Variation in the intensity of inbreeding depression among successive life-cycle stages and generations in gynodioecious Silene vulgaris (Caryophyllaceae)

Inbreeding depression is one of the hypotheses explaining the maintenance of females within gynodioecious plant populations. However, the measurement of fitness components in selfed and outcrossed progeny depends on life-cycle stage and the history of inbreeding. Comparative data indicate that strong inbreeding depression is more likely to occur at later life-cycle stages. We used hermaphrodite individuals of Silene vulgaris originating from three populations located in different valleys in the Swiss Alps to investigate the effect of two generations of self- and cross-fertilization on fitness components among successive stages of the life cycle in a glasshouse experiment. We detected significant inbreeding depression for most life-cycle stages including: the number of viable and aborted seeds per fruit, probability of germination, above ground biomass, probability of flowering, number of flowers per plant, flower size and pollen viability. Overall, the intensity of inbreeding depression increased among successive stages of the life cycle and cumulative inbreeding depression was significantly stronger in the first generation (delta approximately 0.5) compared with the second generation (delta approximately 0.35). We found no evidence for synergistic epistasis in our experiment. Our finding of more intense inbreeding depression during later stages of the life cycle may help to explain the maintenance of females in gynodioecious populations of S. vulgaris because purging of genetic load is less likely to occur.     

THE SIGNIFICANCE OF GENETIC EROSION IN THE PROCESS OF EXTINCTION. IV. INBREEDING DEPRESSION AND HETEROSIS EFFECTS CAUSED BY SELFING AND OUTCROSSING IN SCABIOSA COLUMBARIA

The effects of self-fertilization, within-population crosses (WPC) and between-population crosses (BPC) on progeny fitness were investigated in the greenhouse for Scabiosa columbaria populations of varying size. Plants grown from field collected seeds were hand pollinated to produce selfed, WPC, and BPC progeny. The performance of these progenies was examined throughout the entire life cycle. The different pollination treatments did not significantly affect germination, seedling-to-adult survival, flowering percentage and the number of flower heads. But severe inbreeding depression was demonstrated for biomass production, root development, adult survival, and seed set. Additionally, multiplicative fitness functions were calculated to compare relative fitnesses for progeny. On average, WPC progeny showed a more than 4-fold, and BPC progeny an almost 10-fold, advantage over selfed progeny, indicating that S. columbaria is highly susceptible to inbreeding. No clear relationship was found between population size and level of inbreeding depression, suggesting that the genetic load has not yet been reduced substantially in the small populations. A significant positive correlation was found between plant dry weight and total fitness. In two out of six populations, the differences between the effects of the pollination treatments on dry weight increased significantly when seedlings were grown under competitive conditions. This result is interpreted as an enhancement of inbreeding depression under these conditions. It is argued that improvement of the genetic exchange between populations may lower the probability of population extinction.     

Fitness Traits and Dispersal Ability in the Herb Tragopogon pratensis (Asteraceae): Decoupling the Role of Inbreeding Depression and Maternal Effects

Abstract: Inbreeding depression can decrease several fitness traits and maternal effects can strongly influence the amount of inbreeding depression. Understanding the effects of inbreeding depression on plant fitness is especially important in the context of habitat fragmentation, where plant populations become smaller and more isolated, exhibiting increasing levels of inbreeding depression. We examined the joint influence of inbreeding depression and maternal effects on life cycle traits and dispersal ability in the herb Tragopogon pratensis that grows in fragmented populations in Europe. We conducted experimental crosses to obtain selfed and outcrossed progeny in two contrasted environments. In particular, we produced a first generation of seeds and plants that were self-pollinated again to produce a second generation of seeds. Individual seeds were weighed and their pappuses measured to estimate the dispersal potential. Pollination treatment only had a significant effect on seed mass and dispersal ability. Coefficients of inbreeding depression did not differ between selfed and outcrossed plants. Seed mass had a significant effect on germination date. Environment had a significant effect on mass of the second generation of seeds and the interaction between pollination treatment and family was significant for six traits, indicating the existence of strong maternal effects in T. pratensis. Results suggest population differentiation. Overall, T. pratensis populations exhibited a good performance under selfing, in terms of life cycle traits and dispersal ability, which would allow the species to cope with problems associated with fragmentation.     

