Inbreeding depression is high in a self‐incompatible perennial herb population but absent in a self‐compatible population showing mixed mating

High inbreeding depression is thought to be one of the major factors preventing evolutionary transitions in hermaphroditic plants from self‐incompatibility (SI) and outcrossing toward self‐compatibility (SC) and selfing. However, when selfing does evolve, inbreeding depression can be quickly purged, allowing the evolution of complete self‐fertilization. In contrast, populations that show intermediate selfing rates (a mixed‐mating system) typically show levels of inbreeding depression similar to those in outcrossing species, suggesting that selection against inbreeding might be responsible for preventing the transition toward complete self‐fertilization. By implication, crosses among populations should reveal patterns of heterosis for mixed‐mating populations that are similar to those expected for outcrossing populations. Using hand‐pollination crosses, we compared levels of inbreeding depression and heterosis between populations of Linaria cavanillesii (Plantaginaceae), a perennial herb showing contrasting mating systems. The SI population showed high inbreeding depression, whereas the SC population displaying mixed mating showed no inbreeding depression. In contrast, we found that heterosis based on between‐population crosses was similar for SI and SC populations. Our results are consistent with the rapid purging of inbreeding depression in the derived SC population, despite the persistence of mixed mating. However, the maintenance of outcrossing after a transition to SC is inconsistent with the prediction that populations that have purged their inbreeding depression should evolve toward complete selfing, suggesting that the transition to SC in L. cavanillesii has been recent. SC in L. cavanillesii thus exemplifies a situation in which the mating system is likely not at an equilibrium with inbreeding depression.     

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.     

Family-level covariation between parasite resistance and mating system in a hermaphroditic freshwater snail

Genetic compatibility, nonspecific defenses, and environmental effects determine parasite resistance. Host mating system (selfing vs. outcrossing) should be important for parasite resistance because it determines the segregation of alleles at the resistance loci and because inbreeding depression may hamper immune defenses. Individuals of a mixed mating hermaphroditic freshwater snail, Lymnaea ovata, are commonly infected by a digenetic trematode parasite, Echinoparyphium recurvatum. We examined covariation between quantitative resistance to novel parasites and mating system by exposing snail families from four populations that differed by their inbreeding coefficients. We found that resistance was unrelated to inbreeding coefficient of the population, suggesting that the more inbred populations did not carry higher susceptibility load than the less inbred populations. Most of the variation in resistance was expressed among the families within the populations. In the population with the lowest inbreeding coefficient, resistance increased with outcrossing rate of the family, as predicted if selfing had led to inbreeding depression. In the other three populations with higher inbreeding coefficients, resistance was unrelated to outcrossing rate. The results suggest that in populations with higher inbreeding some of the genetic load has been purged, uncoupling the predicted relationship between outcrossing rate and resistance. Snail families also displayed crossing reaction norms for resistance when tested in two environments that presented low and high immune challenge, suggesting that genotype-by-environment interactions are important for parasite resistance.     

The Integral Role of Genetic Variation in the Evolution of Outcrossing in the Caenorhabditis elegans-Serratia marcescens Host-Parasite System

Outcrossing is predicted to facilitate more rapid adaptation than self-fertilization as a result of genetic exchange between genetically variable individuals. Such genetic exchange may increase the efficacy of selection by breaking down Hill-Robertson interference, as well as promoting the maintenance of within-lineage genetic diversity. Experimental studies have demonstrated the selective advantage of outcrossing in novel environments. Here, we assess the specific role of genetic variation in the evolution of outcrossing. We experimentally evolved genetically variable and inbred populations of mixed mating (outcrossing and self-fertilizing) Caenorhabditis elegans nematodes under novel ecological conditions—specifically the presence of the virulent parasite Serratia marcescens. Outcrossing rates increased in genetically variable host populations evolved in the presence of the parasite, whereas parasite exposure in inbred populations resulted in reduced rates of host outcrossing. The host populations with genetic variation also exhibited increased fitness in the presence of the parasite over eight generations, whereas inbred populations did not. This increase in fitness was primarily the result of adaptation to the parasite, rather than recovery from initial inbreeding depression. Therefore, the benefits of outcrossing were only manifested in the presence of genetic variation, and outcrossing was favored over self-fertilization as a result. As predicted, the benefits of outcrossing under novel ecological conditions are a product of genetic exchange between genetically diverse lineages.     

