DETERMINANTS OF OUTCROSSING RATE IN A PREDOMINANTLY SELF-FERTILIZING WEED,DATURA STRAMONIUM (SOLANACEAE)

We measured outcrossing rates of several North Carolina populations of the annual weed Datura stramonium including both natural populations and experimental populations in which we manipulated plant spatial arrangement. Because capsules of D. stramonium typically produce hundreds of seeds and we used an easily scored genetic marker for flower and hypocotyl color, we could measure outcrossing rates accurately for both individual plants and single flowers. The population-wide estimates of outcrossing rates were surprisingly low for a species with showy, entomophilous flowers and ranged from 1.9% in an experimental population with a “clumped” spatial arrangement to 8.5% in an experimental population with a “dispersed” arrangement. These low values were not produced by pollinator discrimination among flower color morphs, as determined by outcrossing measurements on test plants of different colors and by direct observations of pollinator behavior. For individual plants and single flowers in the experimental populations, variation in outcrossing rates was significantly affected by such population-wide characteristics as plant spatial arrangement and nightly fluctuations in total floral abundance. However, by far the most important factor was stigma position. Flowers with stigmas above the anthers had significantly higher outcrossing rates than did flowers with overlapping stigma and anthers. The strong effect of floral morphology suggests that the very low population-wide levels of outcrossing in D. stramonium may represent a persistent mixing mating system rather than a transition to complete selfing.     

THE EFFECTS OF POPULATION SIZE AND PLANT DENSITY ON OUTCROSSING RATES IN LOCALLY ENDANGERED SALVIA PRATENSIS

Multilocus outcrossing rates were estimated in natural and experimental populations of Salvia pratensis, an entomophilous, gynodioecious, protandrous perennial. Male steriles were used to check the estimation procedure of outcrossing rates in hermaphrodites. Estimates of outcrossing rates in hermaphroditic plants ranged from 38.2% to 81.8% in natural populations and from 71.5% to 95.5% in experimental populations. No correlations were found between outcrossing rates and population size. However, outcrossing in hermaphrodites was promoted by high plant densities and low frequencies of male steriles. It is argued that effective management to preserve genetic variation in populations of S. pratensis should provide for the maintenance of high plant densities.     

INBREEDING EFFECTS IN CLARKIA TEMBLORIENSIS (ONAGRACEAE) POPULATIONS WITH DIFFERENT NATURAL OUTCROSSING RATES

Inbreeding depression is commonly observed in natural populations. The deleterious effects of forced inbreeding are often thought to be less pronounced in populations with self-pollinating mating systems than in primarily outcrossing populations. We tested this hypothesis by comparing the performance of plants produced by artificial self- and cross-pollination from three populations whose outcrossing rate estimates were 0.03, 0.26, and 0.58. Outcrossing rates and inbreeding coefficients were estimated using isozyme polymorphisms as genetic markers. Analysis of F statistics suggests that biparental inbreeding as well as self-fertilization contribute to the level of homozygosity in the seed crop. Biparental inbreeding will reduce the heterozygosity of progeny produced by outcrossing, relative to random outcrossing expectations, and hence will reduce the effects of outcrossing versus self-fertilization. Heterotic selection may increase the average heterozygosity during the life history. Selfed and outcrossed seeds from all three populations were equally likely to germinate and survive to reproduce. However, inbreeding depression was observed in fecundity traits of plants surviving to reproduction in all three populations. Even in the population whose natural self-fertilization rate was 97%, plants grown from seed produced by self-pollination produced fewer fruits and less total seed weight than plants grown from outcrossed seed. There was no detectable inbreeding depression in estimated lifetime fitness. Inbreeding effects for all reproductive yield characters were most severe in the accession from the most outcrossing population and least severe in the accession from the most self-fertilizing population.     

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.”     

