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.     

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.     

Inbreeding depression under intraspecific competition in a highly outcrossing population of Crepis sancta (Asteraceae): evidence for frequency-dependent variation

Inbreeding depression is a major selective factor acting to maintain outcrossing in hermaphroditic plants. Recently it has been shown that environmental conditions may greatly affect the levels of inbreeding depression. In this study, the effects of intraspecific competition, from either crossed or inbred progeny, and plant density on the expression of inbreeding depression were estimated for the allogamous colonizing plant Crepis sancta (Asteraceae). The population used in this experiment showed a very high outcrossing rate (t = 0.99). Inbreeding depression was measured for germination, survival to reproduction, number of capitula, and a multiplicative fitness estimate. We found that inbreeding depression on survival, the number of capitula, and total fitness was the greatest when inbred plants compete with outbred plants. The effect of density on inbreeding depression was less evident. The major implication of our study is that the relative fitness of selfed progeny is strongly influenced by the type of competitors (outbred or inbred). These results support the hypothesis that inbreeding depression varies according to the density and frequency of outbred plants and suggest that it could be considered as a density- and frequency-dependent phenomenon.     

DROUGHT STRESS AND INBREEDING DEPRESSION IN LYCHNIS FLOS-CUCULI (CARYOPHYLLACEAE)

Interactions between drought stress and inbreeding depression were studied in Lychnis flos-cuculi. Four inbreeding levels (F = 0, 0.25, 0.50 and 0.75), and three watering treatments were used. Performance was scored for germination rate and proportion, survival, plant size, proportion of plants flowering, flowering date, stem height, number of flowers, flower size, anther weight, fruiting proportion and number of capsules. Multiplicative fitness values were estimated from these traits. Inbreeding affected most of the traits studied, and a severe inbreeding depression was found for the combined fitness estimates. The higher inbreeding depression found here relative to the same family groups in a former experiment may reflect greater dominance and suppression in the present experiment at higher density.     

Intergenerational effects of inbreeding in Nicrophorus vespilloides: offspring suffer fitness costs when either they or their parents are inbred

Inbreeding depression is the reduction in fitness caused by mating between related individuals. Inbreeding is expected to cause a reduction in offspring fitness when the offspring themselves are inbred, but outbred individuals may also suffer a reduction in fitness when they depend on care from inbred parents. At present, little is known about the significance of such intergenerational effects of inbreeding. Here, we report two experiments on the burying beetle Nicrophorus vespilloides, an insect with elaborate parental care, in which we investigated inbreeding depression in offspring when either the offspring themselves or their parents were inbred. We found substantial inbreeding depression when offspring were inbred, including reductions in hatching success of inbred eggs and survival of inbred offspring. We also found substantial inbreeding depression when parents were inbred, including reductions in hatching success of eggs produced by inbred parents and survival of outbred offspring that received care from inbred parents. Our results suggest that intergenerational effects of inbreeding can have substantial fitness costs to offspring, and that future studies need to incorporate such costs to obtain accurate estimates of inbreeding depression.     

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.     

