Genetic variation in functional traits influences arthropod community composition in aspen (Populus tremula L.)

We conducted a study of natural variation in functional leaf traits and herbivory in 116 clones of European aspen, Populus tremula L., the Swedish Aspen (SwAsp) collection, originating from ten degrees of latitude across Sweden and grown in a common garden. In surveys of phytophagous arthropods over two years, we found the aspen canopy supports nearly 100 morphospecies. We identified significant broad-sense heritability of plant functional traits, basic plant defence chemistry, and arthropod community traits. The majority of arthropods were specialists, those coevolved with P. tremula to tolerate and even utilize leaf defence compounds. Arthropod abundance and richness were more closely related to plant growth rates than general chemical defences and relationships were identified between the arthropod community and stem growth, leaf and petiole morphology, anthocyanins, and condensed tannins. Heritable genetic variation in plant traits in young aspen was found to structure arthropod community; however no single trait drives the preferences of arthropod folivores among young aspen genotypes. The influence of natural variation in plant traits on the arthropod community indicates the importance of maintaining genetic variation in wild trees as keystone species for biodiversity. It further suggests that aspen can be a resource for the study of mechanisms of natural resistance to herbivores.     

Phenology and fecundity in 11 sympatric pioneer species of Macaranga (Euphorbiaceae)in Borneo

Reproductive traits of tropical tree species vary predictably in relation to successional stage, but this variation may be due to the species' phylogenetic histories rather than selective pressures imposed by regeneration requirements. Reproductive phenology, tree size at the onset of reproduction, and fecundity of 11 sympatric, closely related Macaranga species were studied to investigate within-species variation in reproductive traits in relation to resource availability, and among-species variation in relation to other life-history traits (shade tolerance, seed size and maximum tree size, H(max)) and consequently the requirements for forest-gap colonization. Nine species reproduced in synchronous episodes, and two species reproduced continuously over 32 mo. Episodic reproduction was most intense in 1992 following a severe drought. For several species, reproductive trees had greater light availability, lower fecundity in lower light levels, and lower growth rates than nonreproductive trees, reflecting resource-limited reproduction. Among species, H(max) was negatively correlated with shade tolerance and seed size. Tree size at the onset of reproduction and fecundity was strongly linked to this axis of life-history variation, but phenological pattern was not. Absolute tree size at the onset of reproduction was positively correlated with H(max) and negatively correlated with shade tolerance. Relative size at reproductive onset was not correlated with shade tolerance or H(max). Fecundity ranged four orders of magnitude among species and was correlated positively with H(max) and negatively with seed size and shade tolerance. The interrelationships among these reproductive and other life-history traits are strongly correlated with the species' requirements for gap colonization.     

Genetic variability and differentiation of three Russian populations of yellow potato cyst nematode Globodera rostochiensis as revealed by nuclear markers

Genetic variability of yellow potato cyst nematode G. rostochiensis from three Russian populations (Karelia, Vladimir oblast, and Moscow oblast) was investigated using two types of nuclear markers. Using RAPD markers identified with the help of six random primers (P-29, OPA-10, OPT-14, OPA-11, OPB-11, and OPH-20), it was possible to distinguish Karelian population from the group consisting of the populations from two adjacent regions (Moscow oblast and Vladimir oblast). Based on the combined matrix, containing 294 RAPD fragments, dendrogram of genetic differences was constructed, and the indices of genetic divergence and partition (P, H, and G(st)), as well as the gene flow indices N(m) between the nematode samples examined, were calculated. The dendrogram structure, genetic diversity indices, and variations of genetic distances between single individuals in each population from Karelia and Central Russia pointed to genetic isolation and higher genetic diversity of the nematodes from Karelia. Based on polymorphism of rDNA first intergenic spacer ITS1, attribution of all populations examined to the species G. rostochiensis was proved. Small variations of the ITS1 sequence in different geographic populations of nematodes from different regions of the species world range did not allow isolation of separate groups within the species. Possible factors (including interregional transportations of seed potato) affecting nematode population structure in Russia are discussed.     