FITNESS CONSEQUENCES OF OUTCROSSING IN IMPATIENS CAPENSIS: TESTS OF THE FREQUENCY-DEPENDENT AND SIB-COMPETITION MODELS

Using field and greenhouse experiments, we tested two hypotheses that could account for the maintenance of outcrossing in Impatiens capensis. Seedlings derived from cleistogamous (CL) and chasmogamous (CH) flowers were grown under competitive conditions while flanked by neighbors that were either related or unrelated. In both experiments, CH progeny sometimes expressed more phenotypic variability than CL progeny. In the greenhouse experiment, CH progeny attained the same weight as CL progeny, and the relatedness of neighboring plants did not affect the growth of either type. In the field experiment, CL and CH progeny performed similarly when grown with related competitors. However, CH progeny were somewhat larger when planted with nonsibs, while CL progeny were somewhat smaller under those conditions. Thus, there is no evidence that either frequency-dependent selection or the avoidance of competition among siblings favors the maintenance of outcrossing in this species. We also modeled the relative variability of selfed and outcrossed progeny under several reproductive systems. When females mate with one male (progeny are full sibs), selfed progeny are often more variable than outcrossed progeny. When females engage in both selfing and outcrossing, variation among progeny is frequently maximized at an intermediate selfing rate. The sib-competition mechanism, under a range of genetic models, is not apt to promote outcrossing, since selfed progeny are commonly more variable than outcrossed progeny.     

THE EVOLUTION OF INSECT-MEDIATED SELF-POLLINATION FROM AN OUTCROSSING SYSTEM IN CALYPTRIDIUM (PORTULACACEAE)

Calyptridium monospermum (Portulacaceae) comprises two biological races visited by different groups of insects. One race, pollinated by a bumblebee, Bombus vosnesenskii (Apidae), is outcrossing, while the other race is pollinated casually by a variety of other insects whose visits result in insect-mediated self-pollination and geitonogamy within the inflorescence. Insect-mediated selfing is also the mode of pollination in the closely related species, C. umbellatum. While all plants of both species are self-compatible, insect visitation is necessary for good seed set. Selfing populations of C. monospermum exhibit many floral adaptations for insect-mediated self-pollination which parallel those of the selfing C. umbellatum, while retaining vegetative characteristics typical of C. monospermum. These floral traits involve flower color and odor, style and stamen orientation, size of stigmatic surfaces, number of pollen grains produced, density of open flowers on inflorescences, and presence or absence of protogyny. Artifical pollination experiments show that these floral adaptations are effective in increasing the probability of insect-mediated self-pollination. Insect-mediated selfing in C. umbellatum and in some populations of C. monospermum is encouraged and probably made necessary by the inconstant pollinating behavior of the insects which visit these plants, as shown by the large percentage of non-Calyptridium pollen recovered from the bodies of insects captured while visiting these plants.     

OUTCROSSING IN THE WEED,SOLANUM PTYCANTHUM (SOLANACEAE): A COMPARISON OF AGRESTAL AND RUDERAL POPULATIONS

Levels of outcrossing, using a Pgm polymorphism, were estimated for individuals from ruderal and agrestal populations of the predominantly monomorphic weed Solanum ptycanthum, at the northern edge of its range, and compared to agrestal populations sampled from the southern portion of its distribution. Individuals were grown in agricultural habitats in Ontario. Outcrossing estimates of the northern agrestals were very low (< 3%) and did not differ from ruderal estimates. Southern agrestals exhibited much higher outcrossing rates (10%–17%). Of the floral traits measured, degree of stigma exsertion and style position showed interpopulation variation and correlated with outcrossing rate. Outcrossing in northern individuals was largely precluded by the complete overlap in female and male functions and stigmatic surfaces that were in direct contact with anther sacs as the flower opened. The observed flexibility in mating system may have played an important role in colonization success of 5. ptycanthum.     