Exploring the relationship between tychoparthenogenesis and inbreeding depression in the Desert Locust,Schistocerca gregaria

Tychoparthenogenesis, a form of asexual reproduction in which a small proportion of unfertilized eggs can hatch spontaneously, could be an intermediate evolutionary link in the transition from sexual to parthenogenetic reproduction. The lower fitness of tychoparthenogenetic offspring could be due to either developmental constraints or to inbreeding depression in more homozygous individuals. We tested the hypothesis that in populations where inbreeding depression has been purged, tychoparthenogenesis may be less costly. To assess this hypothesis, we compared the impact of inbreeding and parthenogenetic treatments on eight life‐history traits (five measuring inbreeding depression and three measuring inbreeding avoidance) in four laboratory populations of the desert locust, Schistocerca gregaria, with contrasted demographic histories. Overall, we found no clear relationship between the population history (illustrated by the levels of genetic diversity or inbreeding) and inbreeding depression, or between inbreeding depression and parthenogenetic capacity. First, there was a general lack of inbreeding depression in every population, except in two populations for two traits. This pattern could not be explained by the purging of inbreeding load in the studied populations. Second, we observed large differences between populations in their capacity to reproduce through tychoparthenogenesis. Only the oldest laboratory population successfully produced parthenogenetic offspring. However, the level of inbreeding depression did not explain the differences in parthenogenetic success between all studied populations. Differences in development constraints may arise driven by random and selective processes between populations.     

Limited inbreeding depression in a bottlenecked population is age but not environment dependent

Inbreeding depression is known to vary greatly between populations and among species. Some of this variation is due to differences in genetic load between populations, while some is due to differences in the environment (e.g. local weather conditions) or demography of the population (e.g. age structure and breeding experience) in which inbreeding is expressed. Although the effects of these factors in isolation are well understood, there is still relatively little known about the interface between inbreeding on one hand, and environment and demography on the other in wild populations. We examined how environmental and demographic factors mediated the effects of inbreeding in a threatened species of bird. The Stewart Island robin, Petroica australis rakiura, has been subjected to a prolonged bottleneck for over 150 years. A complete pedigree of a reintroduced island population, extending back seven seasons to its founding, was available for analysis along with survival data (at the level of the brood) obtained from intensive monitoring over two breeding seasons. We found no strong support that the degree to which a brood was inbred affected its survival at either the hatching, fledging or recruitment stages. The inbreeding coefficient of the mother did have an effect on brood survival when analysed over all three life history stages, but only as a result of an interaction with female age, with broods of one‐year‐old inbred females suffering greater mortality than those of older inbred females. Although habitat type, temperature, rainfall and year were the best predictors of brood survival for most life history stages, their effects were weak and there were no interactions with inbreeding. Furthermore, there was no strong evidence of inbreeding depression associated with two periods of severe weather. This population is atypical in that inbreeding depression appears to be weak even under severe environmental conditions, and may be indicative that this bottlenecked population has either reduced genetic load or has fixed deleterious alleles.     

Inbreeding at the edge: does inbreeding depression increase under more stressful conditions?

Edge populations are frequently small and subject to stressful conditions that may compromise their long‐term viability. Inbreeding can play an important role in small populations by reducing genetic diversity, leading to the fixation of deleterious mutations and, finally, carrying populations to an extinction vortex through inbreeding depression. Although stressful conditions may enhance the intensity of inbreeding depression, evidence to date is inconclusive in marginal habitats. Local adaptation, promoting native genotypes, and gene flow, reducing allele fixation, are two factors that can have different effects on the intensity of inbreeding depression. Three populations of Silene ciliata distributed across an elevation gradient at the southernmost edge of the species distribution were used for this study. Several fitness components – germination, survival and growth rate – were compared between inbred seedlings and seedlings from within‐ and between‐population outcrosses, both in the field and controlled conditions. Overall, inbred seedlings had lower fitness than outcrossed seedlings. For most of the variables analysed, similar inbreeding depression effects were found in all three populations, but, for seed weight and seedling survival curve, inbreeding depression was only found in the low altitude population. Similarly, inbreeding depression was more evident in the field than in controlled chamber conditions. Outcrosses between populations contributed to an increase in most fitness estimates and populations, suggesting that the benefits of reducing inbreeding depression overrode the potentially deleterious effects of disrupting local adaptation. Our results suggest that inbreeding depression plays an important role in the fitness of early life stages of Silene ciliata at its southernmost distribution limit, but only provided partial support to the hypothesis that stressful conditions enhance the expression of inbreeding depression.     