EFFECTS OF DIFFERENT LEVELS OF INBREEDING ON PROGENY FITNESS IN PLANTAGO CORONOPUS

Inbreeding depression (δ) is a major selective force favoring outcrossing in flowering plants. Many phenotypic and genetic models of the evolution of selfing conclude that complete outcrossing should evolve whenever inbreeding depression is greater than one-half, otherwise selfing should evolve. Recent theoretical work, however, has challenged this view and emphasized (1) the importance of variation in inbreeding depression among individuals within a population; and (2) the nature of gene action between deleterious mutations at different loci (epistasis) as important determinants for the evolution of plant mating systems. The focus of this study was to examine the maintenance of inbreeding depression and the relationship between inbreeding level and inbreeding depression at both the population and the individual level in one population of the partially self-fertilizing plant Plantago coronopus (L.). Maternal plants, randomly selected from an area of about 50 m² in a natural population, were used to establish lines with expected inbreeding coefficients (f) of 0, 0.25, 0.50, 0.75, and 0.875. Inbreeding depression was estimated both in the greenhouse and at the site of origin of the maternal plants by comparing growth, survival, flowering, and seed production of the progeny with different inbreeding coefficients. No significant inbreeding depression for these fitness traits was detected in the greenhouse after 16 weeks. This was in strong contrast to the field, where the traits all displayed significant inbreeding depression and declined with increased inbreeding. The results were consistent with the view that mutation to mildly deleterious alleles is the primary cause of inbreeding depression. At the family level, significantly different maternal line responses (maternal parent × inbreeding level interaction) provide a mechanism for the invasion of a selfing variant into the population through any maternal line exhibiting purging of its genetic load. At the population level, evidence for synergistic epistasis was detected for the probability of flowering, but not for total seed production. At the family level, however, a significant interaction between inbreeding level and maternal families for both traits was observed, indicating that epistasis could play a role in the expression of inbreeding depression among maternal lines.     

The effects of inbreeding, genetic dissimilarity and phenotype on male reproductive success in a dioecious plant

Pollen fate can strongly affect the genetic structure of populations with restricted gene flow and significant inbreeding risk. We established an experimental population of inbred and outbred Silene latifolia plants to evaluate the effects of (i) inbreeding depression, (ii) phenotypic variation and (iii) relatedness between mates on male fitness under natural pollination. Paternity analysis revealed that outbred males sired significantly more offspring than inbred males. Independently of the effects of inbreeding, male fitness depended on several male traits, including a sexually dimorphic (flower number) and a gametophytic trait (in vitro pollen germination rate). In addition, full-sib matings were less frequent than randomly expected. Thus, inbreeding, phenotype and genetic dissimilarity simultaneously affect male fitness in this animal-pollinated plant. While inbreeding depression might threaten population persistence, the deficiency of effective matings between sibs and the higher fitness of outbred males will reduce its occurrence and counter genetic erosion.     

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.     

Fitness drivers in the threatened Dianthus guliae Janka (Caryophyllaceae): disentangling effects of growth context, maternal influence and inbreeding depression

We studied inbreeding depression, growth context and maternal influence as constraints to fitness in the self-compatible, protandrous Dianthus guliae Janka, a threatened Italian endemic. We performed hand-pollinations to verify outcomes of self- and cross-fertilisation over two generations, and grew inbred and outbred D.?guliae offspring under different conditions - in pots, a common garden and field conditions (with/without nutrient addition). The environment influenced juvenile growth and flowering likelihood/rate, but had little effect on inbreeding depression. Significant interactions among genetic and environmental factors influenced female fertility. Overall, genetic factors strongly affected both early (seed mass, seed germination, early survival) and late (seed/ovule ratio) life-history traits. After the first pollination experiment, we detected higher mortality in the selfed progeny, which is possibly a consequence of inbreeding depression caused by over-expression of early-acting deleterious alleles. The second pollination induced a strong loss of reproductive fitness (seed production, seed mass) in inbred D.?guliae offspring, regardless of the pollination treatment (selfing/crossing); hence, a strong (genetic) maternal influence constrained early life-history traits of the second generation. Based on current knowledge, we conclude that self-compatibility does not prevent the detrimental effects of inbreeding in D.?guliae populations, and may increase the severe extinction risk if out-crossing rates decrease.     

