Divergent selection on flowering time contributes to local adaptation in Mimulus guttatus populations

The timing of when to initiate reproduction is an important transition in any organism's life cycle. There is much variation in flowering time among populations, but we do not know to what degree this variation contributes to local adaptation. Here we use a reciprocal transplant experiment to examine the presence of divergent natural selection for flowering time and local adaptation between two distinct populations of Mimulus guttatus. We plant both parents and hybrids (to tease apart differences in suites of associated parental traits) between these two populations into each of the two native environments and measure floral, vegetative, life-history, and fitness characters to assess which traits are under selection at each site. Analysis of fitness components indicates that each of these plant populations is locally adapted. We obtain striking evidence for divergent natural selection on date of first flower production at these two sites. Early flowering is favored at the montane site, which is inhabited by annual plants and characterized by dry soils in midsummer, whereas intermediate (though later) flowering dates are selectively favored at the temperate coastal site, which is inhabited by perennial plants and is almost continually moist. Divergent selection on flowering time contributes to local adaptation between these two populations of M. guttatus, suggesting that genetic differentiation in the timing of reproduction may also serve as a partial reproductive isolating barrier to gene flow among populations.     

Local adaptation in European populations of Arabidopsis lyrata (Brassicaceae)

We studied local adaptation to contrasting environments using an organism that is emerging as a model for evolutionary plant biology-the outcrossing, perennial herb Arabidopsis lyrata subsp. petraea (Brassicaceae). With reciprocal transplant experiments, we found variation in cumulative fitness, indicating adaptive differentiation among populations. Nonlocal populations did not have significantly higher fitness than the local population. Experimental sites were located in Norway (alpine), Sweden (coastal), and Germany (continental). At all sites after one year, the local population had higher cumulative fitness, as quantified by survival combined with rosette area, than at least one of the nonlocal populations. At the Norwegian site, measurements were done for two additional years, and fitness differences persisted. The fitness components that contributed most to differences in cumulative fitness varied among sites. Relatively small rosette area combined with a large number of inflorescences produced by German plants may reflect differentiation in life history. The results of the current study demonstrate adaptive population differentiation in A. lyrata along a climatic gradient in Europe. The studied populations harbor considerable variation in several characters contributing to adaptive population differentiation. The wealth of genetic information available makes A. lyrata a highly attractive system also for examining the functional and genetic basis of local adaptation in plants.     

Molecular and Quantitative Genetic Differentiation in Sitobion avenae Populations from Both Sides of the Qinling Mountains

Quantitative trait differences are often assumed to be correlated with molecular variation, but the relationship is not certain, and empirical evidence is still scarce. To address this issue, we sampled six populations of the cereal aphid Sitobion avenae from areas north and south of the Qinling Mountains, and characterized their molecular variation at seven microsatellite loci and quantitative variation at nine life-history traits. Our results demonstrated that southern populations had slightly longer developmental times of nymphs but much higher lifetime fecundity, compared to northern populations. Of the nine tested quantitative characters, eight differed significantly among populations within regions, as well as between northern and southern regions. Genetic differentiation in neutral markers was likely to have been caused by founder events and drift. Increased subdivision for quantitative characters was found in northern populations, but reduced in southern populations. This phenomenon was not found for molecular characters, suggesting the decoupling between molecular and quantitative variation. The pattern of relationships between F_ST and Q_ST indicated divergent selection and suggested that local adaptation play a role in the differentiation of life-history traits in tested S. avenae populations, particularly in those traits closely related to reproduction. The main role of natural selection over genetic drift was also supported by strong structural differences in G-matrices among S. avenae populations. However, cluster analyses did not result in two groups corresponding to northern and southern regions. Genetic differentiation between northern and southern populations in neutral markers was low, indicating considerable gene flow between them. The relationship between molecular and quantitative variation, as well as its implications for differentiation and evolution of S. avenae populations, was discussed.     

