Genotype-environment interactions in determining fitness in dense,artificial populations of Phlox drummondii

Summary Survivorship and growth of individuals of the annual plant Phlox drummondii were examined in artificial, density-stressed populations composed of five genetically differentiated varieties sown in monocultures and in five pairwise mixtures. All experiments were replicated under two treatments of nutrient availability.Varieties differed significantly in competitive ability, and relative competitive success was shown to be highly habitat (treatment) dependent. In 19 of 20 mixed populations, the relative rankings of pairs of competitors were consistent with a priori expectations based on the morphological and/or historical differences between seed sources, but were not predictable from relative tolerances to density stress in monoculture.Estimated selection coefficients based on relative competitive abilities ranged from 0.04 to 0.94. Despite these large fitness differences among competing varieties, most of the variance in the absolute fitness of individuals resided within varieties. This variation may be mostly environmental.In the high nutrient treatment, where competition for light was most intense, variation in individual plant size was lower in mixed culture as compared to monocultures. Genetic variation for competitive ability may actually lead to greater size uniformity among survivors by promoting an early initiation of thinning, thereby reducing the density stress on survivors.     

Epistasis in monkeyflowers

Epistasis contributes significantly to intrapopulation variation in floral morphology, development time, and male fitness components of Mimulus guttatus. This is demonstrated with a replicated line-cross experiment involving slightly over 7000 plants. The line-cross methodology is based on estimates for means. It thus has greater power than the variance partitioning approaches historically used to estimate epistasis within populations. The replication of the breeding design across many pairs of randomly extracted, inbred lines is necessary given the diversity of multilocus genotypes residing within an outbred deme. Male fitness is shown to exhibit synergistic epistasis, an accelerating decline in fitness with inbreeding. Synergism is a necessary, but not sufficient, condition for a mutational deterministic hypothesis for the evolutionary maintenance of sexual reproduction. Unlike male fitness measures, flower morphology and development time yield positive evidence of epistasis but not of synergism. The results for these traits suggest that epistatic effects are variable across genetic backgrounds or sets of interacting loci.     

Experimental evolution of field populations of Daphnia magna in response to parasite treatment

Although there is little doubt that hosts evolve to reduce parasite damage, little is known about the evolutionary time scale on which host populations may adapt under natural conditions. Here we study the effects of selection by the microsporidian parasite Octosporea bayeri on populations of Daphnia magna. In a field study, we infected replicated populations of D. magna with the parasite, leaving control populations uninfected. After two summer seasons of experimental evolution (about 15 generations), the genetic composition of infected host populations differed significantly from the control populations. Experiments revealed that hosts from the populations that had evolved with the parasite had lower mortality on exposure to parasite spores and a higher competitive ability than hosts that had evolved without the parasite. In contrast, the susceptibility of the two treatment groups to another parasite, the bacterium Pasteuria ramosa, which was not present during experimental evolution of the populations, did not differ. Fitness assays in the absence of parasites revealed a higher fitness for the control populations, but only under low population density with high resource availability. Overall, our results show that, under natural conditions, Daphnia populations are able to adapt rapidly to the prevailing conditions and that this evolutionary change is specific to the environment.     

Experimental evolution exposes female and male responses to sexual selection and conflict in Tribolium castaneum

Between-individual variance in potential reproductive rate theoretically creates a load in reproducing populations by driving sexual selection of male traits for winning competitions, and female traits for resisting the costs of multiple mating. Here, using replicated experimental evolution under divergent operational sex ratios (OSR, 9:1 or 1:6 ♀:♂) we empirically identified the parallel reproductive fitness consequences for females and males in the promiscuous flour beetle Tribolium castaneum. Our results revealed clear evidence that sexual conflict resides within the T. castaneum mating system. After 20 generations of selection, females from female-biased OSRs became vulnerable to multiple mating, and showed a steep decrease in reproductive fitness with an increasing number of control males. In contrast, females from male-biased OSRs showed no change in reproductive fitness, irrespective of male numbers. The divergence in reproductive output was not explained by variation in female mortality. Parallel assays revealed that males also responded to experimental evolution: individuals from male-biased OSRs obtained 27% greater reproductive success across 7-day competition for females with a control male rival, compared to males from the female-biased lines. Subsequent assays suggest that these differences were not due to postcopulatory sperm competitiveness, but to precopulatory/copulatory competitive male mating behavior.     

