847791
Selection on floral traits through male fertility in a natural plant population
Most studies on selection in plants estimate female fitness components and neglect male mating success, although the latter might also be fundamental to understand adaptive evolution. Information from molecular genetic markers can be used to assess determinants of male mating success through parentage analyses. We estimated paternal selection gradients on floral traits in a large natural population of the herb Mimulus guttatus using a paternity probability model and maximum likelihood methods. This analysis revealed more significant selection gradients than a previous analysis based on regression of estimated male fertilities on floral traits. There were differences between results of univariate and multivariate analyses most likely due to the underlying covariance structure of the traits. Multivariate analysis, which corrects for the covariance structure of the traits, indicated that male mating success declined with distance from and depended on the direction to the mother plants. Moreover, there was directional selection for plants with fewer open flowers which have smaller corollas, a smaller anther–stigma separation, more red dots on the corolla and a larger fluctuating asymmetry therein. For most of these traits, however, there was also stabilizing selection indicating that there are intermediate optima for these traits. The large number of significant selection gradients in this study shows that even in relatively large natural populations where not all males can be sampled, it is possible to detect significant paternal selection gradients, and that such studies can give us valuable information required to better understand adaptive plant evolution.
36544045
THE MEASUREMENT OF SELECTION ON CORRELATED CHARACTERS
Natural selection acts on phenotypes, regardless of their genetic basis, and produces immediate phenotypic effects within a generation that can be measured without recourse to principles of heredity or evolution. In contrast, evolutionary response to selection, the genetic change that occurs from one generation to the next, does depend on genetic variation. Animal and plant breeders routinely distinguish phenotypic selection from evolutionary response to selection (Mayo, 1980; Falconer, 1981). Upon making this critical distinction, emphasized by Haldane (1954), precise methods can be formulated for the measurement of phenotypic natural selection. Correlations between characters seriously complicate the measurement of phenotypic selection, because selection on a particular trait produces not only a direct effect on the distribution of that trait in a population, but also produces indirect effects on the distribution of correlated characters. The problem of character correlations has been largely ignored in current methods for measuring natural selection on quantitative traits. Selection has usually been treated as if it acted only on single characters (e.g., Haldane, 1954; Van Valen, 1965a; O'Donald, 1968, 1970; reviewed by Johnson, 1976 Ch. 7). This is obviously a tremendous oversimplification, since natural selection acts on many characters simultaneously and phenotypic correlations between traits are ubiquitous. In an important but neglected paper, Pearson (1903) showed that multivariate statistics could be used to disentangle the direct and indirect effects of selection to determine which traits in a correlated ensemble are the focus of direct selection. Here we extend and generalize Pearson's major results. The purpose of this paper is to derive measures of directional and stabilizing (or disruptive) selection on each of a set of phenotypically correlated characters. The analysis is retrospective, based on observed changes in the multivariate distribution of characters within a generation, not on the evolutionary response to selection. Nevertheless, the measures we propose have a close connection with equations for evolutionary change. Many other commonly used measures of the intensity of selection (such as selective mortality, change in mean fitness, variance in fitness, or estimates of particular forms of fitness functions) have little predictive value in relation to evolutionary change in quantitative traits. To demonstrate the utility of our approach, we analyze selection on four morphological characters in a population of pentatomid bugs during a brief period of high mortality. We also summarize a multivariate selection analysis on nine morphological characters of house sparrows caught in a severe winter storm, using the classic data of Bumpus (1899). Direct observations and measurements of natural selection serve to clarify one of the major factors of evolution. Critiques of the "adaptationist program" (Lewontin, 1978; Gould and Lewontin, 1979) stress that adaptation and selection are often invoked without strong supporting evidence. We suggest quantitative measurements of selection as the best alternative to the fabrication of adaptive scenarios. Our optimism that measurement can replace rhetorical claims for adaptation and selection is founded in the growing success of field workers in their efforts to measure major components of fitness in natural populations (e.g., Thornhill, 1976; Howard, 1979; Downhower and Brown, 1980; Boag and Grant, 1981; Clutton-Brock et
39315063
DIFFERING SELECTION ON PLANT PHYSIOLOGICAL TRAITS IN RESPONSE TO ENVIRONMENTAL WATER AVAILABILITY: A TEST OF ADAPTIVE HYPOTHESES
I used phenotypic selection analysis to test the prediction from functional and comparative studies of plants that smaller leaves and more efficient water use are adaptive in drier environments. I measured selection gradients on leaf size and instantaneous water‐use efficiency (a measure of carbon gain per unit water loss) in experimental populations of Cakile edentula var. lacustris placed into wet and dry environments in the field. Linear and nonlinear selection differed significantly between the two environments as predicted. Water‐use efficiency was selected to be higher, and leaf area was selected toward a small intermediate optimum, in the dry environment. There was also significant positive correlational selection on water‐use efficiency and leaf size, suggesting that the optimum leaf size in the dry environment is greater for plants with higher water‐use efficiency. In contrast, neither leaf size nor water‐use efficiency were selected in the wet environment, though larger leaves resulted in greater vegetative biomass. Path analysis of the linear selection gradients found that water‐use efficiency affected plant fitness primarily because it increased vegetative biomass, as suggested by the hypotheses about the function of physiological traits. These results were not only consistent with the functional hypotheses but also with the observed genetic differentiation in water‐use efficiency and leaf size between wet and dry site populations.
