ABSENCE OF POLLEN DISCOUNTING IN A GENOTYPE OF IPOMOEA PURPUREA EXHIBITING INCREASED SELFING

Throughout southeastern North America, the annual morning glory Ipomoea purpurea exhibits a polymorphism at a locus that influences the intensity of floral pigmentation. Previous studies have shown that when rare, the homozygous white genotype has a greater selfing rate than the homozygous dark genotype. In the absence of pollen discounting (a reduction in transmission of pollen to other plants by genotypes that exhibit increased selfing) and inbreeding depression, this increased selfing rate should favor the white allele. Experiments reported here confirm that the white genotype has elevated selfing rates when rare but indicate pollen discounting is not associated with elevated selfing. Rather, white genotypes contribute more pollen to the outcross pollen pool. The disparity between genotypes in both selfing rates and success at pollen contribution to other plants disappears at intermediate to high frequencies of the white allele. Pollinator movements are consistent with the pattern of selfing. These results suggest that elevated selfing and enhanced success at pollen donation contribute to maintenance of the white allele in natural populations of morning glories.     

COVARIATION OF SELFING RATES WITH PARENTAL GENE FIXATION INDICES WITHIN POPULATIONS OF MIMULUS GUTTATUS

Wright's gene fixation index F and two single-locus effective selfing rates—the selfing rate at loci with fixed alleles, and the selfing rate at loci without fixed alleles—were estimated in five populations of Mimulus guttatus. These two effective selfing rates describe the inbreeding observed at a single locus when both uniparental and biparental inbreeding are practiced. Estimates were made using progeny arrays assayed for six allozyme loci and two morphological loci exhibiting dominance. The average of the two selfing rates computed for subpopulations (ca. 10 m diameter) ranged from 24% to 59%, with a mean of 37%. When computed for populations (ca. 1 km diameter), average selfing rates were about 10% higher. In four populations, the selfing rate at loci with fixed alleles was higher than the selfing rate at loci without fixed alleles. Thus, the covariance of selfing with parental gene fixation was positive. In one of the populations, estimates for individual plants sampled along a transect gave positive correlations for selfing rates and for gene-fixation indices between adjacent plants. A highly positive correlation between selfing rate and gene fixation of individual plants was also observed. In another population, the covariance of selfing with gene fixation was higher for a locus causing leaf spots than for allozyme loci. This covariance is partially caused by 1) variation in homozygosity among neighborhoods and 2) biparental inbreeding within neighborhoods. The consequences of this covariance are discussed.     

Effects of variation at the flower-colour A locus on mating system parameters in Ipomoea purpurea

Although alleles at both the W and A loci in the common morning glory, Ipomoea purpurea, produce similar white-flowered phenotypes, these alleles differ by over an order of magnitude in average frequency. In this initial attempt to determine the causes of this difference, we employed artificial arrays of plants to estimate mating system characteristics (total siring success, selfing rates and contribution to the outcross pollen pool) for the homozygous pigmented and white-flowered genotypes at the A locus. This experiment demonstrates that: (1) at both low and high frequencies, white-flowered plants were visited by pollinators at the same rate as plants with pigmented flowers; (2) at both frequencies, the a allele exhibited a greater total siring success (self and outcross pollen) than the A allele; (3) individuals of both genotypes contributed equally to the outcross pollen pool; and (4) aa plants may have a higher selfing rate than AA plants. Coupled with minimal inbreeding depression in I. purpurea, these observations indicate that the allele producing white flowers enjoys a transmission advantage that would tend to cause this allele to increase in frequency. This transmission advantage is very similar to that shown previously to be operating on the white-flowered allele at the W locus, although the specific causes of the advantage appear to differ between loci. The frequency difference between the two alleles is thus not likely to be due to differences in the effect of flower-colour variation on transmission. Rather, substantially greater deleterious pleiotropic effects associated with the white-flower a allele is likely to be the primary cause of the frequency difference.     

