LIMITS TO REPRODUCTIVE SUCCESS IN A PARTIALLY SELF-INCOMPATIBLE HERB: FECUNDITY DEPRESSION AT SERIAL LIFE-CYCLE STAGES

Like many angiosperms, Crinum erubescens is partially self-compatible, producing fewer seeds upon selfing than after outcrossing. In this paper we test the relative magnitude of the prefertilization and postfertilization effects of self-incompatibility, inbreeding depression, or both in a natural population of this hermaphroditic tropical herb. We characterize prefertilization effects by examining pollen tube growth, while postfertilization effects are characterized by examination of embryo abortion and seed maturation. Statistical methods are developed to test the magnitude of these effects from one life-cycle stage to the next. We find that although pollen performance in selfed flowers is lower than that in outcrossed flowers, pollen performance is low overall. Postfertilization effects attributable to inbreeding depression account for a larger proportion of the reduction in fecundity in selfed compared to outcrossed flowers. Among naturally pollinated plants, despite ample pollen deposition, the numbers of fruits and seeds set are intermediate to selfed and outcrossed treatments.     

Different levels of inbreeding depression between outcrossing and selfing Serapias species

We quantified inbreeding depression for fruit production, embryo vitality and seed germination in three deceptive orchids, Serapias vomeracea, S. cordigera and S. parviflora, which do not provide any reward to their pollinators, and are predicted to experience high outcrossing. Of the three species examined only S. parviflora was autonomously selfing. Both S. vomeracea and S. cordigera showed highly significant differences in fitness between selfed and outcrossed progenies, resulting in high levels of inbreeding depression, which increased in magnitude from seed set to seed germination. Inbreeding depression may promote outcrossing in Serapias by acting as a post-pollination barrier to selfing. Cumulative inbreeding depression across three stages in S. parviflora was lower that in both outcrossing species. The large difference in germination between selfed and outcrossed seeds is an important issue in conservation biology.     

Effects of inbreeding on male function and self-fertility in the partially self-incompatible herb Campanula rapunculoides (Campanulaceae)

We examined the effect of inbreeding on fitness (through both male and female functions) and changes in self-fertility in the partially self-incompatible species Campanula rapunculoides. Individuals in natural populations of C. rapunculoides varied extensively in their strength of self-incompatibility (SI). We crossed 11 individuals that differed in their strength of SI to generate families with four levels of inbreeding (f = 0.0, 0.25, 0.5, and 0.75). Progeny were scored for three traits related to male fitness and for outcrossed and selfed seed production. Analyses of variance revealed significant inbreeding depression for the three male traits and seed set. Families with strong or weak SI differed in their response to inbreeding. Families with weak SI had lower levels of inbreeding depression for most traits than families with strong SI, but strong SI families had a greater increase in selfed seed set, but not self-fertility, with inbreeding. Finally, we found evidence of a significant linear response to inbreeding for all three male reproductive traits and outcrossed seed, indicating that inbreeding depression was primarily caused by partially or fully recessive deleterious alleles. Variation in genetic load was associated with variation in self-fertility, a finding that suggests an evolutionary role for partial self-fertility in natural populations of C. rapunculoides.     

Outcrossing rate and inbreeding depression in the perennial yellow bush lupine, Lupinus arboreus (Fabaceae)

Little is known about the breeding systems of perennial Lupinus species. We provide information about the breeding system of the perennial yellow bush lupine, Lupinus arboreus, specifically determining self-compatibility, outcrossing rate, and level of inbreeding depression. Flowers are self-compatible, but autonomous self-fertilization rarely occurs; thus selfed seed are a product of facilitated selfing. Based on four isozyme loci from 34 maternal progeny arrays of seeds we estimated an outcrossing rate of 0.78. However, when we accounted for differential maturation of selfed seeds, the outcrossing rate at fertilization was lower, ～0.64. Fitness and inbreeding depression of 11 selfed and outcrossed families were measured at four stages: seed maturation, seedling emergence, seedling survivorship, and growth at 12 wk. Cumulative inbreeding depression across all four life stages averaged 0.59, although variation existed between families for the magnitude of inbreeding depression. Inbreeding depression was not manifest uniformly across all four life stages. Outcrossed flowers produced twice as many seeds as selfed flowers, but the mean performance of selfed and outcrossed progeny was not different for emergence, seedling survivorship, and size at 12 wk. Counter to assumptions about this species, L. arboreus is both self-compatible and outcrosses ～78% of the time.     