The Experimental Assessment of Fitness in Drosophila. II. a Comparison of Competitive and Noncompetitive Measures

Twelve diverse strains of Drosophila melanogaster have been examined with respect to their individual fitness components and with respect to their relative performance under competitive and noncompetitive conditions. Individual fitness components included estimates of time until successful copulation (t), fecundity (f) and egg-to-adult viability (v), and a composite index of overall fitness of the form fv/t was used for comparisons among strains. Noncompetitive performance was assessed in terms of the biomass (standing crop) and productivity of equilibrium experimental populations. Competitive performance was assessed in terms of relative competitive ability vis-à-vis a standard compound-autosome-bearing strain in single-generation tests. A significant correlation was found between the composite index of individual fitness components and the competitive compound-autosome test. Although the biomass and productivity of equilibrium populations were correlated with each other, neither of these noncompetitive measures was correlated with individual fitness components or with the composite index. We suggest that the performance of strains in such noncompetitive tests may be related to what Wright has called the "mean selective value" of the populations. Judging from their association with the composite index of individual fitness components, competitive tests such as the compound-autosome test seem to be related more nearly to the average Darwinian fitness of the populations.     

SPERM COMPETITIVE ABILITY EVOLVES IN RESPONSE TO EXPERIMENTAL ALTERATION OF OPERATIONAL SEX RATIO

In naturally polygamous organisms such as Drosophila, sperm competitive ability is one of the most important components of male fitness and is expected to evolve in response to varying degrees of male–male competition. Several studies have documented the existence of ample genetic variation in sperm competitive ability of males. However, many experimental evolution studies have found sperm competitive ability to be unresponsive to selection. Even direct selection for increased sperm competitive ability has failed to yield any measurable changes. Here we report the evolution of sperm competitive ability (sperm defense‐P1, offense‐P2) in a set of replicate populations of Drosophila melanogaster subjected to altered levels of male–male competition (generated by varying the operational sex ratio) for 55–60 generations. Males from populations with female‐biased operational sex ratio evolved reduced P1 and P2, without any measurable change in the male reproductive behavior. Males in the male‐biased regime evolved increased P1, but there was no significant change in P2. Increase in P1 was associated with an increase in copulation duration, possibly indicating greater ejaculate investment by these males. This study is one of the few to provide empirical evidence for the evolution of sperm competitive ability of males under different levels of male–male competition.     

Genetic Adaptation to Captivity and Inbreeding Depression in Small Laboratory Populations of Drosophila Melanogaster

The rate of adaptation to a competitive laboratory environment and the associated inbreeding depression in measures of reproductive fitness have been observed in populations of Drosophila melanogaster with mean effective breeding size of the order of 50 individuals. Two large wild-derived populations and a long-established laboratory cage population were used as base stocks, from which subpopulations were extracted and slowly inbred under crowded conditions over a period of 210 generations. Comparisons have been made of the competitive ability and reproductive fitness of these subpopulations, the panmictic populations produced from them by hybridization and random mating and the wild- or cage-base populations from which they were derived. After an average of ~180 generations in the laboratory, the wild-derived panmictic populations exceeded the resampled natural populations by 75% in fitness under competitive conditions. The cage-derived panmictic population, after a total of 17 years in the laboratory, showed a 90% superiority in competitive ability over the corresponding wild population. In the inbred lines derived from the wild-base stocks, the average rate of adaptation was estimated to be 0.33 +/- 0.06% per generation. However, the gain in competitive ability was more than offset by inbreeding depression at an initial rate of ~2% per generation. The effects of both adaptation and inbreeding on reproductive ability in a noncompetitive environment were found to be minor by comparison. The maintenance of captive populations under noncompetitive conditions can therefore be expected to minimize adaptive changes due to natural selection in the changed environment.     

