Hybridization,recombination, and the genetic basis of fitness variation across environments in <Emphasis Type="Italic">Avena barbata</Emphasis>

We created Recombinant Inbred Lines (RILs) derived from a cross between ecotypes of Avena barbata associated with moist (mesic) and dry (xeric) habitats in California. Traits which were correlated with fitness across RILs mapped to the same Quantitative Trait Loci (QTLs) as fitness. However, different QTL affected fitness in different environments so that fitness was weakly correlated across environments. Recombination released considerable heritable variation both in fitness, and in ecologically relevant traits. Many traits showed transgressive segregation caused by recombination of QTL associated in repulsion phase in the parents. In addition, some traits were uncorrelated, allowing novel combinations of those traits to be created. Recombination also created heritable variation in reaction norms for at least one trait (root allocation). Altogether these results suggest that recombination can combine the most selectively advantageous genes and traits of the parents to produce broadly adapted genotypes that are capable of outperforming the parents. Indeed, two of the RILs showed higher fitness than the parental ecotypes across a range of environmental treatments in the greenhouse, but their superiority was less pronounced in the field. Although late-generation recombinants exhibited hybrid breakdown, being less fit, on average, than the mid-parent, early generation hybrids appear to exhibit hybrid vigour through the expression of dominance effects in the heterozyotes. This vigour may offset the effects of hybrid breakdown in the early generations following a cross, enhancing the opportunity for recombination to create broadly adapted genotypes. We discuss the implications of these findings to the evolution of colonizing species.     

QUANTITATIVE GENETICS OF SIZE,SHAPE, LIFE-HISTORY,AND FRUIT CHARACTERISTICS OF THE SEED-HETEROMORPHIC COMPOSITE HETEROSPERMA PINNATUM. I. VARIATION WITHIN AND AMONG POPULATIONS

We document phenotypic and genetic variation within and among populations of the seed heteromorphic species Heterosperma pinnatum Cav. (Compositae) in the production of seed morphs and in a variety of life-history and morphological characteristics that might be correlated with seed and head traits. Each trait is found to have significant genetic variance in most or, usually, all populations. Significant among-population genetic variation exists for all traits except number of achenes per head and seedling shape, although some traits have much less genetic variation among than within populations. Number and percentage of intermediate achenes per head, total number of achenes per head, and lengths of central and peripheral achenes had little among-population genetic variation compared to within-population variation. Most traits had slightly less genetic variation among than within populations; however, some traits (percentage of central achenes, length of awns, date that the first flowering head opened, date that the first fruiting head opened, and death date) had more among-population genetic variation. The proportions of achene morphs produced had high broad-sense heritabilities and high among-population genetic variance, except in the case of intermediate achenes. All phenological variables had high among-population genetic variation. Within-population heritabilities were high for dates of first flowering head and fruiting head but low for death date and reproductive interval. Family and population means measured in the greenhouse for traits having high broad-sense heritability or among-population genetic variance were closely correlated with field means for the corresponding families or populations. The amounts of phenotypic variation were similar for traits that were measured in both the field and the greenhouse. These lines of evidence suggest that greenhouse results provide reasonable estimates of genetic variation in the field for this species. Numerous studies have reported variation in the proportion of seed morphs for different heteromorphic-seeded species and have discussed adaptive scenarios for the evolution of seed proportions; however, our investigation is one of only a few that have documented the amount of phenotypic and genetic variation within and among populations.     

QUANTITATIVE GENETICS OF FITNESS TRAITS IN A WILD POPULATION OF PHLOX

To determine if the evolution of fitness traits in the annual plant, Phlox drummondii, is constrained by lack of genetic variation, we calculated the heritability and genetic correlation of 16 traits in a field population. Full- and half-sib families of seeds were generated in the greenhouse and planted back into the study population. Of 855 seeds that germinated, 609 survived to produce fruit. For each plant we measured several aspects of plant size and three components of female fecundity: total number of fruits produced, number of seeds per fruit, and mass of individual seeds. Heritability of traits ranged from 0.00 to 0.15. Several traits showed significant levels of additive genetic variance, but we found no evidence of additive genetic variance in components of female fecundity and no evidence of negative genetic correlation between fitness traits. These results suggest that evolution in this population would be constrained by lack of heritable variation in fitness traits.     