Stand Composition,Tree Proximity and Size Have Minimal Effects on Leaf Function of Coexisting Aspen and Subalpine Fir

Forest structural heterogeneity due to species composition, spatial relationships and tree size are widely studied patterns in forest systems, but their impacts on tree function are not as well documented. The objective of this study was to examine how stand composition, tree proximity relationships and tree size influence the leaf functional traits of aspen, an early successional species, and subalpine fir, a climax species. We measured foliar nutrients, nonstructural carbohydrates (aspen only), defense chemistry and xylem water potential of aspen and subalpine fir trees in three size classes growing in close proximity or independently from other trees under three stand conditions: aspen dominant, aspen-conifer mixed, and conifer dominant stands. Close proximity of subalpine fir to aspen reduced aspen’s storage of starch in foliar tissue by 17% suggesting that competition between these species may have small effects on carbon metabolism in aspen leaves. Simple sugar (glucose + sucrose) concentrations in aspen leaves were slightly higher in larger aspen trees than smaller trees. However, no differences were found in stem water potential, foliar concentrations of nitrogen, phosphorus, or secondary defense chemicals of aspen or subalpine fir across the gradients of stand composition, tree proximity or tree size. These results suggest that mechanisms of coexistence allow both aspen and subalpine fir to maintain leaf function across a wide range of stand structural characteristics. For aspen, resource sharing through its clonal root system and high resource storage capacity may partially contribute to its functional stability in mixed aspen-conifer stands.     

<Emphasis Type="Bold">Epigenetic variation in clonal stands of aspen</Emphasis>

Epigenetic mechanisms can affect ecologically important traits, even in the absence of genetic variation. Environmental factors can influence gene regulation through chemical modifications, such as DNA methylation, resulting in acclimation that can be transferred to subsequent cell generations both mitotically and meiotically. Clonal plants such as Populus tremuloides (aspen) show considerable promise as model species for the long-term in situ study of ecological epigenetics. The common replication of identical genotypes across heterogeneous environments permits within- and between-genotype comparisons while controlling for genetic makeup. With a long lifespan and limited natural selection resulting from sexual recombination, it is conceivable that epigenetic acclimation plays an important role in the long-term ecological success of aspen. This case study is the first in a series investigating the role of epigenetics in aspen ecology. We have established long-term permanent plots of aspen, identified (genotyped) clones and established the baseline epigenetic structure. Here we report the in situ epigenetic structure of two aspen stands. We find considerable epigenetic variation and significant differences within and among genotypes and sites, suggesting both genotype and environment influence the epigenotype.     

The genetic basis of smoltification-related traits in Oncorhynchus mykiss

The timing and propensity for migration between fresh- and seawater is a key theme in the diversity of life histories within the salmonid fishes. Across salmonid species, life-history strategies range from wholly freshwater-resident populations, to migratory and nonmigratory variation within populations, to populations and species that are primarily migratory. Despite the central theme of migration to the evolution of these fishes, the genetic architecture of migration-related processes is poorly understood. Using a genetic cross of clonal lines derived from migratory and nonmigratory life-history types of Onchorhynchus mykiss (steelhead and rainbow trout, respectively), we have dissected the genetic architecture of the complex physiological and morphological transformation that occurs immediately prior to seaward migration (termed smoltification). Quantitative trait loci (QTL) analyses were used to identify the number, effects, and genomic location of loci associated with smoltification-related traits, including growth and condition factor, body coloration, morphology, and osmoregulatory enzymes during the smoltification period. Genetic analyses revealed numerous QTL, but one locus in particular is associated with multiple traits in single and joint analyses. Dissecting the genetic architecture of this highly complex trait has profound implications for understanding the genetic and evolutionary basis of life-history diversity within and among migratory fishes.     

Natural hybridization and hybrid zones between Quercus crassifolia and Quercus crassipes (Fagaceae) in Mexico: morphological and molecular evidence

Hybrid zones provide interesting systems to study genetic and ecological interaction between different species. The correct identification of hybrids is necessary to understand the evolutionary process involved in hybridization. An oak species complex occurring in Mexico formed by two parental species, Quercus crassifolia H. & B. and Q. crassipes H. & B., and their putative hybrid species, Q. dysophylla, was analyzed with molecular markers (random amplified polymorphic DNA [RAPDs]) and morphological tools in seven hybrid zones (10 trees per taxa in each hybrid zone) and two pure sites for each parental species (20 trees per site). We tested whether geographic proximity of hybrid plants to the allopatric site of a parental species increases its morphological and genetic similarity with its parent. Seventeen morphological traits were measured in 8700 leaves from 290 trees. Total DNA of 250 individuals was analyzed with six diagnostic RAPD primers. Quercus crassifolia differed significantly from Q. crassipes in all the examined characters. Molecular markers and morphological characters were highly coincident and support the hypothesis of hybridization in this complex, although both species remain distinct in mixed stands. Clusters and a hybrid index (for molecular and morphological data) showed that individuals from the same parental species were more similar among themselves than to putative hybrids, indicating occasional hybridization with segregation in hybrid types or backcrossing to parents. Evidence does not indicate a unidirectional pattern of gene flow.     

