INBREEDING AND FITNESS IN THE FRESHWATER SNAIL LYMNAEA PEREGRA: AN EVALUATION OVER TWO GENERATIONS OF SELF-FERTILIZATION

Inbreeding depression was estimated from an outbreeding population of the freshwater snail Lymnaea peregra, on the basis of two successive generations of enforced selling and outcrossing, and 70 maternal lineages. Outcrossing was analyzed under two treatments, groups of two and five individuals. The fitness parameters measured included fecundity, growth, and survival. In the first generation, we contrasted three treatments (selfers vs. paired outcrossers and group outcrossers). Very similar results were obtained between the two outcrossing treatments. A strong self-fertilization depression (which includes parental fecundity and progeny fitness) was detected in the selling treatment (about 90%). In the second generation, there was again marked evidence for self-fertilization depression, with the highest contributions coming from parental fecundity and progeny hatching rate. Our results suggest that the decreased parental fecundity is a consequence of the mating system in the previous generation, although the role of partial self-incompatibility and the copulation behavior could not be ruled out. Hatching rate and early survival data are suggestive of purging of lethal mutations. Significant variation in fitness among selfing lineages was found for most fitness traits. Our experimental design also allowed to test for interactions among fitness loci. Only one trait of the nine studied behaved as expected under synergistic interactions. However we cannot rule out some purging during the experiment, which could have biased results towards linearity. Inbreeding depression was also inferred from the change of inbreeding level across generations in the same population. We obtained a value similar to the experimental estimate.     

SELF-FERTILIZATION VERSUS CROSS-FERTILIZATION IN THE HERMAPHRODITIC FRESHWATER SNAIL BULINUS GLOBOSUS

Self-fertilization depression of fitness in the freshwater hermaphroditic snail Bulinus globosus, an intermediate host of the parasitic trematode Schistosoma, has been studied in a strain originating from Niger. B. globosus is an outcrosser that can self-fertilize when isolated before any copulation has occurred. The self-fertilization depression has been estimated during two successive generations. In the first generation, selfing was compared to outcrossing. Within each mating system group, selfing and outcrossing were compared again in the second generation. A striking difference was shown in favor of cross-fertilization for the number of eggs laid, the survival at birth of young snails and the number of snails reaching sexual maturity. The overall self-fertilization depression is 0.920 after two generations of selfing. We discuss the relative role of selfing and outcrossing in the evolution of freshwater snail populations.     

HIGH INBREEDING DEPRESSION,SELECTIVE INTERFERENCE AMONG LOCI,AND THE THRESHOLD SELFING RATE FOR PURGING RECESSIVE LETHAL MUTATIONS

The evolutionary dynamics of recessive or slightly dominant lethal mutations in partially self-fertilizing plants are analyzed using two models. In the identity-equilibrium model, lethals occur at a finite number of unlinked loci among which genotype frequencies are independent in mature plants. In the Kondrashov model, lethals occur at an infinite number of unlinked loci with identity disequilibrium produced by partial selfing. If the genomic mutation rate to (nearly) recessive lethal alleles is sufficiently high, such that the mean number of lethals (or lethal equivalents) per mature plant maintained at equilibrium under complete outcrossing exceeds 10, selective interference among loci creates a sharp discontinuity in the mean number of lethals maintained as a function of the selfing rate. Virtually no purging of the lethals occurs unless the selfing rate closely approaches or exceeds a threshold selfing rate, at which there is a precipitous drop in the mean number of lethals maintained. Identity disequilibrium lowers the threshold selfing rate by increasing the ratio of variance to mean number of lethals per plant, increasing the opportunity for selection. This theory helps to explain observations on plant species that display very high inbreeding depression despite intermediate selfing rates.     