Divergence among arctic and alpine populations of the annual, Koenigia islandica: morphology, life-history, and phenology

Arctic and alpine habitats occur along complex environmental gradients, and over an extensive geographical range. Despite some selective forces common to these habitats, evolutionary divergence among populations of arctic and alpine plants along this gradient is expected. Of particular significance, both in the context of life-history theory and for implications of climate change, are the few annual species that have adapted to the constraints of an unpredictable, short growing season. In this study, morphological, life-history and phenological characters were found to differ significantly among six widely distributed populations of the arctic-alpine annual Koenigia islandica. On the basis of morphology and life-history traits, populations from high latitudes, with the exception of Svalbard, performed better in simulated arctic conditions, whereas the low latitude alpine plants from Colorado showed enhanced performance under simulated alpine conditions. On the basis of phenology, the six populations can be clearly grouped into arctic, high latitude alpine and alpine populations: arctic plants were found to develop and flower earliest; alpine plants latest. Because these results were obtained using seeds harvested from plants first grown through a complete generation in growth chambers, they indicate strong genetic differentiation. We discuss possible adaptive explanations for observed differences among the six geographically divergent populations.     

Local adaptation across a fertility gradient is influenced by soil biota in the invasive grass, <Emphasis Type="Italic">Bromus inermis</Emphasis>

Biotic soil factors, such as fungi, bacteria and herbivores affect resource acquisition and fitness in plants, yet little is known of their role as agents of selection. Evolutionary responses to these selective agents could be an important mechanism that explains the success of invasive species. In this study, we tested whether populations of the invasive grass Bromus inermis are adapted to their home soil environment, and whether biotic factors influence the magnitude of this adaptation. We selected three populations growing at sites that differed in soil fertility and grew individuals from each population in each soil. To assess whether biotic factors influence the magnitude of adaptation, we also grew the same populations in sterilized field soil. To further examine the role of one element of the soil biota (fungi) in local adaptation, we measured colonization by arbuscular mycorrhizal (AM) and septate fungi, and tested whether the extent of colonization differed between local and foreign plants. In non-sterilized (living) soil, there was evidence of a home site advantage because local plants produced significantly more biomass than at least one of the two populations of foreign plants in all three soil origins. By contrast, there was no evidence of a home site advantage in sterilized soil because local plants never produced significantly more biomass than either population of foreign plants. Fungal colonization differed between local and foreign plants in the living soil and this variation corresponded with biomass differences. When local plants produced more biomass than foreign plants, they were also less intensively colonized by AM fungi. Colonization by septate fungi did not vary between local and foreign plants. Our results suggest that biotic soil factors are important causes of plant adaptation, and that selection for reduced interactions with mycorrhizae could be one mechanism through which adaptation to a novel environment occurs.     

Fitness in Naturally Occurring and Restored Populations of a Grassland Plant Lychnis flos‐cuculi in a Swiss Agricultural Landscape

Wildflower seed mixtures are widely used for restoration of grasslands. However, the genetic and fitness consequences of using seed mixes have not been fully evaluated. Here, we studied the role of genetic diversity, origin (commercial regional seed mixtures, natural populations), and environmental conditions for the fitness of a grassland species Lychnis flos‐cuculi. First, we examined the relationship between genetic diversity, environmental parameters, and fitness in sown and natural populations of this species in a Swiss agricultural landscape. Second, we established an experiment in the study area and in an experimental garden to study the implications of local adaptation for plant fitness. Third, to examine the response of plants to different soil properties, we conducted an experiment in climate chambers, where we grew plants from sown and natural populations of L. flos‐cuculi as well as from seed suppliers on soils with different nutrient and moisture content. We detected no significant effect of genetic diversity on the fitness of sown and natural populations. There was no clear indication that plants from natural populations were better adapted to local environment than plants from sown populations or seed suppliers. However, plants of natural origin invested more into generative reproduction than plants from sown populations or seed suppliers. Furthermore, in the climate chamber, plants originating from natural populations tended to flower earlier. Our results indicate that using nonlocal seeds for habitat recreation may influence restoration success even if the seeds originate from the same seed zone as the restored site.     

Local adaptation in rotifer populations

The adaptation of organisms to their environment has been a subject of study for a long time. One method to study adaptations in populations involves comparing contemporary populations of the same species under different selective regimes, in what is known as a ??local adaptation?? study. A previous study of the cyclically parthenogenetic rotifer Brachionus plicatilis found high heritabilities for some life-history traits. Some of these life-history traits significantly differed among six populations from Eastern Spain and data suggested some traits to have higher evolutionary rates than neutral genetic markers. Here, by studying the same B. plicatilis populations, we examine the variation and possible local adaptation of their main life-history traits, closely related to fitness, in relation to habitat salinity and temperature. These environmental factors have been shown to play a key role in the ecological differentiation among co-generic species of B. plicatilis. The results obtained in this study show that: (1) the seasonality of rotifer populations from Eastern Spain has profoundly influenced sexual reproduction strategies; (2) salinity is probably a key factor in the ecological specialization of some populations; and (3) rotifer populations harbour high variability in their fitness components.     