EVALUATING A HYPOTHESIS ABOUT HETEROCHRONY: LARVAL LIFE-HISTORY TRAITS AND JUVENILE HIND-LIMB MORPHOLOGY IN HYLA CRUCIFER

This paper reports the results of an investigation into whether selection on genetically based differences in the timing or rate of development (heterochrony) can give rise to nonadaptive morphological differences among individual frogs. We used a quantitative-genetics approach to examine the relationships among the life-history characters time to metamorphosis and larval-growth rate and a functionally significant morphological features, relative hind-limb length, in the spring peeper, Hyla crucifer. Time to metamorphosis and growth rate had low heritabilities in our population. Morphological traits had moderate heritabilities. There were positive genetic correlations between the life-history traits and the components of relative hind-limb length but no significant correlations with the shape variable itself. We used field observations of pond-drying time and experimental results of selection on growth rate to simulate the correlated responses of hind-limb shape to four reasonable selection regimes on the life-history traits. We found little evidence to suggest that relative hind-limb length would display much of a correlated response to such selection. The differences in relative hind-limb length seen among closely related species or among populations of a single species that appear to be unrelated to performance differences are not obviously explicable as neutral correlated responses to selection on larval traits.     

Population genetic structure of wild rice Oryza glumaepatula distributed in the Amazon flood area influenced by its life-history traits

Wild plant species develop their own way of living to adapt to the specific environment of their habitats. Their life-history traits strongly affect the genetic structure of the population. The wild species Oryza glumaepatula Steud. growing in the Amazon basin seems to have characteristic life-history traits suited for the flood condition. At the vegetative growth stage, the culms frequently break at internodes. With no roots anchoring on the ground, plant bodies floating in the water move downriver by water current and wind. To examine the association between the life-history traits and genetic population structure of Amazonian O. glumaepatula , we analysed allozyme variability at 29 loci of 16 enzymes using 37 populations from five regions. Allozymes were not so variable (total gene diversity H _E = 0.044) compared with Asian wild rice, O. rufipogon Griff. The bottleneck effect and rare opportunity of interspecies gene flow may prevent the development of allozyme variability. Population genotypes tended to be differentiated among geographically isolated regions. Observed heterozygosities were much lower than expected heterozygosities, or gene diversity ( H _O = 0.003 for whole population) and F _IS over polymorphic loci was 0.931, indicating that O. glumaepatula has developed an inbreeding system. But, the intrapopulation gene diversity ( H _S) was higher than interpopulation gene diversity ( D _ST), as generally observed in outbreeding populations. The migration ability of O. glumaepatula makes long-distance seed dispersal possible. This might have led to frequent gene flow among populations.     

DIFFERENTIATION AMONG POPULATIONS OF TETRAPLOID GRINDELIA CAMPORUM

Ten populations of 4N Grindelia camporum (Asteraceae) were investigated in a common-garden study of phenotypic and genetic variability. This species is a long-lived perennial herb and an obligate outcrosser which would be expected to display considerable gene flow among populations based on its life-history traits. Each of the 17 morphological, phenological, and physiological traits examined showed significant differences between populations. Average within-population heritability was 0.39. The variation between populations averaged 3.3 times the variation within populations, indicating that the populations are well-differentiated for each of the 17 traits. Two possible explanations for the differentiation are given: populations may have diverged from a common ancestor in very little geologic time, or the species may have a polytopic origin.     

Inbreeding depression in the competitive fertilization success of male crickets

Mating between close relatives generally results in offspring of decreased fitness. Inbreeding depression is generally greater for life-history traits than for morphological traits, and recent studies of traits subject to sexual selection suggest that these may suffer the greatest inbreeding depression. Sexual selection continues after mating in the form of sperm competition and cryptic female choice, imposing strong selection on male competitive fertilization success. Here, I examine the effects of a single generation of full-sib mating on competitive fertilization success in a cricket, Teleogryllus oceanicus. The estimated coefficient of inbreeding depression in competitive fertilization success was 0.37, higher than that for other life-history and morphological traits. Such intense inbreeding depression coupled with little or no additive genetic variance for this trait is consistent with strong directional selection on male competitive fertilization success generating high levels of dominance variance, and provides an adaptive explanation for the evolution of inbreeding avoidance found in this species.     

