DIFFERENTIAL SEED PRODUCTION IN MIMULUS GUTTATUS IN RESPONSE TO INCREASING CONCENTRATIONS OF COPPER IN THE PISTIL BY POLLEN FROM COPPER TOLERANT AND SENSITIVE SOURCES

Selection can occur in the pistil, during a series of stages that include both pre-zygotic (pollen germination, pollen tube growth, and fertilization) and post-zygotic events. This study explores the extent to which selection, at this level, could be due to the environmental conditions under which the maternal parent is growing. Five plants of Mimulus guttatus, tolerant to copper, were vegetatively cloned and each clone was grown in control and in solutions to which copper was added. The maternal plants received pollen from plants either tolerant or sensitive to copper. Seeds and ovules were counted to estimate the number of seeds/capsule, the seed/ovule ratio, the percent fertilization, and the proportion of zygotes aborting for each clone, treatment and pollen source combination. There were large differences among the pollen recipients for each of the measurements. However, there was a consistent pattern to seed production depending on the pollen source and copper treatment. The seed/ovule ratio was unaffected if pollen came from tolerant sources but was reduced by an average of 24% for both copper supplemented treatments if pollen came from copper sensitive sources. Thus, the data indicated that selection due to environmental factors could occur within the pistil. Differences in percent fertilization were not statistically significant, but the seed/zygote ratio showed a pattern that was similar to seed production suggesting that abortion of immature seeds was responsible for most of the difference in seed production.     

Variation of pollen limitation in the early flowering Mediterranean geophyte Narcissus assoanus (Amaryllidaceae)

A recent literature review indicates that pollen limitation of female fertility is a common feature of flowering plants. Despite the ecological and evolutionary significance of pollen limitation, most studies have only examined fertility in a single population at one time. Here we investigate pollen limitation of fruit and seed set in five populations of Narcissus assoanus, a self- sterile, insect-pollinated geophyte, over 2–3 years in southern France. In common with many early spring flowering plants, pollinator visitation to N. assoanus is often infrequent. Supplemental hand-pollination of flowers with outcross pollen significantly increased overall fruit and seed set by 11% and 19%, respectively. Four of the five populations experienced some pollen limitation during the study. For a given year, there was significant variation in pollen limitation among populations. Two of the populations were pollen limited in one year but not in other years in which they were studied. Seed:ovule ratios for open- and hand-pollinated flowers averaged 0.29 and 0.33, respectively. While hand pollination significantly increased the seed:ovule ratio, the low value obtained indicates that the majority of ovules in flowers do not mature seeds despite hand pollination. The role of genetic and environmental factors governing low seed:ovule ratios in N. assoanus is discussed. Received: 28 December 1999 / Accepted: 6 April 2000     

Effects of nutrient availability on primary sexual traits and their response to selection in Spergularia marina (Caryophyllaceae)

Abstract Theoretical models of the evolution of sex allocation generally assume a negative genetic correlation between components of male and female investment or function. To test this assumption, and to determine whether the expression of the correlation is sensitive to environmental conditions, we conducted an artificial selection experiment targeting primary sexual traits in the autogamous herb, Spergularia marina. Mass selection favouring individuals with high (or low) pollen and ovule production per flower had previously been conducted for two generations under uniform conditions. Following a third episode of selection (reported here) within the high‐pollen‐, high‐ovule‐, low‐pollen‐ and low‐ovule‐producing lineages, selected maternal seed families were replicated and cultivated in three nutrient treatments. With this design, we observed the effects of nutrient availability on: floral phenotype, the realized response to selection and the expression of genetic variation in, and covariation between, male and female investment. The first two episodes of selection Mazer et al. 1999 (Evolution 53:717–731) detected evidence for a genetically based negative correlation between ovule and anther production. Following the third episode of selection, phenotypic differences among the selected lines were maintained in all treatments, but evidence for a negative correlation between male and female investment nearly disappeared. Only under low‐nutrient conditions, in which plants selected for low ovule production exhibited elevated anther production, was a genetic trade‐off expressed. Either genetic drift or the combination of novel growing conditions (relative to previous generations) and G × E interactions may have caused the change in the expression of the genetic correlation. This experiment also allowed us to test the prediction that autogamous selfers should exhibit higher canalization of the ratio of male to female investment than of its components. Supporting this prediction, the A : O ratio was more strongly buffered against environmental variation than either anther or ovule production per flower.     