TRANSMISSION ADVANTAGE FAVORS SELFING ALLELE IN EXPERIMENTAL POPULATIONS OF SELF‐INCOMPATIBLE WITHERINGIA SOLANACEA (SOLANACEAE)

The evolution of self‐fertilization is one of the most commonly traversed transitions in flowering plants, with profound implications for population genetic structure and evolutionary potential. We investigated factors influencing this transition using Witheringia solanacea, a predominantly self‐incompatible (SI) species within which self‐compatible (SC) genotypes have been identified. We showed that self‐compatibility in this species segregates with variation at the S‐locus as inherited by plants in F1 and F2 generations. To examine reproductive assurance and the transmission advantage of selfing, we placed SC and SI genotypes in genetically replicated gardens and monitored male and female reproductive success, as well as selfing rates of SC plants. Self‐compatibility did not lead to increased fruit or seed set, even under conditions of pollinator scarcity, and the realized selfing rate of SC plants was less than 10%. SC plants had higher fruit abortion rates, consistent with previous evidence showing strong inbreeding depression at the embryonic stage. Although the selfing allele did not provide reproductive assurance under observed conditions, it also did not cause pollen discounting, so the transmission advantage of selfing should promote its spread. Given observed numbers of S‐alleles and selfing rates, self‐compatibility should spread even under conditions of exceedingly high initial inbreeding depression.     

PATTERNS OF MATING IN WILD SUNFLOWER HYBRID ZONES

Theory predicts that homoploid hybrid speciation will be facilitated by selfing, yet most well-documented hybrid species are outcrossers. One possible explanation for this puzzle is that conditions in hybrid populations may favor selfing, even in otherwise outcrossing species. For example, in self-incompatible plants, mixtures of self and interspecific pollen often induce selfing. Here, we examine patterns of mating in three hybrid zones and four “pure” populations of Helianthus annuus and H. petiolaris, wild, self-incompatible sunflower species that are thought to have parented three homoploid hybrid species. Fourteen to 16 maternal families from each pure population and 44–46 maternal families from each hybrid zone were analyzed for seven polymorphic isozyme loci. Maximum-likelihood (ML) methods were used to estimate multilocus outcrossing rates (T_m) and hybridization frequencies for each maternal family, each phenotypic group within each hybrid zone (annuus-like, hybrid, and petiolaris-like), and each population. As predicted for self-incompatible species, all four parental populations have outcrossing rate ML estimates of 1.0. Within the hybrid zones, outcrossing rates were lowest in the H. annuus–like fraction of the population (0.73, 0.72, and 0.74 in the three hybrid zones, respectively), largely intermediate in the H. petiolaris–like group (0.94, 0.90, and 0.94), and highest in the hybrid group (0.97, 0.93, and 0.97). Although outcrossing rates are lower in hybrid zones than in pure populations, it is unlikely that the observed decrease facilitates hybrid speciation because outcrossing rates in the critical hybrid fraction of the population do not differ significantly from 1.0. Dividing the outcrossed pollen pool into intraspecific and interspecific components revealed that maternal plants are largely fertilized by conspecific pollen, confirming an important role for pollen competition as a reproductive barrier. Highly sterile hybrid plants do not appear to discriminate between parental species pollen, but hybrids with higher fertility tend to be fertilized by pollen from the parental group they resemble genetically. Thus, gametic selection leads to substantial assortative mating in these hybrid zones.     

Linkage between an incompatibility locus and a peroxidase isozyme locus (Prx 7) in rye

Summary Under controlled growth chamber conditions of 30 °C, seed set after selfing is possible in normally self-incompatible rye plants. Within selfed progenies produced by this method, plants homozygous at the peroxidase isozyme locus Prx 7 were crossed to heterozygous individuals. Segregation at the Prx 7 locus in progenies of these crosses provides clear evidence of a close linkage between Prx 7 and one of the two incompatibility loci in rye. A recombination fraction in the range of 0–2% was calculated from the segregation data. In rye, Prx 7 is linked with a phosphoglucoisomerase locus (Pgi). The similarity between the observations in Secale cereale and those made in Lolium perenne is discussed.     

Mating patterns and spatial distribution of conspecific neighbours in the Mediterranean shrub <Emphasis Type="Italic">Myrtus communis</Emphasis> (Myrtaceae)

Mate abundance is one of the most important sources of variation in plant mating systems. We examined within-population heterogeneity in the pollen pool at two spatial scales (sites and plants), and investigated the mating pattern variation in Myrtus communis under diverse situations of conspecific neighbourhood, using allozyme electrophoresis of naturally pollinated progeny arrays. For mating analyses, plants sampled were classified into four neighbourhood groups (from high to low) based on the local density around them and the distance to their nearest neighbour. The pollen pool was much more genetically heterogeneous among mother plants (~21%) than among sites (~2%), probably because of the high levels of selfing found (average s = 0.65). Outcrossing rates differed significantly among neighbourhood groups and showed a marked trend towards higher values from the lowest (t _m = 0.26) to the highest (t _m = 0.45) degree of conspecific aggregation. However, the lowest levels of biparental inbreeding and correlated paternity were found in the most isolated group of plants, indicating that these plants crossed with more and less genetically related fathers. Our study provides a clear demonstration of positive correlation between conspecific aggregation and the outcrossing rates. We discuss the ecological implications of these results in the context of Mediterranean ecosystems.     