THE EFFECT OF INBREEDING IN DIPLOID AND TETRAPLOID POPULATIONS OF EPILOBIUM ANGUSTIFOLIUM (ONAGRACEAE): IMPLICATIONS FOR THE GENETIC BASIS OF INBREEDING DEPRESSION

The partial dominance model for the evolution of inbreeding depression predicts that tetraploids should exhibit less inbreeding depression than their diploid progenitors. We tested this prediction by comparing the magnitude of inbreeding depression in tetraploid and diploid populations of the herbaceous perennial Epilobium angustifolium (Onagraceae). Inbreeding depression was estimated in the greenhouse for three tetraploid and two diploid populations at four life stages. The mating system of a tetraploid population was estimated and compared to a previous estimate for diploids. Tetraploids showed less inbreeding depression than diploids at all life history stages, and these differences were significant for seed-set and cumulative fitness, but not for germination, survival, or plant dry mass at nine weeks. This result suggests that the genetic basis of inbreeding depression may differ among life stages. The primary selfing rate of the tetraploid population was r = 0.43, which is nearly identical to that of a diploid population (r = 0.45), indicating that differences in inbreeding depression between diploids and tetraploids are probably not due to differences in the mating system. Cumulative inbreeding depression, calculated from the four life history stages, was significantly higher for diploids () than for tetraploids (), supporting the partial dominance model of inbreeding depression.     

EVOLUTION OF THE MAGNITUDE AND TIMING OF INBREEDING DEPRESSION IN PLANTS

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

Inbreeding depression and outbreeding depression in Digitalis purpurea: optimal outcrossing distance in a tetraploid

An optimal crossing distance exists within plant populations if inbreeding and outbreeding depression operate simultaneously. In a population of tetraploid Digitalis purpurea, maternal plants were pollinated with donors at four distances: 0 (self-pollination), 1, 6 and 30 m. Lifetime fitness of F1 progeny was investigated in greenhouse experiments, and significant inbreeding and outbreeding depression were detected at five vs. three life history traits. Inbreeding depression increased at later life stages, whereas outbreeding depression was relatively constant. The existence of within-population outbreeding depression suggests substantial genetic structuring at moderate distances in D. purpurea, and corroborates recent findings of significant outbreeding depression in F1 progeny in polyploids. The moderate inbreeding depression found in this predominately outcrossing population supports the notion that effects of inbreeding are less severe in polyploids than in diploids.     

Selfing and resource allocation in Schiedea salicaria (Caryophyllaceae), a gynodioecious species

Abstract Levels of selfing and resource allocation patterns were investigated in Schiedea salicaria (Caryophyllaceae), a gynodioecious species with high levels of inbreeding depression and nuclear control of male sterility. Selfing levels were higher in hermaphrodites than females, especially when adjusted for early acting inbreeding depression. The sexes of S. salicaria were similar in most allocation patterns including number of flowers and capsules per inflorescence, seeds per flower, and seed mass. Seeds produced by females had higher levels of germination than seeds of hermaphrodites, a likely result of high selfing levels and the expression of inbreeding depression in the progeny of hermaphrodites. Invasion of females in populations of S. salicaria is probably related to the expression of inbreeding depression at germination and in later life history stages. Comparisons with related species of Schiedea that also have nuclear control of male sterility suggest that reallocation of resources in hermaphrodites to male function occurs as females increase in frequency, but that resource reallocation is not important for the success of females when they first invade populations.     

Within and between population variation in inbreeding depression in the locally threatened perennial Scabiosa columbaria

Inbreeding depression plays a central role within the conservation genetics paradigm. Until now inbreeding depression is incorporated into models of population viability as a mean value (e.g. number of lethal equivalents) for all traits in a population. In this study of the locally threatened perennial plant species Scabiosa columbaria we investigated both the mean and the variance among families of inbreeding depression in eight life history traits for five natural populations varying in size from 300 to more than 120,000 individuals. Significant inbreeding depression was found in all populations and all traits. The mean inbreeding depression value per trait was never correlated to population size. Within each population, highly significant variation in inbreeding depression between families (VIFLID) was found. Per trait, families with inbreeding depression next to families with outbreeding depression were often found within the same population. Inbreeding depression at the family level was in many cases not correlated among traits and independent of correlations between traits themselves. VIFLID was negatively correlated with population size: in two traits these correlations were significant. The results underline that inbreeding depression is a complex, highly dynamic phenomenon. Models of viability should incorporate inbreeding depression distributions, with a trait specific mean and variance. Moreover, models of metapopulation dynamics should incorporate genotype quality as factor in colonization success.     