Pollen-Ovule Ratio and Gamete Investment in Pedicularis （Orobanchaceae）

The Pedicularis species provides ideal materials to study floral evolution because of their substantial flower variation based on a narrow genetic basis, even though they are almost exclusively pollinated by bumblebee. These traits allow us to detect the evolutionary trends of floral parameters without considering genetic background and the difference of pollination vectors. The pollen-ovule ratio is widely used to estimate the pattern of resource investment in two sexual functions in flowering plants. Forty species representing all of the corolla types in Pedicularls were used to study pollen-ovule ratio, gamete investment, and their correlations. Results show that pollen-ovule ratio does not differ among both different corolla types and taxonomic groups. It is therefore suggested that pollen-ovule ratio should be a parallel evolution. The correlations between pollen-ovule ratio and pollen size （-）, and ovule size （＋） can be successfully explained in terms of sex allocation theory. The biological significance of such relationships was also discussed. Additionally, we analyzed the pattern of resource investment into female gamete, which has been somewhat neglected, and found that plants have different patterns of gamete investment between the two sexual functions.     

Floral sex allocation and reproductive success within inflorescences of Hosta ventricosa,a pseudogamous apomict

Aims Within inflorescences of sexually reproducing hermaphrodites, the production of ovules, fruits and seeds commonly declines from basal (early-opening) to distal (late-opening) flowers, while pollen production remains constant or only changes slightly, with the result that distal (late-opening) flowers become functionally male. However, few empirical studies have specifically examined whether or not changes in allocation to pollen production actually lead to changes in the number of seeds sired, a more direct measure of male fitness. In pseudogamous apomicts, fitness depends on the number of seeds produced; thus, a contrasting pattern of variation in the pollen-to-ovule (P/O) ratio within inflorescences might be expected.Methods We investigated floral sex allocation and reproductive success within racemes of Hosta ventricosa, a pseudogamous apomictic hermaphrodite possessing flowers that open acropetally. We quantified variations in pollen number, ovule number, the P/O ratio and fruit and seed production, from 2007 to 2011, among flowers within racemes of four populations of H. ventricosa in southwest China. Ecological causes for fruit and seed production were evaluated by observing patterns of pollen deposition, flower removal and supplemental pollination.Important findings Pollen number, ovule number and the P/O ratio declined from basal-to-distal positions in all sampled populations (years). Fruit and seed production decreased distally in most populations (years). Low fruit and seed set of distal flowers was not due to pollen limitation because pollen deposition never declined distally and the low fruit and seed set of distal flowers remained even after supplementary pollination was provided. The flower-removal experiment indicated that inter-fruit competition for resources among flowers was common. The low P/O ratio of distal flowers in H. ventricosa might be favored because they were unable to obtain fitness by donating pollen and siring seeds on other plants. Our study may help to understand the adaptive significance of sex allocation among flowers within inflorescences of sexually reproducing hermaphrodites.     

Pollen-limited seed set in pseudogamous blackberries (Rubus L. subgen. Rubus)

Summary Pseudogamous blackberry species are polyploid and usually exhibit meiotic irregularities causing severe reduction in pollen viability. When species means were compared, relative seed set was strongly correlated with the number of good pollen grains produced per flower divided by the number of ovules (good-pollen/ovule ratio).Partial correlation analysis for percentage viable pollen and relative seed set, controlling for good-pollen/ovule ratio, revealed an almost significant relationship, presumably because meiotic irregularities have a similar effect on both pollen viability and the viability of meiotically derived embryo sacs. Seed germination, on the other hand, showed no relationship with relative seed set or with pollen viability.The automatic selfing capacity exhibited by most of these blackberry species does not appear to be related to either pollen production or relative seed set.     

