Patterns of Genetic Differentiation in the Slender Wild Oat Species Avena barbata

Changing patterns of genetic differentiation in the slender wild oat, Avena barbata

The slender wild oat (Avena barbata) was widely studied in California using allozymes in the 1970s and interpreted as a case of ecotypic adaptation to contrasting moisture environments. However, common garden studies suggested that the moist-associated ("mesic") ecotype had high fitness in both moist and dry habitats, thus predicting an adaptive spread into areas occupied by the dry associated ("xeric") ecotype. To test this prediction, we revisited 100 populations of A. barbata that were screened genetically 40 y ago. As expected, mesic allozyme and morphological markers are much more common than in the 1970s. The less-fit xeric ecotype, while still widespread, has declined markedly in range and frequency. Genotyping by sequencing of modern populations reveals striking genetic uniformity within each of the two ecotypes. In recombinants between the two ecotypes, the mesic allele at a major fitness quantitative trait locus (QTL) shows a high frequency but so do many other genomic regions not identified as fitness QTL. Additional introduced genotypes are diverse and more widespread than in the past, and our results show that these have spread into the former range of the xeric ecotype to an even greater extent than the mesic ecotype has. While these results confirm the prediction of contemporary evolution from common gardens, they also suggest that much of the change has been driven by additional waves of introduced genotypes.

Propagule pressure and the establishment of emergent polyploid populations

BACKGROUND

Whereas the incidence or rate of polyploid speciation in flowering plants is modest, the production of polyploid individuals within local populations is widespread. Explanations for this disparity primarily have focused on properties or interactions of polyploids that limit their persistence.

HYPOTHESIS

The emergence of local polyploid populations within diploid populations is similar to the arrival of invasive species at new, suitable sites, with the exception that polyploids suffer interference from their progenitor(s). The most consistent predictor of successful colonization by invasive plants is propagule pressure, i.e. the number of seeds introduced. Therefore, insufficient propagule pressure, i.e. the formation of polyploid seeds within diploid populations, ostensibly is a prime factor limiting the establishment of newly emergent polyploids within local populations. Increasing propagule number reduces the effects of genetic, environmental and demographic stochasticity, which thwart population survival. As with invasive species, insufficient seed production within polyploid populations limits seed export, and thus reduces the chance of polyploid expansion.

CONCLUSION

The extent to which propagule pressure limits the establishment of local polyploid populations remains to be determined, because we know so little. The numbers of auto- or allopolyploid seed in diploid populations rarely have been ascertained, as have the numbers of newly emergent polyploid plants within diploid populations. Moreover, seed production by these polyploids has yet to be assessed.

Comparative linkage mapping of diploid, tetraploid, and hexaploid Avena species suggests extensive chromosome rearrangement in ancestral diploids

The genus Avena (oats) contains diploid, tetraploid and hexaploid species that evolved through hybridization and polyploidization. Four genome types (named A through D) are generally recognized. We used GBS markers to construct linkage maps of A genome diploid (Avena strigosa x A. wiestii, 2n = 14), and AB genome tetraploid (A. barbata 2n = 28) oats. These maps greatly improve coverage from older marker systems. Seven linkage groups in the tetraploid showed much stronger homology and synteny with the A genome diploids than did the other seven, implying an allopolyploid hybrid origin of A. barbata from distinct A and B genome diploid ancestors. Inferred homeologies within A. barbata revealed that the A and B genomes are differentiated by several translocations between chromosomes within each subgenome. However, no translocation exchanges were observed between A and B genomes. Comparison to a consensus map of ACD hexaploid A. sativa (2n = 42) revealed that the A and D genomes of A. sativa show parallel rearrangements when compared to the A genomes of the diploids and tetraploids. While intergenomic translocations are well known in polyploid Avena, our results are most parsimoniously explained if translocations also occurred in the A, B and D genome diploid ancestors of polyploid Avena.

