The effects of stress intensity and stress type on inbreeding depression in Silene vulgaris

Inbreeding depression affects the growth of seedlings of an African timber species with a mixed mating reproductive system, Pericopsis elata (Harms) Meeuwen

Selfing or mating between related individuals can lead to inbreeding depression (ID), which can influence the survival, growth and evolution of populations of tree species. As selective logging involves a decrease in the density of congeneric partners, it could lead to increasing biparental inbreeding or self-fertilization, exposing the population to higher ID. We assessed the influence of inbreeding on the growth of a commercial timber species, Pericopsis elata (Fabaceae), which produced about 54% of self-fertilized seedlings in a natural population of the Congo basin. We followed the survival and growth of 540 plants raised in a plantation along a gradient of plant density (0.07-15.9 plants per m2). Parentage analysis allowed us distinguishing selfed and outcrossed seedlings. The annual growth was higher for outcrossed than selfed plants, on average by 10.8% for diameter and 12.9% for height growth. Based on the difference in above ground biomass between selfed and outcrossed seedlings after 41 months, we estimated the level of ID at δ = 0.33, while a lifetime estimate of ID based on the proportions of selfed plants at seedling and adult stages led to δ = 0.7. The level of ID on growth rate did not change significantly with age but tended to vanish under high competition. Pericopsis elata is a particularly interesting model because inbreeding depression is partial, with about 26% of reproducing adults resulting from selfing, contrary to most tropical tree species where selfed individuals usually die before reaching adulthood. Hence, the risks of ID must be considered in the management and conservation of the species.

An experimental field study of inbreeding depression in an outcrossing invasive plant

Inbreeding depression is likely to play an important role during biological invasion. But relatively few studies have investigated the fitness of selfed and outcrossed offspring in self-incompatible invasive plants in natural environments in their introduced range. Moreover, the majority of studies on inbreeding depression have investigated self-compatible species with mixed mating, and less is known about the intensity of inbreeding depression in outcrossing self-incompatible species. Here, we address these questions experimentally by comparing selfed and outcrossed progeny of purple loosestrife (Lythrum salicaria) over four growing seasons, including three under field conditions in a freshwater marsh in southern Ontario, Canada, a region where L. salicaria is highly invasive. The tristylous mating system of L. salicaria involves disassortative mating among floral morphs enforced by trimorphic incompatibility. However, owing to partial incompatibility, self-fertilized seed can be obtained by manual self-pollination thus facilitating comparisons of selfed and outcrossed progeny. We compared progeny with and without intraspecific competition from selfed or outcrossed neighbours and examined the influence of breeding treatment and competition on fitness correlates by measuring a range of life-history traits including: proportion of seeds germinating, days to germination, survival, proportion of plants flowering, time to flowering, vegetative mass, and inflorescence number and mass. We analysed data for each trait using functions from time series estimates of growth and two multiplicative estimates of fitness. We detected varying intensities of inbreeding depression for several traits in three of the four years of the experiment, including inflorescence mass and reproductive output. Cumulative inbreeding depression over four years averaged δ = 0.48 and 0.68, depending on the method used to estimate multiplicative fitness. The competition treatments did not significantly affect plant performance and the magnitude of inbreeding depression. Given the primarily outcrossing mating system of L. salicaria populations, the detection of inbreeding depression for several key life-history traits was as predicted by theory. Our results suggests that biparental inbreeding and low selfing in colonizing populations may have significant effects on demographic parameters such as population growth.

Epigenetic Changes Occurring in Plant Inbreeding

Inbreeding is the crossing of closely related individuals in nature or a plantation or self-pollinating plants, which produces plants with high homozygosity. This process can reduce genetic diversity in the offspring and decrease heterozygosity, whereas inbred depression (ID) can often reduce viability. Inbred depression is common in plants and animals and has played a significant role in evolution. In the review, we aim to show that inbreeding can, through the action of epigenetic mechanisms, affect gene expression, resulting in changes in the metabolism and phenotype of organisms. This is particularly important in plant breeding because epigenetic profiles can be linked to the deterioration or improvement of agriculturally important characteristics.

Continuous inbreeding affects genetic variation, phenology, and reproductive strategy in ex situ cultivated Digitalis lutea

PREMISE

Ex situ cultivation is important for plant conservation, but cultivation in small populations may result in genetic changes by drift, inbreeding, or unconscious selection. Repeated inbreeding potentially influences not only plant fitness, but also floral traits and interactions with pollinators, which has not yet been studied in an ex situ context.