A genetic map of the Nicotiana alata S locus that includes three pollen-expressed genes

The S locus of solanaceous plants includes separate genes that control the self-incompatibility phenotype of the pistil and of the pollen. The gene controlling the self-incompatibility phenotype of the pistil encodes an extracellular ribonuclease, the S-RNase. The gene(s) controlling the self-incompatibility phenotype of pollen (the pollen-S gene) has yet to be identified. As part of a long-term strategy to clone the pollen-S gene by chromosome walking, a detailed map of the region near the S locus of Nicotiana alata was generated using a total of 251 F₂ plants. The map spans an interval of approximately 2.6 cM and contains five markers as well as the S-RNase gene. Two markers were detected with heterologous probes that also detect sequences linked to the S locus of Solanum tuberosum and the homologous region of the Lycopersicon genome. Three markers were identified by differential display using N. alata pollen RNA as a template. One of these markers is a pollen-expressed sequence, 48A, which detects a polymorphic marker no more than 0.5 cM from the S locus. RNA blot analysis indicates that the 48A gene is expressed primarily during pollen development after the completion of meiosis and is therefore a candidate for the pollen-S gene. The utility of these markers and the possible involvement of 48A in the molecular mechanism of self- incompatibility are discussed. Received: 28 June 1999 / Accepted: 24 September 1999     

Estimation of outcrossing rates at small-scale flowering sites of the dwarf bamboo species, <Emphasis Type="Italic">Sasa cernua</Emphasis>

We estimated the outcrossing rates at small-scale flowering sites of an endemic dwarf bamboo species, Sasa cernua. The multi-locus estimation of the outcrossing rate of the dwarf bamboo population was 0.148 (SD 0.118). Two culms with the highest outcrossing rates had heterozygous genotypes at one locus, but other culms in the locus were homozygotes. Five culms with high outcrossing rates bore 2–17% seeds with homozygous genotypes. Due to predominant selfing, the overall inbreeding coefficient of seeds was high, although it declined in seedlings. This suggests that selection against inbred progenies began early in the establishment process in the natural habitat.     

INDIVIDUAL VARIATION IN THE VIGOR OF SELF POLLEN AND SELFED PROGENY IN HIBISCUS MOSCHEUTOS (MALVACEAE)

In some self-compatible species, self pollen tubes grow more slowly than outcross pollen, presumably leading to low selfing rates when mixtures of self and outcross pollen reach the stigma simultaneously. Here we show that the competitive ability of self pollen differed among individuals of Hibiscus moscheutos. Self pollen tubes grew slower than outcross pollen in three plants, faster than outcross pollen in four plants, and showed no difference in five other plants (based on rates of callose plug formation). Levels of inbreeding depression were examined by comparing progeny from self and outcross pollinations in seven maternal families. Self pollination led to reduced seed number in only one maternal family, and a slight decrease in seed size was seen in two maternal families. Considerable inbreeding depression occurred later in the life cycle, and the degree of inbreeding depression varied among maternal families of 6-week-old plants. Our results demonstrate the potential for unpredictable effects of pollen competition on individual selfing rates, which in turn may affect progeny vigor. This complex situation contrasts with previous reports of species in which outcross pollen consistently outcompetes self pollen (cryptic self-incompatibility).     

Relatively weak inbreeding depression in selfing but also in outcrossing populations of North American Arabidopsis lyrata