Unidirectional grass hairs usher insects away from meristems

Grasses are major agricultural products worldwide and they are critical to ecosystem function in many terrestrial habitats. Despite their global importance, we know relatively little about their defenses against herbivory. Grasses tend to be tolerant of leaf loss because their valuable meristems are located underground, out of reach for above ground herbivores. Many grasses have unidirectional leaf hairs, prickles, and spines that make moving up the leaf blade easy, but make moving down, toward the meristem, difficult. We tested the hypothesis that unidirectional grass hairs direct small arthropod herbivores away from the meristems. In a field survey of the distribution of herbivore damage, we found that leaf tips received five times more damage than leaf bases for Avena barbata. Early-instar grasshoppers fed three times as often on leaf tops as on leaf bases of pubescent individuals in a common garden laboratory experiment. This effect was not observed for glabrous individuals where grasshoppers damaged leaf bases as often as leaf tops. A common generalist caterpillar, Heliothus virescens, was more than twice as likely to turn in the direction of the hairs, away from the meristems, when it encountered pubescent leaves of A. barbata. However, larger caterpillars of the generalist feeder Arctia virginalis showed no directional bias when they encountered pubescent leaves. In common garden experiments, selection on pubescence was weak and inconsistent over space and time. Under some circumstances, individuals of A. barbata with pubescent leaves were more likely to produce seeds than were individuals with fewer hairs. The surveys, behavioral experiments with small insects, and estimates of lifetime reproduction all support the hypothesis that unidirectional leaf hairs on A. barbata, and perhaps other grasses, serve as an unstudied defense that direct small herbivores away from the meristems.

Genetic diversity of Avena ventricosa populations along an ecogeographical transect in Cyprus is correlated to environmental variables

Avena ventricosa Balansa ex Coss. is considered the C-genome donor of the cultivated hexaploid oat and is a ‘priority’ species for conservation, since it has limited geographic distribution and the only recorded populations in Europe are present in Cyprus. The current study attempts to characterize the genetic structure and fragmentation of the species via the application of genotypic markers. It was revealed that the genetic variety was mainly allocated among the populations collected, since clustering obtained was according to the geographic origin of the samples and the habitat. Species distribution modeling showed that the most important climatic variable defining A. ventricosa distribution is the mean diurnal temperature. Furthermore, significant association of the genetic structure to environmental variables was detected; overall, a negative association to precipitation was confirmed, while significant correlations of genetic structure and the temperature at the time of anthesis and germination were established. The safeguarding of this valuable genetic resource is discussed.

CLINAL VARIATION ASSOCIATED WITH EDAPHIC ECOTONES IN HYBRID POPULATIONS OF GAILLARDIA PULCHELLA

The variety pulchella of the outcrossing annual plant species Gaillardia pulchella consists of two edaphic races in central Texas which are divergent for one morphological and four electrophoretic characters. Reduced pollen stainability in F₁ hybrids suggests the races are also divergent in chromosome structure. The recent proliferation of this species on roadsides and in pastures has led to hybridization between these races. An analysis of character variation in three hybrid populations revealed significant clinal variation associated with edaphic ecotones, and the width of these clines was found to vary among characters in a consistent pattern. It is argued that this pattern is the result of different characters experiencing different effective selection regimes, with narrower clines reflecting greater differentials in effective selection. Several mechanisms are discussed by which selection may impede the transgression of alleles across the ecotones in these populations. The results of this study are compared to the results of parallel studies on the autogamous annual species Avena barbata in California, and it is suggested that the difference between these two species in the width of clines separating edaphic ecotypes may be accounted for by their different breeding systems.

Factors affecting egg production in the selfing mangrove rivulus (Kryptolebias marmoratus)

The mangrove rivulus, Kryptolebias marmoratus, is one of two known vertebrate species with preferentially self-fertilizing hermaphrodites. Males also exist, and can outcross with hermaphrodites. Outcrossing events vary across wild populations and occur infrequently in laboratory settings. This study sought to add dimension to our understanding of mangrove rivulus reproductive habits by probing the effects of male presence on hermaphroditic unfertilized egg production. Specifically, we quantified egg production of solitary hermaphrodites compared to hermaphrodites exposed to males and exposed to other hermaphrodites. Hermaphrodites tended to produce more fertilized eggs in the presence of males but unfertilized eggs were produced relatively rarely and did not vary significantly among treatments. The probability that hermaphrodites would produce eggs changed as a function of genetic dissimilarity with their partner and in a season-dependent manner. In the fall, the probability of laying eggs decreased as a function of increased genetic dissimilarity, regardless of the sex of the partner. In the winter/spring, however, the probability of laying eggs increased markedly with increased genetic dissimilarity, regardless of the sex of the partner. Our findings indicate that reproductive decisions are modulated by factors beyond male presence, and we discuss a number of alternative hypotheses that should be tested in future studies.

