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The article suggests a new theory for the evolution of overproduction of ovules in flowers. By overproducing ovules, females select fertilized ovules having similar resource absorption ability to produce seeds of uniform size.

The assumption of a ubiquitous trade-off between male and female investment is challenged by arguing that in highly self-fertilizing species, stabilizing natural selection should favor highly efficient ratios of male to female gametes. Further, an artificial selection on pollen and ovule production per flower in two sister species with contrasting mating systems is conducted.

The magnitude of pre- and postzygotic barriers between naturally occurring sympatric populations of Mimulus guttatus and Mimulus nasutus, is estimated. Findings indicate greater potential for introgression from Mimulus nasutus to Mimulus guttatus than for the reverse direction.

The study examined one potential bias and finds that published studies of outcrossing rates contain far fewer obligate outcrossers than expected. Because this study examines only one potential bias, and finds it to be large, conclusions regarding either the frequency of mixed mating of shape of the distribution of outcrossing rates in nature are premature.

An experiment that demonstrates short-term changes in trait means, variances, and covariances that happen from the interaction of selection, genetic drift, and mating system is described. Results show that genetic variances and covariances were likely to change rather dramatically over a short period of time if the mating system of a population changes at the same time with selection.

The role of inbreeding avoidance in maintaining sexual system variation in Wurmbea biglandulosa was examined. It was found that present sexual system variation in W. Biglandulosa is maintained by high inbreeding depression coupled with differences in selfing rates among monomorphic and dimorphic populations.

Research has been conducted on the monocarpic perennial Carlina vulgaris. The effects of stochastic variation on C. vulgaris life-history evolution and the estimation of the selection pressures on this perennial have been investigated and the details are reported.

Research has been conducted on introgression frequencies between hybrid zones. The comparison of these frequencies among the blocks with the negative, neutral and positive effects in the hybrid genetic background has been carried out and the results are reported.

A phylogenetic approach has been used to study the evolution of reproductive isolation in the northwestern European ectomycorrhizal Hebeloma crustuliniforme aggregate (Basidiomycetes). The goal of the study was to reconstruct the evolution of (partial) incompatibility in the aggregate.

Outbreeding depression has been studied in cohorts of Ipomopsis aggregata planted in nature. the effect of crossing ditance on fitness is discussed. The magnitude of outbreeding depression in 100-m crosses varied from experiment to experiment, not surprising, given likely spatial and temporal variation in gene flow and selection regimes and in other ways. Characterizing outbreedin depression on the shortest sptial scales over which it is expressed and in other ways can shed light on earliest stages of angiosperm spciation.d

The fitness trade-off between reproducing only once in a lifetime, semelparity, or reproducing repeatedly in different seasons, iteroparity, inspires questioning about the evolution of life histories and spatially structured systems. The evolutionary scenario has been extended into a spatially structured system with dispersal linkage of subunits. A rare reproductive strategy can then invade a population dominated by a resident reproductive strategy easily. The parameter space that makes the invasion possible is much more generous with spatially structured evolutionary scenarios than with spatially nonstructured systems.

Phenotypic selection on phenological and morphological reproductive traits has been studied over three flowering season in a Swedish population of Platanthera bifolia (Orchdaceae), a perennial of Eurasia. It has bisexual, long-spurred flowers. Results show patterns of selection may vary between years and between sex functions in perennial hermaphroditic plants. This is one of the first studied to deal with correlational selection in plants, likely a common occurrence.

Research shows that mutation will degrade complex traits such as sex when they no longer enhance a species' fitness. Studies were based on clonal populations of the tristylous aquatic plant Decodon verticillatus and its pollen tube growth, with a single recessive mutation posited as the cause of infertility.

Research is presented on population characteristics of the perennial herb Adenophora grandiflora for both clonal and sexually reproduced genotypes. Clonal genotypes exhibited statistically substantial clustering within regions measuring less than 4 m, while both clonal and sexually reproduced genotypes showed spatial autocorrelation in areas measuring less than 12 m.

The mechanism underlying the selection on corolla width in Campanula punctata is elucidated. Using multiple regression analysis of male reproductive success, it is shown that the advantage of a wide corolla to the male is not linked to the corolla width of recipients, suggesting that the position of pollen deposited on Bombus is not a factor for selection. Analysis reveals that wider corollas are linked to reduced pollen loss through pollen fall, suggesting that more gentle contact between bees and pollen occurs with wider corollas helps improve reproductive success.

The structured linear model (SLM) predicts response to selection on a single trait in partially selfing populations. The SLM was generalized to predict evolutionary changes when multiple characters determine fitness. It can distinguish four different causes of phenotypic correlations, namely, environmental covariance, identity disequilibrium, linkage disequilibium and pleiotropy which are characterized by distinct variables since they have different implications for character evolution.

