GENOTYPE-ENVIRONMENT INTERACTION FOR JUVENILE GROWTH IN THE HARD CLAM MERCENARIA MERCENARIA (L.)

Offspring from half-sib and full-sib families of the hard clam, Mercenaria mercenaria were reared in five locations along the Atlantic Coast to test for the presence of genotype-environment interaction for juvenile growth rate. Location effects upon growth rate variation were prevalent; of the genetic effects, the additive genetic by location variance was predominant with the nonadditive genetic by location component contributing to a lesser degree to the interaction variance. The additive and nonadditive variation over all environments was negligible. Genotype-environment interaction was found to be at least partially due to a change in the amount of genetic variation expressed at each location; with significant additive variation detected at Charleston and Georgetown, SC sites and significant nonadditive variation at Millsboro, DE. Genetic covariance/correlation analysis indicated that reversals in relative family performance across locations were prevalent, implying the possibility of habitat specialization among genotypes. In addition, graphical analysis produced no evidence of a ubiquitously superior genotype. These analyses suggest that genotype-environment interaction should act to constrain the evolution of juvenile growth rate in Mercenaria, preserve any heritable variation associated with this trait and may lead to the development of phenotypic plasticity for growth.     

Genotype-environment interaction for total fitness in <Emphasis Type="Italic">Drosophila</Emphasis>

A fundamental assumption of models for the maintenance of genetic variation by environmental heterogeneity is that selection favours different genotypes in different environments. Here, I use a method for measuring total fitness of chromosomal heterozygotes in Drosophila melanogaster to assess genotype-environment interaction for fitness across two ecologically relevant environments, medium with and without added ethanol. Two-third chromosomes are compared, one from a population selected for ethanol tolerance, and the other from a control population. The results show strong crossing of reaction norms for outbred, total fitness, with the chromosome from the ethanol-adapted population increasing fitness on ethanol-supplemented food, but decreasing fitness on regular food, relative to the chromosome from the control population. Although I did not map the fitness effects below the chromosome level, the method could be adapted for quantitative trait locus mapping, to determine whether a substantial proportion of fitness variation is contributed by loci at which different alleles are favoured in different environments.     

Effects of genotype-environment interactions on genetic correlations

The objective of the work presented here was to investigate the influence of genotype-environment interaction on genetic correlations. In our theoretical models we have considered plant populations consisting of random samples of lines from chromosome-doubled haploids produced from F ₁ gametes, highly inbred SSD-lines, and clones of randomly breeding populations grown in two and multiple environments. The results of our theoretical considerations are that if genotype-environment interaction exists, great differences are expected to occur in the estimates of genetic correlation coefficients obtained in different environments. Based on the variance and covariance components for genotype-environment interaction we suggest a new type of correlation coefficient, called genotype-environment correlation, r _ge . Our theory has been applied to several series of experiments. Estimates are presented from two series, both of which demonstrate clearly the consequences of genotype-environment interaction on the genetic correlations.     

The Genetic Structure of Natural Populations of Drosophila melanogaster. Xx. Comparison of Genotype-Environment Interaction in Viability between a Northern and a Southern Population

In order to examine the operation of diversifying selection as the maintenance mechanism of excessive additive genetic variance for viability in southern populations in comparison with northern populations of Drosophila melanogaster, two sets of experiments were conducted using second chromosomes extracted from the Ogasawara population (a southern population in Japan) and from the Aomori population (a northern population in Japan). Chromosomal homozygote and heterozygote viabilities were estimated in eight kinds of artificially produced breeding environments. The main findings in the present investigation are as follows: (1) Significant genotype-environment interaction was observed using chromosomes extracted from the Ogasawara population. Indeed, the estimate of the genotype-environment interaction variance for heterozygotes was significantly larger than that of the genotypic variance. On the other hand, when chromosomes sampled from the Aomori population were examined, that interaction variance was significant only for homozygotes and its value was no more than one quarter of that for the chromosomes from the Ogasawara population. (2) The average genetic correlation between any two viabilities of the same lines estimated in the eight kinds of breeding environments for the chromosomes sampled from the Ogasawara population was smaller than that for the chromosomes from the Aomori population both in homozygotes and in heterozygotes, especially in the latter. (3) The stability of heterozygotes over homozygotes against fluctuations of environmental conditions was seen in the chromosomes from the Ogasawara population, but not from the Aomori population. (4) From the excessive genotype-environment interaction variance compared with the genotypic variance in heterozygotes, it was suggested for the chromosomes from the Ogasawara population that the reversal of viability order between homozygotes took place in some environments at the locus level. On the basis of these findings, it is strongly suggested that diversifying selection is operating in a southern population of D. melanogaster on some of the viability polygenes which are probably located outside the structural loci, and the excessive additive genetic variance of viability in southern populations is maintained by this type of selection.     