Role of selection and gene flow in population differentiation at the edge vs. interior of the species range differing in climatic conditions

Evaluating the relative importance of neutral and adaptive processes as determinants of population differentiation across environments is a central theme of evolutionary biology. We applied the QST-FST comparison flanked by a direct test for local adaptation to infer the role of climate-driven selection and gene flow in population differentiation of an annual grass Avena sterilis in two distinct parts of the species range, edge and interior, which represent two globally different climates, desert and Mediterranean. In a multiyear reciprocal transplant experiment, the plants of desert and Mediterranean origin demonstrated home advantage, and population differentiation in several phenotypic traits related to reproduction exceeded neutral predictions, as determined by comparisons of QST values with theoretical FST distributions. Thus, variation in these traits likely resulted from local adaptation to desert and Mediterranean environments. The two separate common garden experiments conducted with different experimental design revealed that two population comparisons, in contrast to multi-population comparisons, are likely to detect population differences in virtually every trait, but many of these differences reflect effects of local rather than regional environment. We detected a general reduction in neutral (SSR) genetic variation but not in adaptive quantitative trait variation in peripheral desert as compared with Mediterranean core populations. On the other hand, the molecular data indicated intensive gene flow from the Mediterranean core towards desert periphery. Although species range position in our study (edge vs. interior) was confounded with climate (desert vs. Mediterranean), the results suggest that the gene flow from the species core does not have negative consequences for either performance of the peripheral plants or their adaptive potential.

Genomic variation across landscapes: insights and applications

The distribution of genomic variation across landscapes can provide insights into the complex interactions between the environment and the genome that influence the distribution of species, and mediate phenotypic adaptation to local conditions. High throughput sequencing technologies now offer unprecedented power to explore these interactions, allowing powerful inferences about historical processes of colonization, gene flow and divergence, as well as the identification of loci that mediate local adaptation. These 'landscape genomic' approaches have been validated in model species and are now being applied to nonmodel organisms, including foundation species that have substantial effects on ecosystem processes. Here we review the growing field of landscape genomics from a very broad perspective. In particular, we describe the inferential power that is gained by taking a genome-wide view of genetic variation, strategies for study design to best capture adaptive variation, and how to apply this information to practical challenges, such as restoration.

Population genetics and evolution of the mangrove rivulus Kryptolebias marmoratus, the world's only self-fertilizing hermaphroditic vertebrate

The mangrove rivulus, Kryptolebias marmoratus (Rivulidae, Cyprinodontiformes), is phylogenetically embedded within a large clade of oviparous (egg laying) and otherwise mostly gonochoristic (separate sex) killifish species in the circumtropical suborder Aplocheiloidei. It is unique in its reproductive mode: K. marmoratus is essentially the world's only vertebrate species known to engage routinely in self-fertilization as part of a mixed-mating strategy of selfing plus occasional outcrossing with gonochoristic males. This unique form of procreation has profound population-genetic and evolutionary-genetic consequences that are the subject of this review.

Evolutionary perspectives on clonal reproduction in vertebrate animals

A synopsis is provided of different expressions of whole-animal vertebrate clonality (asexual organismal-level reproduction), both in the laboratory and in nature. For vertebrate taxa, such clonal phenomena include the following: human-mediated cloning via artificial nuclear transfer; intergenerational clonality in nature via parthenogenesis and gynogenesis; intergenerational hemiclonality via hybridogenesis and kleptogenesis; intragenerational clonality via polyembryony; and what in effect qualifies as clonal replication via self-fertilization and intense inbreeding by simultaneous hermaphrodites. Each of these clonal or quasi-clonal mechanisms is described, and its evolutionary genetic ramifications are addressed. By affording an atypical vantage on standard vertebrate reproduction, clonality offers fresh perspectives on the evolutionary and ecological significance of recombination-derived genetic variety.

Evolving California genotypes of Avena barbata are derived from multiple introductions but still maintain substantial population structure

