Quantifying trait selection driving community assembly: a test in herbaceous plant communities under contrasted land use regimes

Plant traits are particularly important in determining plant community structure. However, how can one identify which traits are the most important in driving community assembly? Here we propose a method 1) to quantify the direction and strength of trait selection during community assembly and 2) to obtain parsimonious lists of traits that can predict species relative abundances in plant communities. We tested our method using floristic data from 32 plots experiencing different treatments (fertilisation and grazing) in southern France. Twelve functional traits were measured on 68 species. We determined the direction and strength of selection on these 12 traits using a metric derived from a maximum entropy model (i.e. lambda). We then determined our parsimonious list of traits using a backward selection of traits based on these lambda values (for all treatments and in each treatment separately). We finally compared our method to two other methods: one based on iterative RLQ and the other based on an entropy‐based forward selection of traits. We found major differences in the direction and strength of selection across the 12 traits and treatments. From the 12 traits, plant vegetative and reproductive heights, leaf dry matter content leaf nitrogen content, specific leaf area, and leaf phosphorus content were particularly important for predicting species relative abundances when considering all treatments together. Our method yielded results similar to those produced by the entropy‐based approach but differed from those produced by the iterative RLQ, whose selected traits could not significantly predict species relative abundances. Together these results suggest that the assembly of these communities is primarily driven by a small number of key functional traits. We argue that our method provides an objective way of determining a parsimonious list of traits that together accurately predict community structure and which, despite its complementarities with entropy‐based method, offers significant advantages.     

基于生长与结实评价红松种子园亲本

为评价选择优良的红松育种材料,对26和28年生69个红松半同胞家系生长(树高和胸径)和结实性状(单株球果数、鲜塔重、千粒重和单塔出籽量)进行测定分析。结果表明:不同树龄、不同家系间各指标差异均达极显著水平(P <0.01);各指标遗传力均较高(0.87~0.96),表型变异系数变化范围为6.30%~63.00%;树高、胸径和单株球果数一般配合力变化范围分别为-1.77~1.83、-9.31~8.67和-15.97~16.03;各生长指标之间相关性均达显著水平(0.716~0.998),其他指标与生长指标多数未达显著水平;利用多性状综合评价法,以10%的入选率对69个半同胞家系的生长性状和结实性状分别进行综合评价,入选的7个生长性状优良家系树高和胸径遗传增益分别为15.66%和35.40%;入选的7个结实性状优良家系单株球果数、鲜塔重和单塔出籽量遗传增益分别为87.30%、1.77%和3.39%。本研究可以为红松优良家系的评价与选择提供依据和材料。     

Response to selection for rapid chill-coma recovery in Drosophila melanogaster: physiology and life-history traits

Resistance to low temperatures can vary markedly among invertebrate species and is directly related to their distribution. Despite the ecological importance of cold resistance this trait has rarely been studied genetically, mainly because low and variable fitness of offspring from cold-stressed mothers makes it difficult to undertake selection experiments and compare cold resistance of parents and offspring. One measure of cold resistance that varies geographically in Drosophila melanogaster and that is amenable to genetic analysis is chill-coma recovery. Three replicate lines of D. melanogaster were selected every second generation, for over 30 generations, for decreased recovery time following exposure to 0 degrees C. Correlated responses were scored to characterize underlying physiological traits and to investigate interactions with other traits. Lines responded rapidly to the intermittent selection regime with realized heritabilities varying from 33% to 46%. Selected lines showed decreased recovery time after exposure to a broad range of low temperatures and also had a lower mortality following a more severe cold shock, indicating that a general mechanism underlying cold resistance had been selected. The selection response was independent of plastic changes in cold resistance because the selected lines maintained their ability to harden (i.e. a short-term exposure to cool temperature resulted in decreased recovery time in subsequent chill-coma assays). Changes in cold resistance were not associated with changes in resistance to high temperature exposure, and selected lines showed no changes in wing size, development time or viability. However, there was a decrease in longevity in the selected lines due to an earlier onset of ageing. These results indicate that chill-coma recovery can be rapidly altered by selection, as long as selection is undertaken every second generation to avoid carry-over effects, and suggest that lower thermal limits can be shifted towards increased cold resistance independently of upper thermal limits and without tradeoffs in many life-history traits.     

