MATING SYSTEM AND THE EVOLUTION OF QUANTITATIVE TRAITS: AN EXPERIMENTAL STUDY OF MIMULUS GUTTATUS

Genetics of quantitative traits with dominance under stabilizing and directional selection in partially selfing species

Recurrent self-fertilization is thought to lead to reduced adaptive potential by decreasing the genetic diversity of populations, thus leading selfing lineages down an evolutionary "blind alley." Although well supported theoretically, empirical support for reduced adaptability in selfing species is limited. One limitation of classical theoretical models is that they assume pure additivity of the fitness-related traits that are under stabilizing selection, despite ample evidence that quantitative traits are subject to dominance. Here, we relax this assumption and explore the effect of dominance on a fitness-related trait under stabilizing selection for populations that differ in selfing rates. By decomposing the genetic variance into additional components specific to inbred populations, we show that dominance components can explain a substantial part of the genetic variance of inbred populations. We also show that ignoring these components leads to an upward bias in the predicted response to selection. Finally, we show that when considering the effect of dominance, the short-term evolutionary potential of populations remains comparable across the entire gradient in outcrossing rates, and genetic associations can even make selfing populations more evolvable on the longer term, reconciling theoretical, and empirical results.

Plastic breeding system response to day length in the California wildflower Mimulus douglasii

PREMISE OF THE STUDY

Angiosperms have evolved multiple breeding systems that allow reproductive success under varied conditions. Striking among these are cleistogamous breeding systems, where individuals can produce alternative flower types specialized for distinct mating strategies. Cleistogamy is thought to be environmentally-dependent, but little is known about environmental triggers. If production of alternate flowers is environmentally induced, populations may evolve locally adapted responses. Mimulus douglasii, exhibits a cleistogamous breeding system, and ranges across temperature and day-length gradients, providing an ideal system to investigate environmental parameters that control cleistogamy.

METHODS

We compared flowering responses across Mimulus douglasii population accessions that produce distinct outcrossing and self-pollinating flower morphs. Under controlled conditions, we determined time to flower, and number and type of flowers produced under different temperatures and day lengths.

KEY RESULTS

Temperature and day length both affect onset of flowering. Long days shift flower type from predominantly chasmogamous to cleistogamous. The strength of the response to day length varies across accessions whether temperature varies or is held constant.

CONCLUSIONS

Cleistogamy is an environmentally sensitive polyphenism in Mimulus douglasii, allowing transition from one mating strategy to another. Longer days induce flowering and production of cleistogamous flowers. Shorter days induce chasmogamous flowers. Population origin has a small effect on response to environmental cues.

Effects of mating system on adaptive potential for leaf morphology in Crepis tectorum (Asteraceae)

BACKGROUND AND AIMS

A shift from outcrossing to selfing is thought to reduce the long-term survival of populations by decreasing the genetic variation necessary for adaptation to novel ecological conditions. However, theory also predicts an increase in adaptive potential as more of the existing variation becomes expressed as homozygous genotypes. So far, relatively few studies have examined how a transition to selfing simultaneously affects means, variances and covariances for characters that might be under stabilizing selection for a spatially varying optimum, e.g. characters describing leaf morphology.

METHODS

Experimental crosses within an initially self-sterile population of Crepis tectorum were performed to produce an outbred and inbred progeny population to assess how a shift to selfing affects the adaptive potential for measures of leaf morphology, with special emphasis on the degree of leaf dissection, a major target of diversifying selection within the study species.

KEY RESULTS

Three consecutive generations of selfing had a minor impact on survival, the total number of heads produced and the mean leaf phenotype, but caused a proportional increase in the genetic (co)variance matrix for foliar characters. For the degree of leaf dissection, the lowest 50th percentile of the inbred progeny population showed a disproportionate increase in the genetic variance, consistent with the recessive nature of the weakly lobed phenotype observed in interpopulation crosses. Comparison of inbreeding response with large-scale patterns of variation indicates a potential for selection in a (recently) inbred population to drive a large evolutionary reduction in degree of leaf dissection by increasing the frequency of particular sibling lines.

CONCLUSIONS

The results point to a positive role for inbreeding in phenotypic evolution, at least during or immediately after a rapid shift in mating system.

Does inbreeding promote evolutionary reduction of flower size? Experimental evidence from Crepis tectorum (Asteraceae)

PREMISE OF THE STUDY

Small, autogamous flowers have evolved repeatedly in the plant kingdom. While much attention has focused on the mechanisms that promote the shift to autogamy, there is still a paucity of information on the factors that underlie the reduction of flower size so prevalent in selfing lineages. In this study of Crepis tectorum, I examine the role of inbreeding, acting alone or together with selection, in promoting evolutionary reduction of flower size.

METHODS

Experimental crosses were performed to produce progeny populations that differed in inbreeding and (or) selection history. Progenies were grown in two different environments and scored for flower size and other characters.