Seed sex ratio in dioecious plants depends on relative dispersal of pollen and seeds: an example using a chessboard simulation model

Two principles are important for the optimal sex ratio strategy of plants. (1) Sib mating. Because seed dispersal is restricted, sib mating may occur which selects for a female bias in the seed sex ratio. (2) Local resource competition (LRC). If a plant produces pollen its nuclear genes are dispersed in two steps: first through the pollen and then, if the pollen is successful in fertilizing an ovule on another plant, through the seed. If the plant produces an ovule, its genes are dispersed only through the seed. By making pollen instead of ovules the offspring of a single plant is then spread out over a wider area. This reduces the chance that genetically related individuals are close together and need to compete for the same resource. The effect is the strongest if pollen is dispersed over a much wider area than seeds. Less LRC for paternally vs. maternally derived offspring selects for a male bias in sex allocation. We study the above‐mentioned opposite effects in dioecious plants (with separate male and female individuals), with maternal control over the sex ratio (fraction males) in the seeds. In a two‐dimensional spatial model female‐biased sex ratios are found when both pollen and seed dispersal are severely restricted. If pollen disperses over a wider area than seeds, which is probably the common situation in plants, the seed sex ratio becomes male‐biased. If pollen and seeds are both dispersed over a wide area, the sex ratio approaches 0.5. Our results do not change if the offspring of brother–sister matings are less fit because of inbreeding depression.     

The Haig-Westoby model revisited

The Haig-Westoby model predicted that seed set in flowering plants would be equally limited by both pollen capture and resource supply because the optimal level of pollinator attraction should garner just the number of ovule fertilizations needed to consume the available seed-provisioning resources. Variability in the underlying resource and fertilization functions can disrupt this predicted optimum, a point made but only briefly explored by Haig and Westoby. Here I incorporate stochastic variation in both ovule fertilization and resource availability into the Haig-Westoby model and show that the modified model makes two noteworthy predictions: (1) pollen limitation of seed set (as measured by the response to supplemental pollen) should be common, and (2) the degree of pollen limitation may be greater when plants are more attractive to pollinators. The first prediction accords with recent meta-analyses of pollen limitation; the second remains to be examined.     

Sexual asymmetry in hermaphroditic plants

Populations of hermaphroditic plants show variation in ovule and pollen production, so that the implicit assumption that hermaphrodite individuals function half as male and half as female is not valid. Such variation in gamete production results in sexual asymmetry, defined as nonconstant ratios of pollen to ovule production among plants of a population. This article reviews (1) some of the considerable amount of recent evidence for sexual asymmetry and non-equal malelfemale sex functioning in hermaphroditic seed plants, and (2) some models of asymmetry and sex functioning, with emphasis on their biological and evolutionary relevance.     

Factors affecting fruit and seed production of Paeonia ostii “Feng Dan”, an economically important oil tree

Many factors may affect reproduction of animal-pollinated species. In this study, the effects of pollen limitation, attractive traits (flower number, plant height and flower width) and flowering phenological traits (flowering onset, duration and synchrony) on female reproduction, as well as the patterns of variation in fruit and seed production within plants, were investigated in Paeonia ostii “Feng Dan” over two flowering seasons (2018 and 2019). Fruit set was very high (90%), and pollen supplementation did not increase fruit and seed production in either year, indicating no pollen limitation. Fruit set, ovule number per fruit and mean individual seed weight per fruit were not affected by any of the six attractive and phenological traits in either year, whereas seed number per fruit was related to the three attractive traits in one or both years. Seed number per plant was positively affected by the three attractive traits and best explained by flower number in both years, but the effect of each of the three phenological traits on seed number per plant differed between years. Within plants, the fruit set, ovule number, seed set and seed number per fruit declined from early- to late-opening flowers, presumably because of resource preemption, but the mean individual seed weight did not vary across the flowering sequence. Our study shows that attractive traits of Paeonia ostii “Feng Dan” are more important than flowering phenological traits in the prediction of total seed production per plant.     

OVULE PACKAGING IN STOCHASTIC POLLINATION AND FERTILIZATION ENVIRONMENTS

The modular morphology of plants has important consequences for reproductive strategies. Ovules are packaged in discrete structures (flowers) that usually vary stochastically in pollen capture and ovule fertilization, because of the vagaries of pollen transfer by external agents. Different ovule packaging schemes may use limited reproductive resources more or less effectively, so that some number of ovules per flower may be optimal, given the prevailing probabilities of ovule fertilization. I derive a phenotypic model for ovule number per flower that maximizes the expected total ovule fertilizations on a plant when pollination and fertilization vary randomly among individual flowers. This model predicts that, except for small or inexpensive flowers, ovules should be “oversupplied” relative to the mean receipt of pollen tubes, so that pollen limitation of seed set should be common. Published data are congruent with this prediction. Additional hypotheses on the relation of ovule packaging to floral cost, plant size, and variance in pollen receipt are suggested by the model, but few data exist to evaluate these hypotheses.     