3822437
ADAPTIVE EVOLUTION OF PLASTIC FORAGING RESPONSES IN A CLONAL PLANT
Stoloniferous rosette plants may show horizontal and vertical foraging responses, such as changes in branching frequency, stolon internode length, leaf length, and height growth of stolons. To study whether such plastic foraging responses constitute an adaptation to het- erogeneity in competition, we studied genetic variation in and fitness consequences of plastic foraging responses in the stoloniferous lake-shore plant Ranunculus reptans. Because plastic foraging responses are likely to have been more strongly selected for in heterogeneous envi- ronments, we used 15 genotypes from competitive, heterogeneous microhabitats and 15 from competition-free, homogeneous microhabitats from Lake Constance (central Europe). We planted vegetatively propagated rosettes of the 30 genotypes (totaling 236 rosettes) into a greenhouse environment with spatially heterogeneous competition. Four replicates of each genotype were planted into the competition-free halves of experimental trays, and four other replicates into the halves with the naturally co-occurring grass Agrostis stolonifera. We found significant variation among genotypes in vertical and horizontal foraging responses. In line with the hypothesis of adaptive plasticity, genotypes from the competitive heterogeneous microhabitats more strongly increased the vertical angle of the first stolon internode (126%) and the specific stolon length (166%) in response to competition than genotypes from the competition-free homogeneous microhabitats. Moreover, we found that genotypes that were more plastic in the vertical angle of the first stolon internode, stolon height, and specific internode length produced more rosettes and flowers than less plastic genotypes (all selection gradients for plasticity . 0.316). Our findings strongly suggest that plastic foraging responses constitute an adaptation to environmental heterogeneity, at least in the stoloniferous R. reptans.
5834739
THE INFLUENCE OF FLOWER COLOR ON OUTCROSSING RATE AND MALE REPRODUCTIVE SUCCESS IN IPOMOEA PURPUREA
Experimental populations of the annual plant, Ipomoea purpurea, composed of individuals belonging to two flower color morphs were studied to determine the effect of flower color on outcrossing rate and reproductive success as a male parent. Analyses of parent and offspring genotypes show that the pigmented and white morphs outcross at similar rates, but that the white morph is favored as a pollen donor. The result suggests that the dynamics of selection occurring at the locus coding for white versus pigmented flowers are more complex than previously believed. Factors such as frequency‐dependent outcrossing rates and epistatic effects of the white allele may be operating. The results also suggest that pollinator observations are unreliable indicators of the actual mating system.
19392853
Floral Evolution: Attractiveness to Pollinators Increases Male Fitness
Because availability of resources often limits seed or fruit set, increased visits by pollinators may not always lead to increases in maternal reproduction. This observation has led evolutionary biologists to hypothesize that a plant's ability to attract pollinators may have its primary impact on male fitness achieved through the fertlization of ovules. This interpretation of angiosperm reproductive ecology is supported by field experiments. Pollinating insects strongly discriminated between two Mendelian petal-color morphs in Raphanus raphanistrum, a widespread, self-incompatible crucifer. In experimental populations composed of petal-color homozygotes. color discrimination by naturally occurring pollinators had no statistically significant effect on relative maternal function (fruit and seed production) in the two morphs. In contrast, yellow-flowered individuals were far more successful as fathers (pollen donors) than were the less visited whites. These results suggest that the evolution of floral signals such as petal color may be driven primarily by selection on male function.
30599471
MEASUREMENTS OF SELECTION IN A HERMAPHRODITIC PLANT: VARIATION IN MALE AND FEMALE POLLINATION SUCCESS
I measured phenotypic selection of floral traits through both male and female functions of the hermaphroditic flowers of Ipomopsis aggregata (Pursh) V. Grant subsp. aggregata (Polemoniaceae). Fluorescent powdered dyes were used to track movement of pollen by hummingbirds and to measure pollen delivery to individual plants as well as pollen receipt. A phenotypic selection analysis revealed that selection due to male‐male competition during pollination was capable of delaying flowering date and widening corolla tubes by 0.22 and 0.24 standard‐deviation units, respectively, in a single generation. Several floral traits were highly correlated with each other. Multivariate selection analysis suggested that selection through male function directly favored late flowering as well as a sexual expression characterized by a short pistillate phase and long corollas. Selection intensities through male and female functions were of similar overall magnitude during the pollination stage of the life cycle, but different traits were favored, and selection sometimes acted in opposing directions. In 1985, selection through female function favored increased time spent in the pistillate phase and exserted stigmas (unlike selection through male function). As a result, individual plants varied greatly in functional gender. Plants that had exserted stigmas and narrow corollas and that spent a disproportionately long time in the pistillate phase achieved greater pollination success as females, while plants with the opposite traits achieved greater success as males. Moreover, female pollination success tended to increase, and male pollination success to decrease, with time spent in the pistillate phase, supporting a critical assumption of sex‐allocation theory. Selection in the populations studied fluctuated from year to year and was highly sex‐specific.