Multiple introductions are thought to enhance the chance of successful colonization, in part because recombination may generate adaptive variation to a new environment. Avena barbata (slender wild oat) is a successful colonist in California, historically noted for striking genetic divergence into two multilocus genotypes, but is still undergoing adaptive change. We sought to understand whether multiple introductions might be contributing to this change. We used cpDNA phylogeography of A. barbata within its home range and in its invaded range in California to determine the minimum number of separate introductions, and the spatial distribution of these introduced lineages. We collected from sites throughout the state of California, where it is an invasive species. Accessions from a representative portion of A. barbata’s full native range were obtained from germplasm repositories. We sequenced seven intergenic chloroplast DNA loci for A. barbata individuals both in California (novel geographic range) and its ancestral range. 204 individuals were assayed for chloroplast haplotype within California using single strand conformational polymorphism SSCPs. Genome size was determined by flow cytometry. Californian accessions are tetraploid as expected, but their genome sizes were smaller than the Old World accessions. There were three haplotypes present in California that were identical to haplotypes in the native range. Within California, the presence of multiple haplotypes at a site was observed primarily in Northern and Central populations. Between populations there was still substantial structure with F_ST ∼ 0.33, due to a shallow latitudinal cline caused by a preponderance of xeric haplotypes in Southern California. There was a minimum of three seed introductions to California. Recombination is thus likely to occur, and contribute to adaptation in new range in this highly-selfing, invader.

The ghost of outcrossing past in downy brome, an inbreeding annual grass

We investigated the frequency of outcrossing in downy brome (Bromus tectorum L.), a cleistogamous weedy annual grass, in both common garden and wild populations, using microsatellite and single nucleotide polymorphic (SNP) markers. In the common garden study, 25 lines with strongly contrasting genotypes were planted in close proximity. We fingerprinted 10 seed progeny from 8 individuals of each line and detected 15 first-generation heterozygotes for a t-value (corrected for cryptic crosses) of 0.0082. Different genotypes were significantly overrepresented as maternal versus paternal parents of heterozygotes, suggesting gender-function-dependent genetic control of outcrossing rates. In 4 wild populations (>300 individuals each), expected heterozygosity ranged from 0.149 to 0.336, whereas t-values ranged from 0.0027 to 0.0133, indicating high levels of both genetic diversity and inbreeding. Up to a third of the individuals in each population belonged to groups with identical or nearly identical SNP genotypes, whereas many of the remaining individuals were members of loose clusters of apparently related plants that probably represent descendants from past outcrossing events. Strict inbreeding in some lineages within a population with occasional outcrossing in others may be related to positive selection on adaptive syndromes associated with specific inbreeding lineages, or possibly to among-lineage differences in genetic regulation of outcrossing.

Allard's argument versus Baker's contention for the adaptive significance of selfing in a hermaphroditic fish

Fertilization assurance (Baker's contention) and multilocus coadaptation (Allard's argument) are two distinct hypotheses for the adaptive significance of self-fertilization in hermaphroditic taxa, and both scenarios have been invoked to rationalize isogenicity via incest in various plants and invertebrate animals with predominant selfing. Here we contrast Allard's argument and Baker's contention as applied to the world's only known vertebrate that routinely self-fertilizes. We pay special attention to frequencies of locally most common multilocus genotypes in Floridian populations of the Mangrove Rivulus (Kryptolebias marmoratus). Isogenicity patterns in this fish appear inconsistent with Allard's argument, thus leaving Baker's contention as the more plausible scenario (a result also supported by natural history information for this species). These results contrast with the isogenicity patterns and conclusions previously drawn from several self-fertilizing plants and invertebrate animal species. Thus, the adaptive significance of selfing apparently varies across hermaphroditic taxa.

Microevolutionary distribution of isogenicity in a self-fertilizing fish (Kryptolebias marmoratus) in the Florida Keys

The mangrove rivulus Kryptolebias marmoratus and a closely related species are the world's only vertebrates that routinely self-fertilize. Such uniqueness presents a model for understanding why this reproductive mode, common in plants and invertebrates, is so rare in vertebrates. A survey of 32 highly polymorphic loci in >200 specimens of mangrove rivulus from multiple locales in the Florida Keys, USA, revealed extensive population-genetic structure on microspatial and micro-temporal scales. Observed heterozygosities were severely constrained, as expected for a hermaphroditic species with a mixed-mating system and low rates of outcrossing. Despite the pronounced population structure and the implied restrictions on effective gene flow, isogenicity (genetic identity across individuals) within and among local inbred populations was surprisingly low even after factoring out probable de novo mutations. Results indicate that neither frequent bottlenecks nor directional genetic adaptation to local environmental conditions were the primary driving forces impacting multilocus population-genetic architecture in this self-fertilizing vertebrate species. On the other hand, a high diversity of isogenic lineages within relatively small and isolated local populations is consistent with the action of diversifying selection driven by the extreme spatio-temporal environmental variability that is characteristic of mangrove habitats.

