Comparison of ancestral and current-generation inbreeding in an experimental strawberry breeding population

Progenies from first-generation self, half-sib, full-sib, and cross fertilizations were generated to evaluate the magnitude of inbreeding depression for vegetative and production traits in strawberry. Tests were conducted to determine the linearity of trait mean depression with inbreeding rate (F) over this range of inbreeding values, as an indication of the presence of non-additive epistasis. A control population, for which a similar range of coancestry had accumulated over several cycles of breeding and selection, was also generated to compare the consequences of ancestral and current-generation inbreeding. Trait means for crosses among current-generation half-sibs, full-sibs, and selfs were 2–17%, 3–12%, and 14–45% lower than for unrelated crosses among the same set of parents, respectively. Linear regression of progeny means on current generation F was significantly negative for all traits and explained 17–44% of the variance among progeny means. Mean depression was largely linear over the range of inbreeding rates tested in this population, indicating the absence of epistasis for the traits evaluated. Conversely, (F) regressions of progeny means on pedigree inbreeding coefficients, where coancestry had accumulated over several cycles of breeding and selection, were uniformly non-significant and explained 0–10% of the variance among cross means. Further, multiple regression of progeny means for current-generation relatives on pedigree F failed to improve fit significantly over regression on current-generation F alone for all traits. Together, these results suggest that pedigree inbreeding coefficients are poor predictors of changes in homozygosity when populations are developed through multiple cycles of breeding and selection. They also imply that inbreeding depression will be of minor importance for strawberry breeding populations managed with adequate population sizes and strong directional selection.     

Genetic incompatibility and offspring quality in the tristylous plant Lythrum salicaria (Lythraceae)

The purpose of this study was to determine the incompatibility relationships among the three different morphs of the tristylous plant Lythrum salicaria, and to assess whether there are fitness consequences to breakdown in incompatibility. Twenty-four different types of pollinations were performed using all possible combinations of anthers and stigmas. These can be grouped as legitimate pollinations (pollinations from the appropriate anther level of a compatible morph), illegitimate intermorph pollinations (pollinations from the inappropriate anther level of a compatible morph), intramorph pollinations (pollinations between individuals of the same morph), and self pollinations. Legitimate pollinations produced significantly more seed than illegitimate intermorph pollinations, intramorph pollinations, and self pollinations. The difference between legitimate and illegitimate intermorph pollination success is particularly interesting because pollen for these crosses came from the same genetic individual. Pollination types also differed in offspring quality. In the rare examples where progeny were produced by illegitimate intermorph pollinations, the progeny did not have significantly lower values for seedling growth traits compared to legitimate progeny. Seedlings produced by self pollinations had significantly lower values for four out of six seedling growth traits measured. Comparisons of legitimate and self progeny traits indicate that inbreeding depression for most traits is close to or above 0.5. The finding that some seed were produced from illegitimate pollinations suggests that there is variation among individuals in incompatibility. There was a significant effect of parental morph type on the probability of breakdown in incompatibility with the midstyled maternal parents setting more seed from illegitimate pollinations.     

Maternal heterozygosity and progeny fitness association in an inbred Scots pine population

Associations between heterozygosity and fitness traits have typically been investigated in populations characterized by low levels of inbreeding. We investigated the associations between standardized multilocus heterozygosity (stMLH) in mother trees (obtained from12 nuclear microsatellite markers) and five fitness traits measured in progenies from an inbred Scots pine population. The traits studied were proportion of sound seed, mean seed weight, germination rate, mean family height of one-year old seedlings under greenhouse conditions (GH) and mean family height of three-year old seedlings under field conditions (FH). The relatively high average inbreeding coefficient (F) in the population under study corresponds to a mixture of trees with different levels of co-ancestry, potentially resulting from a recent bottleneck. We used both frequentist and Bayesian methods of polynomial regression to investigate the presence of linear and non-linear relations between stMLH and each of the fitness traits. No significant associations were found for any of the traits except for GH, which displayed negative linear effect with stMLH. Negative HFC for GH could potentially be explained by the effect of heterosis caused by mating of two inbred mother trees (Lippman and Zamir 2006), or outbreeding depression at the most heterozygote trees and its negative impact on the fitness of the progeny, while their simultaneous action is also possible (Lynch. 1991). However,since this effect wasn’t detected for FH, we cannot either rule out that the greenhouse conditions introduce artificial effects that disappear under more realistic field conditions.     