Offspring fitness in relation to population size and genetic variation in the rare perennial plant species Gentiana pneumonanthe (Gentianaceae)

Seeds were sampled from 19 populations of the rare Gentiana pneumonanthe, ranging in size from 5 to more than 50,000 flowering plants. An analysis was made of variation in a number of life-history characters in relation to population size and offspring heterozygosity (based on seven polymorphic isozyme loci). Life-his-tory characters included seed weight, germination rate, proportion of seeds germinating, seedling mortality, seedling weight, adult weight, flower production per plant and proportion of plants flowering per family. Principal component analysis (PCA) reduced the dataset to three main fitness components. The first component was highly correlated with adult weight and flowering performance, the second with germination performance and the third component with seed and seedling weight and seedling mortality. The latter two components were considered as being maternally influenced, since these comprised life-history traits that were significantly correlated with seed weight. Multiple regression analysis showed that variation in the first fitness component was mainly associated with heterozygosity and not with population size, while the third fitness component was only correlated with population size and not with heterozygosity. The latter relationship appeared to be non-linear, which suggests a stronger loss of fitness in the smallest populations. The second (germination) component was neither correlated with population size nor with genetic variation. There was only a weak association between population size, heterozygosity and the population coefficients of variation for each life history character. Most correlation coefficients were negative, however, which suggests that there is more variation among progeny from smaller populations. We conclude that progeny from small populations of Gentiana pneumonanthe show reduced fitness and may be phenotypically more variable. One of the possible causes of the loss of fitness is a combination of unfavourable environmental circumstances for maternal plants in small populations and increased inbreeding. The higher phenotypic variation in small populations may also be a result of inbreeding, which can lead to deviation of individuals from the average phenotype through a loss of developmental stability.     

On the equivalence of host local adaptation and parasite maladaptation: an experimental test

In spatiotemporally varying environments, host-parasite coevolution may lead to either host or parasite local adaptation. Using reciprocal infestations over 11 pairs of plots, we tested local adaptation in the hen flea and its main host, the great tit. Flea reproductive success (number of adults at host fledging) was lower on host individuals from the same plot compared with foreign hosts (from another plot), revealing flea local maladaptation. Host reproductive success (number of fledged young) for nests infested by foreign fleas was lower compared with the reproductive success of controls, with an intermediate success for nests infested by local fleas. This suggests host local adaptation although the absence of local adaptation could not be excluded. However, fledglings were heavier and larger when reared with foreign fleas than when reared with local fleas, which could also indicate host local maladaptation if the fitness gain in offspring size offsets the potential cost in offspring number. Our results therefore challenge the traditional view that parasite local maladaptation is equivalent to host local adaptation. The differences in fledgling morphology between nests infested with local fleas and those with foreign fleas suggest that flea origin affects host resource allocation strategy between nestling growth and defense against parasites. Therefore, determining the mechanisms that underlie these local adaptation patterns requires the identification of the relevant fitness measures and life-history trade-offs in both species.     

Evidence for plasticity,but not local adaptation,in invasive Japanese knotweed (<Emphasis Type="Italic">Reynoutria japonica</Emphasis>) in North America

While local adaptation and phenotypic plasticity are commonly observed in species occupying heterogeneous environments, these phenomena are less well understood in invasive species. However, plant invasions offer the opportunity to study these dynamics as they occur in species colonizing new habitats. In this study, we examined local adaptation and phenotypic plasticity in an invasive plant, Reynoutria japonica, across a broad latitudinal range within North America. We performed full-factorial reciprocal transplants using plants from three sites and examined fitness responses in both sexual and clonal reproductive measures, as well as vegetative responses in height, basal stem diameter, and biomass. For all vegetative traits, there was a significant effect of source population, indicating genetic differentiation among populations. There was also a significant effect of transplant site, suggesting phenotypic plasticity. However, there was no evidence of local adaptation at the North American meta-population level for either measure of sexual or clonal fitness. All three comparisons for sexual fitness failed to show any differences between source populations, indicating a lack of local adaptation. For clonal fitness, two of the three comparisons showed local maladaptation, and only one showed greater fitness at the home compared to foreign sites, but this population had greater fitness at all sites, indicating greater fitness overall for this population rather than local adaptation. The fact that we did not detect consistent patterns of local adaptation in these populations across a broad geographic range is somewhat surprising given that local adaptation appears common in many species, including invasives, and that the populations have been established for over a century. However, the lack of local adaptation observed in this species may indicate that phenotypic plasticity within the species is sufficient to allow the persistence of R. japonica in a variety of environments across its invaded range.     