METHODS

We studied the molecular genetic variation of Digitalis lutea from a botanic garden population cultivated for 30 years, a frozen seed bank conserving the original genetic structure, and two current wild populations including the source population. In a common garden, we studied the effects of experimental inbreeding and between-population crosses on performance, reproductive traits, and flower visitation of plants from the garden and a wild population.

RESULTS

Significant genetic differentiation was found between the garden population and the wild population from which the seeds had originally been gathered. After experimental selfing, inbreeding depression was only found for germination and leaf size of plants from the wild population, indicating a history of inbreeding in the smaller garden population. Moreover, garden plants flowered earlier and had floral traits related to selfing, whereas wild plants had traits related to attracting pollinators. Bumblebees visited more flowers of outbred than inbred plants and of wild than garden plants.

CONCLUSIONS

Our case study suggests that high levels of inbreeding during ex situ cultivation can influence reproductive traits and thus interactions with pollinators. Together with the effects of genetic erosion and unconscious selection, these changes may affect the success of reintroductions into natural habitats.

Environment dependence of the expression of mutational load and species' range limits

Theoretical and empirical research on the causes of species' range limits suggest the contribution of several intrinsic and extrinsic factors, with potentially complex interactions among them. An intrinsic factor proposed by recent theory is mutational load increasing towards range edges because of genetic drift. Furthermore, environmental quality may decline towards range edges and enhance the expression of load. Here, we tested whether the expression of mutational load associated with range limits in the North American plant Arabidopsis lyrata was enhanced under stressful environmental conditions by comparing the performance of within- versus between-population crosses at common garden sites across the species' distribution and beyond. Heterosis, reflecting the expression of load, increased with heightened estimates of genomic load and with environmental stress caused by warming, but the interaction was not significant. We conclude that range-edge populations suffer from a twofold genetic Allee effect caused by increased mutational load and stress-dependent load linked to general heterozygote deficiency, but there is no synergistic effect between them.

Stresses affect inbreeding depression in complex ways: disentangling stress-specific genetic effects from effects of initial size in plants

The magnitude of inbreeding depression (ID) varies unpredictably among environments. ID often increases in stressful environments suggesting that these expose more deleterious alleles to selection or increase their effects. More simply, ID could increase under conditions that amplify phenotypic variation (CV²), e.g., by accentuating size hierarchies among plants. These mechanisms are difficult to distinguish when stress increases both ID and phenotypic variation. We grew in- and outbred progeny of Mimulus guttatus under six abiotic stress treatments (control, waterlogging, drought, nutrient deficiency, copper addition, and clipping) with and without competition by the grass Poa palustris. ID differed greatly among stress treatments with δ varying from 7% (control) to 61% (waterlogging) but did not consistently increase with stress intensity. Poa competition increased ID under nutrient deficiency but not other stresses. Analyzing effects of initial size on performance of outbred plants suggests that under some conditions (low N, clipping) competition increased ID by amplifying initial size differences. In other cases (e.g., high ID under waterlogging), particular environments amplified the deleterious genetic effects of inbreeding suggesting differential gene expression. Interestingly, conditions that increased the phenotypic variability of inbred progeny regularly increased ID whereas variability among outbred progeny showed no relationship to ID. Our study reconciles the stress- and phenotypic variability hypotheses by demonstrating how specific conditions (rather than stress per se) act to increase ID. Analyzing CV² separately in inbred and outbred progeny while including effects of initial plant size improve our ability to predict how ID and gene expression vary across environments.

Life stage-specific inbreeding depression in long-lived Pinaceae species depends on population connectivity

Inbreeding depression (ID) is a fundamental selective pressure that shapes mating systems and population genetic structures in plants. Although it has been shown that ID varies over the life stages of shorter-lived plants, less is known about how the fitness effects of inbreeding vary across life stages in long-lived species. We conducted a literature survey in the Pinaceae, a tree family known to harbour some of the highest mutational loads ever reported. Using a meta-regression model, we investigated distributions of inbreeding depression over life stages, adjusting for effects of inbreeding levels and the genetic differentiation of populations within species. The final dataset contained 147 estimates of ID across life stages from 41 studies. 44 Fst estimates were collected from 40 peer-reviewed studies for the 18 species to aid genetic differentiation modelling. Partitioning species into fragmented and well-connected groups using Fst resulted in the best way (i.e. trade-off between high goodness-of-fit of the model to the data and reduced model complexity) to incorporate genetic connectivity in the meta-regression analysis. Inclusion of a life stage term and its interaction with the inbreeding coefficient (F) dramatically increased model precision. We observed that the correlation between ID and F was significant at the earliest life stage. Although partitioning of species populations into fragmented and well-connected groups explained little of the between-study heterogeneity, the inclusion of an interaction between life stage and population differentiation revealed that populations with fragmented distributions suffered lower inbreeding depression at early embryonic stages than species with well-connected populations. There was no evidence for increased ID in late life stages in well-connected populations, although ID tended to increase across life stages in the fragmented group. These findings suggest that life stage data should be included in inbreeding depression studies and that inbreeding needs to be managed over life stages in commercial populations of long-lived plants.