OPTIMAL OUTCROSSING IN IPOMOPSIS AGGREGAT A: SEED SET AND OFFSPRING FITNEs

Restricted gene flow and localized selection should establish a correlation between physical proximity and genetic similarity in many plant populations. Given this situation, fitness may decline in crosses between nearby plants (inbreeding depression), and in crosses between more widely separated plants (“outbreeding depression”) mostly as a result of disruption of local adaptation. It follows that seed set and offspring fitness may be greatest in crosses over an intermediate “optimal outcrossing distance.” This prediction was supported for Ipomopsis aggregata, a long-lived herbaceous plant pollinated by hummingbirds. In six replicate pollination experiments, mean seed set per flower was higher with an outcrossing distance of 1–10 m than with selfing or outcrossing over 100 m. A similar pattern appeared in the performance of offspring from experimental crosses grown under natural conditions and censused for a seven-year period. Offspring from 10-m crosses had higher survival, greater chance of flowering, and earlier flowering than those from 1-m or 100-m crosses. As a result, 1-m and 100-m offspring achieved only 47% and 68%, respectively, of the lifetime fitness of 10-m offspring. Offspring fitness also declined with planting distance from the seed parent over a range of 1–30 m, so that adaptation to the maternal environment is a plausible mechanism for outbreeding depression. Censuses in a representative I. aggregata population indicated that the herbaceous vegetation changes over a range of 2–150 m, suggesting that there is spatial variation in selection regimes on a scale commensurate with the observed effects of outbreeding depression and planting distance. We discuss the possibility that differences in seed set might in part reflect maternal mate discrimination and emphasize the desirability of measuring offspring fitness under natural conditions in assessing outcrossing effects.     

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.     

EVOLUTION OF GYNODIOECY AND MAINTENANCE OF FEMALES: THE ROLE OF INBREEDING DEPRESSION,OUTCROSSING RATES,AND RESOURCE ALLOCATION IN SCHIEDEA ADAMANTIS (CARYOPHYLLACEAE)

Levels of inbreeding depression, outcrossing rates, and phenotypic patterns of resource allocation were studied to examine their relative importance in the maintenance of high numbers of females in gynodioecious Schiedea adamantis (Caryophyllaceae), an endemic Hawaiian shrub found in a single population on Diamond Head Crater, Oahu. In studies of inbreeding depression in two greenhouse environments, families of hermaphrodites exhibited significant inbreeding depression (δ = 0.60), based on a multiplicative fitness function using seeds per capsule, germination, survival, and the inflorescence biomass of progeny. Differences between inbred and outcrossed progeny were smallest at the early stage of seeds per capsule and more pronounced at the later stages of survival and inflorescence production. These results are consistent with inbreeding depression caused by many mutations of small effect. Using allozyme analyses, the inbreeding coefficient of adult plants in the field was not significantly different from zero, implying that δ in nature may be equal to one. The single locus estimate of the outcrossing rate for hermaphrodites was 0.50 based on progeny that survived to flowering; corrected for the disproportionate loss before flowering of progeny from selfing, the adjusted outcrossing rate at the zygote stage was 0.32, suggesting that considerable selfing occurs in hermaphrodites. Females were totally outcrossed. When females and hermaphrodites were compared for reproductive output in the field, females produced over twice as many seeds per plant as hermaphrodites, primarily because females had far more capsules per inflorescence than hermaphrodites. Females had greater mass per seed than hermaphrodites in the field, either because of greater provisioning or reduced inbreeding depression. There was no significant differential mortality with respect to sex over a seven year period. The higher number of seeds per plant of females, combined with substantial inbreeding depression and relatively high selfing rates for hermaphrodites, are probably responsible for the maintenance of females in this population. The predicted frequency of females based on data for seed production, the adjusted selfing rate, and inbreeding depression is 42%, remarkably close to the observed frequency of 39%. High levels of inbreeding depression suggest that considerable quantitative genetic variation is present for traits affecting fitness in this population, despite low allozyme variability and a presumed founder effect.     

ORIGIN OF APOMICTIC RED ALGAE: OUTCROSSING STUDIES OF DIFFERENT STRAINS IN CALOGLOSSA MONOSTICHA (CERAMIALES,RHODOPHYTA)1

Various red algae lack sexual reproduction and propagate by spore recycling, but it is still unknown how apomixis originates. In previous crossing experiments, we obtained an unusual hybrid of Caloglossa monosticha M. Kamiya through the outcrossing between a male from Australia and a female from Indonesia. This hybrid was morphologically identical to a normal tetrasporophyte, but its tetraspores grew into tetrasporophytes and repeated tetraspore recycling. During 5 years of culture, no sexual reproductive structures have formed on the tetrasporelings from this hybrid or its progenies. Further hybridization experiments revealed that all the five female strains from Indonesia successfully crossed with the male strain from the East Alligator River, Australia, and most of the F₁ sporophytes demonstrated tetraspore recycling, though the germination rates of these tetraspores were quite low. The ploidy level of the hybrid tetrasporophyte was similar to the normal tetrasporophyte, rather than the parental gametophyte, based on the comparison of relative DNA contents of their nuclei. Single strand conformation polymorphism (SSCP) and sequence analyses of the internal transcribed spacer 1 (ITS1) region indicated that the alleles from both parents were present in all the hybrid tetrasporophytes examined. These results suggest that this hybrid does not carry out meiosis during sporogenesis, and heterozygous diploid sporophytes arose from tetraspores. Therefore, we believe that obligate apomixis was generated through outcrossing between genetically different entities of C. monosticha.     