Strong patterns of intraspecific variation and local adaptation in Great Basin plants revealed through a review of 75 years of experiments

Variation in natural selection across heterogeneous landscapes often produces (a) among‐population differences in phenotypic traits, (b) trait‐by‐environment associations, and (c) higher fitness of local populations. Using a broad literature review of common garden studies published between 1941 and 2017, we documented the commonness of these three signatures in plants native to North America's Great Basin, an area of extensive restoration and revegetation efforts, and asked which traits and environmental variables were involved. We also asked, independent of geographic distance, whether populations from more similar environments had more similar traits. From 327 experiments testing 121 taxa in 170 studies, we found 95.1% of 305 experiments reported among‐population differences, and 81.4% of 161 experiments reported trait‐by‐environment associations. Locals showed greater survival in 67% of 24 reciprocal experiments that reported survival, and higher fitness in 90% of 10 reciprocal experiments that reported reproductive output. A meta‐analysis on a subset of studies found that variation in eight commonly measured traits was associated with mean annual precipitation and mean annual temperature at the source location, with notably strong relationships for flowering phenology, leaf size, and survival, among others. Although the Great Basin is sometimes perceived as a region of homogeneous ecosystems, our results demonstrate widespread habitat‐related population differentiation and local adaptation. Locally sourced plants likely harbor adaptations at rates and magnitudes that are immediately relevant to restoration success, and our results suggest that certain key traits and environmental variables should be prioritized in future assessments of plants in this region.     

RESPONSE TO NATURAL ENVIRONMENTAL HETEROGENEITY: MATERNAL EFFECTS AND SELECTION ON LIFE-HISTORY CHARACTERS AND PLASTICITIES IN MIMULUS GUTTATUS

Recent studies in plant populations have found that environmental heterogeneity and phenotypic selection vary at local spatial scales. In this study, I ask if there is evolutionary change in response to environmental heterogeneity and, if so, whether the response occurs for characters or character plasticities. I used vegetative clones of Mimulus guttatus to create replicate populations of 75 genotypes. These populations were planted into the natural habitat where they differed in mean growth, flowering phenology, and life span. This phenotypic variation was used to define selective environments. There was variation in fitness (flower production) among genotypes across all planting sites and in genotype response to the selective environment. Offspring from each site were grown in the greenhouse in two water treatments. Because each population initially had the same genetic composition, variation in the progeny between selective environments reveals either evolutionary change in response to environmental heterogeneity or environmental maternal effects. Plants from experimental sites that flowered earlier, had shorter life spans and were less productive, produced offspring that had more flowers, on average, and were less plastic in vegetative allocation than offspring of longer-lived plants from high-productivity areas. However, environmental maternal effects masked phenotypic differences in flower production. Therefore, although there was evidence of genetic differentiation in both life-history characters and their plasticities in response to small-scale environmental heterogeneity, environmental maternal effects may slow evolutionary change. Response to local-scale selective regimes suggests that environmental heterogeneity and local variation in phenotypic selection may act to maintain genetic variation.     

Effects of pollination by bats on the mating system of Ceiba pentandra (Bombacaceae) populations in two tropical life zones in Costa Rica

The identity and behavior of pollinators are among the main factors that determine the reproductive success and mating system of plants; however, few studies have directly evaluated the relationship between pollinators and the breeding system of the plants they pollinate. It is important to document this relationship because the global decline in pollinators may significantly affect the breeding systems of many animal-pollinated plants, particularly specialized systems. Ceiba pentandra is a tropical tree that has chiropterophilic flowers and a variable breeding system throughout its distribution, ranging from fully self-incompatible, to a mixed system with different degrees of selfing. To determine if regional differences in pollinators may result in regional differences in the outcrossing rate of this species, we used systematic observations of pollinator behavior in two tropical life zones and high-resolution genetic analysis of the breeding system of populations from these two regions using microsatellites. We found a predominantly self-incompatible system in regions with high pollinator visitation, while in environments with low pollinator visitation rates, C. pentandra changed to a mixed mating system with high levels of self-pollination.     