Review: Plant eco-evolutionary responses to climate change: Emerging directions

Contemporary climate change is exposing plant populations to novel combinations of temperatures, drought stress, [CO₂] and other abiotic and biotic conditions. These changes are rapidly disrupting the evolutionary dynamics of plants. Despite the multifactorial nature of climate change, most studies typically manipulate only one climatic factor. In this opinion piece, we explore how climate change factors interact with each other and with biotic pressures to alter evolutionary processes. We evaluate the ramifications of climate change across life history stages,and examine how mating system variation influences population persistence under rapid environmental change. Furthermore, we discuss how spatial and temporal mismatches between plants and their mutualists and antagonists could affect adaptive responses to climate change. For example, plant-virus interactions vary from highly pathogenic to mildly facilitative, and are partly mediated by temperature, moisture availability and [CO₂]. Will host plants exposed to novel, stressful abiotic conditions be more susceptible to viral pathogens? Finally, we propose novel experimental approaches that could illuminate how plants will cope with unprecedented global change, such as resurrection studies combined with experimental evolution, genomics or epigenetics.

Measuring the effect of environmental stress on inbreeding depression alone obscures the relative importance of inbreeding-stress interactions on overall fitness in Callosobruchus maculatus

Environmental stress can have a profound effect on inbreeding depression. Quantifying this effect is of particular importance in threatened populations, which are often simultaneously subject to both inbreeding and environmental stress. But while the prevalence of inbreeding-stress interactions is well known, the importance and broader applicability of such interactions in conservation are not clearly understood. We used seed beetles, Callosobruchus maculatus, as a model system to quantify how environmental stressors (here host quality and temperature stress) interact with inbreeding as measured by changes in the magnitude of inbreeding depression, δ, as well as the relative importance of inbreeding-stress interactions to overall fitness. We found that while both environmental stressors caused substantial inbreeding-stress interactions as measured by change in δ, the relative importance of these interactions to overall survival was modest. This suggests that assessing inbreeding-stress interactions within the framework of δ alone may give an inaccurate representation of the relevance of interactions to population persistence. Furthermore, we found that the effect of environmental stress on fitness, but not inbreeding depression, varied strongly among populations. These results suggest that the outcomes of inbreeding-stress interactions are not easily generalized, an important consideration in conservation settings.

Season-dependent effect of cleistogamy in Lamium amplexicaule: flower type origin versus inbreeding status

PREMISE

Inbreeding depression is traditionally considered a major factor favoring outcrossing in hermaphrodites. Recent experiments have shown that environmental conditions can influence the magnitude of inbreeding depression, but their relevance in natural populations is unclear. To investigate this, we studied the cleistogamous species Lamium amplexicaule, an annual species with individuals experiencing either spring or autumn environmental conditions. In this species, the proportion of cleistogamous/chasmogamous flowers changes according to seasonal cues (e.g., temperature, photoperiod). Our hypothesis was that the plasticity of cleistogamy is an adaptation to seasonal fitness variation in different flower progenies.

METHODS

To test this hypothesis, we produced the three possible types of progenies through controlled crosses: (1) selfed progeny from cleistogamous flowers; (2) chasmogamous selfed progeny; and (3) chasmogamous outcrossed progeny. Progeny fitness was then measured in a common garden in the two reproduction seasons (autumn and spring).

RESULTS

The results showed that season had a major impact on fitness. The fitness of the different progeny types changed across seasons, indicating that the effect of cleistogamy on progeny performance is season-dependent, consistent with a previous study in a similar environment. Surprisingly, the flower from which the progeny issued (cleistogamous or chasmogamous) had more impact on fitness than the inbred status of the progeny (selfed versus outcrossed), suggesting a potential role of epigenetic processes.

CONCLUSIONS

The observed patterns of variation were not consistent either with adaptation to environment-dependent inbreeding depression or to variation in resource availability, but were possibly consistent with adaptation to seasonal pollinator activity.