INBREEDING DEPRESSION IN PARTIALLY SELF-FERTILIZING DECODON VERTICILLATUS (LYTHRACEAE): POPULATION-GENETIC AND EXPERIMENTAL ANALYSES

Inbreeding depression is a major selective force favoring outcrossing in flowering plants. Some self-fertilization, however, should weaken the harmful effects of inbreeding by exposing genetic load to selection. This study examines the maintenance of inbreeding depression in partially self-fertilizing populations of the long-lived, herbaceous wetland plant, Decodon verticillatus (L.) Ell. (Lythraceae). Estimates from ten populations indicate that 30% of offspring are produced through self-fertilization. Population-genetic estimates of inbreeding depression (δ = 1 – relative mean fitness of selfed progeny) based on changes in the inbreeding coefficient for the same ten populations were uniformly high, ranging from 0.49 to 1.79 and averaging 1.11 ± 0.29 SE. Although confidence intervals of individual population estimates were large, estimates were significantly greater than 0 in six populations and greater than 0.5 in four. Inbreeding depression was also estimated by comparing growth, survival, and flowering of experimentally selfed and outcrossed offspring from two of these populations in a 1-yr glasshouse experiment involving three density regimes; after which offspring were transplanted into garden arrays and two field sites and monitored for two consecutive growing seasons. Overall for survival averaged 0.27 ± 0.01 in the glasshouse, 0.33 ± 0.04 in the garden, and 0.46 ± 0.04 in the field. The glasshouse experiment also revealed strong inbreeding depression for growth variables, especially above-soil dry weight ( = 0.42 ± 0.03). The fitness consequences of inbreeding depression for these growth variables approximately doubles if survival to maturity is determined by severe truncation selection. Despite substantial selfing, inbreeding depression appears to be a major selective force favoring the maintenance of outcrossing in D. verticillatus.     

INBREEDING DEPRESSION IN TWO MIMULUS TAXA MEASURED BY MULTIGENERATIONAL CHANGES IN THE INBREEDING COEFFICIENT

In mixed-mating plant populations, one can estimate the relative fitness of selfed progeny w by measuring the inbreeding coefficient F and selfing rate s of adults of one generation, together with F of adults in the following generation (after selection). In the first application of this multigenerational method, we estimated F and s for adults over three consecutive generations in adjacent populations of two annual Mimulus taxa: the outbreeding M. guttatus and the inbreeding M. platycalyx. This gave estimates of w for the last two generations. Although average multilocus selfing rates were high in both taxa (0.63 in M. guttatus; 0.84 in M. platycalyx), the relative fitness of selfed progeny averaged only 0.19 in M. guttatus and 0.32 in M. platycalyx. An alternative estimator for w that incorporates biparental inbreeding gave even lower estimates of w. These values are significantly below the 0.5 threshold thought to favor selfing, and show that partially selfing populations can harbor substantial genetic load. In accordance with the purging hypothesis, the more highly selfing M. platycalyx showed marginally lower inbreeding depression than M. guttatus in both years (P = 0.08). Inbreeding depression and selfing rates also varied among years in concert among taxa. Several sources of bias are discussed, but computer simulations indicate it is unlikely that w is biased downwards by linkage of marker loci to load loci.     

THE INFLUENCE OF SELF-FERTILIZATION AND POPULATION DYNAMICS ON THE GENETIC STRUCTURE OF SUBDIVIDED POPULATIONS: A CASE STUDY USING MICROSATELLITE MARKERS IN THE FRESHWATER SNAIL BULINUS TRUNCATUS

The distribution of neutral genetic variability within and among sets of populations results from the combined actions of genetic drift, migration, extinction and recolonization processes, mutation, and the mating system. We here analyzed these factors in 38 populations of the hermaphroditic snail Bulinus truncatus. The sampling area covered a large part of the species range. The variability was analyzed using four polymorphic microsatellite loci. A very large number of alleles (up to 55) was found at the level of the whole study. Observed heterozygote deficiencies within populations are consistent with very high selfing rates, generally above 0.80, in all populations. These should depress the variability within populations, because of low effective size, genetic hitchhiking, and background selection, whatever the model of mutation assumed. However, that some populations exhibit much more variability than others suggests that historical demographic processes (e.g., population size variation, bottlenecks, or founding events) may play a significant role. A hierarchical analysis of the distribution of the variability across populations indicates a strong pattern of isolation by distance, whatever the geographical scale considered. Our analysis also illustrates how the mutation rate may affect population differentiation, as different mutation rates result in different levels of homoplasy at microsatellite loci. The effects of both genetic drift and gene flow vary with the temporal and spatial scales considered in B. truncatus populations.     

