Between a rock and a hard place: evaluating the relative risks of inbreeding and outbreeding for conservation and management

As populations become increasingly fragmented, managers are often faced with the dilemma that intentional hybridization might save a population from inbreeding depression but it might also induce outbreeding depression. While empirical evidence for inbreeding depression is vastly greater than that for outbreeding depression, the available data suggest that risks of outbreeding, particularly in the second generation, are on par with the risks of inbreeding. Predicting the relative risks in any particular situation is complicated by variation among taxa, characters being measured, level of divergence between hybridizing populations, mating history, environmental conditions and the potential for inbreeding and outbreeding effects to be occurring simultaneously. Further work on consequences of interpopulation hybridization is sorely needed with particular emphasis on the taxonomic scope, the duration of fitness problems and the joint effects of inbreeding and outbreeding. Meanwhile, managers can minimize the risks of both inbreeding and outbreeding by using intentional hybridization only for populations clearly suffering from inbreeding depression, maximizing the genetic and adaptive similarity between populations, and testing the effects of hybridization for at least two generations whenever possible.     

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.     

The different sources of variation in inbreeding depression, heterosis and outbreeding depression in a metapopulation of Physa acuta

Understanding how parental distance affects offspring fitness, i.e., the effects of inbreeding and outbreeding in natural populations, is a major goal in evolutionary biology. While inbreeding is often associated with fitness reduction (inbreeding depression), interpopulation outcrossing may have either positive (heterosis) or negative (outbreeding depression) effects. Within a metapopulation, all phenomena may occur with various intensities depending on the focal population (especially its effective size) and the trait studied. However, little is known about interpopulation variation at this scale. We here examine variation in inbreeding depression, heterosis, and outbreeding depression on life-history traits across a full-life cycle, within a metapopulation of the hermaphroditic snail Physa acuta. We show that all three phenomena can co-occur at this scale, although they are not always expressed on the same traits. A large variation in inbreeding depression, heterosis, and outbreeding depression is observed among local populations. We provide evidence that, as expected from theory, small and isolated populations enjoy higher heterosis upon outcrossing than do large, open populations. These results emphasize the need for an integrated theory accounting for the effects of both deleterious mutations and genetic incompatibilities within metapopulations and to take into account the variability of the focal population to understand the genetic consequences of inbreeding and outbreeding at this scale.     

Evidence of opposing fitness effects of parental heterozygosity and relatedness in a critically endangered marine turtle?

How individual genetic variability relates to fitness is important in understanding evolution and the processes affecting populations of conservation concern. Heterozygosity–fitness correlations (HFCs) have been widely used to study this link in wild populations, where key parameters that affect both variability and fitness, such as inbreeding, can be difficult to measure. We used estimates of parental heterozygosity and genetic similarity (‘relatedness’) derived from 32 microsatellite markers to explore the relationship between genetic variability and fitness in a population of the critically endangered hawksbill turtle, Eretmochelys imbricata. We found no effect of maternal MLH (multilocus heterozygosity) on clutch size or egg success rate, and no single‐locus effects. However, we found effects of paternal MLH and parental relatedness on egg success rate that interacted in a way that may result in both positive and negative effects of genetic variability. Multicollinearity in these tests was within safe limits, and null simulations suggested that the effect was not an artefact of using paternal genotypes reconstructed from large samples of offspring. Our results could imply a tension between inbreeding and outbreeding depression in this system, which is biologically feasible in turtles: female‐biased natal philopatry may elevate inbreeding risk and local adaptation, and both processes may be disrupted by male‐biased dispersal. Although this conclusion should be treated with caution due to a lack of significant identity disequilibrium, our study shows the importance of considering both positive and negative effects when assessing how variation in genetic variability affects fitness in wild systems.     

Genetic rescue persists beyond first-generation outbreeding in small populations of a rare plant

Habitat fragmentation commonly causes genetic problems and reduced fitness when populations become small. Stocking small populations with individuals from other populations may enrich genetic variation and alleviate inbreeding, but such artificial gene flow is not commonly used in conservation owing to potential outbreeding depression. We addressed the role of long-term population size, genetic distance between populations and test environment for the performance of two generations of offspring from between-population crosses of the locally rare plant Ranunculus reptans L. Interpopulation outbreeding positively affected an aggregate measure of fitness, and the fitness superiority of interpopulation hybrids was maintained in the second offspring (F2) generation. Small populations benefited more strongly from interpopulation outbreeding. Genetic distance between crossed populations in neutral markers or quantitative characters was not important. These results were consistent under near-natural competition-free and competitive conditions. We conclude that the benefits of interpopulation outbreeding are likely to outweigh potential drawbacks, especially for populations that suffer from 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.     