Genetic variance in temperature dependent adult size deriving from physiological genetic variation at temperature boundaries

An increase in genetic variation in body size has often been observed under stress; an increase in dominance variance and interaction variance as well as in additive genetic variance has been reported. The increase in genetic variation must be caused by physiological mechanisms that are specific to adverse environments. A model is proposed to explain the occurrence of an increase in genetic variation in body size in Drosophila at extreme temperatures. The model has parameters specific to the low- and high-temperature regions of the viable range. Additive genetic variation in the boundary temperatures leads to a marked increase in additive genetic variation in development rate and body size at extreme temperatures. Additive genetic variation in the temperature sensitivity in the low- and high-temperature regions adds non-additive genetic variation. Development rate shows patterns in additive genetic variation that differ from the patterns of genetic variation in body size; therefore, the genetic correlation between development rate and body size changes sign repeatedly as a function of temperature. The existence of dominance in the genetic variation in the boundary temperatures or in the low- and high-temperature sensitivities leads to a higher total genetic variance due to higher dominance and interaction variance, for both development rate and body size. This revised version was published online in July 2006 with corrections to the Cover Date.     

Genetic structure among populations in the endemic Hawaiian Plantago lineage: insights from microsatellite variation

Endemic Hawaiian species in the genus Plantago show considerable morphological and ecological diversity. Despite their variation, a recent phylogenetic analysis based on DNA sequence data showed that the group is monophyletic and that sequence variation among species and morphotypes is low. This lack of sequence polymorphisms resulted in an inability to resolve species and population affinities within the most recently derived clade of this lineage. To assess species boundaries, population genetic structure and interpopulation connectivity among the morphologically and ecologically distinct populations within this clade, genetic variation was examined using eight microsatellite loci. Within‐population genetic diversity was found to be lowest in the Maunaiu, Hawai'i population of the endangered P. hawaiensis, and highest in the large P. pachyphylla population from 'Eke, West Maui. Isolation by distance across the range of populations was detected and indicated restricted dispersal. This result is likely to be attributable to few interisland dispersal events in the evolutionary history of this lineage. Genetic differentiation within islands tended to be higher among populations occurring in contrasting bog and woodland habitats, suggesting ecological barriers to gene flow and the potential role of ecological divergence in population diversification. Overall, these results are consistent with findings from phylogenetic analysis of the entire lineage. Our data bring new insights regarding patterns of dispersal and population genetic structure to this endemic and endangered group of island taxa. As island environments become increasingly fragmented, information of this type has important implications for the successful management of these fragile populations and habitats.     

Genetic lineages of the invasive <Emphasis Type="Italic">Aegilops triuncialis</Emphasis> differ in competitive response to neighboring grassland species

Competitive dynamics between native and exotic species can influence both the success of exotics in the novel environment as well as diversity and abundance of native species. Invasive species are often characterized by multiple introductions in the novel range, which can lead to population differentiation for invasion characteristics. Here we use two invasive lineages of the exotic grass, Aegilops triuncialis L., to determine if these lineages differ in their response to competitors and in their persistence in the invaded range. We find that one lineage is negatively affected by competitor presence in both flowering phenology and reproductive output, while the other lineage shows no response in either trait. Furthermore, we find that the two lineages were introduced at different times and are each capable of replacing the other as the most abundant lineage of a given county. Our results demonstrate that genetic lineage is a more important determinant of competitive response than neighbor identity, and that the two lineages may employ alternate invasion mechanisms. Because management techniques are decided upon based on traits that confer invasiveness, our results highlight the importance of considering intraspecific variation in the invaded range.     