Individual Variation in Inbreeding Depression: The Roles of Inbreeding History and Mutation

We use mutation-selection recursion models to evaluate the relative contributions of mutation and inbreeding history to variation among individuals in inbreeding depression and the ability of experiments to detect associations between individual inbreeding depression and mating system genotypes within populations. Poisson mutation to deleterious additive or recessive alleles generally produces far more variation among individuals in inbreeding depression than variation in history of inbreeding, regardless of selfing rate. Moreover, variation in inbreeding depression can be higher in a completely outcrossing or selfing population than in a mixed-mating population. In an initially random mating population, the spread of a dominant selfing modifier with no pleiotropic effects on male outcross success causes a measurable increase in inbreeding depression variation if its selfing rate is large and inbreeding depression is caused by recessive lethals. This increase is observable during a short period as the modifier spreads rapidly to fixation. If the modifier alters selfing rate only slightly, it fails to spread or causes no measurable increase in inbreeding depression variance. These results suggest that genetic associations between mating loci and inbreeding depression loci could be difficult to demonstrate within populations and observable only transiently during rapid evolution to a substantially new selfing rate.     

Effect of variation in herkogamy on outcrossing within a population of Gilia achilleifolia

The diverse features of floral morphology are often thought to be well-designed mechanisms to manipulate plant mating systems. We evaluated the effectiveness of one such mechanism, anther-stigma separation (herkogamy), in controlling variation in the level of outcrossing among plants in a population of Gilia achilleifolia. Variation in outcrossing rates within populations has the potential to influence the coevolution between inbreeding depression and mating system. Using four polymorphic allozymes, we compared the outcrossing-rate estimates of two groups of individuals under natural conditions: one group with low herkogamy and another with high herkogamy. The high herkogamy group had a higher outcrossing rate (0.572) than the low herkogamy group (0.335). This suggests that the within-population variation in outcrossing rate could potentially cause the previously observed association between herkogamy and inbreeding depression (Takebayashi and Delph, 2000). A previous study of floral traits among G. achilleifolia populations failed to detect a relationship between herkogamy and outcrossing rate, demonstrating that the functionality of traits may be obscured in among-population studies as a consequence of uncontrolled environmental variation. Additionally, the effect of herkogamy on outcrossing rate in delayed selfers such as G. achilleifolia may be particularly prominent when pollinator availability is low. Our population-level estimate of outcrossing rate (0.444) was somewhat lower than an estimate from the same population, 15 years prior to our study (0.75), suggesting that pollinator availability may fluctuate among years. Both within-year and among-year variation in the outcrossing rate may have a strong influence on mating-system evolution.     

Fine-scale genetic structure enhances biparental inbreeding by promoting mating events between more related individuals in wild soybean (Glycine soja; Fabaceae) populations

Outcrossing between genetically distant individuals in a plant population enhances allelic heterozygosity-an important source for genetic diversity and adaptive evolution. Fine-scale spatial genetic structure (FSGS) can interfere with outcrossing by promoting mating between more related individuals. To test the influence of FSGS on outcrossing, FSGS and outcrossing rates were analyzed with four wild soybean (Glycine soja) populations from different habitats, using simple sequence repeat (SSR) fingerprints. Spatial autocorrelation analysis indicated variable FSGS (15.44-25.87 m) in all four populations. Multilocus mixed-mating analysis of 1605 progeny indicated substantial variation in single-locus outcrossing (T(s) = 6.3-12.6%) although the total outcrossing rates as estimated by multilocus outcrossing (T(m) = 12.8-17%) did not vary significantly among populations. The comparison between FSGS and outcrossing rates demonstrated that strong FSGS with large genetic patch size can enhance biparental inbreeding by promoting mating between more related individuals in a population. The results suggest that patch size management can aid in situ conservation by avoiding formation of strong FSGS and encouraging true outcrossing among individuals.     

FLORAL CORRELATES AND FITNESS CONSEQUENCES OF MATING-SYSTEM VARIATION IN TURNERA ULMIFOLIA

Outcrossing rates varied from 0% to 69% among Jamaican populations of Turnera ulmifolia. A correlation between increasing herkogamy and outcrossing rate occurred among populations. Predictions from sex-allocation theory were tested by estimating allocation to reproductive functions. Significant differences in allocation patterns occurred among populations, but they were not correlated with outcrossing rates. The fitness consequences of inbreeding were assessed in high- and low-density greenhouse experiments for nine populations with variable outcrossing rates. No evidence for inbreeding depression occurred in early portions of the life history, but multiplicative fitness functions provide evidence for inbreeding depression. We tested the prediction that selfing populations have lower levels of inbreeding depression than outcrossing populations but found no significant correlation.     