Natural variation in the regulation of leaf senescence and relation to other traits in Arabidopsis

Leaf senescence results in the recycling of nutrients, thereby providing resources required for growth and reproduction. In this study, the effect of day-length on leaf senescence in eight different Arabidopsis thaliana ecotypes was determined and the relationship between senescence and other morphological and life history traits was analysed. A significant variation in the start and extent of leaf senescence depending on the genetic background and the response to day-length was found. Whereas senescence of early flowering ecotypes was accelerated by long days, no effect of day-length on senescence could be found in late flowering Kas-1 plants. Senescence in the different ecotypes was associated with other traits, such as floral transition, the total number of fruits, the total number of leaves and the maximum chlorophyll content. Plants that bolted early also senesced early, produced fewer leaves, accumulated less chlorophyll, but produced more fruits. The present results indicate that senescence may be a key component in the trade-off between investment in photosynthetic capacity and reproduction. The relationship between senescence and other traits was maintained independent of whether differences in senescence were caused by genetic (ecotype) or environmental (day-length) variation, suggesting that genetic and environmental factors affect these traits through common regulatory pathways.     

Identifying loci under selection across contrasting environments in Avena barbata using quantitative trait locus mapping

We constructed recombinant inbred lines of a cross between naturally occurring ecotypes of Avena barbata (Pott ex Link), Poaceae, associated with contrasting moisture environments. These lines were assessed for fitness in common garden reciprocal transplant experiments in two contrasting field sites in each of two years, as well as a novel, benign greenhouse environment. An AFLP (amplified fragment length polymorphism) linkage map of 129 markers spanned 644 cM in 19 linkage groups, which is smaller, with more linkage groups, than expected. Therefore parts of the A. barbata genome remain unmapped, possibly because they lack variation between the ecotypes. Nevertheless, we identified QTL (quantitative trait loci) under selection in both native environments and in the greenhouse. Across years at the same site, the same loci remain under selection, for the same alleles. Across sites, an overlapping set of loci are under selection with either (i) the same alleles favoured at both sites or (ii) loci under selection at one site and neutral at the other. QTL under selection in the greenhouse were generally unlinked to those under selection in the field because selection acted on a different trait. We found little evidence that selection favours alternate alleles in alternate environments, which would be necessary if genotype by environment interaction were to maintain genetic variation in A. barbata. Additive effect QTL were best able to explain the genetic variation among recombinant inbred lines for the greenhouse environment where heritability was highest, and past selection had not eliminated variation.     

Heritability and correlation structure of nectar and floral morphology traits in Nicotiana alata

The heritability and genetic basis of nectar traits have been rarely studied in the field, where plants are exposed to environmental factors that could mask underlying genetic effects. Heritabilities and variance components were estimated for nectar and morphological traits of Nicotiana alata , using a partial diallel design. The main experiment was conducted in a Missouri experimental garden using a randomized block design with three plant density treatments, whereas a smaller experiment was conducted near native Brazil habitat to compare the environmental variance in traits between Missouri and Brazil. Significant heritability was detected for nectar volume and energy content, and for corolla tube length. Phenotypic correlations were significant between all traits investigated, whereas significant genetic correlations were only found between nectar volume and energy and between corolla limb width and mouth diameter. There were no significant family-by-density interactions detected in the Missouri field environment. All traits differed significantly between Missouri and Brazil environments, but significant genetic by environment (G × E) interactions between Missouri and Brazil were detected for only one trait. This study shows that nectar traits can be heritable despite considerable environmental variation.     