Interplay between age‐based competitive asymmetries within the brood and direct competition between inbred and outbred offspring in a burying beetle

Theory suggests that intraspecific competition associated with direct competition between inbred and outbred individuals should be an important determinant of the severity of inbreeding depression. The reason is that, if outbred individuals are stronger competitors than inbred ones, direct competition should have a disproportionate effect on the fitness of inbred individuals. However, an individual's competitive ability is not only determined by its inbreeding status but also by competitive asymmetries that are independent of an individual's inbreeding status. When this is the case, such competitive asymmetries may shape the outcome of direct competition between inbred and outbred individuals. Here, we investigate the interface between age‐based competitive asymmetries within broods and direct competition between inbred and outbred offspring in the burying beetle Nicrophorus vespilloides. We found that inbred offspring had lower survival than outbred ones confirming that there was inbreeding depression. Furthermore, seniors (older larvae) grew to a larger size and had higher survival than juniors (younger larvae), confirming that there were age‐based competitive asymmetries. Nevertheless, there was no evidence that direct competition between inbred and outbred larvae exacerbated inbreeding depression, no evidence that inbreeding depression was more severe in juniors and no evidence that inbred juniors suffered disproportionately due to competition from outbred seniors. Our results suggest that direct competition between inbred and outbred individuals does not necessarily exacerbate inbreeding depression and that inbred individuals are not always more sensitive to poor and stressful conditions than outbred ones.     

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.     

Differential effects of offspring and maternal inbreeding on egg laying and offspring performance in the burying beetle Nicrophorus vespilloides

We investigate the effect of offspring and maternal inbreeding on maternal and offspring traits associated with early offspring fitness in the burying beetle Nicrophorus vespilloides. We conducted two experiments. In the first experiment, we manipulated maternal inbreeding only (keeping offspring outbred) by generating mothers that were outbred, moderately inbred or highly inbred. Meanwhile, in the second experiment, we manipulated offspring inbreeding only (keeping females outbred) by generating offspring that were outbred, moderately inbred or highly inbred. In both experiments, we monitored subsequent effects on breeding success (number of larvae), maternal traits (clutch size, delay until laying, laying skew, laying spread and egg size) and offspring traits (hatching success, larval survival, duration of larval development and average larval mass). Maternal inbreeding reduced breeding success, and this effect was mediated through lower hatching success and greater larval mortality. Furthermore, inbred mothers produced clutches where egg laying was less skewed towards the early part of laying than outbred females. This reduction in the skew in egg laying is beneficial for larval survival, suggesting that inbred females adjusted their laying patterns facultatively, thereby partially compensating for the detrimental effects of maternal inbreeding on offspring. Finally, we found evidence of a nonlinear effect of offspring inbreeding coefficient on number of larvae dispersing. Offspring inbreeding affected larval survival and larval development time but also unexpectedly affected maternal traits (clutch size and delay until laying), suggesting that females adjust clutch size and the delay until laying in response to being related to their mate.     

Severe inbreeding depression is predicted by the “rare allele load” in Mimulus guttatus*

Most flowering plants are hermaphroditic and experience strong pressures to evolve self-pollination (automatic selection and reproductive assurance). Inbreeding depression (ID) can oppose selection for selfing, but it remains unclear if ID is typically strong enough to maintain outcrossing. To measure the full cost of sustained inbreeding on fitness, and its genomic basis, we planted highly homozygous, fully genome-sequenced inbred lines of yellow monkeyflower (Mimulus guttatus) in the field next to outbred plants from crosses between the same lines. The cost of full homozygosity is severe: 65% for survival and 86% for lifetime seed production. Accounting for the unmeasured effect of lethal and sterile mutations, we estimate that the average fitness of fully inbred genotypes is only 3–4% that of outbred competitors. The genome sequence data provide no indication of simple overdominance, but the number of rare alleles carried by a line, especially within rare allele clusters nonrandomly distributed across the genome, is a significant negative predictor of fitness measurements. These findings are consistent with a deleterious allele model for ID. High variance in rare allele load among lines and the genomic distribution of rare alleles both suggest that migration might be an important source of deleterious alleles to local populations.     