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.     

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.     

Inbreeding depression in Campanula rapunculoides L. I. A comparison of inbreeding depression in plants derived from strong and weak self-incompatibility phenotypes

The evolution of selfing taxa from outcrossing ancestors has occurred repeatedly and is the subject of many theoretical models, yet few empirical studies have examined the immediate consequences of inbreeding in a population with variable expression of self-incompatibility. Because self-incompatibility breaks down with floral age in Campanula rapunculoides, we were able to mate outbred and selfed maternal plants in a crossing design which produced progeny with inbreeding coefficients of 0, 0.25, 0.50 and 0.75. Cumulative inbreeding depression in plants that were selfed for one generation was very high in families derived from strongly self-incompatible plants (average δ = 0.98), and somewhat lower in families derived from plants with weaker expression of self-incompatibility (average δ = 0.90). Relative to outbred progeny, inbred progeny produced fewer seeds, had lower rates of germination, less vegetative growth and fewer flowers per plant. Inbred progeny also took longer to germinate, and longer to produce a first leaf and to flower. Interestingly, inbred plants also produced 40% fewer seeds than outcrossed plants (t-test P < 0.001) even when mated to the same, unrelated pollen donor, suggesting that inbreeding can produce profound maternal effects. Most importantly, our results demonstrate that progeny derived from plants with stronger expression of self-incompatibility exhibited greater levels of inbreeding depression than progeny from plants with weaker expression of self-incompatibility. Moreover, the decline in fitness (cumulative, ln-transformed) over the four inbreeding levels was steeper for the progeny of the strongly self-incompatible lineages. These empirical results suggest that inbreeding depression and mating system phenotype have the potential to coevolve.     

Evidence for selective fruit abortion in Banksia spinulosa (Proteaceae)

Fruit set is consistently low in the genus Banksia (Proteaceae). One explanation for low fruit:flower ratios is that excess flowers allow plants to selectively abort inferior progeny thereby increasing the average quality of the seed crop. We examined whether self-pollinated fruits were aborted selectively in a partially self-compatible population of Banksia spinulosa. Inflorescences were divided in half and pollinated with cross pollen on both sides, self pollen on both sides or cross pollen on one side and self pollen on the other. Seed production was reduced significantly by 38% on fully self-pollinated compared to fully cross-pollinated inflorescences, indicating partial self-incompatibility or inbreeding depression. On inflorescences pollinated with both cross and self pollen, selfed seed set was reduced by 63% compared to crossed seed set. On the selfed half of these inflorescences, fewer fruits and seeds were produced, and more fruits had aborted seeds than on randomly selected halves of fully self-pollinated inflorescences. These results suggest that self-fertilized embryos were developed in the absence of outcrossed progeny but were selectively aborted when outcrossed progeny were present on inflorescences. To determine the amount of outcrossing occurring in the study population, outcrossing rates (t) of naturally pollinated plants were examined by electrophoretic assessment of progeny arrays. Values of I did not differ significantly from one, indicating complete outcrossing. High outcrossing, despite the high probability of self pollen receipt under natural conditions, is consistent with the conclusion that selective abortion occurs in B. spinulosa.     