Inbreeding depression, increased phenotypic variance, and a trade-off between gonads and appendages in selfed progeny of the aphid Prociphilus oriens

Trade-offs are potentially common among two or more traits whose development is dependent on the same resources. To detect genetic trade-offs, the techniques of quantitative genetics, pedigree analyses, and selection experiments have been used. This study demonstrates genetically based trade-offs between gonads and appendages in hatched larvae of the aphid Prociphilus oriens by focusing on enlarged variance among the families of selfed progeny. The selfed and outbred families were compared in respect to the size of morphological traits, gonad volume, and hatch dates as well as egg volume. Selfing not only increased the among-family variance component in all larval traits examined, but it also increased the mean size of all the morphological traits significantly. In contrast, gonad volume, a fitness component, was reduced with selfing. Calculation of the allometry (log-transformed regression) of larval traits to egg volume indicated that in the outbred group, morphological traits grew slowly relative to egg volume with slopes below 0.25, whereas gonads exhibited isometric growth. With selfing, most morphological traits had significantly steeper slopes, whereas the slope for gonads was greatly decreased. When the effect of egg volume was statistically removed from the means of selfed families, significant negative correlation was detected between the adjusted means of gonad volume and those of tibia length. This result suggests genetic trade-offs between gonad volume and tibia length. Thus, the evidence implies that at the loci governing the development of appendages, the dominant alleles function to canalize the development of tibiae into an optimal size, irrespective of egg volume. It is hypothesized that increased homozygosity of the deleterious recessive alleles reduced gonad volume through increasing the resource allocation to tibiae. The hypothesis of the gonad-appendage trade-off could be applied to explain the phenotypic evolution in some aphid species.     

Prediction of informativeness for microsatellite markers among progeny of sires used for detection of economic trait loci in dairy cattle

Individual loci affecting economic traits can be located using genetic linkage. Application of either daughter or granddaughter designs requires determination of allele origin in the progeny. If only the sires and their progeny are genotyped, the paternal allele origin of progeny having the same genotype as the sire cannot be determined. The expected frequency of informative sons can be predicted for each sire and genetic marker from the allele frequencies in the population. The accuracy of a predictor of the frequency of informative progeny was tested on 103 grandsire x microsatellite combinations. Number of sons per grandsire varied from 24 to 129. Allele frequencies in the population were estimated by genotyping seven sires. The regression of the frequency of informative sons on the predicted frequency was 1.04 with a zero intercept model. Thus, considering the large number of genetic markers available for analysis, predicted informative frequency is a useful criterion for selection of genetic markers.     