Mating system, sex ratio, and persistence of females in the gynodioecious shrub Daphne laureola L. (Thymelaeaceae)

Although in gynodioecious populations male steriles require a fecundity advantage to compensate for their gametic disadvantage, southern Spanish populations of the long-lived shrub Daphne laureola do not show any fecundity advantage over hermaphrodites in terms of seed production and early seedling establishment. By using allozyme markers, we assess the mating system of this species in five populations differing in sex ratio, and infer levels of inbreeding depression over the whole life cycle by comparing the inbreeding coefficients at the seed and adult plant stages. Extremely low outcrossing rates (0.001相似文献   

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 depression for various traits in two cultured populations of the American bay scallop, Argopecten irradians irradians Lamarck (1819) introduced into China

This paper examines the effect of inbreeding level of population on the magnitude of inbreeding depression expressed by comparing them between two cultured populations (A and B) in the hermaphroditic animal of the bay scallop Argopecten irradians irradians. Population A is expected to have less genetic variations and higher inbreeding level due to longer cultured history (20 generations) and less “ancestral” individuals (26 individuals) than population B due to shorter cultured history (4 generations) and more “ancestral” individuals (406 individuals). Two groups within each population were produced, one using self-fertilization and one using mass-mating within the same population. Selfed offspring (AS and BS) from two populations both had lower fitness components than their mass-mated counterparts (AM and BM) and exhibited inbreeding depression for all examined traits, e.g. lower hatching, less viability and slower growth, indicating that inbreeding depression is a common feature in this animal. Fitness components in all traits of offspring from population A significantly differed those from population B and the magnitude of inbreeding depression for all traits in population A with higher inbreeding level was significantly smaller than that in population B with lower inbreeding level, indicating that both fitness components and magnitude of inbreeding depression were significantly affected by inbreeding level of populations and genetic load harbored in population A may be partially purged through inbreeding. Moreover, the magnitude of inbreeding depression in the two populations both varied among traits and life history stages. The present results support the partial-dominance hypothesis of inbreeding depression.     

Effects of experimental inbreeding on herbivore resistance and plant fitness: the role of history of inbreeding, herbivory and abiotic factors

Inbreeding is common in plant populations and can affect plant fitness and resistance against herbivores. These effects are likely to depend on population history. In a greenhouse experiment with plants from 17 populations of Lychnis flos-cuculi, we studied the effects of experimental inbreeding on resistance and plant fitness. Depending on the levels of past herbivory and abiotic factors at the site of plant origin, we found either inbreeding or outbreeding depression in herbivore resistance. Furthermore, when not damaged experimentally by snail herbivores, plants from populations with higher heterozygosity suffered from inbreeding depression and those from populations with lower heterozygosity suffered from outbreeding depression. These effects of inbreeding and outbreeding were not apparent under experimental snail herbivory. We conclude that inbreeding effects on resistance and plant fitness depend on population history. Moreover, herbivory can mask inbreeding effects on plant fitness. Thus, understanding inbreeding effects on plant fitness requires studying multiple populations and considering population history and biotic interactions.     

Inbreeding depression and heterosis in populations of Schiedea viscosa, a highly selfing species

Progeny from self-pollinations and intrapopulation crosses were examined in Schiedea viscosa to determine the extent of inbreeding depression in this highly selfing species. Progeny of interpopulation crosses were also investigated to determine whether deleterious alleles have been fixed in populations of S. viscosa. There was no inbreeding depression at early life history stages, including seeds per capsule, seed mass, or germination. Inbreeding depression was detected for the later life history stage of fruit biomass, although not for survival or life span. Heterosis for vegetative biomass and fruit biomass was detected in progeny from crosses between populations. Levels of inbreeding depression in S. viscosa are low relative to out-crossing species of Schiedea, especially when early life history stages are compared.     