Symbiont‐mediated phenotypic variation without co‐evolution in an insect–fungus association

Recent studies have shown that symbionts can be a source of adaptive phenotypic variation for their hosts. It is assumed that co‐evolution between hosts and symbionts underlies these ecologically significant phenotypic traits. We tested this assumption in the ectosymbiotic fungal associate of the gall midge Asteromyia carbonifera. Phylogenetic analysis placed the fungal symbiont within a monophyletic clade formed by Botryosphaeria dothidea, a typically free‐living (i.e. not associated with an insect host) plant pathogen. Symbiont isolates from four divergent midge lineages demonstrated none of the patterns common to heritable microbial symbioses, including parallel diversification with their hosts, substitution rate acceleration, or A+T nucleotide bias. Amplified fragment length polymorphism genotyping of the symbiont revealed that within‐lineage genetic diversity was not clustered along host population lines. Culture‐based experiments demonstrated that the symbiont‐mediated variation in gall phenotype is not borne out in the absence of the midge. This study shows that symbionts can be important players in phenotypic variation for their hosts, even in the absence of a co‐evolutionary association.     

Genetic variation for foot-rot and Fusarium head-blight resistances among full-sib families of a self-incompatible winter rye (Secale cereale L.) population

The amount of genetic variation for resistance to foot rot caused by Pseudocercosporella herpotrichoides, Fusarium spp., and Microdochium nivale and for resistance to head blight caused by Fusarium culmorum are important parameters when estimating selection gain from recurrent selection in winter rye. One-hundred and eighty-six full-sib families of the selfincompatible population variety Halo, representing the Petkus gene pool, were tested for foot-rot resistance at five German location-year combinations (environments) and for head-blight resistance in three environments with artificial inoculation in all but one environment. Foot-rot rating was based on 25 stems per plot scored individually on a 1–9 scale. Head-blight resistance was plotwise scored on a 1–9 scale and, additionally, grain-weight per spike was measured relative to the non-inoculated control plots. Significant estimates of genotypic variance and medium-sized heritabilities (h ²=0.51–0.69) were observed in the combined analyses for all resistance traits. In four out of five environments, the amount of genetic variance was substantially smaller for foot-rot than for head-blight rating. Considerable environmental effects and significant genotype-environment interactions were found for both foot-rot and head-blight resistance. Coefficients of error-corrected correlation among environments were considerably closer than phenotypic correlations. No significant association was found between the resistances to both diseases (r=-0.20 to 0.17). In conclusion, intra-population improvement by recurrent selection should lead to substantial higher foot-rot and head-blight resistances due to significant quantitative genetic variation within Halo. Selection should be carried out in several environments. Lack of correlation between foot-rot and head-blight resistance requires separate infection tests for improving both resistances.     

LIFE-CYCLE COMPONENTS OF SELECTION IN ERIGERON ANNUUS: II. GENETIC VARIATION

Genetic variation for seedling and adult fitness components was measured under natural conditions to determine the relative importance of the seedling stage for lifetime fitness in Erigeron annuus. Variation in lifetime reproductive success can result from both the persistent effects of genetic variation expressed among seedlings and from variation in adult fitness components. Analysis of covariance was used to separate the stage specific from the cumulative effects of genetic variance expressed earlier in the life cycle. E. annuus produces seeds through apomixis, which allowed measurement of the fitness of replicate genotypes from germination through the entire life cycle. There were significant differences among genotypes for date of emergence, seedling size, survivorship and fecundity, but heritabilities were low, indicating slow response to selection. For all characters, environmental components of variance were one to two orders of magnitude larger than genetic variance components, resulting in broad sense heritabilities less than 0.1. For seedling size and fecundity, all of the genetic variance was in the form of genotype-environment interactions, often with large negative genetic correlations across environments. In contrast, genotypes differed in mean survivorship through one year, but there were no genotype-environment interactions for viability. Genetic differences in viability were primarily expressed as differences in overwinter survivorship. Genotype × environment interactions among sites and blocks were generated early in the life cycle while the genotype × environment interactions in response to competitive environment (open, annual cover, perennial cover) first appeared in adult fecundity. Genetic variation in lifetime fitness was not significant, despite a fourfold difference in mean fitness among genotypes.     