Expression of additive genetic variances and covariances for wild radish floral traits: comparison between field and greenhouse environments

Measurements of the genetic variation and covariation underlying quantitative traits are crucial to our understanding of current evolutionary change and the mechanisms causing this evolution. This fact has spurred a large number of studies estimating heritabilities and genetic correlations in a variety of organisms. Most of these studies have been done in laboratory or greenhouse settings, but it is not well known how accurately these measurements estimate genetic variance and covariance expressed in the field. We conducted a quantitative genetic half-sibling analysis on six floral traits in wild radish. Plants were grown from seed in the field and were exposed to natural environmental variation throughout their lives, including herbivory and intra- and interspecific competition. The estimates of heritabilities and the additive genetic variance-covariance matrix (G) obtained from this analysis were then compared to previous greenhouse estimates of the same floral traits from the same natural population. Heritabilities were much lower in the field for all traits, and this was due to both large increases in environmental variance and decreases in additive genetic variance. Additive genetic covariance expressed was also much lower in the field. These differences resulted in highly significant differences in the G matrix between the greenhouse and field environments using two complementary testing methods. Although the G matrices shared some principal components in common, they were not simply proportional to each other. Therefore, the greenhouse results did not accurately depict how the floral traits would respond to natural selection in the field.     

Costs of resistance: genetic correlations and potential trade-offs in an insect immune system

Theory predicts that natural selection will erode additive genetic variation in fitness-related traits. However, numerous studies have found considerable heritable variation in traits related to immune function, which should be closely linked to fitness. This could be due to trade-offs maintaining variation in these traits. We used the Egyptian cotton leafworm, Spodoptera littoralis, as a model system to examine the quantitative genetics of insect immune function. We estimated the heritabilities of several different measures of innate immunity and the genetic correlations between these immune traits and a number of life history traits. Our results provide the first evidence for a potential genetic trade-off within the insect immune system, with antibacterial activity (lysozyme-like) exhibiting a significant negative genetic correlation with haemocyte density, which itself is positively genetically correlated with both haemolymph phenoloxidase activity and cuticular melanization. We speculate on a potential trade-off between defence against parasites and predators, mediated by larval colour, and its role in maintaining genetic variation in traits under natural selection.     

Analysis of two components of flight using recombinant inbred lines of Drosophila melanogaster

Complex activities require precise coordination of their components for successful action. The genetic basis underlying coordination of traits may range from relatively static pleiotropic associations to more flexible genetic associations that recombine in phenotypes under continuous selective modification by the environment. Successful flight in insects depends on the precise integration of numerous component physiological processes. Here we examine the genetic basis of two of its components, flight duration and rate. To study flight we created recombinant inbred populations from stocks of this laboratory known for their significantly longer duration flights. A heritable basis for these traits was found and determined to be positively correlated between sexes. Correlations of flight length with rate were negative within sexes, suggesting a trade-off, but were significant in males only. Composite interval mapping using the recombinant inbred (RI2) design was used to locate the QTLs for these traits and test for pleiotropy. Four QTLs affecting duration or wing beat rate were found on chromosomes II and III. Tests for pleiotropy showed some effects on traits of QTLs were common to both sexes while others were sex-specific. No QTL was pleiotropic for both traits, suggesting that correlations between flight duration and rate of wing beat are determined by a combination of linkage and environmental factors.     

Genetic and environmental effects on morphology and fluctuating asymmetry in nestling barn swallows

A barn swallow Hirundo rustica partial cross‐fostering experiment with simultaneous brood size manipulation was conducted in two years with contrasting weather conditions, to estimate heritable variation in tarsus, tail and wing size and fluctuating asymmetry. Environmental stress had contrasting effects depending on trait type. Significant heritabilities for tarsus, tail and wing size were found only in enlarged broods irrespective of year effects, while tarsus asymmetry was significantly heritable in the year with benign weather conditions irrespective of brood size manipulation effects. Tail, wing and composite (multicharacter) asymmetry were never significantly heritable. The environment with the higher heritability generally had higher additive genetic variance and lower environmental variance, irrespective of trait type. Heritability was larger for trait size than for trait asymmetry. Patterns of genetic variation in nestlings do not necessarily translate to the juvenile or adult stage, as indicated by lack of correlation between nestling and fledgling traits.     