Selection response for part period egg number and egg mass in chickens — a comparison

Summary Two sublines from each of four flocks of White Leghorn were subjected to two cycles of selection for part year egg number and part year egg mass. A family index (combined selection) was the selection criterion for each of the two traits. The generation means of an unselected pedigreed randombred control, bred and maintained along with the selected sub-lines, were used for correcting short term environmental trends. The sublines selected for a common selection criterion were considered as replicates. The control corrected generation means, when averaged over replicates, gave the mean direct and correlated responses for the selected and different unselected traits. The average genetic change per generation was 2.16 eggs in egg number selected sublines and 146 g of egg mass in egg mass selected sublines. The direct response marginally exceeded the correlated response for both part period egg number and egg mass seen in the corresponding sublines. Except for egg weight the correlated responses for different unselected traits were in the same direction in both egg number and egg mass selected sublines. While the egg weight in egg number lines did not change, its response was positive in egg mass selected lines. A comparison between the lines revealed that the egg mass selected lines matured later and laid heavier but slightly less numerous eggs than the egg number selected sub-lines.     

Direct and correlated responses to artificial selection on developmental time and wing length in Drosophila buzzatii

Abstract.— Developmental time and body size are two positively correlated traits closely related to fitness in many organisms including Drosophila . Previous work suggested that these two traits are involved in a trade-off that may result from a negative genetic correlation between their effects on pre-adult and adult fitness. Here, we examine the evolution of developmental time and body size (indexed by wing length) under artificial selection applied to one or both traits in replicated D. buzzatii populations. Directional changes in both developmental time and wing length indicate the presence of substantial additive genetic variance for both traits. The strongest response to selection for fast development was found in lines selected simultaneously to reduce both developmental time and wing length, probably as an expected consequence of a synergistic effect of indirect selection. When selection was applied in the direction opposite to the putative genetic correlation, that is, large wing length but fast development, no responses were observed for developmental time. Lines selected to reduce both wing length and developmental time diverged slightly faster from the control than lines selected to increase wing length and reduce developmental time. However, wing length did not diverge from the control in lines selected only for fast development. These results suggest a complex genetic basis of the correlation between developmental time and wing length, but are generally consistent with the hypothesis that both traits are related in a trade-off. However, we found that this trade-off may disappear under uncrowded conditions, with fast-developing lines exhibiting a higher pre-adult viability than other lines when tested at high larval density.     

Pleiotropic Models of Quantitative Variation

It is widely held that each gene typically affects many characters, and that each character is affected by many genes. Moreover, strong stabilizing selection cannot act on an indefinitely large number of independent traits. This makes it likely that heritable variation in any one trait is maintained as a side effect of polymorphisms which have nothing to do with selection on that trait. This paper examines the idea that variation is maintained as the pleiotropic side effect of either deleterious mutation, or balancing selection. If mutation is responsible, it must produce alleles which are only mildly deleterious (s approximately 10(-3)), but nevertheless have significant effects on the trait. Balancing selection can readily maintain high heritabilities; however, selection must be spread over many weakly selected polymorphisms if large responses to artificial selection are to be possible. In both classes of pleiotropic model, extreme phenotypes are less fit, giving the appearance of stabilizing selection on the trait. However, it is shown that this effect is weak (of the same order as the selection on each gene): the strong stabilizing selection which is often observed is likely to be caused by correlations with a limited number of directly selected traits. Possible experiments for distinguishing the alternatives are discussed.     