Morphology and phylogenetic placement of Endoconidioma, a new endoconidial genus from trembling aspen

Endoconidioma populi gen. et sp. nov. is described from black subicula on twigs of trembling aspen, Populus tremuloides, in Alberta, Canada. Pycnidium-like conidiomata are produced on twigs and in culture, but, unlike pycnidia, conidiomata of E. populi have a closed peridium and a locule filled with conidiogenous cells that form conidia endogenously. These endoconidia are hyaline, unicellular and released by the dissolution of the peridial cell wall. In addition to endoconidia, mostly two-celled conidia that form blastically from undifferentiated hyphae occur often in culture but are observed only occasionally on Populus twigs. No coelomycetous taxa have been reported to produce endoconidia, and both the morphological features and DNA sequence data demonstrate that Endoconidioma is distinct from the previously established endoconidial genera. Parsimony analyses of portions of the nuclear ribosomal RNA gene (SSU and ITS) suggest that Endoconidioma is closely related phylogenetically to members of the Dothideales and allied anamorphs in Hormonema and Kabatina.     

QUANTITATIVE GENETICS OF SURVIVAL AND GROWTH IN IMPATIENS CAPENSIS

When variation in life-history characters is caused by many genes of small effect, then quantitative-genetic parameters may quantify constraints on rate and direction of microevolutionary change. I estimated heritabilities and genetic correlations for 16 life-history and morphological characters in two populations of Impatiens capensis, a partially self-pollinating herbaceous annual. The Madison population had little or no additive genetic variance for any of these characters, while the Milwaukee population had significant narrowsense heritabilities and genetic correlations for several traits, including adult size, which is highly correlated with fitness. All genetic correlations among fitness components were positive, hence there is no evidence for antagonistic pleiotropy among these traits. Dissimilarity of heritabilities in the two populations supports theoretical predictions that long-term changes in genetic variance-covariance patterns may occur when population sizes are small and selection is strong, as may occur in many plant species.     

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.     

Genetic basis of variation in morphological and life-history traits of a wild population of pink salmon

Understanding the genetic basis of phenotypic variation is essential for predicting the direction and rate of phenotypic evolution. We estimated heritabilities and genetic correlations of morphological (fork length, pectoral and pelvic fin ray counts, and gill arch raker counts) and life-history (egg number and individual egg weight) traits of pink salmon (Oncorhynchus gorbuscha) from Likes Creek, Alaska, in order to characterize the genetic basis of phenotypic variation in this species. Families were created from wild-caught adults, raised to the fry stage in the lab, released into the wild, and caught as returning adults and assigned to families using microsatellite loci and a growth hormone locus. Morphological traits were all moderately to highly heritable, but egg number and egg weight were not heritable, suggesting that past selection has eliminated additive genetic variation in egg number and egg weight or that there is high environmental variance in these traits. Genetic correlations were similar for nonadjacent morphological traits and adjacent traits. Genetic correlations predicted phenotypic correlations fairly accurately, but some pairs of traits with low genetic correlations had high phenotypic correlations, and vice versa, emphasizing the need to use caution when using phenotypic correlations as indices of genetic correlations. This is one of only a handful of studies to estimate heritabilities and genetic correlations for a wild population.     

Can population genetic structure be predicted from life-history traits?

Population genetic structure is a key parameter in evolutionary biology. Earlier comparative studies have shown that genetic structure depends on species ecological attributes and life-history traits, but species phylogenetic relatedness had not been accounted for. Here we reevaluate the relationships between genetic structure and species traits in seed plants. Each species is characterized by a set of life-history and ecological features as well as by its geographic range size, its heterozygote deficit, and its genetic structure at nuclear and organelle markers to distinguish between pollen- and seed-mediated gene flow. We use both a conventional regression approach and a method that controls for phylogenetic relationships. Once phylogenetic conservatism and covariation among traits are taken into account, genetic structure is shown to be related with only a few synthetic traits, such as mating system for nuclear markers and seed dispersal mode or geographic range size for organelle markers. Along with other studies on invasiveness or rarity, our work illustrates the fact that predicting the fate of species across a broad taxonomic assemblage on the basis of simple traits is rarely possible, a testimony of the highly contingent nature of evolution.     