85161944
INFLUENCES OF FLORAL VARIATION ON POLLEN REMOVAL AND SEED PRODUCTION IN WILD RADISH
The effects of corolla size, pollen production, and pollinator visitation fre- quency on pollen removal (an estimate of male reproductive success) and pollen deposition and seed production (female success) were studied in wild radish (Raphanus sativus, Bras- sicaceae) in 1987 and 1988. Wild radish exhibits variation in corolla size, petal coloration, and anther size. A previous study (Stanton and Preston 1988) found no correlation between corolla size and estimates of female success, suggesting that flower size may be selected upon through variation in male reproductive success. For flowers receiving a single honey bee visit, corolla size was positively correlated with pollen removal, but not with pollen deposition on stigmas or seed production. Flowers receiving an average of 3.7 honey bee visits (in the 1987 study) also showed a correlation between corolla size and pollen removal (seed production was not monitored in this study). In contrast, when flowers were visited an average of 5.7 times by honey bees (the 1988 study), there was no significant relationship between corolla size and either pollen removal or estimates of female reproductive success (pollen deposition on stigmas and seed pro- duction). Honey bees demonstrated a preference for large flowers, either by significantly overvisiting large flowers or by visiting the large flowers before visiting smaller flowers. Under natural conditions, most radish flowers receive numerous visits, but even with frequent visitation, at least 20% of the pollen a flower produced was not removed. At the same time, flowers with greater pollen production had a larger proportion of pollen re- moved. Pollen removal was a positive decelerating function of honey bee visitation fre- quency. The number of pollen grains removed per visit declined with increasing honey bee visits. The male fitness function (pollen removal plotted against pollen production) was positive and decelerating in two experiments; positive and linear in one study. Large flowers may have greater male success than small flowers by virtue of their greater pollen pro- duction, their greater pollinator visitation frequency, and their earlier pollinator visitation. If selection is acting on floral traits in wild radish through variation in male success, increases in both corolla size and pollen production are likely to be selected for.
53135207
Male and female pollination success in a deceptive orchid, a selection study
We carried out phenotypic selection analyses to assess the relative importance of several floral traits, as well as microhabitat, in determining both male and female pollination success in a nonrewarding orchid (Cypripedium acaule Ait.) in two Nova Scotia populations. Fruit production in these one-flowered, cosexual plants was strongly pollen limited within a season: 100% of individuals set fruit following hand-pollination, but only 5% and 13% did so under natural circumstances in the two populations. Male and female reproductive success were highly correlated, with >90% of plants that received a pollinium also dispersing one. Despite very low visitation rates, the chance of a removed pollinium fertilizing another ovary was 36% and 51%, respectively, in the two populations. There was no evidence for selection acting differently, either through the two sexual functions or in the two populations. In contrast, the strength of directional selection on several traits differed among microhabitats within each population. Univariate and multivariate regressions indicated statistically significant directional selection acting on flower-opening date through male, female, and total (combined) pollination success in one population. For female, male, and total success in this population, we also detected correlational selection acting on opening date and flower height. There was also negative correlational selection acting on flower height and labellum length through female success. Microhabitats within the populations had a larger effect on both female and male success than did floral traits. In particular, the presence of ericaceous shrubs and an open canopy appeared to be closely linked with higher pollination rates.
11276941
The Rise of the Angiosperms: A Genecological Factor
In the primitive angiosperms, closed carpels are believed to have evolved as protection for ovules, which would otherwise be injured by animal pollinators. The hypothesis is presented that, whatever the origin and other functions of angiosperms, insect pollination and closed carpels may, in combination, reduce the influence of random variation on pollen tube competition, thus enhancing the ability of natural selection to act on the gametophytic phase of the life cycle. The microgametophytic phase represented by vast numbers of haploid individuals can then serve, by insect pollination and closed carpels, as a screen against any genome not functioning with a high degree of metabolic vigor. Poorly balanced genomes could thus be eliminated at relatively little cost. Insect-pollinated angiosperms would therefore benefit from positive aspects of sexual recombination. Such a system may have allowed the angiosperms to undergo their rise to dominance.
84770839
Effective Mate Choice in Wild Radish: Evidence for Selective Seed Abortion and Its Mechanism
Maternal plants that can affect the paternity of their seeds may influence the quality of their offspring. However, mate choice in plants has proved difficult to demonstrate because it is usually confounded by the effects of pollen competition and because its mechanisms are not well understood. We show here that stressed wild radish plants selectively abort seeds sired by particular donors. This selection occurs by position-dependent seed abortion and position-dependent resource allocation within fruits, coupled with position-dependent fertilization of ovules by pollen donors. The result is that stressed plants select for pollen donors that sire seeds in the basal and middle, rather than the stylar, thirds of fruits. Thus, in wild radish, the position of seeds fathered by pollen donors may act as a selective arena. Because of this position-dependent abortion, successful fathers sire the seeds most likely to mature and to be larger than average. However, siring these high-quality seeds may reduce the quantity of seeds sired by a donor after mixed pollination because pollen tubes must grow past fertilizable stylar ovules to reach middle and basal ovules, allowing pollen tubes from other donors to sire many seeds in the less favored positions. Since multiple paternity is common in wild radish fruits in the field, selective seed abortion probably occurs there as well. Finally, the position-dependent seed abortion necessary to this effective maternal choice occurs in other plants; thus, the mechanism suggested here may be a general mechanism of effective mate choice in plants that selects for position of fertilization, not just speed of pollen-tube growth.