Heritability, correlations and in silico mapping of locomotor behavior and neurochemistry in inbred strains of mice

The midbrain dopamine system mediates normal and pathologic behaviors related to motor activity, attention, motivation/reward and cognition. These are complex, quantitative traits whose variation among individuals is modulated by genetic, epigenetic and environmental factors. Conventional genetic methods have identified several genes important to this system, but the majority of factors contributing to the variation remain unknown. To understand these genetic and environmental factors, we initiated a study measuring 21 behavioral and neurochemical traits in 15 common inbred mouse strains. We report trait data, heritabilities and genetic and non-genetic correlations between pheno-types. In general, the behavioral traits were more heritable than neurochemical traits, and both genetic and non-genetic correlations within these trait sets were high. Surprisingly, there were few significant correlations between the behavioral and the individual neurochemical traits. However, striatal serotonin and one measure of dopamine turnover (DOPAC/DA) were highly correlated with most behavioral measures. The variable accounting for the most variation in behavior was mouse strain and not a specific neurochemical measure, suggesting that additional genetic factors remain to be determined to account for these behavioral differences. We also report the prospective use of the in silico method of quantitative trait loci (QTL) analysis and demonstrate difficulties in the use of this method, which failed to detect significant QTLs for the majority of these traits. These data serve as a framework for further studies of correlations between different midbrain dopamine traits and as a guide for experimental cross designs to identify QTLs and genes that contribute to these traits.     

Latitudinal variation in plant size and relative growth rate in Arabidopsis thaliana

Latitude is an important determinant of local environmental conditions that affect plant growth. Forty ecotypes of Arabidopsis thaliana were selected from a wide range of latitudes (from 16°N to 63°N) to investigate genetic variation in plant size and relative growth rate (RGR) along a latitudinal gradient. Plants were grown in a greenhouse for 31 days, during which period three consecutive harvests were performed. Plants from high latitudes tended to have smaller plant size in terms of seed size, cotyledon width, rosette size, number of rosette leaves, size (leaf area) of the largest leaves, total leaf area, and total dry weight per plant than those from low latitudes. The mean (±SE) RGR across ecotypes was 0.229 (±0.0013) day⁻¹. There was, however, significant ecotypic variation, with RGR being negatively correlated with latitude. The two main components of RGR, leaf area ratio (LAR) and unit leaf rate (ULR), were also correlated with latitude: LAR increased with increasing latitude while ULR decreased with increasing latitude. It was also found that RGR tended to be negatively correlated with LAR, specific leaf area (SLA) and specific root length (SRL) but to be positively correlated with mean area per leaf (MAL) and ULR. The variation in RGR among ecotypes was relatively small compared with that in the other traits. RGR may be a conservative trait, whose variation is constrained by the trade-off between its physiological (i.e. ULR) and morphological (i.e. LAR) components. Received: 2 November 1997 / Accepted: 28 February 1998     

Seedling development in a Brassica napus diversity set and its relationship to agronomic performance

Brassica napus L. is the leading European oilseed crop and has therefore a high economical importance. The objectives of our study were to examine (1) the patterns of phenotypic diversity in a species-wide B. napus germplasm set of 518 inbreds with respect to various seedling development, agronomic, and seed quality traits as well as (2) the interrelationship of the examined traits and their use in selection on correlated traits. The B. napus germplasm set was evaluated in greenhouse and field trials for several seedling development, agronomic, and seed quality traits. The traits were highly correlated within the individual trait categories and moderately correlated between the different trait categories. We observed differences in phenotypic diversity among the examined eight germplasm types. The reduction of phenotypic diversity was on average more pronounced for the seedling development traits than for the agronomic and seed quality traits, suggesting that plant breeders need to introgress new genetic variation with respect to the former.     