Diversification and asymmetrical gene flow across time and space: lineage sorting and hybridization in polytypic barking frogs

Young species complexes that are widespread across ecologically disparate regions offer important insights into the process of speciation because of their relevance to how local adaptation and gene flow influence diversification. We used mitochondrial DNA and up to 28 152 genomewide single nucleotide polymorphisms from polytypic barking frogs (Craugastor augusti complex) to infer phylogenetic relationships and test for the signature of introgressive hybridization among diverging lineages. Our phylogenetic reconstructions suggest (i) a rapid Pliocene–Pleistocene radiation that produced at least nine distinct lineages and (ii) that geographic features of the arid Central Mexican Plateau contributed to two independent northward expansions. Despite clear lineage differentiation (many private alleles and high between‐lineage F_ST scores), D‐statistic tests, which differentiate introgression from ancestral polymorphism, allowed us to identify two putative instances of reticulate gene flow. Partitioned D‐statistics provided evidence that these events occurred in the same direction between clades but at different points in time. After correcting for geographic distance, we found that lineages involved in hybrid gene flow interactions had higher levels of genetic variation than independently evolving lineages. These findings suggest that the nature of hybrid compatibility can be conserved overlong periods of evolutionary time and that hybridization between diverging lineages may contribute to standing levels of genetic variation.     

Genetic and environmental components of chiasma control

Variation in chiasma frequency within and between individuals has been investigated in Schistocerca gregaria and Stethophyma grossum. By taking sequential samples of the same testis in S. gregaria it has been demonstrated that there is considerable variation in chiasma frequency between times, within individuals, and that this variation is not evenly distributed between individuals of the same population. The response and recovery patterns after exposure to successive temperature treatments also indicate a differential sensitivity of individuals within the experimental population. — An analysis of interfollicular variation in chiasma frequency has revealed significant differences between follicles within individuals both in S. gregaria and a Spanish population of S. grossum. A comparable analysis on individuals of S. grossum carrying supernumerary segments shows that the presence of such segments increases the amount of variation between follicles within individuals and also between individuals within the population. — The distribution of cell chiasma frequencies in S. gregaria is normal whereas in Stethophyma cell frequencies approximate to a Poisson distribution. The structurally different supernumerary segments, present in Austrian and Spanish populations of S. grossum, both increase mean chiasma frequency and they modify the between cell variance in different ways — the former interchromosomally and the latter intrachromosomally. — The differences in chiasma frequency between follicles and between times within individuals and the differential reaction to heat shock reflect differences in the pattern of genotype-environment interaction. Similarly it can be argued that the presence of polymorphisms involving supernumerary material may play an equivalent role by increasing the between cell or between bivalent variance within individuals, a unique form of genotype-environment interaction at the endophenotypic level. — Such variation in genotype sensitivity to environmental modification has important adaptive value especially in organisms which are subject to recurrent, often random, changes in their environment.     

Fuzzy species limits in Mediterranean gorgonians (Cnidaria,Octocorallia): inferences on speciation processes

The study of the interplay between speciation and hybridization is of primary importance in evolutionary biology. Octocorals are ecologically important species whose shallow phylogenetic relationships often remain to be studied. In the Mediterranean Sea, three congeneric octocorals can be observed in sympatry: Eunicella verrucosa, Eunicella cavolini and Eunicella singularis. They display morphological differences and E. singularis hosts photosynthetic Symbiodinium, contrary to the two other species. Two nuclear sequence markers were used to study speciation and gene flow between these species, through network analysis and Approximate Bayesian Computation (ABC). Shared sequences indicated the possibility of hybridization or incomplete lineage sorting. According to ABC, a scenario of gene flow through secondary contact was the best model to explain these results. At the intraspecific level, neither geographical nor ecological isolation corresponded to distinct genetic lineages in E. cavolini. These results are discussed in the light of the potential role of ecology and genetic incompatibilities in the persistence of species limits.     