THE ROLE OF GENES OF LARGE EFFECT ON INBREEDING DEPRESSION IN MIMULUS GUTTATUS

Severe inbreeding depression is routinely observed in outcrossing species. If inbreeding load is due largely to deleterious alleles of large effect, such as recessive lethals or steriles, then most of it is expected to be purged during brief periods of inbreeding. In contrast, if inbreeding depression is due to the cumulative effects of many deleterious alleles of small effect, then it will be maintained in the face of periodic inbreeding. Whether or not inbreeding depression can be purged with inbreeding in the short term has important implications for the evolution of mating systems and the probability that a small population will go extinct. In this paper I evaluate the extent to which the tremendous inbreeding load in a primarily outcrossing population of the wildflower, Mimulus guttatus, is due to alleles of large effect. To do this, I first constructed a large outbred “ancestral” population by randomly mating plants collected as seeds from a natural population. From this population I formed 1200 lines that were maintained by self-fertilization and single seedling descent: after five generations of selling, 335 lines had survived the inbreeding process. Selection during the line formation is expected to have largely purged alleles of large effect from the collection of highly inbred lines. Because alleles with minor effects on fitness should have been effectively neutral, the inbreeding depression due to this class of genes should have been unchanged. The inbred lines were intercrossed to form a large, outcrossed “purged” population. Finally, I estimated the fitness of outbred and selfed progeny from the ancestral and purged populations to determine the contribution of major deleterious alleles on inbreeding depression. I found that although the average fitness of the outcrossed progeny nearly doubled following purging, the limited decline in inbreeding depression and limited increase in inbred fitness indicates that alleles of large effect are not the principle cause of inbreeding depression in this population. In aggregate, the data suggest that lethals and steriles make a minority contribution to inbreeding depression and that the increased outbred fitness is due primarily to adaptation to greenhouse conditions.     

EVOLUTION OF THE MAGNITUDE AND TIMING OF INBREEDING DEPRESSION IN PLANTS

Estimates of inbreeding depression obtained from the literature were used to evaluate the association between inbreeding depression and the degree of self-fertilization in natural plant populations. Theoretical models predict that the magnitude of inbreeding depression will decrease with inbreeding as deleterious recessive alleles are expressed and purged through selection. If selection acts differentially among life history stages and deleterious effects are uncorrelated among stages, then the timing of inbreeding depression may also evolve with inbreeding. Estimates of cumulative inbreeding depression and stage-specific inbreeding depression (four stages: seed production of parent, germination, juvenile survival, and growth/reproduction) were compiled for 79 populations (using means of replicates, N = 62) comprising 54 species from 23 families of vascular plants. Where available, data on the mating system also were collected and used as a measure of inbreeding history. A significant negative correlation was found between cumulative inbreeding depression and the primary selfing rate for the combined sample of angiosperms (N = 35) and gymnosperms (N = 9); the correlation was significant for angiosperms but not gymnosperms examined separately. The average inbreeding depression in predominantly selfing species (δ = 0.23) was significantly less (43%) than that in predominantly outcrossing species (δ = 0.53). These results support the theoretical prediction that selfing reduces the magnitude of inbreeding depression. Most self-fertilizing species expressed the majority of their inbreeding depression late in the life cycle, at the stage of growth/reproduction (14 of 18 species), whereas outcrossing species expressed much of their inbreeding depression either early, at seed production (17 of 40 species), or late (19 species). For species with four life stages examined, selfing and outcrossing species differed in the magnitude of inbreeding depression at the stage of seed production (selfing δ = 0.05, N = 11; outcrossing δ = 0.32, N = 31), germination (selfing δ = 0.02, outcrossing δ = 0.12), and survival to reproduction (selfing δ = 0.04, outcrossing δ = 0.15), but not at growth and reproduction (selfing δ = 0.21, outcrossing δ = 0.27); inbreeding depression in selfers relative to outcrossers increased from early to late life stages. These results support the hypothesis that most early acting inbreeding depression is due to recessive lethals and can be purged through inbreeding, whereas much of the late-acting inbreeding depression is due to weakly deleterious mutations and is very difficult to purge, even under extreme inbreeding.     