The impact of infection on host competition and its relationship to parasite persistence in a Daphnia microparasite system

Evolutionary studies often estimate fitness components with the aim to make predictions about the outcome of selection. Depending on the system and the question, different fitness components are used, but their usefulness for predicting the outcome of selection is rarely tested. Here we estimate host fitness components in different ways with the aim to test how well they agree with each other and how well they predict host fitness at the population level in the presence of the parasite. We use a Daphnia magna-microparasite system to study the competitive ability of host clones in the absence and presence of the parasite, the infection intensity of the parasite in individuals of twelve host clones (an estimate of both host resistance and parasite reproductive success), and parasite persistence in small host populations (an estimate of R ₀ of the parasite). Analysis of host competitive ability and parasite persistence reveals strong host genotype effects, while none are found for infection intensity. Host competitive ability further shows a genotype-specific change upon infection, which is correlated with the relative persistence of the parasite in the competing hosts. Hosts in which the parasite persists better suffer a competitive disadvantage in the parasite’s presence. This suggests that in this system, parasite-mediated selection can be predicted by parasite persistence, but not by parasite infection intensity.     

POTENTIAL FITNESS TRADE‐OFFS FOR THERMAL TOLERANCE IN THE INTERTIDAL COPEPOD TIGRIOPUS CALIFORNICUS

Thermal adaptation to spatially varying environmental conditions occurs in a wide range of species, but what is less clear is the nature of fitness trade‐offs associated with this temperature adaptation. Here, populations of the intertidal copepod Tigriopus californicus are examined at both local and latitudinal scales to determine whether these populations have evolved differences in their survival under high temperature stress. A clear pattern of increasing high temperature stress tolerance is seen with decreasing latitude, consistent with temperature adaptation. Additionally, there is also evidence for significant variation in thermal tolerance on a smaller scale. The competitive fitness of pairs of northern and southern copepod populations were also examined under a series of lower, more moderate temperatures. These fitness assays show that the southern populations that have the best survival under extreme high temperatures have lowered competitive fitness at the lower temperatures tested, whereas the fitness of the southern populations exceeded that of the northern populations at the highest temperatures tested. Combined, these results suggest that there may be evolutionary trade‐offs between performance at high and stressful temperatures and fitness at moderate temperatures in this species.     

Introduced Drosophila subobscura populations perform better than native populations during an oviposition choice task due to increased fecundity but similar learning ability

The success of invasive species is tightly linked to their fitness in a putatively novel environment. While quantitative components of fitness have been studied extensively in the context of invasive species, fewer studies have looked at qualitative components of fitness, such as behavioral plasticity, and their interaction with quantitative components, despite intuitive benefits over the course of an invasion. In particular, learning is a form of behavioral plasticity that makes it possible to finely tune behavior according to environmental conditions. Learning can be crucial for survival and reproduction of introduced organisms in novel areas, for example, for detecting new predators, or finding mates or oviposition sites. Here we explored how oviposition performance evolved in relation to both fecundity and learning during an invasion, using native and introduced Drosophila subobscura populations performing an ecologically relevant task. Our results indicated that, under comparable conditions, invasive populations performed better during our oviposition task than did native populations. This was because invasive populations had higher fecundity, together with similar cognitive performance when compared to native populations, and that there was no interaction between learning and fecundity. Unexpectedly, our study did not reveal an allocation trade‐off (i.e., a negative relationship) between learning and fecundity. On the contrary, the pattern we observed was more consistent with an acquisition trade‐off, meaning that fecundity could be limited by availability of resources, unlike cognitive ability. This pattern might be the consequence of escaping natural enemies and/or competitors during the introduction. The apparent lack of evolution of learning may indicate that the introduced population did not face novel cognitive challenges in the new environment (i.e., cognitive “pre‐adaptation”). Alternatively, the evolution of learning may have been transient and therefore not detected.     