Sources of variation in pollinator contribution within a guild: the effects of plant and pollinator factors

Among plants visited by many pollinator species, the relative contribution of each pollinator to plant reproduction is determined by variation in both pollinator and plant traits. Here we evaluate how pollinator movement among plants, apparent pollen carryover, ovule number, resource limitation of seed set, and pollen output affect variation in contribution of individual pollinator species to seed set in Lithophragma parviflorum (Saxifragaceae), a species visited by a broad spectrum of visitors, including beeflies, bees and a moth species. A previous study demonstrated differences among visitor species in their single-visit pollination efficacy but did not evaluate how differences in visitation patterns and pollen carryover affect pollinator efficacy. Incorporation of differential visitation patterns and pollen carryover effects —commonly cited as potentially important in evaluating pollinator guilds — had minor effects (0–0.6% change) on the estimates of relative contribution based on visit frequency and single-visit efficacy alone. Beeflies visited significantly more flowers per inflorescence than the bees and the moth. Seed set remained virtually constant during the first three visited flowers for beeflies and larger bees, indicating that apparent pollen carryover did not reduce per-visit efficacy of these taxa. In contrast, Greya moth visits showed a decrease in seed set by 55.4% and the smaller bees by 45.4% from first to second flower. The larger carryover effects in smaller bees and Greya were diminished in importance by their small overall contribution to seed set. Three variable plant traits may affect seed set: ovule number, resource limitation on seed maturation, and pollen output. Ovule number per flower declined strongly with later position within inflorescences. Numbers were much higher in first-year greenhouse-grown plants than in field populations, and differences increased during 3 years of study. Mean pollen count by position varied 7-fold among flowers; it paralleled ovule number variation, resulting in a relatively stable pollen:ovule ratio. Resource limitation of seed set increased strongly with later flowering, with seed set in hand-pollinated flowers ranging from 66% in early flowers to 0% in the last two flowers of all plants. Variation in ovule number and resource limitation of seed maturation jointly had a strong effect on the number of seeds per flower. Visitation to early flowers had the potential to cause more seed set than visitation to later flowers. Overall, the most important sources of variation to seed production contribution were differences among pollinators in abundance and absolute efficacy (ovules fertilized on a single visit) and potentially differential phenology among visitor species. These effects are likely to vary among populations and years.     

Fertilization recovery after defective sperm cell release in Arabidopsis

In animal fertilization, multiple sperms typically arrive at an egg cell to "win the race" for fertilization. However, in flowering plants, only one of many pollen tubes, conveying plant sperm cells, usually arrives at each ovule that harbors an egg cell. Plant fertilization has thus been thought to depend on the fertility of a single pollen tube. Here we report a fertilization recovery phenomenon in flowering plants that actively rescues the failure of fertilization of the first mutant pollen tube by attracting a second, functional pollen tube. Wild-type (WT) ovules of Arabidopsis thaliana frequently (～80%) accepted two pollen tubes when entered by mutant pollen defective in gamete fertility. In typical flowering plants, two synergid cells on the side of the egg cell attract pollen tubes, one of which degenerates upon pollen tube discharge. By semi-in vitro live-cell imaging we observed that fertilization was rescued when the second synergid cell accepted a WT pollen tube. Our results suggest that flowering plants precisely control the number of pollen tubes that arrive at each ovule and employ a fertilization recovery mechanism to maximize the likelihood of successful seed set.     

Interspecific size and number variation in pollen grains and seeds

An interspecific correlation between pollen grain size and seed size is demonstrated by means of the phylogenetic regression, which allows for phylogenetic bias. The correlation was not explained by plant size, mass of DNA per cell, style length or breeding system, although the first three of these factors all correlated with both pollen size and seed size. Two interpretations, involving pollen competition and flower size, are discussed. There is also an interspecific correlation between pollen grain number per flower and ovule number per flower. Some consequences of these correlations for the interpretation of pollen-ovule ratios and sex allocation strategies are considered.     