Inbreeding depression and selfing rates in a self-compatible,hermaphroditic species,Schiedea membranacea (Caryophyllaceae)

Inbreeding depression and selfing rates were investigated in Schiedea membranacea (Caryophyllaceae), a hermaphroditic species endemic to the Hawaiian Islands. Most theoretical models predict high inbreeding depression in outcrossing hermaphroditic species and low inbreeding depression in inbreeding species. Although high outcrossing rates and high levels of inbreeding depression are characteristic of many species of Schiedea, self- fertilization is common among relatives of hermaphroditic S. membranacea, and high selfing rates and low levels of inbreeding depression were predicted in this species. Sixteen individuals grown in the greenhouse were used to produce selfed and outcrossed progeny. Inbreeding depression, which was evident throughout the stages measured (percentage viable seeds per capsule, mean seed mass, percentage seed germination, percentage seedling survival, and biomass after 8 mo), averaged 0.70. Inbreeding depression among maternal families varied significantly for all measured traits and ranged from −0.12 to 0.97. Using isozyme analysis, the multilocus selfing rate varied from 0.13 to 0.38 over 4 yr. Contrary to the initial prediction of high selfing and low inbreeding depression based on phylogenetic relationships within Schiedea, low selfing rates and high levels of inbreeding depression were found in S. membranacea. These results indicate that outcrossing is stable in this species and maintained by high levels of inbreeding depression.     

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.     

Inbreeding effects on fitness traits in the heterocarpic herb Leontodon autumnalis L. (Asteraceae)

Heterocarpic plants are characterized by the production of distinct types of fruits that usually differ in their ecological behavior. In the Asteraceae, differences are mainly found between peripheral non-dispersal and central dispersal achenes (single-seeded fruits). Inbreeding depression is considered as an evolutionary force as it may reduce several fitness traits, and in the case of heterocarpic plants, it could influence fitness traits (e.g., seed set, germination rate, growth rate) of each fruit morph, which may have important ecological and evolutionary consequences. In particular, differential effects on fitness traits and dispersal of selfed and outcrossed progeny can strongly determine the viability of extant populations and the potential to colonize new habitats. We conducted a hand-pollination experiment in greenhouse conditions to test whether inbreeding affects the fitness of achene morphs in the heterocarpic herb Leontodon autumnalis (Asteraceae). Results show that achene morphs significantly differ in their ecological behavior, peripheral achenes germinating more and faster than central achenes. The significant interaction between pollination treatment and achene morph for germination probability might indicate a link between dormancy and mating system in L. autumnalis: germination was higher for outcrossed achenes in central achenes whereas the opposite pattern was exhibited by peripheral achenes. Selfing dramatically reduced seed set, probably as a consequence of strong self-incompatibility mechanisms rather than inbreeding effects. Inbreeding depression significantly affected late life-cycle traits, such as growth rate and biomass at flowering. Overall, results suggest that inbreeding depression seems to be an important selective force maintaining outcrossing in L. autumnalis.     

Conservation implications of the reproductive ecology of Agalinis acuta (Scrophulariaceae)

Reproductive ecology of Agalinis acuta was investigated by examining potential for self-fertilization before and at anthesis, reproductive output from outcrossed vs. selfed matings, and effects of browsing, plant size, and conspecific plant density on seed and fruit production. These features of a plant species can provide indirect information pertinent to conservation such as patterns and maintenance of genetic diversity, risk associated with inbreeding depression, and changes in pollinator abundance or effectiveness. The species is self-compatible, with 97% of selfed flowers setting fruit; pollinators were not required for reproduction. However, seed set in self-pollinated fruits averaged 17-20% less than that in open-pollinated fruits. Geitonogamous and facilitated selfing are possible throughout anthesis and autonomous selfing is possible late in anthesis as corollas abscise. Delaying self-pollination until after outcrossing opportunities likely limits selfing rates and thus reduces risks associated with inbreeding but allows reproduction in absence of pollinators. Supplementing pollen on open-pollinated flowers yielded no additional seed set over controls. Neither early-season browsing of primary stems nor conspecific plant density had significant effects on number of fruits per plant, on fruit size, or on number of seeds from open-pollinated flowers. Currently, reproduction appears to be high (about 2400 seeds/plant), and future risks due to lack of genetic diversity are likely low.     