Population genetic analysis of Bromus tectorum (Poaceae) indicates recent range expansion may be facilitated by specialist genotypes

PREMISE OF THE STUDY

The mechanisms for range expansion in invasive species depend on how genetic variation is structured in the introduced range. This study examined neutral genetic variation in the invasive annual grass Bromus tectorum in the Intermountain Western United States. Patterns of microsatellite (SSR) genotype distribution in this highly inbreeding species were used to make inferences about the roles of adaptively significant genetic variation, broadly adapted generalist genotypes, and facultative outcrossing in the recent range expansion of B. tectorum in this region.

METHODS

We sampled 20 individuals from each of 96 B. tectorum populations from historically and recently invaded habitats throughout the region and used four polymorphic SSR markers to characterize each individual.

KEY RESULTS

We detected 131 four-locus SSR genotypes; however, the 14 most common genotypes collectively accounted for 79.2% of the individuals. Common SSR genotypes were not randomly distributed among habitats. Instead, characteristic genotypes sorted into specific recently invaded habitats, including xeric warm and salt desert as well as mesic high-elevation habitats. Other SSR genotypes were common across a range of historically invaded habitats. We observed very few heterozygous individuals (0.58%).

CONCLUSIONS

Broadly adapted, generalist genotypes appear to dominate historically invaded environments, while recently invaded salt and warm desert habitats are dominated by distinctive SSR genotypes that contain novel alleles. These specialist genotypes are not likely to have resulted from recombination; they probably represent more recent introductions from unknown source populations. We found little evidence that outcrossing plays a role in range expansion.

Microgeographical Variation in Allozyme Frequencies in Avena barbata

Microgeographical distribution of alleles of five enzyme loci and one morphological variant has been determined within populations of A. barbata occupying sites that are transitional between different vegetational zones. The results show that the spatial distribution of alleles is nonrandom and highly correlated with habitat in patterns that parallel the distribution of the same alleles in major climatic zones in California. It is concluded that the observed patterns of variation are best explained by Neo-Darwinian evolutionary models, in which selection plays a predominant role.

Quantitative trait locus mapping of genes under selection across multiple years and sites in Avena barbata: epistasis, pleiotropy, and genotype-by-environment interactions

The genetic architecture of variation in evolutionary fitness determines the trajectory of adaptive change. We identified quantitative trait loci (QTL) affecting fitness in a mapping population of recombinant inbred lines (RILs) derived from a cross between moist- and dry- associated ecotypes of Avena barbata. We estimated fitness in 179 RILs in each of two natural environments in each of 4 years. Two loci account for over half of the variation in geometric mean fitness across environments. These loci are associated in repulsion phase in the wild ecotypes, suggesting the potential for strong transgressive segregation, but also show significant epistasis giving hybrid breakdown. This epistasis is the result of sharply lower fitness in only one of the recombinant genotypes, suggesting that the loci may contain synergistically acting mutations. Within each trial (year/site combination), we can explain less of the variation than for geometric mean fitness, but the two major loci are associated with variation in fitness in most environments. Tests for pleiotropic effects of QTL on fitness in different environments reveal that the same loci are under selection in all trials. Genotype-by-environment interactions are significant for some loci, but this reflects variation in the strength, not the direction of selection.

Sowing (and mapping) the wild oats

Both ecological and molecular methods have been applied to the study of locally adapted populations. While Clausen et al. (1940) were the pioneers of field transplant studies, the work of Robert Allard et al. on the invasive species Avena barbata is a classic example of using molecular evidence to infer adaptation via correlation of particular alleles with environmental gradients. In a new study published in this issue of Molecular Ecology, Latta (2009) combines ecological methods (reciprocal environment studies) with quantitative genetic techniques (recombinant inbred lines and quantitative trait analysis) to provide new evidence for local adaptation within this well known system. The conclusions are orthogonal to the original hypothesis, and instead, provide evidence that factors other than local adaptation were likely responsible for the historically observed patterns. This new evidence suggests that one ecotype is generally more fit than the other in both a moist and a dry environment, and accordingly, it appears to be increasing in frequency in historically surveyed populations.