Changing environments and genetic variation: natural variation in inbreeding does not compromise short-term physiological responses

Selfing plant lineages are surprisingly widespread and successful in a broad range of environments, despite showing reduced genetic diversity, which is predicted to reduce their long-term evolutionary potential. However, appropriate short-term plastic responses to new environmental conditions might not require high levels of standing genetic variation. In this study, we tested whether mating system variation among populations, and associated changes in genetic variability, affected short-term responses to environmental challenges. We compared relative fitness and metabolome profiles of naturally outbreeding (genetically diverse) and inbreeding (genetically depauperate) populations of a perennial plant, Arabidopsis lyrata, under constant growth chamber conditions and an outdoor common garden environment outside its native range. We found no effect of inbreeding on survival, flowering phenology or short-term physiological responses. Specifically, naturally occurring inbreeding had no significant effects on the plasticity of metabolome profiles, using either multivariate approaches or analysis of variation in individual metabolites, with inbreeding populations showing similar physiological responses to outbreeding populations over time in both growing environments. We conclude that low genetic diversity in naturally inbred populations may not always compromise fitness or short-term physiological capacity to respond to environmental change, which could help to explain the global success of selfing mating strategies.

Does gene flow aggravate or alleviate maladaptation to environmental stress in small populations?

Environmental change can expose populations to unfamiliar stressors, and maladaptive responses to those stressors may result in population declines or extirpation. Although gene flow is classically viewed as a cause of maladaptation, small and isolated populations experiencing high levels of drift and little gene flow may be constrained in their evolutionary response to environmental change. We provide a case study using the model Trinidadian guppy system that illustrates the importance of considering gene flow and genetic drift when predicting (mal)adaptive response to acute stress. We compared population genomic patterns and acute stress responses of inbred guppy populations from headwater streams either with or without a recent history of gene flow from a more diverse mainstem population. Compared to "no-gene flow" analogues, we found that populations with recent gene flow showed higher genomic variation and increased stress tolerance-but only when exposed to a stress familiar to the mainstem population (heat shock). All headwater populations showed similar responses to a familiar stress in headwater environments (starvation) regardless of gene flow history, whereas exposure to an entirely unfamiliar stress (copper sulfate) showed population-level variation unrelated to environment or recent evolutionary history. Our results suggest that (mal)adaptive responses to acutely stressful environments are determined in part by recent evolutionary history and in part by previous exposure. In some cases, gene flow may provide the variation needed to persist, and eventually adapt, in the face of novel stress.

Release from natural enemies mitigates inbreeding depression in native and invasive Silene latifolia populations

Inbreeding and enemy infestation are common in plants and can synergistically reduce their performance. This inbreeding ×environment (I × E) interaction may be of particular importance for the success of plant invasions if introduced populations experience a release from attack by natural enemies relative to their native conspecifics. Here, we investigate whether inbreeding affects plant infestation damage, whether inbreeding depression in growth and reproduction is mitigated by enemy release, and whether this effect is more pronounced in invasive than native plant populations. We used the invader Silene latifolia and its natural enemies as a study system. We performed two generations of experimental out- and inbreeding within eight native (European) and eight invasive (North American) populations under controlled conditions using field-collected seeds. Subsequently, we exposed the offspring to an enemy exclusion and inclusion treatment in a common garden in the species' native range to assess the interactive effects of population origin (range), breeding treatment, and enemy treatment on infestation damage, growth, and reproduction. Inbreeding increased flower and leaf infestation damage in plants from both ranges, but had opposing effects on fruit damage in native versus invasive plants. Inbreeding significantly reduced plant fitness; whereby, inbreeding depression in fruit number was higher in enemy inclusions than exclusions. This effect was equally pronounced in populations from both distribution ranges. Moreover, the magnitude of inbreeding depression in fruit number was lower in invasive than native populations. These results support that inbreeding has the potential to reduce plant defenses in S. latifolia, which magnifies inbreeding depression in the presence of enemies. However, future studies are necessary to further explore whether enemy release in the invaded habitat has actually decreased inbreeding depression and thus facilitated the persistence of inbred founder populations and invasion success.

Responses of a legume to inbreeding and the intensity of novel and familiar stresses

It is often assumed that the negative effects of inbreeding on fitness (inbreeding depression, ID) are particularly strong under stressful conditions. However, ID may be relatively mild under types of stress that plant populations have experienced for a long time, because environment-specific deleterious alleles may already have been purged. We examined the performance of open- and self-pollinated progeny of the short-lived calcareous grassland plant Anthyllis vulneraria under three intensities of each of five types of stress. Drought, nutrient deficiency, and defoliation were chosen as stresses typical for the habitat of origin, while shade and waterlogging were expected to be novel, unfamiliar stresses for A. vulneraria. The stresses reduced plant biomass by up to 91%, and the responses of the plants were mostly in line with the functional equilibrium hypothesis. There was significant ID in biomass (δ = 0.17), leaf chlorophyll content, and the number of root nodules of the legume, but the magnitude of ID was independent of the stress treatments. In particular, there was no significant interaction between inbreeding and the intensity of any stress type, and ID was not higher under novel than under familiar stresses. In addition, phenotypic plasticity in biomass allocation, leaf functional traits and in root nodulation of the legume to the various stress treatments was not influenced by inbreeding. Our findings do not support the common hypothesis of stronger ID under stressful environments, not even if the stresses are novel to the plants.