INBREEDING DEPRESSION IN SCHIEDEA GLOBOSA AND S. SALICARIA (CARYOPHYLLACEAE), SUBDIOECIOUS AND GYNODIOECIOUS HAWAIIAN SPECIES

Progeny produced by inbreeding were compared to progeny derived from outcrosses for gynodioecious Schiedea salicaria and subdioecious S. globosa to assess fitness consequences of breeding system on parental fecundity (seeds per capsule) and progeny measures of fitness (germination, survival, biomass, and number of flowers). Results from both species indicated that inbreeding depression occurred at all measured stages of the life history. In both species, different females showed different levels of inbreeding depression. Multiplicative fitness functions of the ratio of values for selfed and outcrossed progeny in S. salicaria resulted in inbreeding depression values of 0.62–0.94. Within- vs. between-family crosses of S. globosa also resulted in inbreeding depression values as high as 0.49. These values suggest that inbreeding depression may promote the evolution of dioecy within S. globosa and S. salicaria, depending on the levels of natural outcrossing.     

Costs of selfing prevent the spread of a self‐compatibility mutation that causes reproductive assurance

In flowering plants, shifts from outcrossing to partial or complete self‐fertilization have occurred independently thousands of times, yet the underlying adaptive processes are difficult to discern. Selfing's ability to provide reproductive assurance when pollination is uncertain is an oft‐cited ecological explanation for its evolution, but this benefit may be outweighed by costs diminishing its selective advantage over outcrossing. We directly studied the fitness effects of a self‐compatibility mutation that was backcrossed into a self‐incompatible (SI) population of Leavenworthia alabamica, illuminating the direction and magnitude of selection on the mating‐system modifier. In array experiments conducted in two years, self‐compatible (SC) plants produced 17–26% more seed, but this advantage was counteracted by extensive seed discounting—the replacement of high‐quality outcrossed seeds by selfed seeds. Using a simple model and simulations, we demonstrate that SC mutations with these attributes rarely spread to high frequency in natural populations, unless inbreeding depression falls below a threshold value (0.57 ≤ δ_threshold ≤ 0.70) in SI populations. A combination of heavy seed discounting and inbreeding depression likely explains why outcrossing adaptations such as self‐incompatibility are maintained generally, despite persistent input of selfing mutations, and frequent limits on outcross seed production in nature.     

Effects of cleistogamy and pollen source on seed production and offspring performance in three endangered violets

In rare plants that often occur in small or isolated populations the probability of selfing between close relatives is increased as a consequence of demographic stochasticity. The mode of pollination (selfing, outcrossing) may have considerable effects on seed traits and offspring performance and hence potential viability. Since current efforts aiming at the restoration of floodplain grasslands through the transfer of plant material from species-rich source stands may lead to the establishment of initially small populations consisting of founders from different populations, the present paper experimentally investigated the effects of pollen source and floral types (i.e. chasmogamous (CH) and cleistogamous (CL) flowers) on seed traits and offspring performance in three highly endangered violet species (Viola elatior, V. pumila, V. stagnina) of these grasslands. We estimated inbreeding depression and tested the performance of selfed and outcrossed offspring in two microbial environments, i.e. in soil inoculated with (i) non-sterile substrate from the same species (‘home’-conditions) and (ii) sterilised substrate.Plants produced more CL capsules than CH flowers. Pollinator exclusion had only small effects on CH seed production. CL seeds had a significantly lower mass per seed than CH seeds. This may be related to constraints in allocation or environmental conditions. Seedling growth was reduced in plants grown under ‘home’-conditions as compared to control soils. Under ‘home’-conditions, relative fitness of selfed seedlings of V. stagnina was significantly higher than that of crossed progeny. Our results suggest that high genetic differentiation among populations as a consequence of isolation may result in outbreeding depression, e.g., through biochemical or physiological incompatibilities between genes or the breaking of coadapted gene complexes. In V. stagnina, offspring fitness differed considerably between environments, but in general we found no indications for inbreeding depression in these rare species.     