Quantile regression estimates of animal population trends

Ecologists often estimate population trends of animals in time series of counts using linear regression to estimate parameters in a linear transformation of multiplicative growth models, where logarithms of rates of change in counts in time intervals are used as response variables. We present quantile regression estimates for the median (0.50) and interquartile (0.25, 0.75) relationships as an alternative to mean regression estimates for common density-dependent and density-independent population growth models. We demonstrate that the quantile regression estimates are more robust to outliers and require fewer distributional assumptions than conventional mean regression estimates and can provide information on heterogeneous rates of change ignored by mean regression. We provide quantile regression trend estimates for 2 populations of greater sage-grouse (Centrocercus urophasianus) in Wyoming, USA, and for the Crawford population of Gunnison sage-grouse (Centrocercus minimus) in southwestern Colorado, USA. Our selected Gompertz models of density dependence for both populations of greater sage-grouse had smaller negative estimates of density-dependence terms and less variation in corresponding predicted growth rates (λ) for quantile than mean regression models. In contrast, our selected Gompertz models of density dependence with piecewise linear effects of years for the Crawford population of Gunnison sage-grouse had predicted changes in λ across years from quantile regressions that varied more than those from mean regression because of heterogeneity in estimated λs that were both less and greater than mean estimates. Our results add to literature establishing that quantile regression provides better behaved estimates than mean regression when there are outlying growth rates, including those induced by adjustments for zeros in the time series of counts. The 0.25 and 0.75 quantiles bracketing the median provide robust estimates of population changes (λ) for the central 50% of time series data and provide a 50% prediction interval for a single new prediction without making parametric distributional assumptions or assuming homogeneous λs. Compared to mean estimates, our quantile regression trend estimates for greater sage-grouse indicated less variation in density-dependent λs by minimizing sensitivity to outlying values, and for Gunnison sage-grouse indicated greater variation in density-dependent λs associated with heterogeneity among quantiles.     

Clonal variation for tolerance to polyethylene glycol-induced water stress in cultured tomato cells

Cell clones were isolated from a population of cultured tomato (Lycopersicon esculentum Mill cv VFNT-cherry) cells and their tolerance to polyethylene glycol (PEG)-induced water stress was measured. Considerable variation for tolerance among the clones was found. Tolerance differences between clones appeared to be spontaneous and were different from tolerance differences between adapted and unadapted cells. Unlike adapted (selected by exposure to PEG) cells, cell clones retained their relative tolerance for many generations in the absence of selection pressure, and tolerance of both relatively tolerant and intolerant clones was very dependent on growth cycle stage and inoculum density. Analysis of subclones isolated from relatively tolerant and intolerant parent clones revealed that each parent clone gives rise to progeny with tolerances near the mean tolerance of both parents. However, progeny populations of both tolerant and intolerant parents are enriched with individuals with phenotypes nearer the mean response of their respective parent populations. When exposed to PEG, relatively tolerant and intolerant clones alike become adapted to the level of PEG to which they are exposed, and have the same phenotypic level of tolerance. Thus, selection by exposure to stress is unable to discriminate (on the basis of growth) between the innately tolerant and intolerant cell types within the population. This is indicated also by the fact that clones isolated from a population of cells adjusted to growth on 25% PEG do not show an enriched frequency of tolerant phenotypes when grown in the absence of PEG compared to the nonselected normal cell population which has never been adjusted to growth on PEG.     

Fluctuating asymmetry of meristic traits: an isofemale line analysis in an invasive drosophilid, <Emphasis Type="Italic">Zaprionus indianus</Emphasis>

Metric (e.g., body size) and meristic (e.g., bristle number) traits are of general use in quantitative genetic studies, and the phenotypic variance is subdivided into a genetic and a non-genetic environmental component. The non-genetic variance may have two origins: a common garden effect between individuals and a developmental instability within the same individual. Developmental instability may be studied by considering the fluctuating asymmetry (FA) between the two sides of the body. The isofemale line technique is a convenient method for investigating the architecture of natural populations but has been rarely implemented for investigating FA. In this paper, we use this experimental design for analyzing four meristic traits in eight populations of the cosmopolitan Zaprionus indianus. A study of the correlation between left and right side of each line revealed that almost 90% of the variability was due to a developmental noise, while a much higher correlation among the means of the lines from the same population was observed. A slight trend toward a directional asymmetry was observed: more thoracic bristles on the left side. Four kinds of indices, scaled or non-scaled to the mean were used for comparing the different traits. Unscaled values (mean absolute values or standard deviation of each line) revealed a linear increase with the means. Interestingly the results of ovariole number were included in the same regression. With the scaled indices (mean absolute divided by each individual value or stadard deviation devided by the mean), the differences among traits were considerably decreased, but still remained significant. The mean FA of the various traits were not correlated, suggesting that each trait harbors its own developmental stability. The CVs of FA were high with a magnitude similar to those of the trait themselves, slightly less than 10%. Finally, even with the isofemale line design, which is a powerful means for unravelling slight genetic variations, we did not to find any clear indication of a genetic component of FA under the optimal environmental conditions used in this study.     

