legs that are used as weapons. RESULTS: We show that evolution at 18 °C resulted in populations with a higher prevalence of this SST compared to evolution at 24 °C. We further demonstrate that temperature modified male reproductive success in a way that was consistent with these changes. There was a genotype-by-environment interaction for reproductive success - at 18 °C the relative reproductive success of armored males competing with unarmored ones was higher than at the moderate temperature of 24 °C. However, male morph did not have interactive effects with temperature with respect to other life history traits (development time and longevity). CONCLUSIONS: A male genotype that is associated with the expression of a SST interacted with temperature in determining male reproductive success. This interaction caused an elaborate SST to evolve in different directions (more or less prevalent) depending on the thermal environment. The implication of this finding is that seasonal temperature fluctuations have the potential to maintain male polymorphism within populations. Furthermore, spatial heterogeneity in thermal conditions may cause differences among populations in SST selection. This could potentially cause selection against male immigrants from populations in different environments and thus strengthen barriers to gene flow.

Proximate mechanisms of the differences in reproductive success of males bearing different alleles of Pgdh - a gene involved in a sexual conflict in bulb mite.

Enzyme polymorphism in phosphogluconate dehydrogenase (Pgdh) is a striking example of single gene polymorphism involved in sexual conflict in bulb mite Rhizoglyphus robini. Males homozygous for the S Pgdh allele were shown to achieve higher reproductive success than FF homozygous males, while negatively influencing fecundity of their female partners. Here, we investigate proximate mechanisms responsible for the increased reproductive success of SS males and find that the S allele is associated with shorter time until copulation, higher copulation frequency and increased sperm production. We also show that Pgdh alleles are probably codominant, with SS males gaining the highest reproductive success, FF males - the lowest - and FS-heterozygous males taking an intermediate position in all fitness parameters differentiating males of different genotypes. Additionally, we confirm the negative effect that S-bearing males impose on the fecundity of females they mate with, showing a clear pattern of interlocus sexual conflict. We discuss that this effect is probably associated with increased copulation frequency. Whereas, contrary to what we have predicted, the S allele does not cause increased general male mobility, we speculate that the S allele-bearing males are more efficient in forcing copulation and/or detecting females.

Complex response in size-related traits of bulb mites (Rhizoglyphus robini) under elevated thermal conditions - an experimental evolution approach.

Temperature is a key environmental factor affecting almost all aspects of life history in ectotherms. Theory predicts that they grow faster, reach smaller sizes and produce smaller offspring when temperature increases. In addition, temperature changes, through their effects on metabolism, may also influence the expression of alternative reproductive phenotypes (ARPs) in ectotherms. Although many studies have investigated the phenotypic plasticity of life history traits in relation to temperature change, little is known about how those traits and phenotypic plasticity may evolve together. In our study we subjected bulb mites (non-model, soil organisms that normally experience rather stable thermal conditions) to experimental evolution in two temperature treatments: control (24°C) and elevated (28°C). After 18 generations, we measured adult body size, egg size and development time of both treatments at control as well as at elevated temperatures (test temperatures). Thus, we were able to detect genetic changes (the effects of selection temperature) and environmental effects (the effects of test temperature). We also observed the ARP expression throughout the experimental evolution. Our results revealed quite complex patterns of life history in traits response to temperature. Mites developed faster and reached smaller sizes at increased temperature, but evolutionary responses to increased temperature were not always parallel to the observed phenotypic plasticity. Additionally, despite smaller body sizes, females laid larger eggs at higher temperature. This effect was more pronounced in animals evolving at elevated temperature. Evolution at increased temperature also affected ARP expression, with the proportion of armored fighters decreasing from generation to generation. We propose that this could be the consequence of temperature sensitivity of the cost-to-benefits ratio of expressing ARPs.

Mating system affects population performance and extinction risk under environmental challenge.