EXAMINING SELECTION ON THE MULTIVARIATE PHENOTYPE: PLANT RESISTANCE TO HERBIVORES

Cost–benefit theory posits that stabilizing selection, produced by a trade-off between associated costs and benefits, often maintains phenotypic traits at intermediate equilibrium values. Measurement of selection on one type of trait, resistance to herbivory, should provide evidence to test this prediction. However, most plants host more than one species of herbivore, and resistance to various herbivores may be phenotypically correlated. Consequently, selection must be measured on a multivariate phenotype, which may produce a very complex selection gradient. Canonical analysis of the matrix of quadratic coefficients of the phenotypic selection gradient is presented as a method to simplify the interpretation of the estimated multidimensional adaptive surface. Because this analytical method finds the canonical axes of second-order effects on the selection surface, it shows where nonlinear, and possibly stabilizing, selection is strongest. The resulting index traits may generate hypotheses regarding underlying resistance traits to which more than one type of herbivore responds.     

Polygenic inheritance is not necessary for sympatric speciation by sexual selection

 The theoretical possibility of sympatric speciation by sexual selection has been demonstrated by a number of mathematical models. Although these models assumed that sexually selected traits are additively determined by many genes, recent empirical studies suggest that many sexually selected traits are determined by major gene inheritance. Thus, using a mathematical simulation model, this article examines whether sympatric speciation by sexual selection can occur when sexually selected traits are determined by major gene inheritance. The model reveals that speciation can occur with major gene inheritance of sexually selected traits. Simulations show that speciation from an initially monomorphic population occurs via two successive Fisher's runaway processes of sexual selection. The first runaway causes the unidirectional evolution of male secondary sexual character toward one extreme in the trait space and of female mate preference for such a character. The second runaway then drives the male character and female preference of a part of the population toward the other extreme in the trait space, splitting the population into two reproductively isolated subgroups. The current results reinforce the plausibility of sympatric speciation by sexual selection. Received: January 30, 2002 / Accepted: June 27, 2002     

Expected gain and status number following restricted individual and combined-index selection.

Imposition of restrictions on number of individuals selected from a family and number of families from which superior individuals are selected could markedly alter the consequences of individual and combined-index selection. Predicted genetic gain and diversity measured as status number following selection were studied to draw general conclusions. Selection and its prediction were applied to two sets of real-life data. Theoretical prediction gave results close to those from factual selection. Gain and status number varied with initial family number and size, sib type, heritability, selection proportion, restriction type and intensity, and selection criteria. Proper restriction on the number of individuals selected can control the reduction of status number to an acceptable level, particularly when breeding values are used as the selection criterion. Restriction on the number of families selected would effectively improve the gain efficiency of selection based on phenotypic values. Choosing combinations of both restrictions might produce higher gain without the loss of status number. Given constant population size, family number should be large enough to ensure that restricted selection will yield higher gain and status number.     

Sperm mobility: mechanisms of fertilizing efficiency, genetic variation and phenotypic relationship with male status in the domestic fowl, Gallus gallus domesticus

When females are sexually promiscuous, sexual selection continues after insemination through sperm competition and cryptic female choice, and male traits conveying an advantage in competitive fertilization are selected for. Although individual male and ejaculate traits are known to influence paternity in a competitive scenario, multiple mechanisms co-occur and interact to determine paternity. The way in which different traits interact with each other and the mechanisms through which their heritability is maintained despite selection remain unresolved. In the promiscuous fowl, paternity is determined by the number of sperm inseminated into a female, which is mediated by male social dominance, and by the quality of the sperm inseminated, measured as sperm mobility. Here we show that: (i) the number of sperm inseminated determines how many sperm reach the female sperm-storage sites, and that sperm mobility mediates the fertilizing efficiency of inseminated sperm, mainly by determining the rate at which sperm are released from the female storage sites, (ii) like social status, sperm mobility is heritable, and (iii) subdominant males are significantly more likely to have higher sperm mobility than dominant males. This study indicates that although the functions of social status and sperm mobility are highly interdependent, the lack of phenotypic integration of these traits may maintain the variability of male fitness and heritability of fertilizing efficiency.     