"Ant" and "grasshopper" life-history strategies in Saccharomyces cerevisiae

From the evolutionary and ecological points of view, it is essential to distinguish between the genetic and environmental components of the variability of life-history traits and of their trade-offs. Among the factors affecting this variability, the resource uptake rate deserves particular attention, because it depends on both the environment and the genetic background of the individuals. In order to unravel the bases of the life-history strategies in yeast, we grew a collection of twelve strains of Saccharomyces cerevisiae from different industrial and geographical origins in three culture media differing for their glucose content. Using a population dynamics model to fit the change of population size over time, we estimated the intrinsic growth rate (r), the carrying capacity (K), the mean cell size and the glucose consumption rate per cell. The life-history traits, as well as the glucose consumption rate, displayed large genetic and plastic variability and genetic-by-environment interactions. Within each medium, growth rate and carrying capacity were not correlated, but a marked trade-off between these traits was observed over the media, with high K and low r in the glucose rich medium and low K and high r in the other media. The cell size was tightly negatively correlated to carrying capacity in all conditions. The resource consumption rate appeared to be a clear-cut determinant of both the carrying capacity and the cell size in all media, since it accounted for 37% to 84% of the variation of those traits. In a given medium, the strains that consume glucose at high rate have large cell size and low carrying capacity, while the strains that consume glucose at low rate have small cell size but high carrying capacity. These two contrasted behaviors may be metaphorically defined as "ant" and "grasshopper" strategies of resource utilization. Interestingly, a strain may be "ant" in one medium and "grasshopper" in another. These life-history strategies are discussed with regards to yeast physiology, and in an evolutionary perspective.     

Selection for uniformity in livestock by exploiting genetic heterogeneity of residual variance

In some situations, it is worthwhile to change not only the mean, but also the variability of traits by selection. Genetic variation in residual variance may be utilised to improve uniformity in livestock populations by selection. The objective was to investigate the effects of genetic parameters, breeding goal, number of progeny per sire and breeding scheme on selection responses in mean and variance when applying index selection. Genetic parameters were obtained from the literature. Economic values for the mean and variance were derived for some standard non-linear profit equations, e.g. for traits with an intermediate optimum. The economic value of variance was in most situations negative, indicating that selection for reduced variance increases profit. Predicted responses in residual variance after one generation of selection were large, in some cases when the number of progeny per sire was at least 50, by more than 10% of the current residual variance. Progeny testing schemes were more efficient than sib-testing schemes in decreasing residual variance. With optimum traits, selection pressure shifts gradually from the mean to the variance when approaching the optimum. Genetic improvement of uniformity is particularly interesting for traits where the current population mean is near an intermediate optimum.     

供氮水平对番茄苗期Q型烟粉虱成虫种群动态的影响

烟粉虱(Bemisia tabaci)是重要的农业害虫。在栽培管理中,为了减少化学农药的施用,对该害虫采取生态防控措施日益得到重视。以Q型烟粉虱为研究对象,根据温室大棚内9次烟粉虱成虫种群数量调查和番茄苗期5项农艺指标测量,结合主成分分析、线性逐步回归分析及Pearson相关性分析,从"氮素-番茄-Q型烟粉虱"互作角度开展生态防控措施研究,旨在探明番茄苗期的烟粉虱成虫种群发生数量与氮素水平关系。结果表明:3个氮水平(高氮1134 mg·L^-1,标准氮945 mg·L^-1,低氮756 mg·L^-1)下的5个番茄指标(株高、茎粗、叶面积、叶绿素相对含量、叶片全氮含量)最大值基本呈现高氮>标准氮>低氮的规律;调查的9次烟粉虱虫口数量在最大峰值时的高低排序为高氮>标准氮>低氮。主成分分析表明,3种氮素水平下叶绿素相对含量是影响烟粉虱成虫虫口数量的主要因子。线性回归与相关性分析表明,3种氮素水平下的叶片全氮含量、叶绿素相对含量与虫口种群数量相关性较高。氮营养是影响植物和烟粉虱的关键因素之一,高氮水平下番茄长势较好,Q型烟粉虱虫口数量居多;作物叶片全氮含量和叶绿素相对含量可作为检测Q型烟粉虱种群发生消长的生态防控指标。     

