OUTCROSSING AND FECUNDITY ADVANTAGE OF FEMALES IN GYNODIOECIOUS CHIONOGRAPHIS JAPONICA VAR. KUROHIMENSIS (LILIACEAE)

The selling rate of hermaphrodites, inbreeding depression, and relative fecundity of females compared to hermaphrodites were estimated for a gynodioecious population of Chionographis japonica var. kurohimensis to test the models for the evolution of gynodioecy. In spite of the high level of selling of hermaphrodites, significant amounts of inbreeding depression were found in seed germination and seedling growth. In addition, females produced more fertilized ovules than hermaphrodites. Nevertheless, the conditions theoretically required for maintaining gynodioecy were not satisfied even if the combined effect of these two factors was considered. Additional causes for the evolution of gynodioecy, including biparental inbreeding, are discussed.     

The maintenance of gynodioecy and androdioecy in angiosperms

Algebraic models of gynodioecy show that the effects on the equilibrium sex ratio of the relative survival and seed production of the sexes and of inbreeding of male-fertile plants are identical for all genic modes of inheritance, provided that different genotypes among male-fertile plants (or among females) do not differ in average fitness. The effects of three modes of inbreeding on equilibrium sex ratios are examined. If there is competition between self- and cross-fertilization of male-fertile individuals, a stable sexual dimorphism can be maintained by an outbreeding advantage of females if both the proportion of cross-fertilized seeds among those borne on male-fertile individuals,t, and the inbreeding depression (fitness inbred/outbred seeds),i, are less than one half. A lower frequency of females is obtained for the same values oft andi if self-fertilization precedes cross-fertilization. If self-fertilization follows cross-fertilization, gynodioecy cannot be maintained by an outbreeding advantage of females. When the sex phenotypes of gynodioecious populations are determined by cytoplasmic inheritance, females need only a slight advantage over males in survival, ovule production or outbreeding to persist at equilibrium. When determined by nuclear genes, androdioecy can be maintained by greater fecundity or a higher survival rate of males than of female-fertile plants, but not by an outbreeding advantage. Androdioecy cannot be maintained with cytoplasmic inheritance of sex. The models suggest explanations for the more frequent occurrence of gynodioecy than of andrdioecy, the high frequency of gynodioecy in Hawaii and New Zealand, and the origin of gynodioecy from hermaphrodite but not from monoecious ancestors.     

GENETIC CONSEQUENCES OF OUTCROSSING IN THE CLEISTOGAMOUS ANNUAL,IMPATIENS CAPENSIS. II. OUTCROSSING RATES AND GENOTYPIC CORRELATIONS

The genetic consequences of a plant's mating system depend on both the degree of outcrossing and the genetic relationship between mates. We examined the electrophoretic genotypes of seeds derived from cleistogamous (CL) and chasmogamous (CH) flowers in six populations of the facultatively cleistogamous annual, Impatiens capensis. Multilocus estimates of the outcrossing rates for the strongly protandrous CH flowers ranged from 0.29 to 0.71 and were higher than estimates based on single-locus data. Such results suggest that the CH flowers experience variable levels of both geitonogamous self-fertilization and biparental inbreeding. A new and generally applicable technique based on the relative level of inbreeding within progeny groups provided direct estimates of the correlation between the genotypic values of outcrossed mates. These correlations varied widely among populations and contributed up to half of the inbreeding observed among the CH progeny. Such biparental inbreeding biases estimates of the outcrossing rate based on the mixed-mating model downward and influences mating-system evolution by decreasing the “cost of meiosis.”     

MIXED MATING SYSTEMS IN HAWAIIAN BIDENS (ASTERACEAE)

Floral features related to the breeding system were studied for 11 species of Hawaiian Bidens. Protandry and male sterility promote outcrossing, while self-compatibility and geitonogamy contribute to inbreeding. The combination of these floral mechanisms results in a mixed mating system in all species studied. Outcrossing rates of 15 populations of these species ranged from 0.43 to 0.88, averaging 0.65. Apparent selling rates of females ranged from 0 to 0.25 in seven gynodioecious populations surveyed, suggesting that there is variation in the level of biparental inbreeding among populations. The presence of females increased the level of outcrossing by an average of 9% in gynodioecious populations. This study indicates that the efficiency of gynodioecy as an outcrossing mechanism largely depends on the current outcrossing rate of hermaphrodites, the frequency of females, and the extent of genetic substructuring in populations. On average, autogamy contributed 4%, geitonogamy contributed 24%, and consanguineous mating contributed 15% to the realized selfing rate (43%) in the hermaphrodites of these species.     