A TEST OF THE SIB-COMPETITION HYPOTHESIS FOR OUTCROSSING ADVANTAGE IN IMPATIENS CAPENSIS

One of the potential selective mechanisms invoked in discussions of breeding-system evolution is that competition within sibships increases the fitness of outcrossed progeny relative to selfed progeny. We tested this sib-competition hypothesis using cleistogamous (CL) and chasmogamous (CH) seeds of Impatiens capensis in a large greenhouse experiment. The experimental design was a double replacement series which also allowed us to test for inbreeding depression and overall resource partitioning among sibships. We found no evidence for strong inbreeding depression in the study population; although plants from CH seeds had a slight advantage over plants from CL seeds in total flower and pod production, CL plants had slightly higher growth. We also could not detect significant resource partitioning among sibships nor any evidence to support the sib-competition hypothesis for outcrossing advantage. CH sibships were not significantly more variable than CL sibships in any of the phenotypic traits measured. These results suggest that sibling competition may have little importance in the evolution of Impatiens breeding systems.     

SIZES OF NATURAL MICROGAMETOPHYTE POPULATIONS IN PISTILS OF PHLOX DRUMMONDII

The numbers of microgametophytes in the stigmas of the outcrossing Phlox drummondii were determined in 19 natural populations. Microgametophyte numbers per stigma ranged from 0 to over 40 in each population. The number of flowers per plant, distance to the nearest-neighbor, and number of plants within 1 m of the focal plants explained about 23% of the variation in microgametophyte numbers within 2 of the populations. The mean number per stigma across all populations was 14.34; and the mean number per population varied from 7.77 to 19.52. Given that there are 3 ovules per flower, there was an average of 4.73 microgametophytes per ovule. Sixty-nine percent of all pistils contained more than 3 microgametophytes, thereby presenting an opportunity for intergametophytic competition.     

Morphological adaptation in host races of Tephritis conura

The present study investigates morphological differentiation among host races of the fruit fly Tephritis conura Loew (Diptera: Tephritidae) for two fitness‐related traits and whether these traits are host induced or genetically determined. Flies were analyzed from independent sympatric regions, and from one syntopic site where parental host plants [Cirsium heterophyllum (L.) Hill. and Cirsium oleraceum (L.) Scop. (Cardueae)] and hybrid plants (C. heterophyllum×C. oleraceum) co‐occur. As both host races may oviposit on hybrid plants and hybrid plants provide an identical environment for larvae of both host races, flies emerging from C. heterophyllum×C. oleraceum hybrids were used to assess whether host‐race morphological differences are genetically determined or due to phenotypic plasticity. No significant size (wing length) differences were found among host races, whereas flies emerging from C. heterophyllum had on average 8.4% longer ovipositors than flies emerging from C. oleraceum. The mean size‐corrected ovipositor length (i.e., the ratio ovipositor/wing length) was 10.3% longer. These proportions were repeated among host races emerging from hybrid plants. Although flies of the C. heterophyllum host race from hybrid plants were smaller than on parental host plants, the ratio ovipositor/wing length was constant. Hybrid flies (which emerged only on hybrid plants) were intermediate in relative and absolute ovipositor length. Thus, ovipositor‐length differences among T. conura host races most likely have a genetic basis. This suggests that host‐related differences in ovipositor length reflect adaptations to the respective host‐plant species, most likely to the host's flower‐head size, whereas both host races experience similar selection regimes on body size.     