86086376
Reproductive Traits and Male Fertility in Plants: Empirical Approaches*
Within-population variability in plant reproductive traits can influence both male and female fitness, but research on the male function of flowers has been hindered by the difficulty of measuring male fertility. Here we evaluate studies that employ paternity analysis to examine how specific plant traits affect male reproductive success (RS) in both natural and artificial populations. These studies illustrate the risks of assuming that male RS is correlated with female RS or with components of male fitness, such as the amount of pollen produced per plant. In some studies, paternity was assigned by simple genetic exclusion using unique multilocus allozyme profiles. More powerful methods involve statistical procedures that assign paternity to the most likely father of each offspring. Lack of genetic markers is a common problem in paternity analysis, and we discuss the types of molecular markers that may soon become more widely used in small, natural populations.
10590072
PATERNITY ANALYSIS IN A NATURAL POPULATION OF ASCLEPIAS EXALTATA: MULTIPLE PATERNITY, FUNCTIONAL GENDER, AND THE “POLLEN‐DONATION HYPOTHESIS”
Recently, some evolutionary biologists have argued that selection on the male component of fitness shapes the evolution of reproductive characters in angiosperms. Floral features, such as inflorescence size, that lead to increased insect visitation without a concomitant increase in seed production are viewed as adaptations to enhance the probability of fathering seeds on other plants. In tests of this “pollen donation hypothesis,” male reproductive success has usually been measured indirectly by flower production, pollinator visitation, or pollen removal. We tested the pollen donation hypothesis directly by quantifying the number of seeds sired by individual genotypes in a natural population of poke milkweed, Asclepias exaltata, in southwestern Virginia. Multiple paternity was low within fruits, a fact which allowed us to use genotypes of progeny arrays to identify a unique pollen parent for 85% of the fruits produced in the population. Seeds sired (male success) and seeds produced (female success) were significantly correlated with flower number per plant (for male success, r = 0.32, P > 0.05; for female success, r = 0.66, P > 0.001). While the number of pollinaria removed, the usual estimator of male success in milkweeds, was highly correlated with numbers of seeds sired (r = 0.47; P > 0.001), it was even more highly correlated with numbers of seeds produced (r = 0.71, P > 0.001). Analysis of functional gender indicated that plants with many flowers did not behave primarily as males. In fact, individuals with the highest total reproductive success contributed equally as males and females. Furthermore, estimates of gender based on numbers of flowers produced or pollinaria removed overestimated the number of functional males in the population. In pollen‐limited species, such as many milkweeds, proportional increases in both male and female reproductive success indicate the potential for selection to shape the evolution of large floral displays through both male and female functions.
84257410
Parentage analysis in Chamaelirium luteum (L.) Gray (Liliaceae): why do some males have higher reproductive contributions?
Much of the contemporary study of adaptation in natural populations involves the regression of some component of fitness, usually survivorship or viability, on one or more characters of interest. It is difficult to apply this approach to measures of paternal reproduction, however, because paternity is typically estimated indirectly from genetic markers, rather than being measured directly from progeny counts. Here, we present maximum likelihood methods for modelling relative male reproductive success as a log‐linear function of one or more potentially predictive features, as well as providing a framework for the assessment of pairwise (male:female) effects, as they affect male reproductive performance. We also provide nonparametric statistical tests for alternative models.
36262361
VARIATION IN MALE REPRODUCTIVE INVESTMENT AND MALE REPRODUCTIVE SUCCESS IN WHITE SPRUCE
An experimental population composed of ramets of white spruce clones was studied to determine the influence of different levels of clonal male reproductive investment on clonal male reproductive success, where the male reproductive success of different clones in the population was defined as the proportion of the seed crop sired by each clone. We present a multilocus estimation procedure which provides unbiased estimates of the proportion of seed sired by each parental genotypic class from pollen gamete frequency data, whenever gametic segregation frequencies are known.
35598978
MEASUREMENTS OF NATURAL SELECTION ON FLORAL TRAITS IN WILD RADISH (RAPHANUS RAPHANISTRUM). II. SELECTION THROUGH LIFETIME MALE AND TOTAL FITNESS
It has often been suggested that selection on floral traits in hermaphroditic plants should occur primarily through differences in male fitness. However, measurements of selection on floral traits through differences in lifetime male fitness have been lacking. We measured selection on a variety of wild radish floral traits using lifetime male fitness measures derived from genetic paternity analysis. These male fitness estimates were then combined with estimates of lifetime female fitness of the same plants to produce measurements of selection based on lifetime total fitness. Contrary to the prediction above, there was no strong evidence for selection on floral morphology through male fitness differences in any of the three years of the study, but there was strong selection for increased flower size through female fitness differences in one year. The main determinant of both male and female fitness in all years was flower number; this lead to moderately positive correlations between male and female fitness in all three years.