Mapping of quantitative trait loci controlling adaptive traits in coastal Douglas fir. III. Quantitative trait loci-by-environment interactions

Quantitative trait loci (QTL) were mapped in the woody perennial Douglas fir (Pseudotsuga menziesii var. menziesii [Mirb.] Franco) for complex traits controlling the timing of growth initiation and growth cessation. QTL were estimated under controlled environmental conditions to identify QTL interactions with photoperiod, moisture stress, winter chilling, and spring temperatures. A three-generation mapping population of 460 cloned progeny was used for genetic mapping and phenotypic evaluations. An all-marker interval mapping method was used for scanning the genome for the presence of QTL and single-factor ANOVA was used for estimating QTL-by-environment interactions. A modest number of QTL were detected per trait, with individual QTL explaining up to 9.5% of the phenotypic variation. Two QTL-by-treatment interactions were found for growth initiation, whereas several QTL-by-treatment interactions were detected among growth cessation traits. This is the first report of QTL interactions with specific environmental signals in forest trees and will assist in the identification of candidate genes controlling these important adaptive traits in perennial plants.     

Facilitation and competition interact with seed dormancy to affect population dynamics in annual plants

Seed dormancy increases population size via bet-hedging and by limiting negative interactions (e.g., competition) among individuals. On the other hand, individuals also interact positively (e.g., facilitation), and in some systems, facilitation among juveniles precedes competition among adults in the same generation. Nevertheless, studies of the benefits of seed dormancy typically ignore facilitation. Using a population growth model, we ask how the facilitation–competition balance interacts with seed dormancy rate to affect population dynamics in constant and variable environments. Facilitation increases the growth rate and equilibrium size (in both constant and variable environments) and reduces the extinction rate of populations (in a variable environment), and a higher rate of seed dormancy allows populations with facilitation to reach larger sizes. However, the combined benefits of facilitation and a high dormancy rate only occur in large populations. In small populations, weak facilitation does not affect the growth rate, but does induce a weak demographic Allee effect (where population growth decreases with decreasing population size). Our results suggest that facilitation within populations can interact with bet-hedging traits (such as dormancy) or other traits that mediate density to affect population dynamics. Further, by ensuring survival but limiting reproduction, ontogenetic switches from facilitation to competition may enable populations to persist but limit their maximum size in variable environments. Such intrinsic regulation of populations could then contribute to the maintenance of similar species within communities.     

Genetic variation for aerial dispersal behavior in the Banks grass mite

Banks grass mites, Oligonychus pratensis (Banks), exhibit a behavior that facilitates their dispersal as aerial plankton over long distances. The propensity to display this behavior and its latency varies among individual mites. Some of this variation can be attributed to genetic differences among individuals. Mother-daughter resemblance and bi-directional selection were used to estimate the genetic component of latency of the aerial dispersal behavior in a wild population collected from corn plants. Mother-daughter regression analysis suggested that up to 10% of variation in initiation of aerial dispersal behavior was attributable to an additive genetic component. Subsequent selection for shorter and longer latency resulted in significant divergence between selected lines after 9 generation, but the response was not symmetrical. Realized heritability for shorter latency was 0.09, while that for longer latency was not different from 0. This response to selection was consistent with results of the mother-daughter analysis, and the response asymmetry may explain the high variability in the mother-daughter regression. In both their natural habitat and in a corn agroecosystem, mites survive by using a succession of temporarily available hosts. Aerial dispersal behavior by Banks grass mites facilitates colonization of new hosts. The genetic basis and selection response of behavior initiating aerial dispersal is consistent with its role in mites population dynamics in this setting.     