INBREEDING DEPRESSION IN A SELF-COMPATIBLE,ANDRODIOECIOUS CRUSTACEAN,EULIMNADIA TEXANA

The observation that offspring produced by the mating of close relatives are often less fit than those produced by matings between unrelated individuals (i.e., inbreeding depression) has commonly been explained in terms of the increased probability of expressing deleterious recessive alleles among inbred offspring (the partial dominance model). This model predicts that inbreeding depression should be limited in regularly inbreeding populations because the deleterious alleles that cause inbreeding depression (i.e., the genetic load) should be purged by regularly exposing these alleles to natural selection. We indirectly test the partial dominance model using four highly inbred populations of an androdioecious crustacean, the clam shrimp Eulimnadia texana. These shrimp are comprised of males and hermaphrodites, the latter capable of either self-fertilizing or mating with a male (i.e., outcrossing between hermaphrodites is impossible). Hermaphrodites are further subdivided into monogenics (produced via self-fertilization) and amphigenics (produced via self-fertilization or outcrossing). Electrophoretic evidence suggests significant differences in heterozygosity among populations, but that selfing rates were not statistically different (average s = 0.67). Additional electrophoretic analyses reveal that three previously described sex-linked loci (Fum, Idh-1, and Idh-2) are all tightly linked to each other, with crossing over on the order of 1% per generation. Although selfing rates are clearly high, we present evidence that early inbreeding depression (hatching rates, juvenile survival, and age at sexual maturity) exists in all four populations. For all of these factors, inbreeding depression was inferred by the positive correlation of multilocus heterozygosity and fitness. Cumulative inbreeding depression (8) is between 0.41 and 0.47 across all populations, which appears to be too low to limit the effects of purging via identity disequilibrium. Instead, we suggest that the maintenance of inbreeding depression in these populations is due to the observed linkage group, which we suggest contains a large number of genes including many related to fitness. Segregation of such a large linkage group would explain our observations of the predominance of amphigenic hermaphrodites in our field samples and of survival differences between monogenics and amphigenics within selfed clutches. We propose that a modified form of the overdominance model for inbreeding depression operating at the level of linkage groups maintains the observed levels of inbreeding depression in these populations even in the face of high rates of selfing.     

INBREEDING DEPRESSION IN HYDROPHYLLUM APPENDICULATUM: ROLE OF MATERNAL EFFECTS,CROWDING, AND PARENTAL MATING HISTORY

This paper examines several aspects of the expression of inbreeding depression in an outcrossing, obligately biennial plant, Hydrophyllum appendiculatum (Hydrophyllaceae). The amount of inbreeding depression detected was small during the first year of life but increased with age and had significant effects on adult size and reproductive traits. The lack of significant inbreeding depression during early growth is likely due to the overriding influence of maternal environmental effects on seed size and seedling growth. However, as maternal effects decreased with age, the seedling's own genotype became a more important determinant of its fate. To examine whether the expression of inbreeding depression was sensitive to ecological conditions, selfed and outcrossed seedlings were grown alone or with other H. appendiculatum seedlings. No inbreeding depression was detected in the plants grown alone. In contrast, under competitive conditions, outcrossed seedlings were significantly larger than selfed seedlings by the end of the first growing season. To address whether parental mating history influences the amount of inbreeding depression expressed, I examined the consequences of two successive generations of selfing on seed set and seed weight. The amount of inbreeding depression increased following the second generation of selfing. In the first generation, seed set and seed weight differed by less than 5% between selfed and outcrossed progeny. However, both traits were 15% greater for outcrossed plants after two generations. These results indicate that the alleles responsible for the reductions in these traits were not purged and suggest the action of multiple loci with deleterious effects.     