The cytological origin of the polyads and their significance in the reproductive biology of Mimosa bimucronata

The mechanism of polyad formation in Mimosa bimucronata (DC.) Kuntze is described and its significance in the reproductive biology is discussed. In this species c . 500 complexes of two coupled pollen mother cells (PMCs) per pollen sac are formed after the last mitotic division of the sporogenous tissue. All the pollen grains of each complex remain attached and form a bitetrad. This mechanism leads to the formation of many polyads per pollen sac. On this basis, we propose that in Mimosa the number of pollen grains per polyad (eight or 16) is determined by the number of cells that remain attached after the last mitotic divisions of the sporogenous tissue. Concerning the biological significance of the polyads in M. bimucronata , we find that the number of pollen grains per polyad is correlated with the number of ovules per ovary, and that the internal dimensions of the pit-shaped stigma are sufficient just for one polyad to be localized in it. Based on the number of polyads per flower this species has a high pollen/ovule ratio characteristic of xenogamous plants, in opposition to that expected for plants with composite pollen dispersal units. In contrast, it has low pollination efficiency, low fruit set, a 1/1 relationship between the pollen grains of a polyad and the number of ovules per ovary, and high seed set per fruit. These facts support the hypothesis that the occurrence of polyads in M. bimucronata would be an adaptation helping to ensure high seed set following a single cross pollination event.  © 2004 The Linnean Society of London, Botanical Journal of the Linnean Society , 2004, 144 , 343–349.     

The structure of phenotypic variation in gender and floral traits within and among populations of Spergularia marina (Caryophyllaceae)

We sampled four wild populations of the highly autogamous Spergularia marina (Caryophyllaceae) in California to detect and to measure the magnitude of within- and among-population sources of phenotypic variation in gender and floral traits. From flowers and fruits collected from field and greenhouse-raised plants, we measured ovule number, seed number, mean seed mass, pollen production (greenhouse families only), mean pollen grain volume (greenhouse families only), anther number, anther/ovule ratio, pollen/ovule ratio (estimated using different flowers for pollen than for ovules; greenhouse families only), petal number, and petal size. Using greenhouse-raised genotypes, variation among maternal families nested within populations was evaluated for each trait to determine whether populations differ in the degree of maternally transmitted phenotypic variation. For each population, we used 15 greenhouse-raised maternal families to estimate the broad-sense heritability and genetic coefficient of variation of each floral trait. The magnitude and statistical significance of broad-sense heritability estimates were trait- and population-specific. Each population was characterized by a different combination of floral traits that expressed significant maternally transmitted (presumably genetic) variation under greenhouse conditions. Flowers representing two populations expressed low levels of maternally transmitted variation (three or fewer of nine measured traits exhibited significant maternal family effects on phenotype), while flowers representing the other two populations exhibited significant maternal family effects on phenotype for five or more traits. Our ability to detect statistically significant differences among the four populations depended upon the conditions under which plants were grown (field vs. greenhouse) and on the floral trait observed. Field-collected flowers exhibited significant differences among population means for all traits except anther number. Flowers sampled from greenhouse-raised maternal families differed among populations for all traits except ovule number, seed number, and petal size. We detected negligible evidence that genetic correlations constrain selection on floral traits in Spergularia marina.     

Factors limiting seed production of Taxus brevifolia (Taxaceae) in Western Oregon

Seed production of Pacific yew (Taxus brevifolia), an understory conifer, was studied at four sites in western Oregon over 2 yr. The effects of pollen supplementation, overstory canopy, and predator exclusion on ovule attrition were examined. Supplemental hand-pollination of ten trees at two sites resulted in significantly increased rates of ovule development and a doubling of seed efficiency (ratio of seeds to ovules). However, seed efficiency still averaged <15% on branches receiving supplemental pollen, so pollination was not a primary factor limiting seed production. The number of developing ovules was positively associated with overstory openness, but seed production was not. Seed efficiency was negatively associated with overstory openness. Branches bagged to exclude vertebrate seed predators had higher seed production than unbagged branches at three of four sites for 2 yr. In contrast to unbagged branches, seed production on bagged branches was positively associated with overstory openness, as was the effectiveness of bagging. Therefore, both vertebrate predation and overstory were important in limiting seed production, and these factors interacted. Factors limiting seed production varied in importance among the four sites and between years, illustrating the importance of examining multiple limiting factors over several sites and years.     