THE RELATIVE IMPORTANCE OF INBREEDING AND MATERNAL SEX IN DETERMINING PROGENY FITNESS IN SIDALCEA OREGANA SSP. SPICATA,A GYNODIOECIOUS PLANT

Gynodioecious plant populations contain both hermaphrodite and female individuals. For females to be maintained they must compensate for their loss of reproductive fitness through pollen. Females may achieve compensation by producing more and/or higher quality seeds than hermaphrodites. In this study, I investigated the independent and interactive effects of maternal sexual identity and inbreeding level on fitness of the progeny of hermaphrodites and females of Sidalcea oregana ssp.spicata. Seeds produced by selling hermaphrodites and by outcrossing or sib-crossing hermaphrodites and females, were planted in the field and greenhouse. Maternal-sex effects were substantial at the juvenile stages of the life cycle; seeds of females germinated in higher proportions and produced seedlings that grew significantly faster. Inbreeding effects were manifested primarily at the adult stage of the life cycle. Outcrossed plants were significantly larger and produced more flowers per plant than sib-crossed and selfed plants growing in the greenhouse. Progeny of hermaphrodites and females appeared to respond similarly to sib-matings. The maternal-sex effects observed in Sidalcea may have been related to cytoplasmically inherited factors and could be a driving force in the maintenance of females. Inbreeding depression could play a role in determining the fitness of both sex morphs, if females experience biparental inbreeding in the field. Frequent inbreeding of hermaphrodites may not be necessary to explain the maintenance of gynodioecy in this species.     

EXPERIMENTAL EVIDENCE FOR FREQUENCY DEPENDENT SELF-FERTILIZATION IN THE GYNODIOECIOUS PLANT, SILENE VULGARIS

After over a half century of empirical and theoretical research regarding the evolution and maintenance of gynodioecy in plants, unexplored factors influencing the relative fitnesses of females and hermaphrodites remain. Theoretical studies suggest that hermaphrodite self-fertilization (selfing) rate influences the maintenance of gynodioecy and we hypothesized that population sex ratio may influence hermaphrodite selfing rate. An experimental test for frequency-dependent self-fertilization was conducted using replicated populations constructed with different sex ratios of the gynodioecious plant Silene vulgaris . We found that hermaphrodite selfing increased with decreased hermaphrodite frequency, whereas evidence for increased inbreeding depression was equivocal. We argue that incorporation of context dependent inbreeding into future models of the evolution of gynodioecy is likely to yield novel insights into sex ratio evolution.     

INBREEDING DEPRESSION IN MORPHOLOGICAL AND PHYSIOLOGICAL TRAITS OF SCHIEDEA LYDGATEI (CARYOPHYLLACEAE) IN TWO ENVIRONMENTS

We compared inbreeding depression in hermaphroditic Schiedea lydgatei and its gynodioecious sister species, S. salicaria, to infer the level of inbreeding depression in their common ancestor. With measurements of selfing rates, this information can be used to assess the importance of inbreeding depression in the evolution of breeding systems in S. lydgatei and S. salicaria. Morphological and physiological characters related to fitness were compared for inbred and outcrossed S. lydgatei in high- and low-fertilizer environments in the greenhouse. Seed mass, number of seeds per capsule, germination, survival, biomass, number of flowers, and age at first flowering were compared for inbred versus outcrossed progeny. We also measured inbreeding depression in maximal rates of photosynthetic carbon assimilation and stomatal conductance to water vapor, traits that affect fitness through their influence on plant carbon balance and water-use efficiency (ratio of carbon gain to water loss). All traits except number of seeds per capsule in parents and survival showed inbreeding depression, with the magnitude depending on family and environment. High inbreeding depression is likely in the ancestor of S. lydgatei and S. salicaria, indicating that, with sufficiently high selfing rates, females could spread in populations. Hermaphroditism in S. lydgatei is probably favored by low selfing rates. In contrast, the evolution of gynodioecy in S. salicaria apparently has been favored by relatively high selfing rates in combination with high inbreeding depression.     