In his classic paper on the correlation between relatives, R. A. Fisher (1918) introduced the notion of 'dual epistacy' to account for deviations from linearity due to interactions between distinct [...]

In his monograph on plant speciation, Grant (1981) described several mechanisms by which the breeding behavior of hybrids could be stabilized, thus providing the potential for hybrid speciation. These include: [...]

It has been 30 years since Baker (1967) reemphasized in these pages the significance of what Stebbins (1957) had termed 'Baker's Law,' namely, that it is more likely for self-compatible [...]

A key feature of self-incompatibility (SI) systems in plants is the mate availability or proportion of compatible matings in a population (Bateman 1952; de Nettancourt 1977). A mating is the [...]

Theoretical (e.g., Maynard Smith 1971; Lloyd 1979; Charlesworth and Charlesworth 1979, 1987, 1990; Lande and Schemske 1985; Campbell 1986) and empirical (e.g., see Charlesworth and Charlesworth 1987) studies have illustrated [...]

The majority of outcrossing angiosperms have bisexual flowers, a condition from which self-pollination can evolve directly through the modification of self-incompatibility or other floral traits that prevent self-pollination. Indeed, theoretical [...]

Many plant species seed at intervals of several years during their lifetime, with such reproduction often being synchronized over a large forest area and termed 'mast seeding' (Silvertown 1980). Mast [...]

Hybridization and introgression are common phenomena in many plant and animal groups and have been proposed as important processes in evolution. Interspecific gene flow has received much attention not only [...]

Key words.--Allozymes, apomixis, clonal variation, dandelions, multilocus genotypes, pollen flow, Taraxacum. Received October 24, 1993. Accepted September 19, 1994. The evolutionary potential of a population or species depends largely on [...]

Intermediate selfing rates often are observed in populations of animal-pollinated plants (e.g., Aide 1985; Waller 1986). However, many theoretical studies did not predict the evolution of intermediate selfing, but rather [...]

The study of plant mating-system evolution has provided fertile ground for the interaction of theoretical and empirical workers. Extensive empirical work began with Darwin (1876) and has continued at an [...]

Allocation theory and ESS models have been used to predict the optimal allocation to male and female function and to pollinator attraction in hermaphrodite plants (e.g., Charnov et al. 1976; [...]

The resources a plant invests in sexual reproduction are partitioned among multiple units, such as gametes within a flower, flowers within an inflorescence, and inflorescences on a plant. The manner [...]

In hermaphroditic plants, reproductive success is achieved through pollen donation (male success) and seed production (female success). Pollinators respond to variation in floral features, such as nectar quality, flower color, [...]

Pollination of flowering plants is followed by the germination of pollen, growth of pollen tubes, fertilization of ovules, and development of resulting embryos. Variation in performance during any of these [...]

[W]ith hermaphrodite plants that are strongly proterandrous, the stamens in the flowers which open first sometimes abort; and this seems to follow from their being useless, as no pistils are [...]

In population genetic studies, knowledge of the mating system is of major importance, because patterns of mating influence the amount and distribution of genetic variability and hence evolutionary processes in [...]

The relationship between current and future reproduction is one of the most frequently cited examples of evolutionary trade-offs (Fisher 1958; Stearns 1976; Partridge and Harvey 1985; Reznick 1985). Reproductive schedules [...]

Abstract. - Most models of mating-system evolution predict inbreeding depression to be low in inbred populations due to the purging of deleterious recessive alleles. This paper presents estimates of outcrossing [...]

Abstract. -While gene flow can be an important force in evolution, few direct measures are available in the plant literature. Descriptions of gene movement within populations are more common, but [...]

Abstract. - To test several predictions of a model of linear, size-dependent reproductive output in plants, we analyzed the relationship between shoot vegetative (v) and reproductive (r) mass in five [...]

A study was conducted to study the evolutionary aspects of the maintenance of gynodioecy in Phacelia linearis. Results show that nuclear genes control gender in P. linearis, and females produce significantly more fruits and seeds than hermaphrodites in natural populations. Moreover, gender-specific patterns of resource allocation to growth and reproduction is responsible for the greater seed production of females relative to hermaphrodites in P. linearis. Therefore, resource compensation by females is a major factor in the evolution of gynodioecy in P. linearis, in the absence of a large outcrossing advantage of females.