Self‐incompatibility and post‐fertilization maternal regulation cause low fecundity in Aegle marmelos (Rutaceae)

Outbreeding confers an evolutionary advantage, and flowering plants have evolved a variety of contrivances for its maximization. However, neither fruit set nor seed set is realized to its fullest potential for a variety of reasons. The causes of low flower to fruit and seed to ovule ratios were investigated in naturally occurring bael trees (Aegle marmelos) at two sites for three seasons. The study established that the mass effect of synchronized flowering attracted a variety of insect pollinators to the generalist flowers; Apis dorsata was the most efficient pollinator. The seed to ovule ratio was low despite high natural pollination efficiency (c. 2400 pollen per stigma). Although pollination‐induced structural and histochemical changes in the style allowed many (9.5 ± 2.1) pollen tubes to grow, only cross‐pollen tubes could grow through the style. Gametophytic self‐incompatibility, manifested in the stylar zone, resulted in a significantly slower growth rate of self‐pollen tubes. The occurrence of obligate self‐incompatibility, coupled with increased self‐pollen deposition (geitonogamy), caused a significant number of flowers to abort. Fruit retention in the trees declined from 40% to 12% as a result of abortion of fruits at different stages of development. The number of mature fruits on a tree was negatively correlated (r = ?0.82) with their size. It is inferred that low natural fecundity in A. marmelos is primarily a result of obligate self‐incompatibility and strong post‐fertilization maternal regulation of allocation of resources to the developing fruits. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2013, 172 , 572–585.     

Factors influencing inter-population variation in Ranunculus acris seed production in an alpine area of southwestern Norway

There have been few attempts to examine the relationship between mean seed production of populations and the environmental conditions experienced by these populations. This study uses redundancy, analysis (RDA) to explore which environmental factors influence the patterns of inter-population variation in mean seed production (seed:ovule ratio and seed weight) and mean plant height in 25 alpine populations of Ranunculus acris L at Finse, southwestern Norway. The mean seedrovule ratio and height of the plants differed more than three-fold among populations situated close to each other, whereas mean seed weight showed less variation. In two RDAs differences in site elevation and pH accounted for a substantial part of the among-population variation in mean plant height, seed:ovule ratio, and seed weight. Mean plant height and seed weight were negatively correlated with the altitude of the populations, whereas mean seed:ovule ratio showed a high positive correlation with pH. When pooling all data from the populations, simple linear regressions showed that seed weight was positively correlated with plant height, whereas plant height had little influence on seed:ovule ratio. Seed weight was positively correlated with the numbers of seeds that a flower produced. The results indicate that mean seediovule ratio of a population is influenced primarily by the pH-conditions at the site, whereas mean seed weight and plant height are more strongly influenced by altitude which largely reflects the climatic severity at the site. Inter-population variation in average reproductive success in R. acris at Finse appears to be determined by several environmental and intrinsic factors that interact in a complex fashion.     

Variation in ovule and seed size and associated size-number trade-offs in angiosperms

Unlike pollen and seed size, the extent and causes of variation in ovule size remain unexplored. Based on 45 angiosperm species, we assessed whether intra- and interspecific variation in ovule size is consistent with cost minimization during ovule production or allows maternal plants to dominate conflict with their seeds concerning resource investment. Despite considerable intraspecific variation in ovule volume (mean CV = 0.356), ovule production by few species was subject to a size-number trade-off. Among the sampled species, ovule volume varied two orders of magnitude, whereas seed volume varied four orders of magnitude. Ovule volume varied positively among species with flower mass and negatively with ovule number. Tenuinucellate ovules were generally larger that crassinucellate ovules, and species with apical placentation (which mostly have uniovulate ovaries) had smaller ovules than those with other placentation types. Seed volume varied positively among species with fruit mass and seed development time, but negatively with seed number. Seeds grew a median 93-fold larger than the ovules from which they originated. Our results provide equivocal evidence that selection minimizes ovule size to allow efficient resource allocation after fertilization, but stronger evidence that ovule size affords maternal plants an advantage in parent-offspring conflict.