Variability and correlations in muskmelon in relation to the cultivation method

Summary Six fruit characters have been measured in 23 cultivars of Cucumis melo, representing a wide geographical range. Plants were grown both in the greenhouse and in the field. When the 23 cultivars were analyzed together, the largest component of variance was found between cultivars under both growth conditions, suggesting the existence of large genetic diversity for all the characters studied. Generally, variance between plants within cultivars was less than or equal to variance between fruits within plant. This indicates that environmental variation is the most important part of the variation within cultivars. Correlations between pairs of characters at cultivar, plant and fruit levels were calculated from the variance-covariance components. In the majority of paired traits, the correlation values indicated that genetic and environmental factors may act in the same direction.     

Genotype by environment interactions for fitness in hybrid genotypes of Avena barbata

We examined genotype (G) by environment (E) interactions for fitness in mesic and xeric ecotypes of the self-fertilizing annual grass, Avena barbata and their recombinant inbred hybrid progeny. Fitness was assayed (1) in experimental water and nutrient treatments in the greenhouse and (2) in common gardens in each ecotype's native habitat. G x E interactions were significant in the greenhouse. Nevertheless, the same recombinant genotypes tended to have high fitness across all water and nutrient treatments. G x E interactions were less pronounced in the field, and were driven by the contrast between the uniformly low survivorship at the mesic site in 2004 and genetic variation in fitness at the other years/site combinations. Moreover, the mesic ecotype consistently outperformed the xeric in both field and greenhouse. Several of the recombinant genotypes outperformed the parents in the novel greenhouse treatments, but these genotypes did not outperform the mesic parent in field trials. Indeed, it is only in the comparison between field and greenhouse environments that there was a noticeable change in the identity of the most-fit genotype. The results provide evidence that hybridization can create genotypes that are better adapted to newer environments such as those imposed in our greenhouse experiments.     

Genetic variation in variability: Phenotypic variability of fledging weight and its evolution in a songbird population

Variation in traits is essential for natural selection to operate and genetic and environmental effects can contribute to this phenotypic variation. From domesticated populations, we know that families can differ in their level of within‐family variance, which leads to the intriguing situation that within‐family variance can be heritable. For offspring traits, such as birth weight, this implies that within‐family variance in traits can vary among families and can thus be shaped by natural selection. Empirical evidence for this in wild populations is however lacking. We investigated whether within‐family variance in fledging weight is heritable in a wild great tit (Parus major) population and whether these differences are associated with fitness. We found significant evidence for genetic variance in within‐family variance. The genetic coefficient of variation (GCV) was 0.18 and 0.25, when considering fledging weight a parental or offspring trait, respectively. We found a significant quadratic relationship between within‐family variance and fitness: families with low or high within‐family variance had lower fitness than families with intermediate within‐family variance. Our results show that within‐family variance can respond to selection and provides evidence for stabilizing selection on within‐family variance.     

Environmental conditions and genetic differentiation: what drives the divergence of coexisting Leymus chinensis ecotypes in a large-scale longitudinal gradient?