KEY RESULTS

Inbreeding depressed flower and fruit size, but also caused changes in flowering time and the number of heads produced. Despite some inconsistencies in the results for the last progeny generation, the decline in flower size was persistent over generations, consistent across environments, and similar in magnitude to the effects of selection for small flower size and the floral reduction inferred to have taken place during the shift toward autogamy within the study species. The floral size reduction was largely independent of changes in overall vigor, and there was considerable adaptive potential in flower size (measured by sib analyses and parent-offspring comparisons) after inbreeding.

CONCLUSIONS

The results of this study indicate that inbreeding can promote evolutionary reduction of flower size and highlight the close, persistent association between flower and fruit size in the study species.

Variation in inbreeding depression and plasticity across native and non-native field environments

BACKGROUND AND AIMS

Since the early 1990s, research on genetic variation of phenotypic plasticity has expanded and empirical research has emphasized the role of the environment on the expression of inbreeding depression. An emerging question is how these two evolutionary ecology mechanisms interact in novel environments. Interest in this area has grown with the need to understand the establishment of populations in response to climate change, and to human-assisted transport to novel environments.

METHODS

We compare performance in the field of outcrossed (O) and inbred lines (S1, S2) from 20 maternal families from each of two native populations of Mimulus guttatus. The experiment was planted in California in each population's home site, in the other populations's home site, in a novel site within the native range of M. guttatus, and in a novel site within the non-native range in North America. The experiment included nearly 6500 individuals. Survival, sexual reproduction and above-ground biomass were examined in order to evaluate inbreeding depression, and stem diameter and plant height were examined in order to evaluate phenotypic plasticity.

KEY RESULTS

Across all field sites, approx. 36 % of plants survived to flowering. Inbreeding depression differed among sites and outcrossed offspring generally outperformed selfed offspring. However, in the native-novel site, self-progeny performed better or equally well as outcross progeny. Significant phenotypic plasticity and genetic variation in plasticity was detected in the two architectural traits measured. The absolute value of plasticity showed the most marked difference between home and non-native novel site or non-native-novel site. Evidence was detected for an interaction between inbreeding and plasticity for stem diameter.

CONCLUSIONS

The results demonstrate that during initial population establishment, both inbreeding depression and phenotypic plasticity vary among field sites, and may be an important response to environments outside a species' currently occupied range. However, the interaction between inbreeding and plasticity may be limited and environment-dependent.

Rapid evolution caused by pollinator loss in Mimulus guttatus

Anthropogenic perturbations including habitat loss and emerging disease are changing pollinator communities and generating novel selection pressures on plant populations. Disruption of plant-pollinator relationships is predicted to cause plant mating system evolution, although this process has not been directly observed. This study demonstrates the immediate evolutionary effects of pollinator loss within experimental populations of a predominately outcrossing wildflower. Initially equivalent populations evolved for five generations within two pollination treatments: abundant bumblebee pollinators versus no pollinators. The populations without pollinators suffered greatly reduced fitness in early generations but rebounded as they evolved an improved ability to self-fertilize. All populations diverged in floral, developmental, and life-history traits, but only a subset of characters showed clear association with pollination treatment. Pronounced treatment effects were noted for anther-stigma separation and autogamous seed set. Dramatic allele frequency changes at two chromosomal polymorphisms occurred in the no pollinator populations, explaining a large fraction of divergence in pollen viability. The pattern of phenotypic and genetic changes in this experiment favors a sequential model for the evolution of the multitrait "selfing syndrome" observed throughout angiosperms.

The quantitative genetics of incipient speciation: heritability and genetic correlations of skeletal traits in populations of diverging Favia fragum ecomorphs

Recent speciation events provide potential opportunities to understand the microevolution of reproductive isolation. We used a marker-based approach and a common garden to estimate the additive genetic variation in skeletal traits in a system of two ecomorphs within the coral species Favia fragum: a Tall ecomorph that is a seagrass specialist, and a Short ecomorph that is most abundant on coral reefs. Considering both ecomorphs, we found significant narrow-sense heritability (h(2) ) in a suite of measurements that define corallite architecture, and could partition additive and nonadditive variation for some traits. We found positive genetic correlations for homologous height and length measurements among different types of vertical plates (costosepta) within corallites, but negative correlations between height and length within, as well as between costosepta. Within ecomorphs, h(2) estimates were generally lower, compared to the combined ecomorph analysis. Marker-based estimates of h(2) were comparable to broad-sense heritability (H) obtained from parent-offspring regressions in a common garden for most traits, and similar genetic co-variance matrices for common garden and wild populations may indicate relatively small G × E interactions. The patterns of additive genetic variation in this system invite hypotheses of divergent selection or genetic drift as potential evolutionary drivers of reproductive isolation.