Spatial genetics of a high elevation lineage of Rhytididae land snails in New Zealand: the Powelliphanta Kawatiri complex

ABSTRACT

Powelliphanta is a genus of large carnivorous land snails endemic to New Zealand which display phenotypic variation within comparatively small geographic distances. The diversity within these snails has become a matter of high interest to conservation, as many lineages occupy small (or highly fragmented) ranges that render them vulnerable to ongoing habitat loss and predation by exotic pests. Combining Powelliphanta mitochondrial sequence data and genotypes of microsatellite loci we document the genetic structure within a species complex dubbed ‘Kawatiri’. All populations (with one exception) within the Kawatiri lineage are restricted to subalpine habitat (at elevations over 600?m above sea level). The ranges of some Kawatiri complex populations are adjacent to the congeneric lowland species Powelliphanta lignaria. Improved understanding of the distribution of this complex and the level and structure of genetic diversity provided a picture of a naturally fragmented lineage, restricted to a particular ecological zone. We identified six genetic clusters associated with population connectivity orientated north–south along mountain ranges, with east–west divisions between ranges. Future management should aim to retain the evolutionary potential within this young radiation by actively conserving the variation encompassed by each of the six clusters identified here.     

Exploring the role of Micronesian islands in the maintenance of coral genetic diversity in the Pacific Ocean

Understanding how genetic diversity is maintained across patchy marine environments remains a fundamental problem in marine biology. The Coral Triangle, located in the Indo‐West Pacific, is the centre of marine biodiversity and has been proposed as an important source of genetic diversity for remote Pacific reefs. Several studies highlight Micronesia, a scattering of hundreds of small islands situated within the North Equatorial Counter Current, as a potentially important migration corridor. To test this hypothesis, we characterized the population genetic structure of two ecologically important congeneric species of reef‐building corals across greater Micronesia, from Palau to the Marshall Islands. Genetic divergences between islands followed an isolation‐by‐distance pattern, with Acropora hyacinthus exhibiting greater genetic divergences than A. digitifera, suggesting different migration capabilities or different effective population sizes for these closely related species. We inferred dispersal distance using a biophysical larval transport model, which explained an additional 15–21% of the observed genetic variation compared to between‐island geographical distance alone. For both species, genetic divergence accumulates and genetic diversity diminishes with distance from the Coral Triangle, supporting the hypothesis that Micronesian islands act as important stepping stones connecting the central Pacific with the species‐rich Coral Triangle. However, for A. hyacinthus, the species with lower genetic connectivity, immigration from the subequatorial Pacific begins to play a larger role in shaping diversity than input from the Coral Triangle. This work highlights the enormous dispersal potential of broadcast‐spawning corals and identifies the biological and physical drivers that influence coral genetic diversity on a regional scale.     

Pleiotropic Stabilizing Selection Limits the Number of Polymorphic Loci to at Most the Number of Characters

We demonstrate that, in a model incorporating weak Gaussian stabilizing selection on n additively determined characters, at most n loci are polymorphic at a stable equilibrium. The number of characters is defined to be the number of independent components in the Gaussian selection scheme. We also assume linkage equilibrium, and that either the number of loci is large enough that the phenotypic distribution in the population can be approximated as multivariate Gaussian or that selection is weak enough that the mean fitness of the population can be approximated using only the mean and the variance of the characters in the population. Our results appear to rule out antagonistic pleiotropy without epistasis as a major force in maintaining additive genetic variation in a uniform environment. However, they are consistent with the maintenance of variability by genotype-environment interaction if a trait in different environments corresponds to different characters and the number of different environments exceeds the number of polymorphic loci that affect the trait.     