Failure of organisms to adapt to sudden environmental changes may lead to extinction. The type of mating system, by affecting fertility and the strength of sexual selection, may have a major impact on a population's chances to adapt and survive. Here, we use experimental evolution in bulb mites (Rhizoglyphus robini) to examine the effects of the mating system on population performance under environmental change. We demonstrate that populations in which monogamy was enforced suffered a dramatic fitness decline when evolving at an increased temperature, whereas the negative effects of change in a thermal environment were alleviated in polygamous populations. Strikingly, within 17 generations, all monogamous populations experiencing higher temperature went extinct, whereas all polygamous populations survived. Our results show that the mating system may have dramatic effects on the risk of extinction under environmental change.

Sexual conflict in a changing environment.

Sexual conflict has extremely important consequences for various evolutionary processes including its effect on local adaptation and extinction probability during environmental change. The awareness that the intensity and dynamics of sexual conflict is highly dependent on the ecological setting of a population has grown in recent years, but much work is yet to be done. Here, we review progress in our understanding of the ecology of sexual conflict and how the environmental sensitivity of such conflict feeds back into population adaptivity and demography, which, in turn, determine a population's chances of surviving a sudden environmental change. We link two possible forms of sexual conflict - intralocus and interlocus sexual conflict - in an environmental context and identify major gaps in our knowledge. These include sexual conflict responses to fluctuating and oscillating environmental changes and its influence on the interplay between interlocus and intralocus sexual conflict, among others. We also highlight the need to move our investigations into more natural settings and to investigate sexual conflict dynamics in wild populations.

Male-limited secondary sexual trait interacts with environment in determining female fitness.

Selection for secondary sexual trait (SST) elaboration may increase intralocus sexual conflict over the optimal values of traits expressed from shared genomes. This conflict can reduce female fitness, and the resulting gender load can be exacerbated by environmental stress, with consequences for a population's ability to adapt to novel environments. However, how the evolution of SSTs interacts with environment in determining female fitness is not well understood. Here, we investigated this question using replicate lines of bulb mites selected for increased or decreased prevalence of a male SST-thickened legs used as weapons. The fitness of females from these lines was measured at a temperature to which the mites were adapted (24°C), as well as at two novel temperatures: 18°C and 28°C. We found the prevalence of the SST interacted with temperature in determining female fecundity. At 28°C, females from populations with high SST prevalence were less fecund than females from populations in which the SST was rare, but the reverse was true at 18°C. Thus, a novel environment does not universally depress female fitness more in populations with a high degree of sexually selected dimorphism. We discuss possible consequences of the interaction we detected for adaptation to novel environments.

Fitness Effects of Thermal Stress Differ Between Outcrossing and Selfing Populations in Caenorhabditis elegans.

The maintenance of males and outcrossing is widespread, despite considerable costs of males. By enabling recombination between distinct genotypes, outcrossing may be advantageous during adaptation to novel environments and if so, it should be selected for under environmental challenge. However, a given environmental change may influence fitness of male, female, and hermaphrodite or asexual individuals differently, and hence the relationship between reproductive system and dynamics of adaptation to novel conditions may not be driven solely by the level of outcrossing and recombination. This has important implications for studies investigating the evolution of reproductive modes in the context of environmental changes, and for the extent to which their findings can be generalized. Here, we use Caenorhabditis elegans-a free-living nematode species in which hermaphrodites (capable of selfing but not cross-fertilizing each other) coexist with males (capable of fertilizing hermaphrodites)-to investigate the response of wild type as well as obligatorily outcrossing and obligatorily selfing lines to stressfully increased ambient temperature. We found that thermal stress affects fitness of outcrossers much more drastically than that of selfers. This shows that apart from the potential for recombination, the selective pressures imposed by the same environmental change can differ between populations expressing different reproductive systems and affect their adaptive potential.

Do males pay for sex? Sex-specific selection coefficients suggest not.