Mating system contributes only slightly to female maintenance in gynodioecious Geranium maculatum (Geraniaceae)

Gynodioecy, the co-occurrence of female and hermaphroditic individuals within a population, is an important intermediate in the evolution of separate sexes. The first step, female maintenance, requires females to have higher seed fitness compared with hermaphrodites. A common mechanism thought to increase relative female fitness is inbreeding depression avoidance, the magnitude of which depends on hermaphroditic selfing rates and the strength of inbreeding depression. Less well studied is the effect of biparental inbreeding on female fitness. Biparental inbreeding can affect relative female fitness only if its consequence or frequency differs between sexes, which could occur if sex structure and genetic structure both occur within populations. To determine whether inbreeding avoidance and/or biparental inbreeding can account for female persistence in Geranium maculatum, we measured selfing and biparental inbreeding rates in four populations and the spatial genetic structure in six populations. Selfing rates of hermaphrodites were low and did not differ significantly from zero in any population, leading to females gaining at most a 1–14% increase in seed fitness from inbreeding avoidance. Additionally, although significant spatial genetic structure was found in all populations, biparental inbreeding rates were low and only differed between sexes in one population, thereby having little influence on female fitness. A review of the literature revealed few sexual differences in biparental inbreeding among other gynodioecious species. Our results show that mating system differences may not fully account for female maintenance in this species, suggesting other mechanisms may be involved.     

Inbreeding effect on male and female fertility and inheritance of male sterility in Nemophila menziesii (Hydrophyllaceae)

Models of the evolution of gynodioecy assume that inbreeding affects male and female fertility equally and ignore quantitative variation in sex expression. The objectives of this study were to assess inbreeding effects, genetic background, and plant maturity on male and female fertility and the mechanism of male sterility inheritance for Nemophila menziesii (Hydrophyllaceae). Frequency of male-sterile flowers, number of anthers and ovules, and percentage of viable pollen were measured on plants from different pedigrees and five inbreeding levels (F = 0, 0.0625, 0.25, 0.5, and 0.75). Quantitative variation in male sterility was evident. As inbreeding increased, anther and ovule number decreased; the effect on anther number was greater than on ovule number. Pedigrees varied in number of male-sterile flowers and inbreeding effects. Frequency of male-sterile flowers was greatest among first flowers. No trade-off between male and female fertility was detected. A model attributing male sterility to a cytoplasmic locus and restoration to male fertility to a nuclear locus accounted for the distribution of complete sterility and hermaphroditism over the pedigrees. This study suggests that models of the evolution and maintenance of gynodioecy should allow for quantitative variation in male and female fertility components due to inbreeding, pedigree, and plant maturity.     

Functional characterization of gynodioecy in Fragaria vesca ssp. bracteata (Rosaceae)

Background and Aims

Gynodioecy is a phylogenetically widespread and important sexual system where females coexist with hermaphrodites. Because dioecy can arise from gynodioecy, characterization of gynodioecy in close relatives of dioecious and sub-dioecious species can provide insight into this transition. Thus, we sought to determine whether Fragaria vesca ssp. bracteata, a close relative to F. chiloensis and F. virginiana, exhibits the functional and population genetic hallmarks of a gynodioecious species.

Methods

We compared reproductive allocation of females and hermaphrodites grown in the greenhouse and estimated genetic diversity (allelic diversity, heterozygosity) and inbreeding coefficients for field-collected adults of both sexes using simple sequence repeat (SSR) markers. We estimated mating system and early seed fitness from open-pollinated families of both sex morphs.

Key Results

Under greenhouse conditions, females and hermaphrodites allocated similarly to all reproductive traits except flower number, and, as a consequence, females produced 30 % fewer seeds per plant than hermaphrodites. Under natural conditions, hermaphrodites produce seeds by self-fertilization approx. 75 % of the time, and females produced outcrossed seeds with very little biparental inbreeding. Consistent with inbreeding depression, seeds from open-pollinated hermaphrodites were less likely to germinate than those from females, and family-level estimates of hermaphrodite selfing rates were negatively correlated with germination success and speed. Furthermore, estimates of inbreeding depression based on genetic markers and population genetic theory indicate that inbreeding depression in the field could be high.

Conclusions

The joint consideration of allocation and mating system suggests that compensation may be sufficient to maintain females given the current understanding of sex determination. Fragaria vesca ssp. bracteata exhibited similar sex morph-dependent patterns of mating system and genetic diversity, but less reproductive trait dimorphism, than its sub-dioecious and dioecious congeners.     