Reduced inbreeding depression in peripheral relative to central populations of a monocarpic herb

Many temperate taxa were confined to warmer latitudes during the last glacial maximum. As their ranges expanded when climates warmed, genetic drift and inbreeding in relatively small peripheral populations are expected to have reduced genetic diversity and the segregating genetic load. Therefore, inbreeding depression in peripheral populations might be lower than in centrally located sites. We evaluated the consequences of inbreeding for fitness traits in six central and six northern peripheral populations of the herb Campanulastrum americanum. Inbreeding reduced performance for all traits. Inbreeding depression in peripheral populations was lower than in central populations. This difference increased across the life cycle from similar levels for germination, to central populations having three times the inbreeding depression for adult traits. Geographical patterns of inbreeding depression suggest that mating system variation and potential future mating system evolution in many temperate taxa might reflect, at least in part, nonequilibrium conditions associated with historic range changes.     

Evidence for inbreeding depression in the food-deceptive colour-dimorphic orchid Dactylorhiza sambucina (L.) Soò

About one third of all orchid species are deceptive, i.e., not providing any reward to their pollinator. Such species often have lower visitation rates compared to rewarding relatives. This could result in lower levels of geitonogamous selfing and thus would provide an advantage in term of progeny fitness through inbreeding avoidance. This hypothesis could be tested by comparing the level of inbreeding depression between deceptive and rewarding orchids. However, due to the difficulty to raise orchids from seeds, few studies of inbreeding depression are available, and most are focused on very early life stages, such as seed mass or embryo viability. Here, we present the results from an experimental investigation of inbreeding depression in the deceptive flower-colour dimorphic Dactylorhiza sambucina, from in vitro cultivation to greenhouse soil transplantation. We found strong inbreeding depression at all recorded stages (i.e., germination and survival), with estimates ranging from 0.47 to 0.75. Our study finally proposes a simple and suitable experimental protocol to raise orchids from seeds with high germination rates.     

Maternal sex and mate relatedness affect offspring quality in the gynodioecious Silene acaulis

In gynodioecious species, females sacrifice fitness by not producing pollen, and hence must have a fitness advantage over hermaphrodites. Because females are obligately outcrossed, they may derive a fitness advantage by avoiding selfing and inbreeding depression. However, both sexes are capable of biparental inbreeding, and there are currently few estimates of the independent effects of maternal sex and multiple levels of inbreeding on female advantage. To test these hypotheses, females and hermaphrodites from six Alaskan populations of Silene acaulis were crossed with pollen from self (hermaphrodites only), a sibling, a random plant within the same population, and a plant from a different population. Germination, survivorship and early growth revealed inbreeding depression for selfs and higher germination but reduced growth in sib-crosses, relative to outcrosses. Independent of mate relatedness, females germinated more seeds that grew faster than offspring from hermaphrodites. This indicates that inbreeding depression as well as maternal sex can influence breeding system evolution. The effect of maternal sex may be explained by higher performance of female genotypes and a greater abundance of female genotypes among the offspring of female mothers.     

Seasonal stress drives predictable changes in inbreeding depression in field-tested captive populations of Drosophila melanogaster

Recent meta-analyses conducted across a broad range of taxa have demonstrated a strong linear relationship between the change in magnitude of inbreeding depression under stress and stress level, measured as fitness loss in outbred individuals. This suggests that a general underlying response may link stress and inbreeding depression. However, this relationship is based primarily on laboratory data, and it is unknown whether natural environments with multiple stressors and fluctuating stress levels alter how stress affects inbreeding depression. To test whether the same pattern persists in the field, we investigated the effect of seasonal variation on stress level and inbreeding depression in a 3-year field study measuring the productivity of captive populations of inbred and outbred Drosophila melanogaster. We found cold winter temperatures were most stressful and induced the greatest inbreeding depression. Furthermore, these data, collected under natural field conditions, conformed to the same predictive linear relationship seen in Drosophila laboratory studies, with inbreeding depression increasing by 0.17 lethal equivalents for every 10 per cent increase in stress level. Our results suggest that under natural conditions stress level is a primary determinant of the magnitude of inbreeding depression and should be considered when assessing extinction vulnerability in small populations.