THE EFFECT OF COEXISTENCE ON COMPETITIVE OUTCOME IN TRIBOLIUM CASTANEUM AND TRIBOLIUM CONFUSUM

We describe an experiment exploring the effects of coexistence and population differentiation on the competitive outcome of two species of Tribolium flour beetles, T. castaneum and T. confusum. The only manipulation was whether the populations used in the competitive phase of the study were raised initially in mixed-species communities, single-species populations, or in the standard culture conditions used to maintain stocks in the laboratory. Any treatment effects observed were due to natural selection acting within populations and genetic drift. In the competitive phase, we examined 10 mixed-species communities and 10 pairs of single-species populations. We replicated each community 15 times to provide an assessment of the distribution of competitive outcome. Statistical analysis demonstrates the lineages within the treatments became highly differentiated for all measures of competitive outcome: the outcome of competition (which species won), time to extinction of one of the competing species, and census history. The fraction of the variance that is among lineages has been referred to as the group or community heritability. All of these measures of competitive outcome had high community level heritabilities indicating that competitive ability would evolve rapidly as a result of group or community level selection. In contrast, competitive outcome was not affected by whether the two species had coexisted prior to the competitive phase. This indicates that the outcome of competition was not systematically changed by processes acting within the two species communities.     

LABORATORY EVOLUTION OF LONGEVITY AND REPRODUCTIVE FITNESS COMPONENTS IN MALE FRUIT FLIES: MATING ABILITY

Populations of Drosophila melanogaster that had been selected for divergent rates of senescence were compared with respect to age-specific male mating ability. The competitive mating ability of males from populations with delayed senescence was inferior to that of males from populations with higher rates of senescence when males were young. This relationship was reversed when males were older. For noncompetitive mating ability and for recovery of fertility after an exhaustive mating bout, there was no difference between populations with different rates of senescence when males were young. However when males were older, flies from populations selected for delayed senescence again had superior mating ability. Thus, rates of male reproductive senescence can be altered in predictable ways by natural selection. The results for the competitive mating tests are consistent with the hypothesis that antagonistic gene action between early- and late-life fitness components influences the evolution of senescence in these populations.     

Inclusive fitness,asymmetric competition and kin selection in plants

The findings that some plants alter their competitive phenotype in response to genetic relatedness of its conspecific neighbour (and presumed competitor) has spurred an increasing interest in plant kin‐interactions. This phenotypic response suggests the ability to assess the genetic relatedness of conspecific competitors, proposing kin selection as a process that can influence plant competitive interactions. Kin selection can favour restrained competitive growth towards kin, if the fitness loss from reducing own growth is compensated by increased fitness in the related neighbour. This may lead to positive frequency dependency among related conspecifics with important ecological consequences for species assemblage and coexistence. However, kin selection in plants is still controversial. First, many studies documenting a plastic response to neighbour relatedness do not estimate fitness consequences of the individual that responds, and when estimated, fitness of individuals grown in competition with kin did not necessarily exceed that of individuals grown in non‐kin groups. Although higher fitness in kin groups could be consistent with kin selection, this could also arise from mechanisms like asymmetric competition in the non‐kin groups. Here we outline the main challenges for studying kin selection in plants taking genetic variation for competitive ability into account. We emphasize the need to measure inclusive fitness in order to assess whether kin selection occurs, and show under which circumstances kin selected responses can be expected. We also illustrate why direct fitness estimates of a focal plant, and group fitness estimates are not suitable for documenting kin selection. Importantly, natural selection occurs at the individual level and it is the inclusive fitness of an individual plant – not the mean fitness of the group – that can capture if a differential response to neighbour relatedness is favoured by kin selection.     