Assessment of sorghum genetic resources of Ethiopia for anthracnose (Colletotrichum sublineolum Henn.) resistance and agronomic traits

Sorghum anthracnose caused by Colletotrichum sublineolum Henn. is one of the key diseases limiting sorghum production and productivity. Development of anthracnose‐resistant sorghum genotypes possessing yield‐promoting agronomic traits is an important breeding goal in sorghum improvement programs. The objective of this study was to determine the responses of diverse sorghum genetic resources for anthracnose resistance and agronomic traits to identify desirable lines for breeding. A total of 366 sorghum collections and three standard checks were field evaluated during the 2016 and 2017 cropping seasons. Lines were artificially inoculated with a virulent pure isolate of the pathogen. Anthracnose disease severity was assessed to calculate the area under disease progress curve (AUDPC). Agronomic traits such as panicle length (PL), panicle width (PW), head weight (HW) and thousand grain weight (TGW) were measured. Lines showed highly significant differences (p < .001) for anthracnose severity, AUDPC and agronomic traits. Among the collections 32 lines developed levels of disease severity between 15% and 30% in both seasons. The following sorghum landraces were selected: 71708, 210903, 74222, 73955, 74685, 74670, 74656, 74183, 234112, 69412, 226057, 214852, 71420, 71484, 200126, 71557, 75120, 71547, 220014, 228179, 16212, 16173, 16133, 69088, 238388, 16168 and 71570. These landraces had a relatively low anthracnose severity possessing farmer‐preferred agronomic traits. The selected genotypes are useful genetic resources to develop anthracnose‐resistant sorghum cultivars.     

Estimating and controlling for spatial structure in the study of ecological communities

Aim Variation partitioning based on canonical analysis is the most commonly used analysis to investigate community patterns according to environmental and spatial predictors. Ecologists use this method in order to understand the pure contribution of the environment independent of space, and vice versa, as well as to control for inflated type I error in assessing the environmental component under spatial autocorrelation. Our goal is to use numerical simulations to compare how different spatial predictors and model selection procedures perform in assessing the importance of the spatial component and in controlling for type I error while testing environmental predictors. Innovation We determine for the first time how the ability of commonly used (polynomial regressors) and novel methods based on eigenvector maps compare in the realm of spatial variation partitioning. We introduce a novel forward selection procedure to select spatial regressors for community analysis. Finally, we point out a number of issues that have not been previously considered about the joint explained variation between environment and space, which should be taken into account when reporting and testing the unique contributions of environment and space in patterning ecological communities. Main conclusions In tests of species‐environment relationships, spatial autocorrelation is known to inflate the level of type I error and make the tests of significance invalid. First, one must determine if the spatial component is significant using all spatial predictors (Moran's eigenvector maps). If it is, consider a model selection for the set of spatial predictors (an individual‐species forward selection procedure is to be preferred) and use the environmental and selected spatial predictors in a partial regression or partial canonical analysis scheme. This is an effective way of controlling for type I error in such tests. Polynomial regressors do not provide tests with a correct level of type I error.     

Variation in the expression of Hsp70, the major heat-shock protein, and thermotolerance in larval and adult selection lines of Drosophila melanogaster

(1) Lines of Drosophila melanogaster were “laboratory naturally” or artificially selected under five thermal regimes. (2) Hsp70 expression per unit protein after heat hardening and heat-shock resistance with and without prior hardening were measured. (3) Differences between the selection regimes in the responses of these traits suggest that thermal resistance can be changed independently of inducible Hsp70 expression. (4) Adult males had higher survival than females but did not differ in inducible Hsp70 expression per unit protein after heat hardening. (5) Larvae expressed less Hsp70 per unit protein than adults after heat hardening.     

Converting multi-trait breeding objectives into operative selection indexes to ensure genetic gains in low-input sheep and goat breeding programmes