The consequences of mating over a range of parental genetic similarity in a selfing allopolyploid plant species

In diploids, F(1) offspring performance is expected to increase with increasing genetic dissimilarity between the parents until an optimum is reached because outbreeding mitigates inbreeding depression and maximizes heterosis. However, many flowering plant species are derived through allopolyploidization, i.e. interspecific hybridization with genome doubling. This mode of plant speciation can be expected to considerably alter the consequences of inbreeding and outbreeding. We investigated the F1 fitness consequences of mating over a range of (genetic) distances in the allohexaploid plant species Geum urbanum. Offspring was raised under controlled conditions (632 plants). The performance of outcrossed progeny was not significantly better than that of their selfed half-siblings and did not increase with parental genetic dissimilarity (0-0.83). Our findings support low, if any, inbreeding depression and heterosis. We attribute this to the peculiar state of quasi-permanent heterozygosity in allopolyploids and frequent selfing.     

INBREEDING DEPRESSION IN FOUR POPULATIONS OF COLLINSIA HETEROPHYLLA NUTT (SCROPHULARIACEAE)

The effects of one and two generations of inbreeding were studied in plants from four natural populations of the annual plant, Collinsia heterophylla, using inbred and outcrossed plants generated by hand pollinations to create expected inbreeding coefficients ranging from 0–0.75. The selfing rates of the populations were estimated using allozyme markers to range from 0.37–0.69. Inbreeding depression was mild, ranging from 5–40%, but significant effects were detected for characters measured at all stages of the life cycle. Fitness components declined significantly with the inbreeding coefficient, and regression of fitness characters on inbreeding coefficients gave no evidence of any strongly synergistic effects attributable to the different genetic factors that contribute to decline in fitness under inbreeding. The magnitude of inbreeding depression did not clearly decrease with the populations' levels of inbreeding. This is not surprising because the selfing rates are similar enough that it is unlikely that the populations have been characterized for long periods of time by these different inbreeding levels.     

Sibling competition, size variation and frequency-dependent outcrossing advantage in Plantago coronopus

Many plants display limited seed dispersal, thereby creating an opportunity for sibling competition, i.e. fitness-determined interactions between related individuals. Here I investigated the consequences of intra-specific competition, by varying density and genetic composition of neighbors, on the performance of seedlings derived by selfing or outcrossing of the partially self-fertilizing plant Plantago coronopus (L.). Seedlings from eight plants, randomly selected from an area of about 50 m² in a natural population, were used in (i) a density series with either one, four or eight siblings of each cross type per pot and (ii) a replacement series with eight plants per pot where selfed and outcrossed siblings were grown intermixed in varying frequencies. Density had a pronounced effect on plant performance. But, except for singly grown individuals, no differences were detected between selfed and outcrossed progenies in vegetative and reproductive biomass. When grown intermixed, selfed offspring were always inferior to their outcrossed relatives. The magnitude of reduction in performance was dependent on the number of outcrossed relatives a selfed seedling had to compete with, giving rise to a frequency-dependent fitness advantage to outcrossed seedlings. The major result of this study is (i) that the relative fitness of inbred progeny is strongly affected by the type of competitors (inbred or outbred) and (ii) that inbreeding depression varies according to the density and frequency of outbred plants and could be considered as a density- and frequency-dependent phenomenon. It is argued that sibling competition, due to the small genetic neighborhood of P. coronopus, might be an important selective force in natural populations of this species.     