Population genetics and morphological comparisons of migratory European (Hirundo rustica rustica) and sedentary East-Mediterranean (Hirundo rustica transitiva) barn swallows

Speciation processes are largely determined by the relative strength of divergent selection versus the magnitude of gene flow. The barn swallow (Hirundo rustica) has a broad geographic distribution that encompasses substantial geographic variation in morphology and behavior. The European (H. r. rustica) and East-Mediterranean (H. r. transitiva) subspecies are closely related, despite differing in morphological and life-history traits. To explore patterns of genetic differentiation and gene flow, we compared morphological and genetic variation among the nonmigratory breeding population of H. r. transitiva from Israel and the migratory population of H. r. rustica that passes through Israel and compared it with the genetic differentiation between H. r. transitiva from Israel and a breeding population of H. r. rustica from the United Kingdom that uses a different migratory flyway. Mitochondrial haplotype network analysis suggests that the European and East-Mediterranean populations are intermixed, although there was low but significant genetic differentiation between the subspecies based on both mitochondrial (F(ST) = 0.025-0.033) and microsatellite (F(ST) = 0.009-0.014) loci. Coalescent-based analyses suggest recent divergence and substantial gene flow between these populations despite their differences in morphological and behavioral traits. The results suggest that these subspecies are undergoing a differentiation process in the face of gene flow, with selection possibly operating on sexually selected traits.     

Epistasis and dominance: evidence for differential effects in life-history versus morphological traits

Dominance and epistatic effects are predicted to be larger in life-history than in morphological traits. We test these predictions using published results from line cross analyses. We find that dominance is found in more than 95% of traits, regardless of the type of trait, but that the magnitude of the effect in relation to the additive effect is much greater in life-history than in morphological traits. Epistatic effects were detected more often in life-history than in morphological traits (79% and 67%, respectively). We also test for a difference in the magnitude of the effects by comparing the ratio of the nonadditive components separately to the additive component. For both dominance and epistatic components, the ratio of the nonadditive component to additive effects in life-history traits is approximately twice as large as that for morphological traits.     

Admixture and Gene Flow from Russia in the Recovering Northern European Brown Bear (Ursus arctos)

Large carnivores were persecuted to near extinction during the last centuries, but have now recovered in some countries. It has been proposed earlier that the recovery of the Northern European brown bear is supported by migration from Russia. We tested this hypothesis by obtaining for the first time continuous sampling of the whole Finnish bear population, which is located centrally between the Russian and Scandinavian bear populations. The Finnish population is assumed to experience high gene flow from Russian Karelia. If so, no or a low degree of genetic differentiation between Finnish and Russian bears could be expected. We have genotyped bears extensively from all over Finland using 12 validated microsatellite markers and compared their genetic composition to bears from Russian Karelia, Sweden, and Norway. Our fine masked investigation identified two overlapping genetic clusters structured by isolation-by-distance in Finland (pairwise F_ST = 0.025). One cluster included Russian bears, and migration analyses showed a high number of migrants from Russia into Finland, providing evidence of eastern gene flow as an important driver during recovery. In comparison, both clusters excluded bears from Sweden and Norway, and we found no migrants from Finland in either country, indicating that eastern gene flow was probably not important for the population recovery in Scandinavia. Our analyses on different spatial scales suggest a continuous bear population in Finland and Russian Karelia, separated from Scandinavia.     

QUANTITATIVE GENETICS OF SIZE AND PHENOLOGY OF LIFE-HISTORY TRAITS IN CHAMAECRISTA FASCICULATA

Despite numerous adaptive scenarios concerning the evolution of plant life-history phenologies few studies have examined the heritable basis for and genetic correlations among these phenologies. Documentation of genetic variation for and covariation among reproductive phenologies is important because it is this variation/covariation that will determine the potential for response to evolutionary forces. To address this problem, I conducted a breeding experiment to determine narrow-sense heritabilities for and genetic correlations among the phenologies of life-history events and plant size in Chamaecristafasciculata, a temperate summer annual plant species. Paternal families showed no evidence of heritable variation for two estimates of plant size, six measures of reproductive phenology or two fitness components. Similarly, paternal estimates of genetic correlations among these traits were low or zero. In contrast, maternal estimates of heritability suggested the influence of maternal parent on one estimate of plant size and four phenological traits. Likewise, maternal effects influenced maternal estimates of genetic correlations. These maternal effects can arise from three sources: endosperm nuclear, cytoplasmic genetic and/or maternal phenotypic. The degree to which the phenology of one life-history trait acts as a constraint on the evolution of other phenological traits depends on the source of the maternal influence in this species.