Transgenerational plasticity in Silene vulgaris in response to three types of stress

The environment experienced by plants can influence the phenotype of their offspring. Such transgenerational plasticity can be adaptive when it results in higher fitness of the offspring under conditions correlated with those experienced by the mother plant. However, it has rarely been tested if such anticipatory parental effects may be induced with different environments. We grew clonal replicates of Silene vulgaris under control conditions and three types of stress (nutrient deficiency, copper addition and drought), which are known from natural populations of the species. We then subjected offspring from differently treated mother plants to each of the different stress treatments to analyse the influence of maternal and offspring environment on performance and several functional traits. Current stress treatments strongly influenced biomass and functional traits of the plants, mostly in line with responses predicted by the theory of functional equilibrium. Plant performance was also influenced by maternal stress treatments, and some effects independent of initial size differences remained until harvest. In particular, stressed mothers produced offspring of higher fitness than control plants. However, there was no evidence for treatment-specific adaptive transgenerational plasticity, as offspring from a mother plant that had grown in a specific environment did not grow better in that environment than other plants. Our results indicate that the maternal environment may affect offspring traits and performance, but also that this transgenerational plasticity is not necessarily adaptive.

Aridity weakens population-level effects of multiple species interactions on Hibiscus meyeri

Predicting how species' abundances and ranges will shift in response to climate change requires a mechanistic understanding of how multiple factors interact to limit population growth. Both abiotic stress and species interactions can limit populations and potentially set range boundaries, but we have a poor understanding of when and where each is most critical. A commonly cited hypothesis, first proposed by Darwin, posits that abiotic factors (e.g., temperature, precipitation) are stronger determinants of range boundaries in apparently abiotically stressful areas ("stress" indicates abiotic factors that reduce population growth), including desert, polar, or high-elevation environments, whereas species interactions (e.g., herbivory, competition) play a stronger role in apparently less stressful environments. We tested a core tenet of this hypothesis-that population growth rate is more strongly affected by species interactions in less stressful areas-using experimental manipulations of species interactions affecting a common herbaceous plant, Hibiscus meyeri (Malvaceae), across an aridity gradient in a semiarid African savanna. Population growth was more strongly affected by four distinct species interactions (competition with herbaceous and shrubby neighbors, herbivory, and pollination) in less stressful mesic areas than in more stressful arid sites. However, contrary to common assumptions, this effect did not arise because of greater density or diversity of interacting species in less stressful areas, but rather because aridity reduced sensitivity of population growth to these interactions. Our work supports classic predictions about the relative strength of factors regulating population growth across stress gradients, but suggests that this pattern results from a previously unappreciated mechanism that may apply to many species worldwide.

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.

Interactions of inbreeding and stress by poor host quality in a root hemiparasite

BACKGROUND AND AIMS

Populations of many hemiparasitic plants are fragmented and threatened by inbreeding depression (ID). In addition, they may also be strongly affected by a lack of suitable host species. However, nothing is known about possible interactive effects of inbreeding and host quality for parasitic plants. Poor host quality represents a special type of biotic stress and the magnitude of ID is often expected to be higher in more stressful environments.

METHODS

We studied the effects of inbreeding and the quality of host species for the declining root hemiparasite Rhinanthus alectorolophus Selfed and open-pollinated parasites from two natural populations were grown (1) with 13 potential host species and (2) with 15 four-species mixtures.

KEY RESULTS

ID differed among host species and mixtures. In the first experiment, ID was highest in parasites grown with good hosts and declined with stress intensity. In the second experiment, ID was not influenced by stress intensity, but was highest in mixtures of hosts from only one functional group and lowest in mixtures containing three functional groups. Both parasite performance with individual host species and the damage to these host species differed between parasites from the two study populations.

CONCLUSIONS

Our results contradict the common assumption that ID is generally higher in more stressful environments. In addition, they support the importance of diverse host communities for hemiparasitic plants. The differences in host quality between the two parasite populations indicate genetic variation in the adaptation to individual hosts and in host-specific virulence. However, inbreeding did not affect specific host-parasite interactions.