A meta‐analysis of latitudinal variations in life‐history traits of roach,Rutilus rutilus,over its geographical range: linear or non‐linear relationships?

1. We collated information from the literature on life history traits of the roach (a generalist freshwater fish), and analysed variation in absolute fecundity, von Bertalanffy parameters, and reproductive lifespan in relation to latitude, using both linear and non-linear regression models. We hypothesized that because most life history traits are dependent on growth rate, and growth rate is non-linearly related with temperature, it was likely that when analysed over the whole distribution range of roach, variation in key life history traits would show non-linear patterns with latitude.
2. As fecundity depends strongly on length, and the length structure of females varied among populations, latitudinal patterns in fecundity were examined based on residuals from the length–fecundity relationship. The reproductive lifespan of roach was estimated as the difference between age at maturity and maximum age of females in each population.
3. The three life history traits of roach analysed all varied among populations and were non-linearly related to latitude. Only the relationship between reproductive lifespan and latitude was a better fit to a linear that to a quadratic model, although Loess smoothing curves revealed that this relationship was actually closer to biphasic than linear in form. A latitude of 50°N formed a break point in all three life history traits.
4. The negative relationships we have described between (i) fecundity and reproductive lifespan and (ii) fecundity and egg mass suggest that lower fecundity is compensated for by longer lifespan, while lower fecundity is compensated for by an increased egg mass, when analysed independently of location.     

A morphological classification capturing functional variation in phytoplankton

1. A logical way of distinguishing functional groups of phytoplankton is to cluster species according to their functional traits, such as growth rate and nutrient assimilation constants. However, data for such an approach are lacking for the vast majority of the species. 2. In this study, we show that a classification based on simple morphological traits may capture much of the variability in functional properties among the phytoplankton. We used information on more than 700 freshwater species, from more than 200 lakes situated in climate zones ranging from subpolar to tropical. 3. Morphological characteristics correlated well with functional properties, such as growth rate and sinking rate, and also with the population size and biomass attained in the field. This suggests that morphology is a good predictor of the functional characteristics of species. 4. Cluster analysis was used to define seven species groups based on morphology. Although some of the clusters are taxonomically homogeneous, others include species of several separate divisions. Functional traits (not used for the classification) differed significantly among the clusters, suggesting that the clusters may indeed represent meaningful functional groups. 5. Advantages of our morphological approach to classification include its objectivity, its independence from taxonomic affiliations, and the relative ease of its application to the majority of species for which physiological traits are unknown and are not readily determined.     

Variation in migration pattern,broodstock origin,and family productivity of coho salmon hatchery populations in British Columbia,Canada, derived from parentage‐based tagging

In salmonid parentage‐based tagging (PBT) applications, entire hatchery broodstocks are genotyped, and subsequently, progeny can be nonlethally sampled and assigned back to their parents using parentage analysis, thus identifying their hatchery of origin and brood year (i.e., age). Inter‐ and intrapopulation variability in migration patterns, life history traits, and fishery contributions can be determined from PBT analysis of samples derived from both fisheries and escapements (portion of a salmon population that does not get caught in fisheries and returns to its natal river to spawn). In the current study of southern British Columbia coho salmon (Oncorhynchus kisutch) populations, PBT analysis provided novel information on intrapopulation heterogeneity among males in the total number of progeny identified in fisheries and escapements, the proportion of progeny sampled from fisheries versus escapement, the proportion of two‐year‐old progeny (jacks) produced, and the within‐season return time of progeny. Fishery recoveries of coho salmon revealed heterogeneity in migration patterns among and within populations, with recoveries from north and central coast fisheries distinguishing “northern migrating” from “resident” populations. In northern migrating populations, the mean distance between fishery captures of sibs (brothers and sisters) was significantly less than the mean distance between nonsibs, indicating the possible presence of intrapopulation genetic heterogeneity for migration pattern. Variation among populations in productivity and within populations in fish catchability indicated that population selection and broodstock management can be implemented to optimize harvest benefits from hatcheries. Application of PBT provided valuable information for assessment and management of hatchery‐origin coho salmon in British Columbia.     