17631925
Predicting evolution of floral traits associated with mating system in a natural plant population
Evolution of floral traits requires that they are heritable, that they affect fitness, and that they are not constrained by genetic correlations. These prerequisites have only rarely been examined in natural populations. For Mimulus guttatus, we found by using the Riska‐method that corolla width, anther length, ovary length and number of red dots on the corolla were heritable in a natural population. Seed production (maternal fitness) was directly positively affected by corolla width and anther size, and indirectly so by ovary length and number of red dots on the corolla. The siring success (paternal fitness), as estimated from allozyme data, was directly negatively affected by anther–stigma separation, and indirectly so by the corolla length–width ratio. Genetic correlations, estimated with the Lynch‐method, were positive between floral size measures. We predict that larger flowers with larger reproductive organs, which generally favour outcrossing, will evolve in this natural population of M. guttatus.
19047616
Fractional paternity assignment: theoretical development and comparison to other methods
SummaryThere has recently been a burgeoning interest in the analysis of paternity patterns for natural populations because of its relevance to population genetic phenomena such as the distance between successful mates, relative male reproductive success and gene flow. In this paper we develop a method of analyzing populational patterns of paternity, the fractional paternity method, and compare its performance to two other commonly used methods of paternity analysis (simple exclusion and the most-likely methods). We show that the fractional method is the most accurate method for determining populational patterns of paternity because it assigns paternity to all progeny examined, and because it avoids biases inherent in the other paternity analysis methods when model assumptions are met. In particular, it avoids a systematic bias of the most-likely paternity assignment method, which has a tendency to over-assign paternity of progeny to certain male parents with a greater than average number of homozygous marker loci. We also demonstrate the effect of linkage of some of the marker loci on paternity assignment, showing how the knowledge of the linkage phase of male and female parents in the population can significantly improve the accuracy of the estimates of populational patterns of paternity. Knowledge of the linkage phase of individuals in a population is usually unknown and difficult to assess without progeny testing, which involves considerable labor. However, we show how the linkage phase of hermaphroditic individuals in a population can be obtained in conjunction with the paternity analysis if progeny can be obtained from each hermaphroditic individual in the population, thereby avoiding the problem of traditional progeny testing. Applications of the fractional paternity approach developed herein should contribute significantly to our understanding of the mating patterns in, and hence the evolution of, natural populations.
124799265
Application of Maximum Likelihood Methods to Population Genetic Data for the Estimation of Individual Fertilities
SUMMARY The fitness of plants or animals within a population is largely determined by the number of offspring they produce. In natural populations lacking familial structure either one or both parents are often unknown. To circumvent this problem, parents and offspring can be genotyped for a set of genetic markers. Likelihood models are proposed to estimate the fertility of either male or male and female parents in a population where fertility is defined broadly as a fraction of progeny in the population fathered or mothered by some individual. Models are developed for three cases: the mother is known and the male fertilities differ depending on the maternal parent; the mother is known and the male fertilities are the same for all maternal parents; neither parent is known and fertilities are estimated for parent pairs. It is established that a unique maximum likelihood solution exists under conditions that are commonly met. For situations in which these conditions are not met, a method is presented to determine estimates of a set of independent linear combinations of the parameters that can be uniquely estimated. Two algorithms are examined which can be used to find the parameter estimates. The variance of the estimator, likelihood ratio tests for constraints on the fertility parameters, and a goodness-of-fit test are developed. Finally, a worked example is presented.
32067183
Complex patterns of mating revealed in a Eucalyptus regnans seed orchard using allozyme markers and the neighbourhood model
The neighbourhood model apportions offspring of individual mother plants to self‐fertilization, outcrossing to males within a circumscribed area around the mother plant (the neighbourhood), and outcrossing to males outside the neighbourhood. Formerly the model was applied only to haploid pollen gametes in the offspring of conifers, but is extended so that it can be used with genotypic data from diploid offspring of both angiosperms and gymnosperms. In addition, it is shown that the mating parameters can be estimated without independent estimates of allele frequencies in the pollen pools outside the neighbourhood; thus the model might be applied effectively to natural populations exposed to unknown external pollen sources. Parameters of the neighbourhood mating model were estimated for a 10‐year‐old seed orchard population of the insect‐pollinated tree, Eucalyptus regnans, in southeast Australia, which contained a mixture of two geographical provenances (Victoria and Tasmania). The mating patterns revealed were complex. Crosses between trees of the same provenance occurred three times more often than crosses between trees of different provenances. Levels of self‐fertilization and patterns of mating within neighbourhoods were influenced by provenance origin, crop fecundity and orchard position (central vs. edge) of mother trees. Gene dispersal, however, was extensive, with approximately 50% of effective pollen gametes coming from males more than 40 m away from mother trees (average distance between neighbouring trees was 7.4 m). Thus, insect pollinators are efficient promoters of cross‐fertilization in this orchard, with the result that the effective number of males mating with each female is large.