OUTCROSSING RATE AND INBREEDING DEPRESSION IN TWO ANNUAL MONOECIOUS HERBS,BEGONIA HIRSUTA AND B. SEMIOVATA

Most models of mating-system evolution predict inbreeding depression to be low in inbred populations due to the purging of deleterious recessive alleles. This paper presents estimates of outcrossing rates and inbreeding depression for two highly selfing, monoecious annuals Begonia hirsuta and B. semiovata. Outcrossing rates were estimated using isozyme polymorphisms, and the magnitude of inbreeding depression was quantified by growing progeny in the greenhouse produced through controlled selfing and outcrossing. The estimated single-locus outcrossing rate was 0.03 ± 0.01 (SE) for B. hirsuta and 0.05 ± 0.02 for B. semiovata. In both species, the seed production of selfed flowers was on average 12% lower than that of outcrossed flowers (B. hirsuta P = 0.07, B. semiovata P < 0.05, mixed model ANOVAs). There was no significant effect of crosstype on germination rate or survival, but selfed offspring had a lower dry mass than outcrossed offspring 18 weeks after planting in both species (on average 18% lower in B. hirsuta and 31% lower in B. semiovata). Plants that were the products of selfing began flowering later than plants produced through outcrossing in B. semiovata, but not in B. hirsuta. The effects of crosstype on seed production (B. semiovata), days to first flower and offspring dry mass (both species) varied among maternal parents, as indicated by significant crosstype x maternal parent interactions for these characters. Both species showed significant inbreeding depression for total fitness (estimated as the product of seed production, germination rate, survival and dry mass at 18 weeks). In B. hirsuta, the average total inbreeding depression was 22% (range -57%-98%; N = 23 maternal parents), and in B. semiovata, it was 42% (-11%-84%; N = 21). This study demonstrates that highly selfing populations can harbor substantial inbreeding depression. Our findings are consistent with the hypothesis that a high mutation rate to mildly deleterious alleles contributes to the maintenance of inbreeding depression in selfing populations.     

DEVELOPMENTAL STABILITY IN LEAVES OF CLARKIA TEMBLORIENSIS (ONAGRACEAE) AS RELATED TO POPULATION OUTCROSSING RATES AND HETEROZYGOSITY

Four natural populations of Clarkia tembloriensis, whose levels of heterozygosity and rates of outcrossing were previously found to be correlated, are examined for developmental instability in their leaves. From the northern end of the species range, we compare a predominantly selfing population (t? = 0.26) with a more outcrossed population (t? = 0.84), which is genetically similar. From the southern end of the range, we compare a highly selfing population (t? = 0.03) with a more outcrossed population (t? = 0.58). We measured developmental stability in the populations using two measures of within-plant variation in leaf length as well as calculations of fluctuating asymmetry (FA) for several leaf traits. Growth-chamber experiments show that selfing populations are significantly more variable in leaf length than more outcrossed populations. Developmental instability can contribute to this difference in population-level variance. Plants from more homozygous populations tend to have greater within-plant variance over developmentally comparable nodes than plants from more heterozygous populations, but the difference is not significant. At the upper nodes of the plant, mature leaf length declines steadily with plant age, allowing for a regression of leaf length on node. On average, the plants from more homozygous populations showed higher variance about the regression (MSE) and lower R² values, suggesting that the decline in leaf length with plant age is less stable in plants from selfing populations than in plants from outcrossing populations. Fluctuating asymmetry (FA) was calculated for four traits within single leaves at up to five nodes per plant. At the early nodes of the plant where leaf arrangement is opposite, FA was also calculated for the same traits between opposite leaves at a node. Fluctuating asymmetry is significantly greater in the southern selfing population than in the neighboring outcrossed population. Northern populations do not differ in FA. Fluctuating asymmetry can vary significantly between nodes. The FA values of different leaf traits were not correlated. We show that developmental stability can be measured in plants using FA and within-plant variance. Our data suggest that large differences in breeding system are associated with differences in stability, with more inbred populations being the least stable.     