Path analysis of natural selection via survival and fecundity across contrasting environments in Avena barbata

We employed path analysis to analyse natural selection through two major fitness components in each of three contrasting environments. Using a randomized block design, 188 Recombinant Inbred Lines (RILs) derived from a cross between contrasting ecotypes of Avena barbata were planted in common gardens in the greenhouse, and in two field sites typical of each ecotype's native habitat. Individuals were monitored for germination phenology, early growth, survival, final size, flowering phenology, reproductive allocation, fecundity and lifetime reproductive success. The variance/covariance matrix of the RIL (genotype) means was fit to a path model in which total fitness was made up of survival and fecundity (of survivors) components. In the greenhouse, all fitness variation was determined by fecundity variation (with no mortality), which was itself primarily determined by reproductive allocation mediated by date of first flowering. By contrast, in the field, early growth was the major determinant of survival, and final size was the major determinant of fecundity. Both components of fitness affected lifetime reproductive success equally in the field. Thus the major difference between greenhouse and field seems to be a shift from selection on allocation patterns in adults, to selection on resource acquisition, especially at earlier life stages. The pattern of selection was similar in the two field sites, despite the contrasting environments.

Natural selection on pleiotropic quantitative trait Loci affecting a life-history trade-off in Avena barbata

We applied QTL mapping to fitness variation of Avena barbata under well-watered greenhouse conditions. One hundred eighty recombinant inbred lines were assayed for flowering time, total size, mass allocation, and fitness. Composite Interval Mapping identified two to five loci affecting these traits. These were well supported in more powerful Multiple and Bayesian interval mapping analyses that indicated that additional QTL, as well as epistatic interactions also affect the traits. The posterior distribution of the number of QTL peaked at five to eight additive loci and one to two interactions, but the specific locations of the additional loci could not be determined with certainty. In most cases in which loci for separate traits mapped to similar locations, explicit tests supported pleiotropy over close linkage of separate loci. Alleles that hastened first flowering generally reduced vegetative mass, increased reproductive mass, and were associated with high fitness. Because effects on mass allocation generally cancelled one another, few loci affected total plant size. Only one QTL affected vegetative mass independent of reproductive mass and this locus had little effect on fitness. Thus selection acts to shift the mass allocation toward greater reproductive allocation, because the correlated decrease in vegetative mass poses only a minor fitness cost.

Bromus tectorum (Poaceae) in midcontinental United States: Population genetic analysis of an ongoing invasion

Biological invasions can be substantially influenced by the genetic sampling associated with a species' introduction. As a result, we assessed the genetic and evolutionary consequences of the entry and spread of the invasive grass Bromus tectorum (cheatgrass) across the United States midcontinent through an analysis of 54 populations, using enzyme electrophoresis. On average, these populations display 1.04 alleles per locus (A), 4.1% percent polymorphic loci per population (%P) and an expected mean heterozygosity (H(exp)) value of 0.009. Heterozygotes, which have been rarely reported for B. tectorum in North America, occur in three populations in the midcontinent and are likely novel multilocus genotypes that arose postimmigration. The midcontinent distribution of multilocus genotypes suggests that plant immigrants came directly from either the native range or the eastern United States, or both. Continued dispersal of preadapted genotypes and the assembly of populations that are genetic admixtures may enhance this invasion by increasing both the genetic diversity within populations and the selection of novel genotypes arising from occasional outcrossing. The potential for postimmigration evolution in most species points to the largely unrecognized need to block the introduction of new, potentially aggressive genotypes of an alien species already in the United States.

Heritable variation and genetic correlation of quantitative traits within and between ecotypes of Avena barbata

We examined heritable variation for quantitative traits within and between naturally occurring mesic and xeric ecotypes of the slender wild oat (Avena barbata), and in 188 recombinant inbred lines derived from a cross between the ecotypes. We measured a suite of seedling and adult traits in the greenhouse, as well as performance-related traits in field sites native to the two ecotypes. Although the ecotypes were genetically diverged for most traits, few traits showed significant heritable variation within either ecotype. In contrast, considerable heritable variation was released in the recombinant progeny of the cross, and transgressive segregation was apparent in all traits. Heritabilities were substantially greater in the greenhouse than in the field, and this was associated with an increase in environmental variance in the field, rather than a decrease in genetic variance. Strong genetic correlations were evident among the recombinants, such that 22 measured traits could be well represented by only seven underlying factors, which accounted for 80% of the total variation. The primary axis of variation in the greenhouse described a trade-off between vegetative and reproductive allocation, mediated by the date of first flowering, and fitness was strongly correlated with this trade-off. Other factors in the greenhouse described variation in size and in seedling traits. Lack of correlation among these factors represents the release of multivariate trait variation through recombination. In the field, a separate axis of variation in overall performance was found for each year/site combination. Performance was significantly correlated across field environments, but not significantly correlated between greenhouse and field.