198158228
USING GENETIC MARKERS TO DIRECTLY ESTIMATE MALE SELECTION GRADIENTS
We present an analysis of Raphanus raphanistrum and simulations illustrating the utility of directly estimating male phenotypic selection gradients using genetic markers. The method offers a much more refined characterization of selection than attempting to assign paternity to individual progeny. Our analysis of R. raphanistrum reveals selection on remarkably fine features of floral morphology, including anther exsertion, that were opaque to previous approaches. The new results also undermine a previous conclusion that selection on wild radish floral morphology acts primarily through female fitness. Simulation results show that selection gradients on the order of b 5 0.1–0.2 can be readily detected with allozyme markers in moderate-sized (, 200 paternal individuals) populations. Highly polymorphic (e.g., microsatellite) markers will likely detect fine scale selection (b , 0.1) in larger populations ($ 400 individuals). Increased progeny sample size, by sampling either additional maternal families or more progeny per maternal parent, partly compensates for low exclusion probability. Increasing the number of possible fathers without changing progeny sample size decreases the ability to detect selection, especially at lower exclusion probabilities. Sampling only some male genotypes reduces the power to detect selection and biases (underestimates) the magnitude of the selection gradient estimate.
86658915
Flower and Fruit Abortion: Proximate Causes and Ultimate Functions
During the past decade ecologist have devoted considerable energy to demographic studies of flowers and fruits between anthesis and dispersal. These efforts have led to the growing realization that the number of flowers and fruits and the way they are clustered in time and space influence pollinator attraction...
86805458
VARIATION OF SEX ALLOCATION AMONG EIGHT TAXA OF THE MIMULUS GUTTATUS SPECIES COMPLEX (SCROPHULARIACEAE)
Characters related to sex allocation and the mating system were studied in eight California taxa of the Mimulus guttatus complex: M. guttatus, M. nasutus, M. glaucescens, M. Tilingii, M. nudatus, M. laciniatus, M. platycalyx, and M. micranthus, ranked in approximate decreasing levels of outbreeding. Dry weights and lengths of floral parts, pollen and ovule number, and timing of stigmatic closure were measured on plants in the growth chamber. As percent of total flower weight, allocation to stamens and corollas was lowest in M. micranthus (28%), intermediate in M. platycalyx, M. Tilingii, and M. laciniatus (50%), and high in other taxa (60%). Among M. micranthus, M. platycalyx, and M. laciniatus, pollen-ovule ratios ranged from 3.9 to 12.0; ratios for other taxa were 19.3 to 26.6. Taxa with increased male biomass allocation generally show increased outcrossing and increased P/O ratios. Stigma-anther separation and closure of stigma lobes upon touch were positively correlated with outcrossing ratios, P/O ratios, and male allocation. Isozyme variation indicates the inbreeding taxa have been independently derived; thus, these associations of maleness with outbreeding are significant trends in the Mimulus guttatus complex. HERMAPHRODITIC FLOWERS serve both male and female functions. In hermaphroditic-flowered species with higher selfing rates, the theory of local mate competition predicts increased allocation of resources to floral structures that are female in function (Charnov, 1979; Charlesworth and Charlesworth, 1981). The exact nature of this relationship is undetermined (Charnov, 1987), and more fundamentally, the gender of individual floral structures is subject to dispute (Cruden and Miller-Ward, 1981; Goldman and Willson, 1986). Some parts of the hermaphroditic flower, such as the anthers, pollen, pistils and ovules, are of unequivocal gender, while others, such as the corolla and the calyx, are not. However, if few pollinator visits are sufficient to fertilize almost all eggs while many pollinator visits are required to disperse pollen to other plants, the corolla should be predominantly male (Bell, 1985). In contrast, the calyx, with its permanent attachment to the ovary and its role in seed production and dispersal, has characteristics that are more female. Besides the prob
26650660
Selection on floral characters in natural Spanish populations of Silene latifolia
In insect‐pollinated plants, floral characters are expected to play an important role in paternal and maternal reproductive success. Bateman's principle states that male reproductive success increases with more mating opportunities, while female reproductive success is limited by the amount of resources available to produce progeny, thus there should be greater selection on male floral characters than on female. In the case of the dioecious plant Silene latifolia, floral characters are likely to be influenced by its association within its native European range with moths of the genus Hadena, which serve as both pollinators and seed predators. The present study addresses relationships between male and female reproductive success, spatial location and floral characters (corolla, calyx and claw) over a 2‐year period in two Spanish populations of S. latifolia in the presence of Hadena moths. A maximum likelihood paternity analysis using genetic variation at six allozyme markers showed heterogeneity in male reproductive success. There was much less variation in female reproductive success. When this analysis was extended to include interplant distance as a causal factor underlying variation in male success, we found that successful pollination tended to be limited to nearby females, in accordance with exponential decay of pollen dispersal with increasing distance. When the paternity analysis included floral characters as a causal factor underlying variation in male success, our data showed little evidence for directional selection, but there was stabilizing selection in one of the two years for calyx diameter. Selection on female characters varied widely between sites and years, in most of the site/year combinations there was little selection on female floral characters; however, in one site/year there was evidence for selection on all three floral characters. We conclude that pollinators visit flowers that are close together, and that while floral characters are important for the attraction of pollinators, larger flowers do not necessarily attract more pollinators at all sites and that variation among sites and years makes difficult any conclusions about the long‐term importance of the predictions suggested by Bateman's principle.