Aims The two coexisting Leymus chinensis ecotypes exhibit remarkable divergences in adaptive strategies under drought and salinity in semi-humid meadows and semi-arid steppes. In order to detect the major genetic and environmental factors dominating the intraspecific phenotype variations and ecotype formation, the questions regarding the two distinct phenotypic forms (ecotypes) in L. chinensis were addressed: (i) did environments drive the L. chinensis ecotype formation? (ii) was there a molecular basis for the morphological divergence between the two ecotypes? (iii) which driving force dominated the intraspecies divergence, divergent natural selection, genetic drift or stabilizing selection?Methods We applied a series experiments on demographical, morphological and physiological traits of two Leymus chinensis ecotypes with gray green (GG) and yellow green (YG) leaf color in nine wild sites along a longitudinal gradient from 114° to 124°E in northeast China. The environmental data including mean annual precipitation, mean annual temperature, elevation and soil properties were collected. We compared the differences of morphological, physiological and genetic differentiations between the two ecotypes.Important findings The GG type exhibited stronger fitness than YG type from the population densities, morphological traits (e.g. shoot height, leaf area, leaf and seed weights et al.), leaf mass per area (LMA) and physiological traits [relative water content (RWC), proline, soluble sugar contents]. Most of above phenotypes (e.g. total shoot densities, spike length et al.) were significantly correlated with mean annual precipitation, mean annual temperature and soil water content (SWC), rarely a correlated with soil pH and soil nutrient. Transplanted populations showed convergence trend by their leaf chlorophyll contents and osmotic adjustments (proline and soluble sugar contents) in the greenhouse, but still exhibited their divergences between two ecotypes in the outdoor transplantation, suggesting that whether L. chinensis ecotype differentiated could be largely affected by the environmental conditions. Furthermore, by the comparison result of quantitative genetic variation (Q ST) values from phenotypes with theoretical neutral genetic differentiation (F ST), differentiation in phenotypic traits greatly surpassed neutral predictions, implying that directional natural selection played a crucial role in L. chinensis ecotype differentiation. In addition, microsatellite analysis from Neighbor-joining tree and Bayesian assignment generated into two groups according to ecotypes, indicating molecular genetic differentiation also propelled the two ecotypes divergence. We conclude that L. chinensis population variations were driven by combing divergent natural selection (precipitation, temperature and SWCs) along the large-scale gradient and significantly intrinsic genetic differentiation.     

Sexual selection in the cricket <Emphasis Type="Italic">Gryllus bimaculatus</Emphasis>: no good genes?

Recent studies have suggested that females of the field cricket Gryllus bimaculatus exercise post-copulatory choice over the paternity of their offspring. There is evidence that these choices are made in relation to the genetic compatibility of mates rather than their absolute quality, but the magnitude of heritable differences in males has not been thoroughly examined. Using a half-sib breeding design we measured additive genetic variance and dam effects in a suite of reproductive and non-reproductive traits. Both components explained relatively little of the phenotypic variance across traits. The dam component in our design contains variance caused by both maternal effects and dominance. If maternal effects are negligible as suggested by previous studies, our data suggest that dominance variance is an important source of variation in these traits. The lack of additive genetic variation, but possible existence of large amounts of non-additive genetic variation is consistent with the idea that female mate choice and multiple mating may be driven by differences in genetic compatibility between potential mates rather than by differences in genetic quality.     

Basal metabolic rate: heritability and genetic correlations with morphological traits in the zebra finch

Studies of genetic variation in metabolic traits have so far not focused on birds. In our study population of captive zebra finches we found evidence for a significant heritable genetic component in basal metabolic rate (BMR). Heritability of all morphological traits investigated (body mass, head length, tars length and wing length) was significantly larger than zero. All traits were positively phenotypically correlated. Eight of 10 genetic correlations presented in this study differed significantly from zero, all being positive, suggesting the possibility of correlated responses to any selection acting on the traits. When conditioned on the genetic variance in body mass, the heritability of BMR was reduced from 25% to 4%. Hence, our results indicate that genetic changes in BMR through directional selection are possible, but the potential for adaptation independent of body mass may be limited.     

小麦种子活力性状的遗传变异和相关研究

本研究利用12个普通小麦品种对10个种子活力性状的遗传变异和相关研究,表明除正常幼苗百分率外,其余种子活力性状在品种间均存在显著的差异。种子贮藏物质转换效率、电导率两个性状问及与其它性状均无显著的遗传相关,因此对他们的选择不会影响到其它性状。通径分析表明幼苗干重主要取决于种子贮藏物质转换效率、种子贮藏物质利用速率;发芽指数主要由平均发芽时间决定。电导率、发芽势、幼苗于重、种子干重、发芽指数、种子贮藏物质消耗比率6个性状表现中到高的遗传力、遗传变异系数和相对遗传进展,指明通过遗传育种手段改良这些性状是可能的。