Long-term selection for a quantitative character in large replicate populations of Drosophila melanogaster

Summary Six replicate lines of Drosophila melanogaster, which had been selected for increased abdominal bristle number for more than 85 generations, were assayed by hierarchical analysis of variance and offspring on parent regression immediately after selection ceased, and by single-generation realised heritability after more than 25 generations of subsequent relaxed selection.Half-sib estimates of heritability in 5 lines were as high as in the base population and much higher than observed genetic gains would suggest, excluding lack of sufficient additive genetic variance as a cause of ineffective selection in these lines. Also, there was considerable diversity among the six lines in composition of phenotypic variability: in addition to differences in the additive genetic component, one or more of the components due to dominance, epistasis, sex-linkage or genotype-environment interaction appeared to be important in different lines.Even after relaxed selection, single-generation realised heritabilities in four lines were as high as in the base population. As a large proportion of total genetic gain must have been made by fixation of favourable alleles, the compensatory increase of genetic variability has been sought in a genetic model involving genes at low initial frequencies, enhancement of gene effects during selection and/or new mutations.     

EVOLUTION OF BROAD AND SPECIFIC COMPETITIVE ABILITY IN NOVEL VERSUS FAMILIAR ENVIRONMENTS IN DROSOPHILA SPECIES

We used nine pairs of competing Drosophila melanogaster and Drosophila simulans populations to test three hypotheses. (1) Weaker competitors undergo greater evolutionary increases in competitive ability, compared with stronger ones. (2) Increased competitive ability against a specific competitor population causes a correlated increase in competitive ability against other competitor populations. (3) In a novel environment, adaptation to the abiotic environment contributes more to competitive ability than adaptation to the competitor population. After 11 generations of competition, initially weaker competitor populations showed relatively greater increases in competitive ability. Broad and specific competitive abilities, the latter being specific to a particular competitor population, were positively correlated in both familiar and novel environments. Adaptation to the abiotic environment seemed to be a more important component of competitive ability in the novel environments. We conclude that in geographically structured species, biotic and abiotic factors affecting the evolution of competitive ability may interact to help create a mosaic of outcomes that can affect the evolutionary dynamics of the interaction over the range of the competing species.     

Variation in the relative magnitude of intraspecific and interspecific competitive effects in novel versus familiar environments in two<Emphasis Type="Italic">Drosophila</Emphasis> species

Models of competitor coevolution, especially the genetic feedback hypothesis, suggest that a negative correlation between intraspecific and interspecific competitive effects may be important in sustaining competitor coexistence, and can give rise to oscillatory dynamics with repeated reversals of competitive superiority. I reanalyzed previously published census data from an experiment in which populationsof Drosophila melanogaster andD. simulans underwent competitive coevolution in one familiar and two novel environments, to specifically look for any evidence of a negative relationship between intraspecific and interspecific competitive effects on population growth rates, and for any indication of short period cycling in the relative magnitude of intraspecific and interspecific competitive effects. While there was considerable variation in the relative magnitude of intraspecific and interspecific competitive effects over generations, among both populations and environments, there was no clear evidence supporting the genetic feedback hypothesis. Intraspecific and interspecific competitive effects on population growth rates were strongly positively correlated in novel environments, and uncorrelated in the familiar environment. Data from the familiar environment indicated that indices of competition of populations of the initially superior competitor,D. melanogaster, might be showing some cyclic behaviour, but I argue that this is likely to be transient, and not suggestive of sustained oscillatory dynamics predicted by the genetic feedback model. I discuss the results in the context of the importance of the genetic architecture of intraspecific and interspecific competitive abilities in determining the coevolutionary trajectory of competitive interactions.     

ON THE LOW HERITABILITY OF LIFE-HISTORY TRAITS

Life-history traits such as longevity and fecundity often show low heritability. This is usually interpreted in terms of Fisher's fundamental theorem to mean that populations are near evolutionary equilibrium and genetic variance in total fitness is low. We develop the causal relationship between metric traits and life-history traits to show that a life-history trait is expected to have a low heritability whether or not the population is at equilibrium. This is because it is subject to all the environmental variation in the metric traits that affect it plus additional environmental variation. There is no simple prediction regarding levels of additive genetic variance in life-history traits, which may be high at equilibrium. Several other patterns in the inheritance of life-history traits are readily predicted from the causal model. These include the strength of genetic correlations between life-history traits, levels of nonadditive genetic variance, and the inevitability of genotype-environment interaction.