Genetic variation and covariation of aphid life-history traits across unrelated host plants

A central paradigm of life-history theory is the existence of resource mediated trade-offs among different traits that contribute to fitness, yet observations inconsistent with this tenet are not uncommon. We previously found a clonal population of the aphid Myzus persicae to exhibit positive genetic correlations among major components of fitness, resulting in strong heritable fitness differences on a common host. This raises the question of how this genetic variation is maintained. One hypothesis states that variation for resource acquisition on different hosts may override variation for allocation, predicting strong fitness differences within hosts as a rule, but changes in fitness hierarchies across hosts due to trade-offs. Therefore, we carried out a life-table experiment with 17 clones of M. persicae, reared on three unrelated host plants: radish, common lambsquarters and black nightshade. We estimated the broad-sense heritabilities of six life-history traits on each host, the genetic correlations among traits within hosts, and the genetic correlations among traits on different hosts (cross-environment genetic correlations). The three plants represented radically different environments with strong effects on performance of M. persicae, yet we detected little evidence for trade-offs. Fitness components were positively correlated within hosts but also between the two more benign hosts (radish and lambsquarters), as well as between those and another host tested earlier. The comparison with the most stressful host, nightshade, was hampered by low survival. Survival on nightshade also exhibited genetic variation but was unrelated to fitness on other hosts. Acknowledging that the number of environments was necessarily limited in a quantitative genetic experiment, we suggest that the rather consistent fitness hierarchies across very different plants provided little evidence to support the idea that the clonal variation for life-history traits and their covariance structure are maintained by strong genotypexenvironment interactions with respect to hosts. Alternative explanations are discussed.     

Inbreeding depression and mating-distance dependent offspring fitness in large and small populations of Lychnis flos-cuculi (Caryophyllaceae)

The spatial structure of four Lychnis flos-cuculi populations, varying in size and degree of isolation, was studied by comparing the fitness of offspring resulting from self-pollination and pollinations by neighbouring plants, plants within the same population, and plants from other populations. Selfed offspring had the lowest fitness of the four offspring groups. No significant difference was found between the performance of offspring from pollinations by neighbouring plants and offspring pollinated by plants further apart but within the same population. A lower fitness of offspring from pollinations between neighbours would be expected if these matings, on average, yielded inbred offspring which suffered from inbreeding depression. These results imply that either a tight neighbourhood structuring is not present, or that the inbreeding depression for offspring by neighbours is too low to detect, although these are inbred. Crossings between populations produced offspring with a significantly higher fitness than offspring sired within populations. There were no significant differences in response to inbreeding among the populations, and differences in mean fitness among populations had no clear relation to the population size or degree of isolation. A reduced fitness of small populations due to inbreeding depression or a less severe response to experimental inbreeding due to purging of deleterious alleles is therefore not supported by our results.     

Adaptive inter-population differences in blue tit life-history traits on Corsica

Few studies of natural populations have investigated how phenotypic variation across populations relates to key factors in the environment and landscape structure. In the blue tits of southern France, inter-population differences in reproductive life-history traits (e.g. laying date and clutch size) are small, whatever the timing of maximum caterpillar availability, a key factor for offspring survival in tits. These small differences are attributed to gene flow between local populations occupying different habitat types. In contrast, in blue tits on the island of Corsica, we noted large differences in reproductive life-history traits between two populations, where each population is synchronized with the peak-date of caterpillar abundance. These occur over a short geographical distance (25km). Considering our study within a framework of long-term population studies in tits, our results support the hypothesis that different blue tit populations on Corsica show adaptive differences in life-history traits, and suggest that landscape structure at a small spatial scale can have profound effects on adaptive between-population differentiation in life-history traits that are closely linked with fitness.     