84073439
Male and Female Fertility Variation in Wild Radish, a Hermaphrodite
We estimated male and female fertilities in a naturally occurring population of 41 hermaphroditic wild radish plants. Male fertilities were estimated by analyzing the paternity of 2,428 seeds (from 39 of the plants), coupled with a maximum-likelihood method to estimate fertility from the paternity data. Female fertilities were estimated as the fraction of the seed yield of the population produced by each individual. We detected a 15-fold range in mean male fertility, nearly a 200-fold range in female fertility, and a 25-fold range in total fertility. Male fertility was correlated with the distance among mates and with a number of phenotypic features of the plants: the total number of flowers produced, flower color, peak flowering time, whether the peak flowering times of the male and female parents overlapped, and some of the interactions of these variables. An additional portion of the variance of male fertility, however, was accounted for by a class variable denoting each male parent, suggesting that unmeasured features of the plants influenced male fertility. Female fertility was distinctly bimodal. Within mode, most of the variance of female fertility was accounted for by the number of flowers produced by each plant and its peak flowering period. Male and female fertilities were only weakly correlated, permitting a wide range of functional genders, 0.03-0.91 (possible range, 0-1), to be expressed. Because wild radishes are annuals, the lifetime fertility variation that we report must have important fitness consequences. In addition, contrary to many researchers' expectations, in this population the variance of female fertility exceeds that of male fertility. Consequently, phenotypic selection would have been greatest on traits correlated with female fertility or total fertility in this 1986 population.
4445499
Sex Allocation and Reproductive Success in the Andromonoecious Perennial Solanum carolinense (Solanaceae). II. Paternity and Functional Gender
According to Bateman's principle, male fitness in entomophilous plant species should be limited by mating opportunity, which is influenced by the size or number of flowers. We determined male‐specific fitness consequences of floral phenotype in andromonoecious Solanum carolinense, examined the relationship between male and female reproductive success within plants, and evaluated the distribution of functional gender among plants. A maximum likelihood‐based paternity analysis, based on multilocus allozyme phenotypes of parents and offspring from four experimental plots, was used to determine male reproductive success and its relationship to floral phenotype. Male success was enhanced by an increase in the proportion of male flowers produced but not by an increase in total flower number, even though all flowers contain male parts. Larger flower size increased male success in only one plot. Male and female reproductive success were negatively correlated, and plants varied in functional gender from completely female to completely male. This gender specialization may occur because hermaphroditic and male flowers differ in their ability to contribute to male and female success. Although sex allocation theory predicts a positive relationship between the size or number of plant parts and reproductive success, this study indicates that aspects of floral morphology that affect gender specialization should also be considered.
40471796
REGRESSION ANALYSIS OF NATURAL SELECTION: STATISTICAL INFERENCE AND BIOLOGICAL INTERPRETATION
Recent theoretical work in quantitative genetics has fueled interest in measuring natural selection in the wild. We discuss statistical and biological issues that may arise in applications of Lande and Arnold's (1983) multiple‐regression approach to measuring selection. We review assumptions involved in estimation and hypothesis testing in regression problems, and we note difficulties that frequently arise as a result of violation of these assumptions. In particular, multicollinearity (extreme intercorrelation of characters) and extrinsic, unmeasured factors affecting fitness may seriously complicate inference regarding selection. Further, violation of the assumption that residuals are normally distributed vitiates tests of significance. For this situation, we suggest applications of recently developed jackknife tests of significance. While fitness regression permits direct assessment of selection in a form suitable for predicting selection response, we suggest that the aim of inferring causal relationships about the effects of phenotypic characters on fitness is greatly facilitated by manipulative experiments. Finally, we discuss alternative definitions of stabilizing and disruptive selection.
9204868
PATTERNS AND MECHANISMS OF SELECTION ON A FAMILY‐DIAGNOSTIC TRAIT: EVIDENCE FROM EXPERIMENTAL MANIPULATION AND LIFETIME FITNESS SELECTION GRADIENTS
Abstract Plant traits that show little variation across higher taxa are often used as diagnostic traits, but the reason for the stasis of such traits remains unclear. Wild radish, Raphanus raphanistrum, exhibits tetradynamous stamens (four long and two short, producing a dimorphism in anther height within each flower), as do the vast majority of the more than 3000 species in the Brassicaceae. Here we examine the hypothesis that selection maintains the stasis of dimorphic anther height by investigating the effects of this trait on pollen removal, seed siring success, and seed set in R. raphanistrum using both experimental and observational methods. Observational selection gradient analysis based on lifetime seed siring success provided evidence for an optimum dimorphism that was greater than zero in one of three years. In both experimentally manipulated and unmanipulated flowers, more pollen was removed in single visits from flowers with less dimorphism. There was no significant effect of anther dimorphism on female fitness (seed set). Therefore, there is some evidence to suggest that selection is maintaining anther dimorphism in wild radish, and that higher male fitness might result from restriction of single‐visit pollen removal. We discuss these results in light of pollen presentation theory.