The genetics of phenotypic plasticity in plant defense: trichome production in Mimulus guttatus

Insect herbivory is a major driving force of plant evolution. Phenotypic plasticity and developmental variation provide a means for plants to cope with variable herbivory. We characterized the genetics of developmental variation and phenotypic plasticity in trichome density, a putative defensive trait of Mimulus guttatus (yellow monkeyflower). Our results are evaluated in relation to the optimal defense theory, which provides testable predictions for plastic and developmental patterns in defense traits. We found that both developmental stage and simulated insect damage affected trichome production, but in different ways. Plants were more likely to produce at least some trichomes on later leaves than on earlier leaves, regardless of damage. Damage did not affect the average probability of producing trichomes, but it did increase the density of hairs on trichome-positive plants. We mapped trichome quantitative trait loci (QTL) by selectively genotyping a large panel of recombinant inbred lines derived from two highly divergent populations. Several highly pleiotropic QTL influenced multiple aspects of the trichome phenotype (constitutive, developmental, and/or plastic responses). Only one of the QTL influenced trichome induction following damage. In a result that is consistent with a central prediction of optimal defense theory, the high allele at this location was from the ancestral population with low constitutive trichome production.

Patterns of selection of two North American native and nonnative populations of monkeyflower (Phrymaceae)

To better understand invasion dynamics, it is essential to determine the influence of genetics and ecology in species persistence in both native and nonnative habitats. One approach is to assess patterns of selection on floral and growth traits of individuals in both habitats. Mimulus guttatus (Phrymaceae) has a mixed mating system and grows under variable water conditions across its native and nonnative range in North America. Field investigations of patterns of selection of floral and plant size traits were conducted in two native and two nonnative populations. Field-collected seed was grown and crossed in the glasshouse using a paternal half-sib design. The resulting offspring were grown in saturated and dry-down low-water conditions and the same traits were measured in both environments. Patterns of selection varied across years in the native range. Nonnative populations exhibited selection for increased floral size, consistent with the hypothesis that selection favors larger size in nonnative habitats. In the glasshouse, we detected genetic variation for traits across population/treatment combinations. However, size hierarchy in the glasshouse was dependent on water conditions. Our results suggest that both variable selection pressures and local adaptation probably influence the persistence of both native and nonnative populations.

Contribution of chromosomal polymorphisms to the G-matrix of Mimulus guttatus

Evolution of genetic (co)variances (the G-matrix) fundamentally influences multitrait divergence. Here, we isolated the contribution of two chromosomal quantitative trait loci (QTLs), a meiotic drive locus and a polymorphic inversion, to the overall G-matrix for a suite of floral, phenological and male fitness traits in a population of Mimulus guttatus. This allowed us to predict the evolution of trait means and genetic (co)variances as a function of allele frequencies, and to evaluate theories about the maintenance of genetic variation in fitness. Individuals generated using a replicated F(2) breeding design were grown under common conditions, genotyped and measured for trait values. Significant additive genetic variance existed for all traits, and most genetic covariances were significantly nonzero. Both QTLs contribute to the additive genetic (co)variances of multiple traits. Pleiotropy was not generally consistent, either between QTLs or with the genetic background. Shifts in allele frequencies at either QTL are predicted to result in substantial changes in the G-matrix. Both QTLs contribute substantially to the genetic variation in pollen viability. The Drive QTL, and perhaps also the inversion, demonstrates the contribution of balancing selection to the maintenance of genetic variation in fitness.

Within and between generation phenotypic plasticity in trichome density of Mimulus guttatus

Mimulus guttatus (yellow monkeyflower) frequently produce glandular trichomes, a trait that may resist herbivory. Constitutive production of trichomes is variable both within and among populations of M. guttatus and most of this variation is genetic. This study demonstrates that damage on early leaves can induce increased trichome production on later leaves, a plastic response that is likely adaptive. Moreover, this study shows that this induction can be maternally transmitted, increasing trichome density in progeny before they experience herbivory. This transgenerational response must involve a yet undescribed epigenetic mechanism. These experiments also show genetic variation among plants in the capacity for both within and between plant generation induction. Despite the clear evolutionary importance of variation in constitutive and induced herbivory-resistance traits, few other studies have noted genetic variation in both within a plant species.

Sex ratio variation in gynodioecious Lobelia siphilitica: effects of population size and geographic location

Variation in population sex ratio can be influenced by natural selection on alternate sex phenotypes as well as nonselective mechanisms, such as genetic drift and founder effects. If natural selection contributes to variation in population sex ratio, then sex ratio should covary with resource availability or herbivory. With nonselective mechanisms, sex ratio should covary with population size. We estimated sex ratio, resource availability, herbivory and size of 53 populations of gynodioecious Lobelia siphilitica. Females were more common in populations with higher annual temperatures, lower soil moisture and lower predation on female fruits, consistent with sex-specific selection. Females were also more common in small populations, consistent with drift, inbreeding or founder effects. However, small populations occurred in areas with higher temperatures than large populations, suggesting that female frequencies in small populations could be caused by sex-specific selection. Both selective and nonselective mechanisms likely affect sex ratio variation in this gynodioecious species.