Reduced mate availability leads to evolution of self‐fertilization and purging of inbreeding depression in a hermaphrodite

Basic models of mating‐system evolution predict that hermaphroditic organisms should mostly either cross‐fertilize, or self‐fertilize, due to self‐reinforcing coevolution of inbreeding depression and outcrossing rates. However transitions between mating systems occur. A plausible scenario for such transitions assumes that a decrease in pollinator or mate availability temporarily constrains outcrossing populations to self‐fertilize as a reproductive assurance strategy. This should trigger a purge of inbreeding depression, which in turn encourages individuals to self‐fertilize more often and finally to reduce male allocation. We tested the predictions of this scenario using the freshwater snail Physa acuta, a self‐compatible hermaphrodite that preferentially outcrosses and exhibits high inbreeding depression in natural populations. From an outbred population, we built two types of experimental evolution lines, controls (outcrossing every generation) and constrained lines (in which mates were often unavailable, forcing individuals to self‐fertilize). After ca. 20 generations, individuals from constrained lines initiated self‐fertilization earlier in life and had purged most of their inbreeding depression compared to controls. However, their male allocation remained unchanged. Our study suggests that the mating system can rapidly evolve as a response to reduced mating opportunities, supporting the reproductive assurance scenario of transitions from outcrossing to selfing.     

Pollination biology and outcrossing rates in hermaphroditic Schiedea lydgatei (Caryophyllaceae)

The influence of outcrossing and pollination biology on the maintenance of hermaphroditism was studied for Schiedea lydgatei (Caryophyllaceae: Alsinoideae), a species endemic to Moloka`i in the Hawaiian Islands. Schiedea lydgatei is the only hermaphroditic species in an otherwise dimorphic clade and hermaphroditism is likely the result of a reversal from a gynodioecious ancestor. Both wind and native moths in the family Pyralidae are responsible for pollination in S. lydgatei. Outcrossing rates were generally high (0.80), especially in years when the greatest number of plants were flowering. The combination of high outcrossing rates and substantial inbreeding depression indicates that at present females would not be favored in the population. Pollination by both wind and insects is consistent with the hypothesis that hermaphroditism is the result of a relatively recent reversal, as the ancestor of S. lydgatei was probably wind pollinated and gynodioecious with few females in the populations. A shift from wind to predominately insect pollination on Moloka`i may have resulted in increased outcrossing rates and prevented the expression of high inbreeding depression among progeny of hermaphrodites, a condition that would select against females and favor a reversal to hermaphroditism. Because few females were likely to have been present in ancestral populations that colonized Moloka`i, founder effect is another potential explanation for loss of females. In either case, current high levels of outcrossing prevent re-establishment of females in populations of S. lydgatei.     

MIXED MATING SYSTEMS IN HAWAIIAN BIDENS (ASTERACEAE)

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

THE EVOLUTION OF SELF-FERTILIZATION AND INBREEDING DEPRESSION IN PLANTS. II. EMPIRICAL OBSERVATIONS

A bimodal distribution of outcrossing rates was observed for natural plant populations, with more primarily selfing and primarily outcrossing species, and fewer species with intermediate outcrossing rate than expected by chance. We suggest that this distribution results from selection for the maintenance of outcrossing in historically large, outcrossing populations with substantial inbreeding depression, and from selection for selfing when increased inbreeding, due to pollinator failure or population bottlenecks, reduces the level of inbreeding depression. Few species or populations are fixed at complete selfing or complete outcrossing. A low level of selfing in primarily outcrossing species is unlikely to be selectively advantageous, but will not reduce inbreeding depression to the level where selfing is selectively favored, particularly if accompanied by reproductive compensation. Similarly, occasional outcrossing in primarily selfing species is unlikely to regularly provide sufficient heterosis to maintain selection for outcrossing through individual selection. Genetic, morphological and ecological constraints may limit the potential for outcrossing rates in selfers to be reduced below some minimum level.     