EXPERIMENTAL STUDIES OF PLEIOTROPY AND EPISTASIS IN ESCHERICHIA COLI. II. COMPENSATION FOR MALADAPTIVE EFFECTS ASSOCIATED WITH RESISTANCE TO VIRUS T4

Mutations in Escherichia coli that confer resistance to virus T4 also have maladaptive effects that reduce competitive fitness. After resistant populations had evolved for 400 generations in the absence of T4, their fitness approached that of sensitive populations allowed to evolve under identical conditions. However, the resistant populations had not reverted to sensitivity. Instead, this convergence in fitness resulted from genetic changes that compensated for maladaptive pleiotropic effects of the resistance mutations. An allele selected in an evolving resistant population reduced the competitive disadvantage associated with resistance by almost half. Interestingly, this allele was also beneficial in sensitive populations, although its fitness advantage was only about one-fifth as great as it was in the resistant population. These results run counter to a commonly held view that trade-offs between components of fitness should become more pronounced as populations approach their “selective equilibria.” If a trade-off derives from some limiting energetic or material currency, then it is likely to become more pronounced as a population becomes more finely adapted. If a trade-off derives from the disruption of genetic integration, then it is likely to be diminished with further adaptation.     

Rapid fitness recovery in mutationally degraded lines of Caenorhabditis elegans

Abstract Deleterious mutation accumulation has been implicated in many biological phenomena and as a potentially significant threat to human health and the persistence of small populations. The vast majority of mutations with effects on fitness are known to be deleterious in a given environment, and their accumulation results in mean population fitness decline. However, whether populations are capable of recovering from negative effects of prolonged genetic bottlenecks via beneficial or compensatory mutation accumulation has not previously been tested. To address this question, long-term mutation-accumulation lines of the nematode Caenorhabditis elegans , previously propagated as single individuals each generation, were maintained in large population sizes under competitive conditions. Fitness assays of these lines and comparison to parallel mutation-accumulation lines and the ancestral control show that, while the process of fitness restoration was incomplete for some lines, full recovery of mean fitness was achieved in fewer than 80 generations. Several lines of evidence indicate that this fitness restoration was at least partially driven by compensatory mutation accumulation rather than a result of a generic form of laboratory adaptation. This surprising result has broad implications for the influence of the mutational process on many issues in evolutionary and conservation biology.     

Relaxation of herbivore‐mediated selection drives the evolution of genetic covariances between plant competitive and defense traits

Insect herbivores are important mediators of selection on traits that impact plant defense against herbivory and competitive ability. Although recent experiments demonstrate a central role for herbivory in driving rapid evolution of defense and competition‐mediating traits, whether and how herbivory shapes heritable variation in these traits remains poorly understood. Here, we evaluate the structure and evolutionary stability of the G matrix for plant metabolites that are involved in defense and allelopathy in the tall goldenrod, Solidago altissima. We show that G has evolutionarily diverged between experimentally replicated populations that evolved in the presence versus the absence of ambient herbivory, providing direct evidence for the evolution of G by natural selection. Specifically, evolution in an herbivore‐free habitat altered the orientation of G , revealing a negative genetic covariation between defense‐ and competition‐related metabolites that is typically masked in herbivore‐exposed populations. Our results may be explained by predictions of classical quantitative genetic theory, as well as the theory of acquisition‐allocation trade‐offs. The study provides compelling evidence that herbivory drives the evolution of plant genetic architecture.     

THE EFFECT OF NUCLEAR AND CYTOPLASMIC GENES ON FITNESS AND LOCAL ADAPTATION IN AN ANNUAL LEGUME,CHAMAECRISTA FASCICULATA

The role of nuclear genes in local adaptation has been well documented. However, the role of maternally inherited cytoplasmic genes to the evolution of natural populations has been relatively unstudied. To evaluate the contribution of cytoplasmic and nuclear genomes and their interactions to local adaptation we created second-generation backcross hybrids between a Maryland and an Illinois population of the annual legume Chamaecrista fasciculata. Backcross progeny were planted in the sites native to each population for two years and we quantified germination, survivorship, fruit production, vegetative biomass, and cumulative fitness. We found limited evidence for the contribution of either cytoplasmic or nuclear genes to local adaptation. In Maryland plants had greater survivorship, biomass, fruit production, and cumulative fitness if their nuclear genome was composed predominately of native Maryland genes; cytoplasmic genes did not affect fitness. In Illinois local cytoplasm marginally enhanced fitness, whereas Maryland nuclear genes outperformed local nuclear genes. Interactions between cytoplasmic and nuclear genes influenced seed weight, vegetative biomass, and fitness and therefore may affect evolution of these characters. Genetic effects were stronger acting through seed size than directly on characters. However, seed size differences between the two populations were largely genetic and therefore selection on fitness components is likely to result in evolutionary change. The contribution of nuclear and cytoplasmic genes to fitness components varied across sites and years, suggesting that experiments should be replicated and conducted under natural conditions to understand the influence of these genomes and their interactions to population differentiation.     