MAGNITUDE AND TIMING OF INBREEDING DEPRESSION IN SCOTS PINE (PINUS SYLVESTRIS L.)

Inbreeding depression is a major selective force favoring outcrossing in flowering plants. However, some self-fertilization should weaken the harmful effects of inbreeding by exposing deleterious alleles to selection. This study examines the maintenance of inbreeding depression in the predominantly outcrossing species Pinus sylvestris L. (Scots pine). Open-pollinated and self-fertilized progeny of 23 maternal trees, originating from a natural stand in southern Finland, were grown at two sites. We observed significant inbreeding depression in two of the four life stages measured. Inbreeding depression was largest for seed maturation (δ = 0.74), where seedset in open-pollinated strobili (70.9%) was about four times higher than in selfed strobili (18.3%). Inbreeding depression in postgermination survival (upto an age of 23 years) was also high (δ = 0.62–0.75). No significant differences in height (δ = 0.05) or flowering (δ = 0.14) of the trees after 23 years were observed. Cumulative inbreeding depression was high (δ = 0.90–0.94) and differed significantly among maternal families (range 0.45–1.00). The magnitude of inbreeding depression among the 23 maternal parents was not significantly correlated between early (seed maturation) and later (postgermination survival) life stages, suggesting that its genetic basis varies across the life cycle. Size differences among the progeny types diminished in time due to nonrandom size-specific mortality, causing a decrease in the inbreeding depression estimates for height over time. Our results indicate that Scots pine exhibits high levels of inbreeding depression during both early and later stages of the life cycle. It is argued that self-fertilization in Scots pine is inefficient in purging the genetic load caused by highly deleterious mutations because of the nearly complete loss of selfed individuals over time. This results in an effectively random mating outcrossing population.     

A COMPARISON OF INBREEDING DEPRESSION IN LIFE-HISTORY AND MORPHOLOGICAL TRAITS IN ANIMALS

The current study tests the hypothesis that life-history traits (closely related to fitness) show greater inbreeding depression than morphological traits (less closely related to fitness). The mean and median slope of the standardized coefficient of inbreeding depression (the slope of the linear relationship between F and the trait value) for life-history and morphological traits were compared. Slopes for life-history traits were higher than those for morphological traits. At F = 0.25 (full-sibling mating), life-history traits experienced a median reduction of 11.8% in trait value, whereas morphological traits showed a depression in trait value of approximately 2.2%.     

黄山钓桥青冈种群的交配系统与近交衰退

采用垂直板型不连续聚丙烯酰胺凝胶电泳检测了黄山钓桥青冈种群的遗传变异、交配系统及近交衰退程度。黄山钓桥青冈群维持有中等程度的遗传变异,多态位点百分比和期望杂合度分别为５０％和０．１８１４。采用ＰＯＤ－２、ＥＳＴ－１和ＥＳＴ－２等３个多态位点,利用ＭＬＴ程序估计的花粉库和母株基因频率之间存在一些差异,可能是由于成熟个体花粉产量不同和种群内个体非随机分布造成的。单位点杂交率平均为０．３９２,略高于多位     

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.     