THE RELATIONSHIP BETWEEN MATING-SYSTEM CHARACTERS AND INBREEDING DEPRESSION IN MIMULUS GUTTATUS

Several recent theoretical considerations of mating-system evolution predict within-population covariation between levels of inbreeding depression and genetically controlled mating-system characters. If inbreeding depression is caused by deleterious recessive alleles, families with characters that promote self-pollination should show lower levels of inbreeding depression than families with characters that promote outcrossing. The converse is expected if inbreeding depression is due to overdominant allelic interactions. Whether these associations between mating-system and viability loci evolve will have important consequences for mating-system evolution. The evolution of selfing within the genus Mimulus is associated with a reduction in stigma-anther separation (i.e., a loss of herkogamy) and high autogamous seed set. In this study we compared families from two M. guttatus populations that differed genetically in their degree of stigma-anther separation. In one of these populations we also compared families that differed genetically in the degree to which they autogamously set seed in a pollinator-free greenhouse. Dams often differed significantly in levels of inbreeding depression for aboveground biomass and flower production, but variation in inbreeding depression was never explained by herkogamy class or autogamy class. Several factors might account for why families with traits associated with selfing did not show lower inbreeding depression, and these are discussed. Our study also demonstrated significant variation among self progeny from a given female likely due to differences in pollination date and position of fruit maturation. The detection of significant dam × sire interactions suggests biparental inbreeding or differences in combining ability for specific pairs of parents.     

HOW SELFING,INBREEDING DEPRESSION,AND POLLEN LIMITATION IMPACT NUCLEAR‐CYTOPLASMIC GYNODIOECY: A MODEL

Gynodioecy, the co‐occurrence of females and hermaphrodites, is often due to conflicting interactions between cytoplasmic male sterility genes and nuclear restorers. Although gynodioecy often occurs in self‐compatible species, the effect of self‐pollination, inbreeding depression, and pollen limitation acting differently on females and hermaphrodites remains poorly known in the case of nuclear‐cytoplasmic gynodioecy (NCG). In this study, we model NCG in an infinite population and we study the effect of selfing rate, inbreeding depression, and pollen limitation on the maintenance of gynodioecy and on sex ratios at equilibrium. We found that selfing and inbreeding depression have a strong impact, which depends on whether restorer cost acts on male or female fitness. When cost affects male fitness, the strength of cost has no effect, whereas selfing and inbreeding depression only impact gynodioecy by modifying the value of the female advantage. When cost affects female fitness, selfing facilitates NCG and reduces the role of strength of the cost, even when no inbreeding depression occurs, whereas inbreeding depression globally restricts the maintenance of the polymorphism. Finally, we found that pollen limitation could additionally strongly modify the dynamic of gynodioecy. We discuss our findings in the light of empirical data available in gynodioecious species.     

Incest versus abstinence: reproductive trade‐offs between mate limitation and progeny fitness in a self‐incompatible invasive plant

Plant mating systems represent an evolutionary and ecological trade‐off between reproductive assurance through selfing and maximizing progeny fitness through outbreeding. However, many plants with sporophytic self‐incompatibility systems exhibit dominance interactions at the S‐locus that allow biparental inbreeding, thereby facilitating mating between individuals that share alleles at the S‐locus. We investigated this trade‐off by estimating mate availability and biparental inbreeding depression in wild radish from five different populations across Australia. We found dominance interactions among S‐alleles increased mate availability relative to estimates based on individuals that did not share S‐alleles. Twelve of the sixteen fitness variables were significantly reduced by inbreeding. For all the three life‐history phases evaluated, self‐fertilized offspring suffered a greater than 50% reduction in fitness, while full‐sib and half‐sib offspring suffered a less than 50% reduction in fitness. Theory indicates that fitness costs greater than 50% can result in an evolutionary trajectory toward a stable state of self‐incompatibility (SI). This study suggests that dominance interactions at the S‐locus provide a possible third stable state between SI and SC where biparental inbreeding increases mate availability with relatively minor fitness costs. This strategy allows weeds to establish in new environments while maintaining a functional SI system.     

Recent advances in the study of gynodioecy: the interface of theory and empiricism

Background

In this review we report on recent literature concerned with studies of gynodioecy, or the co-occurrence of female and hermaphrodite individuals in natural plant populations. Rather than review this literature in its entirety, our focus is on the interplay between theoretical and empirical approaches to the study of gynodioecy.

Scope

Five areas of active inquiry are considered. These are the cost of restoration, the influence of population structure on spatial sex-ratio variation, the influence of inbreeding on sex expression, the signature of cyto-nuclear coevolution on the mitochondrial genome, and the consequences of mitochondrial paternal leakage.