Dioecy is unusually common in the Hawaiian Islands, yet little is known about the evolutionary biology of this breeding system. A native shrub, Wikstroemia, has an unusually diverse array of breeding systems: two forms of dioecy, cryptic and morphological dioecy, as well as hermaphroditism (perfect flowers). The existence of two forms of dioecy is significant for three reasons: 1) the presence of cryptic unisexuals that are functionally unisexual, but retain the appearance of hermaphroditism in both sexes, is strong evidence for ancestral status of hermaphroditism; 2) the production of nonfunctional pollen, by female cryptic unisexuals, is a new instance of a phenomenon which has previously been reported for a few other species; 3) the two forms of dioecy are morphological markers which are useful in hybridization studies for tracing the genetic basis of their inheritance. Crosses were made between cryptically unisexual individuals (C), between morphologically unisexual individuals (M), and between the two types of unisexuality. The offspring of crosses between individuals with the same sex type usually resulted in offspring with that sex type, but most of the progeny of between-sex type crosses were, unexpectedly, perfectly-flowered hermaphrodites. These results show that genetic control of sex determination is not homologous in all populations, suggesting that dioecy has evolved at least twice in Hawaiian Wikstroemia. The genetic data further suggest that males are the heterozygous sex.

Clarkia tembloriensis exhibits a wide range of variation among its natural populations in outcrossing rate and in separation of male and female function in space (anther-stigma separation or herkogamy) and in time (protandry). Here we show that outcrossing rate is highly correlated with protandry and anther-stigma separation. Both genetic and environmental variation contribute to inter- and intrapopulation variation in protandry and anther-stigma separation. Interpopulation differentiation for protandry and anther-stigma separation was found to be polygenic. Genetic variation for protandry and anther-stigma separation within populations was demonstrated by a significant among-family variance in two populations with contrasting breeding systems. Environmental effects on the expression of mating system traits were manifested in two ways. First, significant variation among lathhouse benches suggests that small-scale environmental heterogeneity may affect the development of floral traits. Second, protandry was shortened under hot summer conditions. Hence, hotter and drier habitats, typical of the more self-pollinating populations of C. tembloriensis, can promote self-pollination purely through environmental effects.

In this study, we sought to determine if Phaseolus coccineus normally aborts potentially viable seeds, and whether seed abortion is nonrandom with respect to progeny vigor. The ovaries of Phaseolus coccineus typically have six linearly arranged ovules. The three ovule positions at the stylar end are more likely to mature seeds than the three ovules at the basal end of the ovary. When we destroyed the developing ovules at the stylar end of the fruit after fertilization but before seed abortion, there was a significant increase in the probability that the ovules in the three basal positions would produce a mature seed. The probability of seed maturation in control fruits (no ovules destroyed) ranged from 38.3 to 42.7% over the three field seasons, whereas in the experimental fruits is ranged from 64.3 to 79.7% (similar to that of ovules at the stylar end in control fruits). We did not find any significant change in the probability of seed production in the three ovule positions in the stylar end of the ovary (the positions with high probability of seed maturation) when the basal ovules were experimentally destroyed. These results indicate that potentially viable seeds are regularly aborted in P. coccineus, with seed abortion more frequent in the basal ovule positions than at the stylar positions. In two greenhouse studies and one field study, we compared the vigor of progeny produced in the control fruits with the vigor of progeny produced in fruits where three ovules (either stylar or basal end) were destroyed. We found that the performance of the progeny from the three positions at the stylar end of the control fruits did not differ from the performance f the progeny from experimental fruits in which the three basal ovule positions were destroyed. In contrast, the progeny from the basal positions of the control fruits outperformed the progeny from the experimental fruits when the seeds were produced in the three basal ovules (stylar ovules destroyed). Our findings indicate that when the experimental treatment increased the probability of seed maturation, there was a significant decrease in the average vigor of the progeny in the remaining (basal) ovule positions. Phaseolus coccineus, progeny performance, seed abortion.

The consequences of selfing were examined for a population of self-compatible, protandrous, Sabatia angularis L. (Grentianaceae). Field-collected plants were hand-pollinated in the greenhouse to produce selfed progeny and outcrossed progeny from parents separated by a maximum of 5 m (near-outcross) and 85 m (far-outcross) in the field. Self, near-outcross, and far-outcross half sib progeny were grown in the greenhouse, a garden plot, and their native habitat. Progeny were compared by multiplicative fitness functions based on seed production per hand-pollination, seed germination, rosette formation, survival to reproduction, and reproduction in each environment. Variation in reproduction among progeny groups was influenced by the environment in which they were grown. Significant inbreeding depression was detected between the self and far-outcross progeny in each environment. Only the natural environment demonstrated a greater than 50% reduction in relative fitness of self compared to near-outcross progeny. This is of biological relevance since near-outcross hand-pollinations occurred within the range of pollen and seed dispersal suggesting that inbreeding depression in S. angularis is strong enough to maintain outcrossing in the study population. In the field, the far-outcross progeny outperformed the near-outcross progeny suggesting local population substructure. The magnitude of the inbreeding depression expressed among the self progeny was the greatest in the field, intermediate in the garden plot, and the least in the greenhouse.