CROSSING-DISTANCE EFFECTS IN DELPHINIUM NELSONII: OUTBREEDING AND INBREEDING DEPRESSION IN PROGENY FITNESS

Depending on its genetic causes, outbreeding depression in quantitative characters may occur first in the free-living F₁ generation produced by a wide cross. In 1981–1985, we generated F₁ progenies by hand-pollinating larkspurs (Delphinium nelsonii) with pollen from 1-m, 3-m, 10-m, or 30-m distances. From the spatial genetic structure indicated by previous electrophoretic and reciprocal transplantation studies, we estimate that these crosses range from being inbred (f ≈ 0.06) to outbred. We planted 594 seeds from 66 maternal sibships under natural conditions. As of 1992, there was strong evidence for both inbreeding depression and outbreeding depression. Progeny from intermediate crossing distances grew approximately twice as large as more inbred or outbred progeny in the first 5 yr after planting (P = 0.013, repeated measures ANOVA), and survived almost 1 yr longer on average (contrast of 3-m and 10-m treatments versus 1 m and 30 m; P = 0.028, ANOVA). Twenty maternal sibships produced flowering individuals; only four and two of these represented 1-m and 30-m crossing distances, respectively (P = 0.021, G-test). The cumulative fitness of intermediate distance sibships averaged about twice that of 1-m sibships, and five to eight times that of 30-m sibships (P = 0.017, ANOVA). Thus, even though progeny of 1-m crosses were inbred to a degree only about one-eighth that of selling, inbreeding depression approximated 50%, and outbreeding depression equaled or exceeded 50% for all fitness components.     

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.     

SEVERE INBREEDING DEPRESSION AND RAPID FITNESS REBOUND IN THE BUTTERFLY BICYCLUS ANYNANA (SATYRIDAE)

We established inbred laboratory lines of the satyrid Bicyclus anynana with one, three and 10 pairs of butterflies, which were subsequently allowed to increase freely to a maximum size of 300 butterflies. Minimally inbred control lines were established with 300 randomly selected virgin butterflies of equal sex ratio. We measured fecundity, egg weight, egg hatching, adult emergence, adult size, and the proportion of crippled adults in generations F₂, F₃, F₅, and F₇ (the latter two for the one pair bottleneck lines only). The most striking result was an unexpectedly large decrease in egg hatching with increase in inbreeding (25% per 10% increase in inbreeding). Such a level of inbreeding depression has not been reported previously for any insect. The distribution of egg hatching rate for individual clutches within inbred lines was markedly skewed, with a large fraction of clutches producing no eggs at all. This is interpreted as a relatively lower ratio of detrimental to lethal (or sterile) mutation loads than is found in Drosophila, the only insects for which mutation loads have been well characterized. Possible explanations for this severe inbreeding depression include a relatively high rate of mutation to recessive alleles with substantial damaging effects and infrequent episodes of inbreeding in nature. In the experiments, average egg hatching rate recovered rapidly between F₂ and F₇ in three of the six one-pair lines. We discuss the implications of these results for survival of populations through extreme bottlenecks in nature and in captivity.     

SELECTION ON A FLORAL COLOR POLYMORPHISM IN THE TALL MORNING GLORY (IPOMOEA PURPUREA): TRANSMISSION SUCCESS OF THE ALLELES THROUGH POLLEN

The W locus, a codominant locus influencing floral pigment intensity in the tall morning glory, Ipomoea purpurea, is polymorphic throughout the southeastern United States. Previous studies suggest that this polymorphism is actively maintained by balancing selection, and that increased selfing accompanied by lack of pollen discounting (“Fisher effect”) may act to protect the white allele when it is rare. Processes that act to protect the dark allele and thus stabilize the polymorphism in conjunction with the Fisher effect have not been previously detected. The goal of this study was to determine whether any of three such processes might operate in I. purpurea. Estimates of breeding system parameters in a large experimental population in which the white allele was in higher than normal frequency (0.5) provided little evidence that either dark- or light-flowered plants were more successful as pollen parents than white-flowered plants. In addition, no evidence was found for a transmission bias favoring the dark allele in the ovules produced by light heterozygotes. In contrast, a strong transmission bias favoring the dark allele in pollen of heterozygotes was observed. A simple model using parameter estimates derived from this and previous studies indicates that a balance between the Fisher effect and biased transmission in heterozygote pollen could account for many properties of the polymorphism.