Hermaphroditic plants can potentially self‐fertilize, but most possess adaptations that promote outcrossing. However, evolutionary transitions to higher selfing rates are frequent. Selfing comes with a transmission advantage over outcrossing, but self‐progeny may suffer from inbreeding depression, which forms the main barrier to the evolution of higher selfing rates. Here, we assessed inbreeding depression in the North American herb Arabidopsis lyrata, which is normally self‐incompatible, with a low frequency of self‐compatible plants. However, a few populations have become fixed for self‐compatibility and have high selfing rates. Under greenhouse conditions, we estimated mean inbreeding depression per seed (based on cumulative vegetative performance calculated as the product of germination, survival and aboveground biomass) to be 0.34 for six outcrossing populations, and 0.26 for five selfing populations. Exposing plants to drought and inducing defences with jasmonic acid did not magnify these estimates. For outcrossing populations, however, inbreeding depression per seed may underestimate true levels of inbreeding depression, because self‐incompatible plants showed strong reductions in seed set after (enforced) selfing. Inbreeding‐depression estimates incorporating seed set averaged 0.63 for outcrossing populations (compared to 0.30 for selfing populations). However, this is likely an overestimate because exposing plants to 5% CO₂ to circumvent self‐incompatibility to produce selfed seed might leave residual effects of self‐incompatibility that contribute to reduced seed set. Nevertheless, our estimates of inbreeding depression were clearly lower than previous estimates based on the same performance traits in outcrossing European populations of A. lyrata, which may help explain why selfing could evolve in North American A. lyrata.     

Transgene copy number can be positively or negatively associated with transgene expression

Two different types of T-DNA insert were found in tobacco plants transformed with Agrobacterium tumefaciens. High-expressing (H) types had one copy of the T-DNA at a locus and produced high expression of the transgene uidA, as measured by uidA RNA levels and -glucuronidase activity; low-expressing (L) types had inverted repeats of the T-DNA at a locus and produced low uidA expression. H-types from different transformants acted additively, and cross-fertilization between two different homozygous transformants with H-type inserts produced F₁ plants with GUS activity that equalled the parents and individual F₂ plants with 50%, 100%, 150% and 200% of parental values. However, the L-type inserts worked in trans to suppress uidA expression from H-type inserts when both were present in the same genome. Hence when a transformant homozygous for the L-type insert was crossed to one homozygous for the H-type, all plants in the F₁ and F₂ generations with both types of insert had low GUS activity while F₂ segregants that only had the H-type inserts had high GUS activity again. Suppression of the H-type gene was associated with increased methylation of the insert. Particle acceleration was used to introduce further copies of uidA into tissues of the transformants. Regardless of the promoter used, those plants with endogenous L-type inserts showed none of the distinct loci of GUS activity readily visible in material with no inserts, showing that L-type inserts could suppress not only the uidA expression of genomic homologues, but also of copies added in vitro.     

EVALUATING A SIMPLE APPROXIMATION TO MODELING THE JOINT EVOLUTION OF SELF‐FERTILIZATION AND INBREEDING DEPRESSION

A comprehensive understanding of plant mating system evolution requires detailed genetic models for both the mating system and inbreeding depression, which are often intractable. A simple approximation assuming that the mating system evolves by small infrequent mutational steps has been proposed. We examine its accuracy by comparing the evolutionarily stable selfing rates it predicts to those obtained from an explicit genetic model of the selfing rate, when inbreeding depression is caused by partly recessive deleterious mutations at many loci. Both models also include pollen limitation and pollen discounting. The approximation produces reasonably accurate predictions with a low or moderate genomic mutation rate to deleterious alleles, on the order of U = 0.02–0.2. However, for high mutation rates, the predictions of the full genetic model differ substantially from those of the approximation, especially with nearly recessive lethal alleles. This occurs because when a modifier allele affecting the selfing rate is rare, homozygous modifiers are produced mainly by selfing, which enhances the opportunity for purging nearly recessive lethals and increases the marginal fitness of the allele modifying the selfing rate. Our results confirm that explicit genetic models of selfing rate and inbreeding depression are required to understand mating system evolution.     

A cDNA encoding an S-locus specific glycoprotein from Brassica oleracea plants containing the S5 self-incompatibility allele

Summary A cDNA sequence homologous to the Brassica self-incompatibility locus specific glycoprotein (SLSG) sequence was isolated from stigmas of B. oleracea plants homozygous for the S₅ allele. The nucleotide sequence of this cDNA was obtained and compared with the S₆ allelic form of the SLSG. Evidence is presented which indicates that this sequence does not specify the self-incompatibility response of pollen.Abbreviations SDS sodium dodecyl sulphate - PVP polyvinylpyrrolidone - BSA bovine serum albumin - SLSG self-incompatibility locus specific glycoprotein     

Selection caused by self-fertilization I. Four measures of self-fertilization and their effects on fitness