Inbreeding effects on mating system traits for two species of Lupinus (Leguminosae)

The consequences of inbreeding for reproductive traits were investigated for two closely related annual lupines that differ in their mating system. Lupinus bicolor (Leguminosae) is a primarily selling species while Lupinus nanus outcrosses at intermediate rates. A controlled crossing program was used for each species to produce selfed and outcrossed progeny. These progeny were then grown in a greenhouse and scored for the date of first flower, flower morphology, and autofertility. Selfed progeny of L. bicolor produced significantly smaller flowers but did not differ from outcrossed progeny for the remaining traits. Selfed progeny of L. nanus produced flowers that significantly differed in shape and had fewer ovules than the flowers of outcrossed progeny. Selfed progeny of L. nanus also had significantly lower rates of autofertility in comparison to outcrossed progeny. The significant effects of inbreeding on these mating system traits may indicate the presence of directional dominance at the loci underlying these characters. The consequences of these direct effects of inbreeding on reproductive traits for plants growing in natural populations may include nonadaptive changes in the outcrossing rate between generations.     

THE RELATIVE COSTS OF SELF- AND CROSS-FERTILIZED SEEDS IN IMPATIENS CAPENSIS (BALSAMINACEAE)

By using a generally applicable technique that involves monitoring the development and survivorship of flowers and seed capsules, I estimated the material and energetic costs of producing self- and cross-fertilized seeds in Impatiens capensis. All flowers and fruits on six plants were censused intensively for the two-month period of reproduction. Cleistogamous (selfing) flowers ripened seed in about 24 days, compared to about 36 days for the chasmogamous (outcrossing) flowers. In terms of dry weight, selfed seeds cost about two-thirds as much as outcrossed seeds: 12.4 versus 18.4 mg dry weight per seed. When adjusted to the currency of calories, and including an independent estimate of pollen and nectar production in outcrossing flowers, I estimate the costs to be about 65 and 135 calories per selfed or outcrossed seed. Sources of error include the accuracy of the estimates of flower and fruit weight, and possible differences among the developmental stages in respiratory costs. The cost discrepancy implies that outcrossed seeds should possess a countervailing fitness advantage large enough to offset their greater energetic cost.     

Experimental and genetic analyses reveal that inbreeding depression declines with increased self‐fertilization among populations of a coastal dune plant

Theory predicts that inbreeding depression (ID) should decline via purging in self‐fertilizing populations. Yet, intraspecific comparisons between selfing and outcrossing populations are few and provide only mixed support for this key evolutionary process. We estimated ID for large‐flowered (LF), predominantly outcrossing vs. small‐flowered (SF), predominantly selfing populations of the dune endemic Camissoniopsis cheiranthifolia by comparing selfed and crossed progeny in glasshouse environments differing in soil moisture, and by comparing allozyme‐based estimates of the proportion of seeds selfed and inbreeding coefficient of mature plants. Based on lifetime measures of dry mass and flower production, ID was stronger in nine LF populations [mean δ = 1?(fitness of selfed seed/fitness of outcrossed seed) = 0.39] than 16 SF populations (mean δ = 0.03). However, predispersal ID during seed maturation was not stronger for LF populations, and ID was not more pronounced under simulated drought, a pervasive stress in sand dune habitat. Genetic estimates of δ were also higher for four LF (δ = 1.23) than five SF (δ = 0.66) populations; however, broad confidence intervals around these estimates overlapped. These results are consistent with purging, but selective interference among loci may be required to maintain strong ID in partially selfing LF populations, and trade‐offs between selfed and outcrossed fitness are likely required to maintain outcrossing in SF populations.     

The cost of selfing inEncyclia cochleata (Orchidaceae)

To determine whether inbreeding depression accounts for the maintenance of outcrossing in populations of the self-compatible orchidEncyclia cochleata, the estimated selective advantage of selfing was compared to a measure of inbreeding depression. Individuals from three populations ofE. cochleata and some of their progeny were phenotyped using isozyme analysis. The electrophoretic data were used to estimate the outcrossing rate and the theoretical cost of outcrossing. Inbreeding depression was estimated by comparing the fitness of the progeny resulting from both types of pollinations. The seeds from outcrossed and selfed hand-pollinations and naturally pollinated seeds from a population of the triandrous form ofE. cochleata were grown aseptically on culture media, and their development over the next three years recorded. Inbreeding was common, particularly in one population (outcrossing rate 40%). However, the level of inbreeding depression was only 1–2%, considerably less inbreeding depression than expected.     

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

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