EVOLUTION OF A POLYMORPHISM FOR OUTCROSSING RATE IN SENECIO VULGARIS: INFLUENCE OF GERMINATION BEHAVIOR

A difference in germination behavior between the highly selling, nonradiate variant of Senecio vulgaris and the more outcrossing, radiate form had a large effect on the relative female fitness of these two morphs raised in mixed stands under conditions similar to those experienced in the wild. Of particular significance was the finding that female fitness differences between morphs were reversed in early- and late-sown plots. This was because a greater proportion of nonradiate than radiate seed germinated directly after sowing, and in early-sown plots a relatively large proportion of early-germinated nonradiate seedlings survived winter to produce large, highly fecund plants the following summer, that contributed greatly to the total female fitness of the nonradiate morph. In contrast, in late-sown plots (i.e., sown two weeks later), survivorship of early germinated seedlings was much reduced, and the radiate morph had an advantage because most radiate seed delayed germination until spring, therefore avoiding seedling mortality during winter. The effect of the association between morph type and germination behavior on morph female fitness in S. vulgaris is clearly important in regard to the evolution of the polymorphism for outcrossing rate in the species. This, in turn, emphasizes the point that an understanding of factors responsible for the evolution and maintenance of polymorphisms for outcrossing rate in the wild should be based on a detailed examination of the ecological genetics of such polymorphisms that extend beyond traditional studies of pollen discounting and inbreeding depression.     

TEMPORAL DYNAMICS OF OUTCROSSING AND HOST MORTALITY RATES IN HOST–PATHOGEN EXPERIMENTAL COEVOLUTION

Cross‐fertilization is predicted to facilitate the short‐term response and the long‐term persistence of host populations engaged in antagonistic coevolutionary interactions. Consistent with this idea, our previous work has shown that coevolving bacterial pathogens (Serratia marcescens) can drive obligately selfing hosts (Caenorhabditis elegans) to extinction, whereas the obligately outcrossing and partially outcrossing populations persisted. We focused the present study on the partially outcrossing (mixed mating) and obligately outcrossing hosts, and analyzed the changes in the host resistance/avoidance (and pathogen infectivity) over time. We found that host mortality rates increased in the mixed mating populations over the first 10 generations of coevolution when outcrossing rates were initially low. However, mortality rates decreased after elevated outcrossing rates evolved during the experiment. In contrast, host mortality rates decreased in the obligately outcrossing populations during the first 10 generations of coevolution, and remained low throughout the experiment. Therefore, predominant selfing reduced the ability of the hosts to respond to coevolving pathogens compared to outcrossing hosts. Thus, we found that host–pathogen coevolution can generate rapid evolutionary change, and that host mating system can influence the outcome of coevolution at a fine temporal scale.     

SEX ALLOCATION AND OUTCROSSING RATE: A TEST OF THEORETICAL PREDICTIONS USING BROMEGRASSES (BROMUS)

Predictions of sex-allocation theory were tested by comparisons among hermaphroditic bromegrass (Bromus) species that differed in outcrossing rate. Relative maternal and paternal investment were calculated using both the ratio of pollen to seed production, and absolute allocations in units of energy, nitrogen, phosphorus, potassium, magnesium, and calcium. Outcrossing rate had a large effect on sex allocation; species having greater outcrossing rates had relatively more paternal reproductive effort. Bromus inermis was obligately outcrossing, and nearly half of its reproductive effort was devoted to pollen production. Three partially outcrossed species, B. kalmii, B. ciliatus, and B. latiglumis, invested between 5% and 11% of reproductive effort in pollen production. Paternal investment was less than 2% in the selling species B. tectorum. Estimates of sex allocation were relatively unaffected by the resource currency used in calculation. The differences among species in sex allocation were mostly due to differences in anther size and seed set.>     

QUANTITATIVE VARIATION IN PHLOX: COMPARISON OF SELFING AND OUTCROSSING SPECIES

Variation of 20 quantitative characters was examined within and among 10 populations of the predominantly outcrossing Phlox drummondii and 4 populations of the predominantly selfing P. cuspidata grown in a greenhouse. Multivariate analysis of variance, considering all characters simultaneously, indicated that there were significant differences among populations in both species while analysis of individual characters demonstrated that there were significant population differences for 19 characters in P. drummondii and 13 characters in P. cuspidata. On average, 16% of the total phenotypic variation in P. drummondii occurred among populations compared to less than 4% of the total variation in P. cuspidata. In addition, P. drummondii exhibited significant differences among families within populations more frequently than P. cuspidata. Most observed variation in both species occurred within families where environmental and genetic sources of variation could not be partitioned. There was a trend for P. drummondii to have higher heritabilities than P. cuspidata for most characters even when assumptions about breeding systems were relaxed. Thus, the outbreeding species exhibited larger genetic differences among populations and among families within populations than the selfing species in the greenhouse environment. These data suggest that P. drummondii has the greater evolutionary potential of the two species and are consistent with the hypothesis that differences in population structure result from differences in the breeding systems of the two species.