Genetic variation in tolerance of competition and neighbour suppression in Arabidopsis thaliana

Intraspecific competitive interactions can profoundly influence phenotypic evolution. However, prior studies have rarely evaluated the evolutionary potential of the two components of competitive ability, tolerance of competition and suppression of neighbours. Here, we grow a set of 20 Arabidopsis thaliana recombinant inbred lines in three competitive treatments (noncompetitive, intra‐genotypic competition and inter‐genotypic competition) to examine if there is genetic variation for the components of competitive ability and whether neighbour relatedness has an effect on fitness. We find evidence for genetic variation in tolerance of competition and neighbour suppression and that these two competitive strategies are correlated, such that genotypes that tolerate competition will also strongly suppress neighbours. We further observe that the effect of neighbour relatedness on fitness of target individuals depends on neighbour identity, i.e. whether target individuals perform better when competing against self vs. nonself individuals depends on the genotypic identity of the nonself neighbour. The results are particularly relevant to evolutionary responses under multi‐level selection.     

Fitness consequences of allorecognition-mediated agonistic interactions in the colonial hydroid Hydractinia [GM

In sessile and sedentary organisms, competition for space may have fitness consequences that depend strongly on ecological context. Colonial hydroids in the genus Hydractinia use an inducible defense when encountering conspecifics, and intraspecific competition is common in natural populations, often resulting in complete overgrowth of subordinate competitors. My goal in this study was to quantify the impacts of agonistic interactions in Hydractinia [GM] (an undescribed species from the Gulf of Mexico) in terms of three primary fitness components: colony survival, growth rate, and immature gonozooid production. The results demonstrate that the fitness consequences of intraspecific competition depend on the size at which competitive encounters are initiated and the growth form (an indicator of competitive ability) of the competitors. Moreover, some competing colonies consistently produced more immature gonozooids than the controls without competition, and they exhibited extremely low mortality even after 90 days of growth. These results have several ramifications. First, agonistic interactions do not always proceed to competitive elimination. Second, the increase in production of immature gonozooids--an investment in future reproduction--in response to intraspecific competition supports the hypothesis that indeterminately growing organisms increase sexual reproductive effort when growth becomes limiting. Lastly, in light of known ontogenetic variation in the ability of Hydractinia to differentiate among genetically related colonies, strongly size-dependent fitness consequences are consistent with an adaptive, kin-discriminating allorecognition system.     

Selection against immigrants in wild seabird populations

Immigration is a major demographic parameter shaping population dynamics and is an important driver of eco‐evolutionary patterns, but the fitness consequences for individuals following their settlement to a new population (immigrants) remain poorly tested in wild animal populations, particularly among long‐lived species. Here we show that immigrants have a lower fitness than residents in three wild seabird populations (wandering albatross Diomedea exulans, southern fulmar Fulmarus glacialoides, snow petrel Pagodroma nivea). Across all species and during a 32‐year period, immigrants made on average ?9 to 29% fewer breeding attempts, had 5–31% fewer fledglings, had 2–16% lower breeding success and produced 6–46% fewer recruits. Female immigration and male residency were also favored through differences in breeding performance. We provide evidence for selection against immigrants in wild populations of long‐lived species and our results are consistent with female‐biased dispersal in birds being driven by asymmetric limiting resources and the competitive ability of dispersers vs. non‐dispersers.