Conclusions

Recent advances in the study of gynodioecy have been made by considering both the ecology of female:hermaphrodite fitness differences and the genetics of sex expression. Indeed theory has guided empiricism and empiricism has guided theory. Future advances will require that some of the methods currently available only for model organisms be applied to a wider range of species.Key words: Breeding system, gynodioecy, cytoplasmic male sterility, restoration, sex ratio, inbreeding, population structure, genetic conflict     

BIPARENTAL INBREEDING DEPRESSION IN THE SELF-INCOMPATIBLE ANNUAL PLANT GAILLARDIA PULCHELLA (ASTERACEAE)

Biparental inbreeding is thought to be a common feature of plant populations with restricted pollen dispersal. It is generally assumed that the inbreeding depression frequently observed to accompany self-fertilization can be extrapolated to the lesser degrees of consanguinity involved in biparental inbreeding, but this is virtually untested. To test this assumption, seeds collected from a single natural population of the self-incompatible annual Gaillardia pulchella were used to generate full-sib families derived by crossing either noninbred full-sibs (inbred families) or noninbred nonrelatives (outbred families). Members of each family were divided between high-stress and low-stress treatments that differed in soil volume and nutrient level. Inbred seedlings had a lower chance of survival, were more likely to be morphologically abnormal, and grew more slowly than outbred seedlings, indicating the presence of biparental inbreeding depression. Stress treatment had no significant effect on inbreeding depression, and no family stress-environment interactions were detected. Inbreeding did not increase the among-family variance in growth rate, suggesting that inbreeding depression of growth rate is caused by many genes with small individual effects. Relative to direct estimates of inbreeding depression, observed levels of near-neighbor outcrossing depression, presumed to be biparental inbreeding depression, are surprisingly high in many plant species.     

INBREEDING DEPRESSION IN GYNODIOECIOUS LOBELIA SIPHILITICA: AMONG-FAMILY DIFFERENCES OVERRIDE BETWEEN-MORPH DIFFERENCES

If inbreeding depression is caused by deleterious recessive alleles, as suggested by the partial dominance hypothesis, a negative correlation between inbreeding and inbreeding depression is predicted. This hypothesis has been tested several times by comparisons of closely related species or comparisons of populations of the same species with different histories of inbreeding. However, if one is interested in whether this relationship contributes to mating-system evolution, which occurs within populations, comparisons among families within a population are needed; that is, inbreeding depression among individuals with genetically based differences in their rate of selfing should be compared. In gynodioecious species with self-compatible hermaphrodites, hermaphrodites will have a greater history of potential inbreeding via both selfing and biparental inbreeding as compared to females and may therefore express a lower level of inbreeding depression. We estimated the inbreeding depression of female and hermaphrodite lineages in gynodioecious Lobelia siphilitica in a greenhouse experiment by comparing the performance of selfed and outcrossed progeny, as well as sibling crosses and crosses among subpopulations. We did not find support for lower inbreeding depression in hermaphrodite lineages. Multiplicative inbreeding depression (based on seed germination, juvenile survival, survival to flowering, and flower production in the first growing season) was not significantly different between hermaphrodite lineages (δ = 0.30 ± 0.08) and female lineages (δ = 0.15 ± 0.18), although the trend was for higher inbreeding depression in the hermaphrodite lineages. The population-level estimate of inbreeding depression was relatively low for a gynodioecious species (δ = 0.25) and there was no significant inbreeding depression following biparental inbreeding (δ = 0.01). All measured traits showed significant variation among families, and there was a significant interaction between family and pollination treatment for four traits (germination date, date of first flowering, number of flowers, and aboveground biomass). Our results suggest that the families responded differently to selfing and outcrossing: Some families exhibited lower fitness following selfing whereas others seemed to benefit from selfing as compared to outcrossing. Our results support recent simulation results in that prior inbreeding of the lineages did not determine the level of inbreeding depression. These results also emphasize the importance of determining family-level estimates of inbreeding depression, relative to population-level estimates, for studies of mating-system evolution.     