Small ruminant breeding programmes in low-input production systems are best organised at the community level. Participant farmers have to agree on goal traits and their relative importance. When BLUP breeding values of goal traits are not available in time, appropriate selection indexes can be used to aid visual selection. Taking Ethiopian Abergelle goat and Bonga sheep community-based breeding programmes (CBBPs) as an example, breeding objective functions were defined and selection indexes were constructed and evaluated. Breeding goals for Abergelle goats included early sale weight, survival and milk production. Breeding goals for Bonga included the number of offspring born, sale weight and survival. Economic weights of objective traits can be used in several ways depending on measured traits and the reliability of their genetic parameters. Selection indexes included combinations of objective traits measured on candidates and their dams and situations when Abergelle communities prefer to restrict genetic changes in number of offspring born or adult weight and when Bonga communities prefer to restrict changes in adult weight. Genetic and economic gains were evaluated as well as sensitivity to feed cost assumptions and to repeated dam records. After independent culling on preponderant traits such as coat colour and horn/tail type, sires in Abergelle goat community breeding programmes should be selected on indexes including at least own early live weight and their dams average milk production records. Sires for Bonga sheep programmes should be selected on own early live weight and desirably also on their dam’s number of offspring born. Sensitivity to feed cost assumptions was negligible but repeated measurements of dam records improved index accuracies considerably. Restricting genetic changes in number of offspring born or adult weight is not recommended.     

Response of a complex foraging phenotype to artificial selection on its component traits

In nature, where predators must often track dynamic and dispersed prey populations, predator consumption rate, conversion efficiency, dispersal, and prey finding are likely to be important links between foraging and predator–prey population dynamics. Small differences in predator foraging caused by variation in any of the abovementioned traits might lead to significant differences in predator success as well as population dynamics. We used artificial selection to create lines of the predatory mite, Phytoseiulus persimilis in order to determine the potential for or constraints on the evolution of predator foraging behaviors. All four foraging traits demonstrated considerable phenotypic variation. They also exhibited significant realized heritabilities after artificial selection, except that prey finding did not respond to downward selection. Lines that responded to selection did so rapidly, and high-consumption, high-conversion efficiency, and high- and low-dispersal were stable for at least four generations after artificial selection was relaxed. There were some indirect responses to selection among the foraging traits. For example, there was positive correlation between consumption and dispersal. However, none of the correlated responses were of the magnitude of the direct responses we measured on the same trait. We also observed some correlations between foraging traits and life-history traits such as low-consumption and development time (negative), high-consumption and fecundity (positive), and high-conversion efficiency and fecundity (positive), but these were more likely to represent non-genetic constraints. Intrinsic rates of increase in low-consumption and low-conversion efficiency lines were lower than in their respective high lines and the unselected control, whereas rates of increase in dispersal and olfactory response lines did not differ from the unselected control. Thus, traits that make up foraging share partially overlapping genetic architectures with highly heritable phenotypic components, suggesting that each foraging trait will be able to respond rapidly to changes in the density and distribution of resources.     

Effect of <Emphasis Type="Italic">Lr34/Yr18</Emphasis> on agronomic and quality traits in a spring wheat mapping population and implications for breeding

The locus Lr34/Yr18 plays an important role in conferring resistance to a number of fungal diseases and is thus an important component of global wheat breeding efforts. We investigated the differences in disease response and agronomic traits of the ‘CDC Teal’ × ‘CDC Go’ spring wheat population of 187 recombinant inbred lines (RILs) in relation to the presence/absence of the rust resistance gene Lr34/Yr18. Lines carrying the resistant allele of Lr34/Yr18 were taller, matured earlier, and yielded less grain with lower test weights than lines without Lr34/Yr18. Lines with or without the resistant allele of Lr34/Yr18 did not differ for grain protein content, SDS sedimentation volume, and for resistance to leaf spotting and common bunt. Lines with Lr34/Yr18 exhibited lower leaf and stripe rust infection than lines without it. We selected superior lines from the population based on high yield, protein content, SDS sedimentation, and the presence of the resistant allele of Lr34/Yr18 and grew them with continued selection in replicated yield trials over nine site-years. We attempted to combine Lr34/Yr18 with high yield, protein content, and SDS sedimentation suitable for the Canadian western red spring wheat class. Our results suggested that the population size we used was not large enough to obtain recombinants with high yield potential, high grain protein, and acceptable quality attributes. Moreover, selection for Lr34/Yr18 resulted in the elimination of lines with high yield potential. We therefore suggest using a population size of at least 310 to increase the potential of pooling Lr34/Yr18 with high grain yield and desirable agronomic and end-use quality attributes.     