MUTATIONAL MELTDOWN IN SELFING ARABIDOPSIS LYRATA

The majority of plant species and many animals are hermaphrodites, with individuals expressing both female and male function. Although hermaphrodites can potentially reproduce by self‐fertilization, they have a high prevalence of outcrossing. The genetic advantages of outcrossing are described by two hypotheses: avoidance of inbreeding depression because selfing leads to immediate expression of recessive deleterious mutations, and release from drift load because self‐fertilization leads to long‐term accumulation of deleterious mutations due to genetic drift and, eventually, to extinction. I tested both hypotheses by experimentally crossing Arabidopsis lyrata plants (self‐pollinated, cross‐pollinated within the population, or cross‐pollinated between populations) and measuring offspring performance over 3 years. There were 18 source populations, each of which was either predominantly outcrossing, mixed mating, or predominantly selfing. Contrary to predictions, outcrossing populations had low inbreeding depression, which equaled that of selfing populations, challenging the central role of inbreeding depression in mating system shifts. However, plants from selfing populations showed the greatest increase in fitness when crossed with plants from other populations, reflecting higher drift load. The results support the hypothesis that extinction by mutational meltdown is why selfing hermaphroditic taxa are rare, despite their frequent appearance over evolutionary time.     

Digest: Exposing the role of rare alleles in inbreeding depression in monkeyflower*

Outcrossing is maintained in many hermaphroditic species despite theoretical work suggesting that alleles increasing selfing should invade outcrossing populations. Brown and Kelly (2019) identify reasons why this may not have occurred in an outcrossing population of monkeyflower, namely that inbreeding depression causes strong reductions in fitness, resulting in selection for the maintenance of outcrossing. They find that genetic load imposed by rare alleles is inversely correlated with fitness-associated traits, providing evidence that recessive, deleterious alleles contribute to inbreeding depression.     

THE EVOLUTION OF SELF-FERTILIZATION AND INBREEDING DEPRESSION IN PLANTS. I. GENETIC MODELS

The amounts of inbreeding depression upon selfing and of heterosis upon outcrossing determine the strength of selection on the selfing rate in a population when this evolves polygenically by small steps. Genetic models are constructed which allow inbreeding depression to change with the mean selfing rate in a population by incorporating both mutation to recessive and partially dominant lethal and sublethal alleles at many loci and mutation in quantitative characters under stabilizing selection. The models help to explain observations of high inbreeding depression (> 50%) upon selfing in primarily outcrossing populations, as well as considerable heterosis upon outcrossing in primarily selfing populations. Predominant selfing and predominant outcrossing are found to be alternative stable states of the mating system in most plant populations. Which of these stable states a species approaches depends on the history of its population structure and the magnitude of effect of genes influencing the selfing rate.     

Increased inbreeding but not homozygosity in small populations of Sabatia angularis (Gentianaceae)

Understanding how the mating system varies with population size in plant populations is critical for understanding their genetic and demographic fates. We examined how the mating system, characterized by outcrossing rate, biparental inbreeding rate, and inbreeding coefficient, and genetic diversity varied with population size in natural populations of the biennial Sabatia angularis. We found a significant, positive relationship between outcrossing and population size. Selfing was as high as 40% in one small population but was only 7% in the largest population. Despite this pattern, observed heterozygosity did not vary with population size, and we suggest that selection against inbred individuals maintains observed heterozygosity in small populations. Consistent with this hypothesis, we found a trend of lower inbreeding coefficients in the maternal than progeny generation in all of the populations, and half of the populations exhibited significant excesses of adult heterozygosity. Moreover, genetic diversity was not related to population size and was similar across all populations examined. Our results suggest that the consequences of increased selfing for population fitness in S. angularis, a species that experiences significant inbreeding depression, will depend on the relative magnitude and consistency of inbreeding depression and the demographic cost of selection for outcrossed progeny in small populations.     