INBREEDING AND THE COST OF MEIOSIS: THE EVOLUTION OF SELFING IN POPULATIONS PRACTICING BIPARENTAL INBREEDING

The effect of biparental inbreeding on the conditions governing the evolution of selfing is examined using recursions in mating-type frequencies. Sibmating in combination with random outcrossing influences two key determinants of the adaptive value of selfing: 1) the meiotic cost of biparental reproduction and 2) the level of inbreeding depression due to deleterious mutations. Biparental inbreeding serves to maintain biparental reproduction by increasing relatedness between parents and their biparentally derived offspring and introduces the possibility of an optimal mating system that incorporates both modes of reproduction. Biparental inbreeding serves to promote uniparental reproduction by reducing the relative inbreeding depression suffered by uniparental offspring. The net effect of these two antagonistic trends depends upon the extent to which mutational load accounts for differences in the numbers of the two types of offspring. A brief summary of the empirical literature suggests that: 1) biparental inbreeding may occur in populations exhibiting mixed mating systems; 2) while inbreeding depression represents an important factor, it does not account entirely for differences in offspring number between the two modes of reproduction.     

Experimental analysis of biparental inbreeding in a self-fertilizing plant

Abstract.— Localized dispersal and mating may genetically structure plant populations, resulting in matings among related individuals. This biparental inbreeding has significant consequences for the evolution of mating systems, yet is difficult to estimate in natural populations. We estimated biparental inbreeding in two populations of the largely self-fertilizing plant Aquilegia canadensis using standard inference as well as a novel experiment comparing apparent selfing between plants that were randomly relocated within populations to experimental control plants. Using two allozyme markers, biparental inbreeding ( b ) inferred from the difference between single-locus and multilocus estimates of selfing ( b = s_s – s_m ) was low. Less than 3% of matings involved close relatives (mean b = 0.029). In contrast, randomly relocating plants greatly reduced apparent selfing (mean s_s = 0.674) compared to control plants that had been dug up and replanted in their original locations ( s_s = 0.953, P = 0.002). Based on this difference in s_s , we estimated that approximately 30% of all matings involved close relatives (mean b = 0.279, 95% CL = 0.072–0.428). Inference from s_s – s_m underestimated b in these populations by more than an order of magnitude. Biparental inbreeding is thought to influence the evolution of self-fertilization primarily through reducing the genetic cost of outcrossing. This is unlikely to be of much significance in A. canadensis because inbreeding depression (a major cost of selfing) is much stronger than the cost of outcrossing. However, biparental inbreeding combined with strong inbreeding depression may influence selection on dispersal.     

BIPARENTAL INBREEDING AND INTERREMNANT MATING IN A PERENNIAL PRAIRIE PLANT: FITNESS CONSEQUENCES FOR PROGENY IN THEIR FIRST EIGHT YEARS

Despite fundamental importance to population dynamics, mating system evolution, and conservation management, the fitness consequences of breeding patterns in natural settings are rarely directly and rigorously evaluated. We experimentally crossed Echinacea angustifolia, a widespread, perennial prairie plant undergoing radical changes in distribution and abundance due to habitat fragmentation. We quantified the effects of both biparental inbreeding and crossing between remnant populations on progeny survival and reproduction in the field over the first eight years. Lifetime fitness is notoriously difficult to assess particularly for iteroparous species because of the long sequence and episodic nature of selection events. Even with fitness data in hand, analysis is typically plagued by nonnormal distributions of overall fitness that violate the assumptions of the usual parametric statistical approaches. We applied aster modeling, which integrates the measurements of separate, sequential, nonnormally distributed annual fitness components, and estimated current biparental inbreeding depression at 68% in progeny of sibling‐mating. The effect of between‐remnant crossing on fitness was negligible. Given that relatedness among individuals in remnant populations is already high and dispersal very limited, inbreeding depression may profoundly affect future dynamics and persistence of these populations, as well as their genetic composition.     

Modes of self-fertilization in Mimulus guttatus (Scrophulariaceae): a field experiment

We inferred Lloyd's modes of selfing in a natural population of the common monkeyflower, Mimulus guttatus. Estimates were obtained using floral manipulations combined with seed counts and isozyme analyses of selfing rates. Of the 25% selfing estimated from isozyme markers, about one-half was competing, about one-third was geitonogamous, and at least one-fifth (perhaps twice this) was due to biparental inbreeding. Estimates of prior and delayed selfing were small and did not significantly differ from zero. These results were obtained using plants with the characteristic pair of open flowers at an inflorescence node. The second-opening flower showed twice the rate of selfing, presumably because of protogynous-based geitonogamy differences. Solitary-flowered plants, which have smaller flowers but no geitonogamy, showed about 50% selfing, consisting of about equal components of competing selfing and biparental inbreeding. While geitonogamy and biparental inbreeding might be unavoidable by-products of adaptations for outcrossing, competing selfing is subject to more direct natural selection and warrants adaptive explanations.