The standard models of selfing in seed plants consider only the ovules, which are assumed to have a constant selfing rate. It has recently become clear, however, that hermaphrodite or monoecious populations frequently show sexual asymmetry (nonconstant pollen:ovule fertilities among individuals). Such asymmetry usually results in pollen selfing rates which differ from those for the ovules and are frequency-dependent even for constant ovule selfing rates. A recent study of selfing rates for all gametes of an individual is extended here to include four selfing rates (for ovules, pollen, all gametes, and zygotes), and simple mathematical relationships linking the four rates are obtained. Unlike earlier models of selfing, it is not assumed that the ovule selfing rate is constant, but instead that this rate, like all the others, is determined by the mobility of the pollen, which in turn is determined by the floral biology and ecology. It is found that all four selfing rates are usually frequency-dependent. The selfing rate for all gametes (the combined selfing rate) is usually intermediate between those for the ovules and pollen, and the zygotic rate is usually the smallest of the four. The exceptions to the above statements occur for relatively extreme situations, such as complete selfing for pollen or ovules, no selfing, or sexual symmetry. Three modes of selfing are considered: prior (PS), competing (CS), and delayed (DS) self-fertilization. It is shown that if there are at least two types with different selfing rates in the population, then the ranking of their selfing rates may depend upon the frequencies of the types (for the combined and the zygotic rates), may be frequency-independent (ovule rate), or may be dependent or independent, according to the mode of selfing (pollen rate). The effects of the various influences on the amount of selfing are by no means negligible. Thus a numerical study shows pollen selfing rates for one type which vary from 0.09 to 0.96, according to its frequency. Another numerical result shows a change in combined selfing rate from 0.13 to 0.86, depending solely on the mode of selfing. Results for Scots Pine show that an ovule selfing rate of 0.5 was accompanied by a combined rate of 0.143.The population selfing rate is not the same as the mean of individual selfing rates, and can only be obtained if female fitnesses as well as ovule selfing rates are known for each type.Previous models of selfing have failed to distinguish between the effects of increased selfing and increased pollen fertility, with the result that increased selfing always resulted in greater fitness. In the present models the two effects are distinguishable, and it is found that increased selfing may result in increased or decreased fitness, depending also on population density and on a form of pollen density. Thus the old dogma that in the absence of viability and fertility selection increased selfing always results in increased fitness is finally refuted, and the importance of the influence of ecological parameters on selfing and fitness is emphasized, since population density and pollen density influence the selfing rates.     

S allele discrimination in styles of Petunia hybrida bearing stylar-conditioned pseudo-self-compatibility

Summary A cross between a 0% pseudo-self-compatible (PSC) plant (S_3.3) and a 100% PSC plant (S_1.1) yielded an F₁ population which, when selfed, produced a high mean seed set which was not significantly different than that produced when the F₁ was backcross pollinated by the 100% PSC parent. Backcross pollinating the F₁ with the 0% PSC parent yielded no seed. No S_3.3 plants were recovered in the F₂ populations, indicating that pollen tubes containing the S₃ allele were inhibited during pollen tube growth of the selfed F₁ plants. Apparently stylar-conditioned PSC does not remove all discriminatory power from these petunia styles. Crossing the F₁ (S_1.3) with an self-incompatible (SI) plant (S_2.2) produced plants which were used for computation of a standard linkage test. An approximate map distance of 28 units was found between the S specificity locus and the major gene(s) which influenced its expression. Other generalized PSC modifying genes apparantly were not linked with the S locus.Scientific Journal Series Paper Number 10,606 of the Minnesota Agricultural Experiment Station     

Pollen Dispersal Between Isolated Trees in the Brazilian Savannah: A Case Study of the Neotropical Tree Hymenaea stigonocarpa

This case study examines the pollen dispersal distance, pollen dispersal patterns and intra‐family genetic structure for isolated trees in pastures of the bat‐pollinated Neotropical tree species Hymenaea stigonocarpa using six microsatellite loci and parentage analysis. The sampling included 28 grouped trees (referred to as the population) and six isolated trees in pastureland along a highway in Mato Grosso do Sul State, Brazil. From the population, we sampled 137 seeds from 12 seed‐trees, and from the isolated trees, we sampled 34 seeds from two seed‐trees. The results showed that pollen was dispersed over long distances (reaching 7353 m) and therefore the spatially isolated trees were not reproductively isolated. The pollen immigration rate in the population was also high (31%). Isolated trees presented a higher selfing rate (s=26%) than trees in the population (s=12%), suggesting that the spatial isolation of the trees increased selfing. However, selfing was responsible for only 30 percent of the inbreeding in offspring and mating among relatives was 70 percent. In the population, excluding selfing, ca 72 percent of the pollen was dispersed over distances <1000 m (average: 860 m). For the two isolated seed‐trees, excluding selfing, the average pollen dispersal distance was 5229 m. The results demonstrate that although pollen can be dispersed over long distances for H. stigonocarpa isolated trees, a high percentage of pollen comes from the same tree (selfing) and mating was correlated. Consequently, seeds must be collected from a large number of seed‐trees for conservation purposes.     