Selection acting on males can reduce mutation load of sexual relative to asexual populations, thus mitigating the twofold cost of sex, provided that it seeks and destroys the same mutations as selection acting on females, but with higher efficiency. This could happen due to sexual selection-a potent evolutionary force that in most systems predominantly affects males. We used replicate populations of red flour beetles (Tribolium castaneum) to study sex-specific selection against deleterious mutations introduced with ionizing radiation. We found no evidence for selection being stronger in males than in females; in fact, we observed a nonsignificant trend in the opposite direction. This suggests that selection on males does not reduce mutation load below the level expected under the (hypothetical) scenario of asexual reproduction. Additionally, we employed a novel approach, based on a simple model, to quantify the relative contributions of sexual and offspring viability selection to the overall selection observed in males. We found them to be similar in magnitude; however, only the offspring viability component was statistically significant. In summary, we found no support for the hypothesis that selection on males in general, and sexual selection in particular, contributes to the evolutionary maintenance of sex.

Transcriptomics of Intralocus Sexual Conflict: Gene Expression Patterns in Females Change in Response to Selection on a Male Secondary Sexual Trait in the Bulb Mite.

Intralocus sexual conflict (IASC) prevents males and females from reaching their disparate phenotypic optima and is widespread, but little is known about its genetic underpinnings. In Rhizoglyphus robini, a mite species with alternative male morphs, elevated sexual dimorphism of the armored fighter males (compared to more feminized scramblers males) was previously reported to be associated with increased IASC. Because IASC persists if gene expression patterns are correlated between sexes, we compared gene expression patterns of males and females from the replicate lines selected for increased proportion of fighter or scrambler males (F- and S-lines, respectively). Specifically, we tested the prediction that selection for fighter morph caused correlated changes in gene expression patterns in females. We identified 532 differentially expressed genes (FDR < 0.05) between the F-line and S-line males. Consistent with the prediction, expression levels of these genes also differed between females from respective lines. Thus, significant proportion of genes differentially expressed between sexually selected male phenotypes showed correlated expression levels in females, likely contributing to elevated IASC in F-lines reported in a previous study.

Selection for alternative male reproductive tactics alters intralocus sexual conflict.

Intralocus sexual conflict (IASC) arises when fitness optima for a shared trait differ between the sexes; such conflict may help maintain genetic variation within populations. Sex-limited expression of sexually antagonistic traits may help resolve the conflict, but the extent of this resolution remains a subject of debate. In species with alternative male reproductive tactics, unresolved conflict should manifest more in a more sexually dimorphic male phenotype. We tested this prediction in the bulb mite (Rhizoglyphus robini), a species in which aggressive fighters coexist with benign scramblers. To do this, we established replicated lines in which we increased the proportion of each of the alternative male morphs using artificial selection. After approximately 40 generations, the proportion of fighters and scramblers stabilized at >0.9 in fighter- and scrambler-selected lines, respectively. We then measured several female fitness components. As predicted by IASC theory, female fecundity and longevity were lower in lines selected for fighters and higher in lines selected for scramblers. This finding indicates that sexually selected phenotypes are associated with an ontogenetic conflict that is not easily resolved. Furthermore, we suggest that IASC may be an important mechanism contributing to the maintenance of genetic variation in the expression of alternative reproductive tactics.

No evidence for reproductive isolation through sexual conflict in the bulb mite Rhizoglyphus robini.

Sexual conflict leading to sexual antagonistic coevolution has been hypothesized to drive reproductive isolation in allopatric populations and hence lead to speciation. However, the generality of this speciation mechanism is under debate. We used experimental evolution in the bulb mite Rhizoglyphusrobini to investigate whether sexual conflict promotes reproductive isolation measured comprehensively to include all possible pre- and post-zygotic mechanisms. We established replicate populations in which we either enforced monogamy, and hence removed sexual conflict by making male and female evolutionary interests congruent, or allowed promiscuity. After 35 and 45 generations of experimental evolution, we found no evidence of reproductive isolation between the populations in any of the mating systems. Our results indicate that sexual conflict does not necessarily drive fast reproductive isolation and it may not be a ubiquitous mechanism leading to speciation.