THE RELATIONSHIP BETWEEN INBREEDING DEPRESSION AND PRIOR INBREEDING AMONG POPULATIONS OF FOUR MIMULUS TAXA

When recessive mutations are the primary cause of inbreeding depression, a negative relationship between the levels of prior inbreeding and inbreeding depression is expected. We tested this prediction using 15 populations chosen a priori to represent a wide range of prior inbreeding among four closely related taxa of the Mimulus guttatus species complex. Artificially selfed and outcrossed progeny were grown under controlled growth-chamber conditions, and inbreeding depression was estimated for each population as one minus the ratio of the fitness of selfed to outcrossed progeny. Estimates of inbreeding depression varied from 0% to 68% among populations. Inbreeding coefficients, estimated from electrophoretic assay of field-collected progenies, ranged from 0.02 to 0.76. All five fitness traits displayed a negative association between inbreeding depression and the inbreeding coefficient, but only height showed a statistically significant correlation. Inbreeding depression was also not correlated with the level of genetic variability. In addition, populations with similar levels of prior inbreeding showed significant differences of inbreeding depression, whereas populations with different levels of prior inbreeding showed similar inbreeding depression. Within populations, inbreeding depression did not differ between progeny selfed one versus two generations. Our results are weakly consistent with the recessive mutation model of inbreeding depression, but suggest that additional factors, including genotype-by-environment interaction and complex modes of inheritance, may influence the expression of inbreeding depression.     

Seed production from the mixed mating system of Chesapeake Bay (USA) eelgrass (Zostera marina; Zosteraceae)

In monoecious plants, gametes can be exchanged in three ways: among unrelated genets (outbreeding), with close relatives (inbreeding), or within individuals (geitonogamous selfing). These different mating systems may have consequences for population demography and fitness. The experiment presented herein used artificial crosses to examine the mating system of Chesapeake Bay, Virginia, USA eelgrass (Zostera marina L; Zosteraceae), a bisexual submerged aquatic plant that can outbreed, inbreed, and self. Genetic data indicate severe heterozygosity deficiencies and patchy genotype distribution in these beds, suggesting that plants therein reproduce primarily by vegetative propagation, autogamy, or geitonogamy. To clarify eelgrass reproductive strategies, flowers from three genetically and geographically distinct beds were hand-pollinated in outbred, inbred, and selfed matings. Fertilization success and seed production, life history stages which contribute greatly to the numeric maintenance of populations, were monitored. We found no evidence that inbreeding had negative consequences for seed production. On the contrary, selfed matings produced seeds significantly more frequently than outcrossed matings and produced significantly larger numbers of seeds than either inbred or outbred matings. These results contrast with patterns for eelgrass in other regions but might be expected for similar populations in which pollen limitation or a short reproductive season renders selfing advantageous.     

Effects of inbreeding on traits that influence dispersal and progeny density in Cakile edentula var. lacustris (Brassicaceae)

Inbreeding may influence the intensity of sibling competition by altering the number of offspring produced or by changing plant morphology in ways that influence seed dispersion patterns. To test this possibility, effects of inbreeding on seed production and on traits that influence progeny density were measured using experimental pollinations of flowers of Cakile edentula var. lacustris. Different flowers on a plant were either hand pollinated with self pollen (with and without emasculation) or foreign pollen, or they were allowed to be pollinated naturally. Selfed flowers matured significantly fewer viable seeds than outcrossed flowers (10.3% less seed maturation with inbreeding depression of 19.2%), due in large part to a greater percentage of proximal seed abortions and lower germination success. Plants grown from selfed seeds tended to have lower seed production (37 fewer seeds on average, with inbreeding depression of 16.2%), caused in part by an increase in the percentage of fruits with proximal seed abortions, although this effect was not significant. Inbreeding depression in total fitness was 29.0%, which corresponds to a difference of 46 seeds per pollinated ovule. Selfing rate estimates were usually intermediate to high, indicating that inbreeding effects observed in this study would be present in naturally pollinated progeny. Although the influence of inbreeding directly on dispersal was negligible, the predicted reduction in sibling competition caused by reduced seed production resulted in an estimate of inbreeding depression of 17.5%, which is 11.5% lower than that measured under uniform conditions. Consequently, inbreeding depression estimated under natural dispersion patterns may be lower than that estimated under uniform conditions since seeds from self- and cross-pollination may not experience the same competitive environment in the field. Inbreeding in the maternal generation, therefore, could influence progeny fitness not only by determining the genetic composition of progeny, but also by influencing the competitive environment in which progeny grow.