Spatial variation in phenotypic selection on floral characteristics in an epiphytic orchid

Spatial variation in twelve floral characters was examined in an epiphytic orchidLepanthes rupestris to evaluate the strength and direction of phenotypic selection in seven riparian populations along two river basins in the Caribbean National Forest “El Yunque” for a range of 18–34 months. We evaluated selection on floral characters based on male (pollinaria removal) and female fitness (fruit set). Simple linear and quadratic regressions were used to evaluate the strength of directional, disruptive and stabilizing selections. Univariate and multivariate analyses were used to estimate the total strength of the selection acting on a character. Phenotypic selection was inconsistent among characters and populations. Few of the characters appeared to be under selection and none of them was found to be consistent throughout all populations. Inconsistency in selection coefficients among populations could suggest that selection is spatially variable. We only noted one character (column length) which had some consistency in differential selection coefficients among populations. Previous studies have shown that effective population sizes inL. rupestris are small and the observed “fitness differences” among populations could as easily be explained as stochastic events at play. We argue that the observed “fitness differences” in most characters and inconsistency among populations are likely from stochastic noise and not phenotypic selection. Consequently, we propose that random selection on character state support the hypothesis of genetic drift in small orchid populations.     

Offspring sex ratio under inbreeding and outbreeding in a gynodioecious plant

Silene vulgaris is a gynodioecious plant native to Eurasia and now found throughout much of North America. Using hermaphrodite plants from three geographic regions (Stamford, NY; Broadway,VA; and Giles Co., VA) and four local populations within each region, we employed a hierarchical crossing design to explore the geographic structure of sex determining genes. Sex determination in this species is cytonuclear involving multiple cytoplasmic male sterility and nuclear restorer loci. Due to dominance effects within nuclear restorer loci, self-fertilization of hermaphrodites heterozygous at restorer loci should produce some homozygous recessive female offspring. Female offspring may also result from outcrossing among related individuals. At greater geographic and genetic distances, mismatches between cytoplasmic and nuclear sex determining genes should also produce high frequencies of female offspring if coevolution between cytoplasmic and nuclear sex determining alleles occurs independently among widely separated populations. We found evidence of dominance effects among nuclear restorer loci but no evidence of nuclear-cytoplasmic mismatches at the regional level. Of 63 maternal lines, 55 produced at least one female offspring when self-fertilized. Outcrossing within populations produced significantly fewer female offspring than self-fertilization. Outcrossing among regions produced the lowest proportion of female offspring, significantly fewer than outcrossing among populations within regions. Regions responded differently to among-region outcrossing with pollen donors from the two Virginia regions producing far fewer female offspring with New York dams than crosses among New York populations. These results indicate that nuclear restoration is complex, involving multiple loci with epistatic interactions and that most hermaphrodites in nature are heterozygous at one or more restorer locus. Further, regional differences in restorer frequencies indicate significant genetic structure for sex determining genes at large geographic scales, perhaps reflecting invasion history.     

Inherited variability in multiple traits determines fitness in populations of an annual legume from contrasting latitudinal origins

Background and Aims

Variation in fitness depends on corresponding variation in multiple traits which have both genetically controlled and plastic components. These traits are subjected to varying degrees of local adaptation in specific populations and, consequently, are genetically controlled to different extents. In this study it is hypothesized that modulation of different traits would have contrasting relevance for the fitness of populations of diverse origins. Specifically, assuming that environmental pressures vary across a latitudinal gradient, it is suggested that inherited variation in traits differentially determines fitness in annual Lupinus angustifolius populations from contrasting latitudinal origins in western Spain.

Methods

Seeds of L. angustifolius from three contrasting origins were grown in a common garden. Traits related to more plastic vegetative growth and more genetically conserved phenology were measured, together with estimates of reproductive success. Fitness was estimated by the number of viable seeds per plant. Structural Equation Models were used to infer causal relationships among multiple traits and fitness, separating the direct and indirect effects of morphological, phenological and reproductive traits.

Key Results

Phenological, vegetative and reproductive traits accounted for most of the fitness variation. Fitness was highest in plants of southernmost origin, mainly due to earlier flowering. Fitness within each seed origin was controlled by variation in different traits. Southern origin plants that grew to a larger size achieved higher fitness. However, plant size in plants of northernmost origin was irrelevant, but early flowering promoted higher fitness. Variation in fruit and seed set had a greater effect on the fitness of plants of central origin than phenological and size variation.

Conclusions

It is concluded that modulation of a functional trait can be relevant to fitness in a given population (i.e. affecting intensity and direction), but irrelevant in other populations. This points to the need to consider integrated phenotypes when trying to unravel local adaptation effects over single traits.Key words: Lupinus, Structural Equation Models, fitness, phenology, functional traits, reproductive success, SLA, seed size     

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.