Application of a canonical transformation to detection of quantitative trait loci with the aid of genetic markers in a multi-trait experiment

Effects of individual quantitative trait loci (QTLs) can be isolated with the aid of linked genetic markers. Most studies have analyzed each marker or pair of linked markers separately for each trait included in the analysis. Thus, the number of contrasts tested can be quite large. The experimentwise type-I error can be readily derived from the nominal type-I error if all contrasts are statistically independent, but different traits are generally correlated. A new set of uncorrelated traits can be derived by application of a canonical transformation. The total number of effective traits will generally be less than the original set. An example is presented for DNA microsatellite D21S4, which is used as a marker for milk production traits of Israeli dairy cattle. This locus had significant effects on milk and protein production but not on fat. It had a significant effect on only one of the canonical variables that was highly correlated with both milk and protein, and this variable explained 82% of the total variance. Thus, it can be concluded that a single QTL is affecting both traits. The effects on the original traits could be derived by a reverse transformation of the effects on the canonical variable.     

Experimental evidence for genetic heritability of maternal hormone transfer to offspring

In many animal species, embryos are exposed to maternal hormones that affect their development. Maternal hormone transfer varies with environmental conditions of the mother and is often interpreted as being shaped by natural selection to adjust the offspring to prevailing environmental conditions. Such hormone transfer requires genetic variability, which has not yet been experimentally demonstrated. Our study reports direct evidence for additive genetic variance of maternal androgens through a bidirectional selection on yolk testosterone (T) levels in Japanese quail. Lines selected for high egg T (HET) and low egg T (LET) concentration differed in yolk levels of this androgen, resulting in high realized heritability (h2 = 0.42)Correlated responses to selection on other gonadal hormones indicated that selection specifically targeted biologically active androgens. Eggs of HET quail contained higher androstenedione and lower estradiol concentrations than did those of LET quail, with no line differences in yolk progesterone concentration. Plasma T concentrations in adult females were not affected by selection, seriously challenging the hypothesis that transfer of maternal hormones to offspring is constrained by hormone levels in a mother's circulation. Our results suggest that transfer of maternal T represents an indirect genetic effect that has important consequences for the evolution of traits in offspring.     

Wing morphology of the butterfly Coenonympha arcania in Europe: Traces of both historical isolation in glacial refugia and current adaptation

In this study, we examined the evolutionary outcome of and interplay between historic isolation and current selection pressures on traits more or less closely connected to fitness in the Pearly Heath butterfly (Coenonympha arcania) across its range in Europe. We hypothesized that a trait mean is more related to historic events if it has low connection to fitness, while a trait more closely connected with fitness is expected to have a mean that relates more to current selection pressures. In order to test this, we collected 322 butterflies from across the species range in Europe and measured five wing traits relating to size and color patterns. To infer a phylogeographic history for each individual, we sequenced a 594 bp fragment of the COI gene. The morphological data were then analyzed in relation to selected climatic variables and the history of individuals to disentangle which factors best correlated with morphological variation. The results supported our hypothesis in that wing sizes correlated with summer precipitation but not with its inferred location during the last glaciation. Eyespot position, on the other hand, correlated with the history of individuals but not with the analyzed climatic indicators. The sizes of the black spot and the white band, two traits that were expected to have intermediate selection pressure, were associated with both history and current conditions. Thus, this study illustrates the fascinating interplay between events and processes that lead to a specific evolutionary outcome.     

Modes of selection and recombination response in Drosophila melanogaster

A selection experiment for sternopleural bristle number in Drosophila melanogaster was undertaken to analyze the correlated effects on recombination. Replicate lines were subjected to divergent directional selection and to stabilizing selection. Recombination rates for markers on chromosomes 2 (dp-cn-bw) and 3 (se-ss-ro) were compared to those from a control. All lines responded as predicted for bristle number. Lines selected for both increased and decreased bristle number exhibited significantly increased recombination rates. The predicted recombination response from stabilizing selection is suggested by our data, but only one comparison is statistically significant. These results, taken with other studies, support the proposal that genetic recombination enhances individual fitness when populations are experiencing environmental change. Less conclusively, our results suggest that populations undergoing stabilizing selection may respond by reducing their rates of crossing over.