198157587
HIGH MALE REPRODUCTIVE SUCCESS OF HERMAPHRODITES IN THE ANDRODIOECIOUS PHILLYREA ANGUSTIFOLIA
Abstract Androdioecy, the coexistence of males and hermaphrodites within a population, is a rare breeding system, often considered as unlikely to evolve because of restrictive conditions for its maintenance. Phillyrea angustifolia, a wind-pollinated shrub, is one of the handful species reported to be androdioecious. Our previous studies have shown that natural populations of this species in southern France exhibit higher male frequencies (∼50%) than predicted on theoretical grounds. Thus, the male functionality of hermaphrodites is still debated. To assess the functional breeding system of this species in the wild, a paternity analysis was performed with two highly polymorphic microsatellite loci on 729 seeds collected on 10 maternal shrubs in a natural population of 24 mature individuals of P. angustifolia. A large proportion of seeds were found to have been sired by pollen from outside the population. Analysis of seeds sired by individuals within the study population revealed a high male fertility of hermaphrodites resulting in a low male advantage in fertility for male plants. Intermate distances were found to have a strong impact on male reproductive success, whereas sexual morph had no effect, with males and hermaphrodites performing equivalently. This study is the first to unequivocally document the occurrence of a male function of hermaphrodites in a natural population of an androdioecious species.
62175992
The design and analysis of research studies
1. Preview 2. The sample survey 3. Other sampling designs 4. The linear regression model 5. Experimental designs to assess the effect of a treatment 6. Interrupted time series 7. More advanced experimental designs 8. Some special types of data 9. Computer intensive statistics 10. Ethical considerations 11. Synthesis: carrying out a research study References Index.
20661745
Male reproductive success in Pseudotsuga menziesii (Mirb.) Franco: the effects of spatial structure and flowering characteristics
Reproductive patterns and effective pollen dispersal were investigated in a Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) clonal seed orchard using 11 isozyme loci as genetic markers. Progenies from 94 mother trees were analysed by the mixed-mating and the neighbourhood models and outcrossing rates were estimated as 0.96 and 0.97, respectively. The proportion of offspring resulting from pollination by males located outside the neighbourhood of each mother tree (30 m in radius) was estimated as 0.54, with ≈43 per cent of matings resulting from outcrossing with nearby males (within neighbourhoods). The effects of distance and direction of individual males from mother trees, pollen fecundity and phenological synchronization were all significant in determining outcross mating patterns within neighbourhoods. Generally, male reproductive success increased with proximity and phenology overlap with mother tree and pollen fecundity. The effect of different factors influencing male reproductive success varied among the three groups of mother trees divided according to the time of female receptivity (early, intermediate and late). Phenology was important in the early and intermediate groups, whereas fecundity was important in the intermediate and late phenology groups. Both distance and directionality effects were significant in the early and late classes. The directionality effect could be partially explained by the wind patterns existing during the pollination period. The mean effective number of males mating with each female within neighbourhoods was 10.95 (49.31 per cent of males within neighbourhoods).
15400290
Flower Size Preferences of the Honeybee (Apis mellifera) Foraging on Mimulus guttatus (Scrophulariaceae)
For pollinators to exert selection on floral traits, they must be able to detect and respond to differences in floral morphologies. I examined foraging preferences by the bee Apis mellifera (Apidae) visiting naturally occurring flowers of Mimulus guttatus (Scrophulariaceae). The results indicate that A. mellifera preferentially selects larger M. guttatus flowers in their foraging bouts. Since differences in flower size in M. guttatus have previously been shown to have a significant heritable component, I suggest that A. mellifera has the potential to be an important selective force in the floral morphology of M. guttatus.
26801469
Effects of herbivory and inbreeding on the pollinators and mating system of Mimulus guttatus (Phrymaceae).
Most models of mating system evolution predict mixed mating to be unstable, although it is commonly reported from nature. Ecological interactions with mutualistic pollinators can help account for this discrepancy, but antagonists such as herbivores are also likely to play a role. In addition, inbreeding can alter ecological interactions and directly affect selfing rates, which may also contribute to maintaining mating system variation. We explored herbivore and inbreeding effects on pollinator behavior and selfing rates in Mimulus guttatus. First, individual spittlebug (Philaenus spumarius) herbivores were applied to native plants in two populations. Spittlebugs reduced flower size, increased anther-stigma distance, and increased selfing rates. A second experiment factorially crossed spittlebug treatment with inbreeding history (self- vs. cross-fertilized), using potted plants in arrays. Spittlebugs did not affect pollinator behavior, but they reduced flower size and nearly doubled the selfing rate. Inbreeding reduced the frequency of pollinator visits and increased flower-handling time, and this may be the first report that inbreeding affects pollinator behavior. Selfing rates of inbred plants were reduced by one half, which may reflect early inbreeding depression or altered pollinator behavior. The contrasting effects of herbivory and inbreeding on selfing rates may help maintain mating system variation in M. guttatus.
28902309
A method to estimate pollen viability from pollen size variation.
The mean diameter of viable pollen grains is approximately 13 μm greater than the mean diameter of inviable grains in Mimulus guttatus. We show that this difference is large enough to be detected by particle counters and that these machines can be used to obtain a rapid estimate of pollen viability. While requiring a separate calibration, a size-based statistic is also strongly correlated with pollen viability in Collinsia verna. These results suggest that statistics derived from the size distribution of pollen grains may provide an alternative to more labor-intensive methods for estimating pollen viability, particularly in cases where inviability results from inbreeding depression or hybrid failure.