The mean volume of pollen grains and total pollen production varied both within and among plants of Erythronium grandiflorum. The second flowers of two-flowered plants tended to produce smaller and fewer grains than first flowers, but there was no overall relationship between mean pollen grain size and production per flower. I evaluated the effects of pollen size differences within and among plants on two components of male reproductive success: pollen tube growth and postfertilization siring ability. Pollen tubes grown in media were longer for second flowers, but were not correlated with the mean size of pollen grains, suggesting that (1) internal resource content of pollen (i.e., carbohydrates plus lipids) was not associated with the hydrated size of pollen, and that (2) pollen from second flowers contained more resources. I analyzed the growth rate and the fertilization ability of pollen growing in styles. Growth rate differed among donors and recipients, but no effects of pollen or donor characters (i.e., pollen production, grain size, and flower position) were detected. In single donor pollinations, pollen size was negatively correlated with fertilization ability across donors, and positively correlated with postfertilization siring ability of donors. A second experiment used pairs of donors; within-plant differences in pollen size and flower position had effects similar to the single donor experiment on fertilization ability, but among-plant differences were not significant. The results corroborate earlier experiments that suggest that the growth of pollen tubes in the style is probably controlled by the recipient, since donor characters had minimal effects on pollen fertilization ability. Postfertilization siring ability was not affected by within-plant differences in mean grain size and production. For among-donor differences, the number of seeds set for each donor was positively correlated with the mean grain volume, and when a donor producing large pollen fertilized ovules in an ovary, there was increased seed abortion for seeds in the same ovary sired by a second donor. In addition, the total number of seeds produced by a fruit was decreased when both donors had large pollen, apparently due to increased postfertilization abortion. Postfertilization processes appear to be influenced by paternal differences that are expressed through competition among developing seeds for maternal resources.

Growth and reproduction in higher plants depend on meristems, which have three developmental fates. A meristem can become reproductive, but doing so terminates its activity, it can differentiate vegetatively, or it can remain quiescent for extended periods. The first two fates are mutually exclusive, and only the second leads to the production of additional meristems for subsequent growth and reproduction. In Polygonum arenastrum (frequently referred to as P. aviculare in North American Floras), an annual species lacking quiescent meristems, a quantitative genetic analysis of inbred full-sibling families revealed genetic variation in the developmental pattern of axillary meristem commitment to vegetative growth versus reproduction. Developmental variation resulted in family differences in the age of first reproduction, in age-specific fecundity and growth, and in final plant size and reproductive output. Furthermore, there were strong negative genetic correlations between age-specific growth and fecundity. Early commitment of meristems to reproduction favors high early fecundity, but reduces the number of meristems available for vegetative differentiation, and leads to lowered growth rates and fecundity later in life, when meristems are limiting. Conversely, meristem commitment to vegetative growth early in life results in low early fecundity but high late fecundity and growth. Meristem limitation, like resource limitation, is a proximate mechanism that generates trade-offs between life history traits. Differences between meristem limitation and resource limitation are discussed. Meristem limitation leads automatically to a senescent life history because of the determinate fate of reproductive meristems. Developmental characters were also found to be genetically correlated with metamer characters (leaf size, internode length) and seed size in this selfing species. The pattern of correlation is suggestive of selection for particular suites of life history and morphological characters.

Mating systems in plant populations are more accurately modeled by a continuous characterization, rather than a descrete characterization. Data are obtained from both wind- and animal-pollinated species, and target bimodalism and outcrossing.

The impact of selection on the correlation between the mean and variance of the flowering time within, as well as among individuals is determined. The disruptive selection process is shown to lead to a sharp increase in the variance among individuals.

An individual-based model is employed to explain the various effects of the evolutionary feedbacks existing between the reproductive mode and the mutation rate. The evolutionary feedback is shown to lead to a sudden intensification of the paradox of sex.

Several studies are conducted to explain the various effects of the different poliinators on the divergence, as well as the coexistence observed in the African Gladiolus Iongicollis. The floral tube lengths are shown to exhibit bimodal characteristics, hence also exhibiting positive directional selection.

Individual variation in six floral traits and yearly and lifetime total plant seed and fruit production of 758 plants across nine years were measured to study natural populations of ruby-throated hummingbird-pollinated Silene virginica and understand their phenotypic selection on floral traits. The analyses results demonstrated that selection favored trait combinations and integration of floral features of attraction and pollen transfer efficiency that were consistent with the hummingbird pollination syndrome.