Wild sorghum from different eco-geographic regions of Kenya display a mixed mating system

Knowledge of mating systems is required in order to understand the genetic composition and evolutionary potential of plant populations. Outcrossing in a population may co-vary with the ecological and historical factors influencing it. However, literature on the outcrossing rate is limited in terms of wild sorghum species coverage and eco-geographic reference. This study investigated the outcrossing rates in wild sorghum populations from different ecological conditions of Kenya. Twelve wild sorghum populations were collected in four sorghum growing regions. Twenty-four individuals per population were genotyped using six polymorphic simple sequence repeat (SSR) markers to compute their indirect equilibrium estimates of outcrossing rate as well as population structure. In addition, the 12 populations were planted in a field in a randomised block design with five replications. Their progeny (250 individuals per population) were genotyped with the six SSR markers to estimate multi-locus outcrossing rates. Equilibrium estimates of outcrossing rates ranged from 7.0 to 75.0%, while multi-locus outcrossing rates (t _m) ranged from 8.9 to 70.0% with a mean of 49.7%, indicating that wild sorghum exhibits a mixed mating system. The wide range of estimated outcrossing rates in wild sorghum populations indicate that environmental conditions may exist under which fitness is favoured by outcrossing and others under which selfing is more advantageous. The genetic structure of the populations studied is concordant with that expected for a species displaying mixed mating system.     

High selfing and high inbreeding depression in peripheral populations of Juncus atratus

The mating system of a plant is the prime determinant of its population genetic structure. However, mating system effects may be modified by postzygotic mechanisms like inbreeding depression. Furthermore, historical as well as contemporary ecological factors and population characteristics, like the location within the species range can contribute to genetic variability. Using microsatellite markers we assessed the population genetic structure of the wind-pollinated Juncus atratus in 16 populations from peripheral and nearly central areas of the distribution range and studied the mating system of the species. In three peripheral populations, outcrossing rates at seeds stage were low (mean t(m) = 5.6%), suggesting a highly autogamous mating system. Despite this fact, on adult stage both individual heterozygosity (mean H(O) = 0.48) and gene diversity (mean H(E) = 0.58) were high even in small populations. Inbreeding coefficients were consistently low among all populations (mean F(IS) = 0.15). Within the three peripheral populations indirect estimates of lifetime inbreeding depression were surprisingly high (delta(eq) = 0.96) and inbreeding depression could be shown to act mostly on early seedling establishment. Similar conditions of autogamy combined with high inbreeding depression are typical for plants with a large lifetime genomic mutation rate that cannot avoid selfing by geitonogamy. However, the results presented here are unexpected for small-statured, herbaceous plants. Substantial genetic differentiation among all populations was found (mean F(ST) = 0.24). An isolation-by-distance pattern was apparent on large scale but not on local scale suggesting that the overall pattern was largely influenced by historical factors, e.g. colonization, whereas locally genetic drift was of greater importance than gene flow. Peripheral populations exhibited lower genetic diversity and higher inbreeding coefficients when compared with subcentral populations.     

Inbreeding depression and mixed mating in Leptosiphon jepsonii: a comparison of three populations

BACKGROUND AND AIMS: Inbreeding depression is thought to play a central role in the evolution and maintenance of cross-fertilization. Theory indicates that inbreeding depression can be purged with self-fertilization, resulting in positive feedback for the selection of selfing. Variation among populations of Leptosiphon jepsonii in the timing and rate of self-fertilization provides an opportunity to study the evolution of inbreeding depression and mating systems. In addition, the hypothesis that differences in inbreeding depression for male and female fitness can stabilize mixed mating in L. jepsonii is tested. METHODS: In a growth room experiment, inbreeding depression was measured in three populations with mean outcrossing rates ranging from 0.06 to 0.69. The performance of selfed and outcrossed progeny is compared at five life history stages. To distinguish between self-incompatibility and early inbreeding depression, aborted seeds and unfertilized ovules were counted in selfed and outcrossed fruits. In one population, pollen and ovule production was quantified to estimate inbreeding depression for male and female fitness. KEY RESULTS: Both prezygotic barriers and inbreeding depression limited self seed set in the most outcrossing population. Cumulative inbreeding depression ranged from 0.297 to 0.501, with the lowest value found in the most selfing population. Significant inbreeding depression for early life stages was found only in the more outcrossing populations. Inbreeding depression was not significant for pollen or ovule production. CONCLUSIONS: The results provide modest support for the hypothesized relationship between inbreeding depression and mating systems. The absence of early inbreeding depression in the more selfing populations is consistent with theory on purging. Differences in male and female expression of inbreeding depression do not appear to stabilize mixed mating in L. jepsonii. The current estimates of inbreeding depression for L. jepsonii differ from those of previous studies, underscoring the effects of environmental variation on its expression.