Individual and Temporal Variation in Outcrossing Rates and Pollen Flow Patterns in Ceiba pentandra (Malvaceae: Bombacoidea)

Ceiba pentandra is a tropical tree with high rates of selfing in some populations. In mixed‐mating species, variation in selfing is due to changes in adult density or variability of incompatibility systems. The effect of spatial isolation and phenology on selfing rates and pollen flow distances was analyzed using microsatellites in a fragmented population of Ceiba pentandra, in southern Costa Rica. Adult trees within a heterogeneous landscape were classified as grouped or isolated. We compared selfing rates at the individual level, between isolation conditions and 2 yr (2007, 2009), which differed in the number of flowering individuals. Mixed mating was estimated in both years (t_m = 0.624–0.759). Trees mated predominantly by outcrossing, while only a few trees reproduced through selfing. Spatial isolation did not significantly affect outcrossing rates. The progeny of grouped trees was mostly sired by near‐neighbors (<1 km) and by long‐distance pollen flow events in isolated trees. A reduction in the number of flowering individuals in 2009 reduced near‐neighbor matings, increased selfing in grouped trees, and decreased the number of unsampled sires in the progeny. Comparing selfing rates on individuals that flowered in both reproductive periods suggests a flexible mating system. Variation in self‐fertilization rates in this population appears to depend on variation of individual traits, such as genetic variability in self‐incompatibility genes, but it is independent of landscape heterogeneity. In contrast, pollen flow distances depend on local tree density as bats concentrate their foraging between near individuals to maximize energy efficiency.     

The genetics of self-incompatibility inLeavenworthia crassa rollins (Cruciferae)

Six plants of a self-incompatible population ofLeavenworthia çrassa were grown from seed collected in nature and cross-pollinated in all combinations. The incompatibility relationships between sibs were determined in eleven of the F₁ families. A one-locus sporophytic incompatibility system was established. None of the parents was homozygous at the S locus. At least five, and possibly all six, of the parents did not share an S allele. Only one pair of alleles was shown to have different interactions in the pollen and stigmata. The identity and expression of the S alleles were determined in six families. Eight pairs of alleles were independently expressed in both the pollen grains and the stigmata. Sixteen pairs of alleles showed dominance of one allele over the other in the pollen grains or the stigmata or both.F₁ plants of two crosses between different self-incompatible races were self-incompatible. F₁ plants of six crosses between self-incompatible and self-compatible races were self-incompatible; in five of the families, the frequency of pseudo-compatibility was higher than in the self-incompatible parent. Self-incompatible hybrids from a cross between a self-incompatible and a self-compatible population provide a method for rapidly determining allelic interactions in plants with a sporophytic incompatibility system.The research was carried out at the Biological Laboratories, Harvard University, Cambridge, Massachusetts, U.S.A.     

The reproductive assurance benefit of selfing: importance of flower size and population size

Autonomous selfing can provide reproductive assurance (RA) for flowering plants that are unattractive to pollinators or in environments that are pollen limited. Pollen limitation may result from the breakdown of once-continuous habitat into smaller, more isolated patches (habitat fragmentation) if fragmentation negatively impacts pollinator populations. Here we quantify the levels of pollen limitation and RA among large and small populations of Collinsia parviflora, a wildflower with inter-population variation in flower size. We found that none of the populations were pollen limited, as pollen-supplemented and intact flowers did not differ in seed production. There was a significant effect of flower size on RA; intact flowers (can self) produced significantly more seeds than emasculated flowers (require pollen delivery) in small-flowered plants but not large-flowered plants. Population size nested within flower size did not significantly affect RA, but there was a large difference between our two replicate populations for large-flowered, small populations and small-flowered, large populations that appears related to a more variable pollination environment under these conditions. In fact, levels of RA were strongly negatively correlated with rates of pollinator visitation, whereby infrequent visitation by pollinators yielded high levels of RA via autonomous selfing, but there was no benefit of autonomous selfing when visitation rates were high. These results suggest that autonomous selfing may be adaptive in fragmented habitats or other ecological circumstances that affect pollinator visitation rates.