Adaptive sex ratio variation in pre-industrial human (Homo sapiens) populations?

Sex allocation theory predicts that in a population with a biased operational sex ratio (OSR), parents will increase their fitness by adjusting the sex ratio of their progeny towards the rarer sex, until OSR has reached a level where the overproduction of either sex no longer increases a parent''s probability of having grandchildren. Furthermore, in a monogamous mating system, a biased OSR is expected to lead to lowered mean fecundity among individuals of the more abundant sex. We studied the influence of OSR on the sex ratio of newborns and on the population birth rate using an extensive data set (n = 14,420 births) from pre-industrial (1775-1850) Finland. The overall effect of current OSR on sex ratio at birth was significant, and in the majority of the 21 parishes included in this study, more sons were produced when males were rarer than females. This suggests that humans adjusted the sex ratio of their offspring in response to the local OSR to maximize the reproductive success of their progeny. Birth rate and, presumably, also population growth rate increased when the sex ratio (males:females) among reproductive age classes approached equality. However, the strength of these patterns varied across the parishes, suggesting that factors other than OSR (e.g. socioeconomic or environmental factors may also have influenced the sex ratio at birth and the birth rate.     

Risk spreading as an adaptive strategy in iteroparous life histories

It has long been conjectured, though without satisfactory proof, that life tables with a long reproductive span are advantageous in an environment where fecundity or immature survival rates fluctuate randomly. In the present analysis we recast the nonlinear Leslie matrix problem as an autoregressive time series model for the birth rate, with random addition and removal of newborn. This transformation renders the model linear with respect to the environmental variation, allowing ready solution for the ultimate population size and for the conditions resulting in stationarity of the population distribution. We show that for life tables where the fecundities of all adult age classes are the same (no restrictions are put on the survivorship schedule, or on the age at first reproduction), and where density dependence operates via total adult density, the realized growth rate is less than the growth rate calculated from the mean Leslie matrix associated with the population's growth history. The degree of the discrepancy increases with the environmental variability, and decreases with iteroparity, thus completing a proof which confirms the correctness of the initial conjecture for a class of biologically reasonable lifetable models.     

FACTORS INFLUENCING FREQUENCY OF THE MID-STYLED MORPH IN TRISTYLOUS POPULATIONS OF OXALIS ALPINA

Genetic models and computer simulations suggest that modifications of incompatibility in tristylous populations of Oxalis alpina from southeastern Arizona should result in evolution of distyly, due to the inability of the mid floral morph to compete effectively with the short and long floral morphs. To evaluate this hypothesis, progeny testing of naturally pollinated individuals was carried out over a three-year period for populations with differing mid frequencies. In contrast to expectations, mids appeared in greater than expected proportions in many cases. Detailed analysis of results from two trimorphic populations where the mid form is relatively uncommon revealed that mid morphs produced far more mid progeny among their offspring than expected. Assuming that all compatible pollen has the same probability of deposition on appropriate stigmas, mid excesses were found among progeny of shorts and longs in these populations as well. In a third population where the mid form is abundant, mid excesses were not found among the progeny of mids, nor were mid excesses typical among the progeny of shorts and longs. Mid excesses among progeny of populations where the mid form is uncommon may explain retention of this form, despite incompatibility modifications that should favor evolution of distyly. Over-representation of mids may result from gametophytic selection favoring mid alleles. If this is the case, tristylous populations may be in equilibrium, with the frequencies of the three morphs determined by the balance of the opposing selective forces resulting from incompatibility modifications and gametophytic selection.