Genomic analysis reveals depression due to both individual and maternal inbreeding in a free‐living mammal population

There is ample evidence for inbreeding depression manifested as a reduction in fitness or fitness‐related traits in the focal individual. In many organisms, fitness is not only affected by genes carried by the individual, but also by genes carried by their parents, for example if receiving parental care. While maternal effects have been described in many systems, the extent to which inbreeding affects fitness directly through the focal individual, or indirectly through the inbreeding coefficients of its parents, has rarely been examined jointly. The Soay sheep study population is an excellent system in which to test for both effects, as lambs receive extended maternal care. Here, we tested for both maternal and individual inbreeding depression in three fitness‐related traits (birthweight and weight and hindleg length at 4 months of age) and three fitness components (first‐year survival, adult annual survival and annual breeding success), using either pedigree‐derived inbreeding or genomic estimators calculated using ~37 000 SNP markers. We found evidence for inbreeding depression in 4‐month hindleg and weight, first‐year survival in males, and annual survival and breeding success in adults. Maternal inbreeding was found to depress both birthweight and 4‐month weight. We detected more instances of significant inbreeding depression using genomic estimators than the pedigree, which is partly explained through the increased sample sizes available. In conclusion, our results highlight that cross‐generational inbreeding effects warrant further exploration in species with parental care and that modern genomic tools can be used successfully instead of, or alongside, pedigrees in natural populations.     

Genetic diversity at neutral and adaptive loci determines individual fitness in a long-lived territorial bird

There is compelling evidence about the manifest effects of inbreeding depression on individual fitness and populations' risk of extinction. The majority of studies addressing inbreeding depression on wild populations are generally based on indirect measures of inbreeding using neutral markers. However, the study of functional loci, such as genes of the major histocompatibility complex (MHC), is highly recommended. MHC genes constitute an essential component of the immune system of individuals, which is directly related to individual fitness and survival. In this study, we analyse heterozygosity fitness correlations of neutral and adaptive genetic variation (22 microsatellite loci and two loci of the MHC class II, respectively) with the age of recruitment and breeding success of a decimated and geographically isolated population of a long-lived territorial vulture. Our results indicate a negative correlation between neutral genetic diversity and age of recruitment, suggesting that inbreeding may be delaying reproduction. We also found a positive correlation between functional (MHC) genetic diversity and breeding success, together with a specific positive effect of the most frequent pair of cosegregating MHC alleles in the population. Globally, our findings demonstrate that genetic depauperation in small populations has a negative impact on the individual fitness, thus increasing the populations' extinction risk.     

Unexpected positive and negative effects of continuing inbreeding in one of the world's most inbred wild animals

Inbreeding depression, the reduced fitness of offspring of related individuals, is a central theme in evolutionary biology. Inbreeding effects are influenced by the genetic makeup of a population, which is driven by any history of genetic bottlenecks and genetic drift. The Chatham Island black robin represents a case of extreme inbreeding following two severe population bottlenecks. We tested whether inbreeding measured by a 20‐year pedigree predicted variation in fitness among individuals, despite the high mean level of inbreeding and low genetic diversity in this species. We found that paternal and maternal inbreeding reduced fledgling survival and individual inbreeding reduced juvenile survival, indicating that inbreeding depression affects even this highly inbred population. Close inbreeding also reduced survival for fledglings with less‐inbred mothers, but unexpectedly improved survival for fledglings with highly inbred mothers. This counterintuitive interaction could not be explained by various potentially confounding variables. We propose a genetic mechanism, whereby a highly inbred chick with a highly inbred parent inherits a “proven” genotype and thus experiences a fitness advantage, which could explain the interaction. The positive and negative effects we found emphasize that continuing inbreeding can have important effects on individual fitness, even in populations that are already highly inbred.     

Outbreeding effects in an inbreeding insect,Cimex lectularius

In some species, populations with few founding individuals can be resilient to extreme inbreeding. Inbreeding seems to be the norm in the common bed bug, Cimex lectularius, a flightless insect that, nevertheless, can reach large deme sizes and persist successfully. However, bed bugs can also be dispersed passively by humans, exposing inbred populations to gene flow from genetically distant populations. The introduction of genetic variation through this outbreeding could lead to increased fitness (heterosis) or be costly by causing a loss of local adaptation or exposing genetic incompatibility between populations (outbreeding depression). Here, we addressed how inbreeding within demes and outbreeding between distant populations impact fitness over two generations in this re‐emerging public health pest. We compared fitness traits of families that were inbred (mimicking reproduction following a founder event) or outbred (mimicking reproduction following a gene flow event). We found that outbreeding led to increased starvation resistance compared to inbred families, but this benefit was lost after two generations of outbreeding. No other fitness benefits of outbreeding were observed in either generation, including no differences in fecundity between the two treatments. Resilience to inbreeding is likely to result from the history of small founder events in the bed bug. Outbreeding benefits may only be detectable under stress and when heterozygosity is maximized without disruption of coadaptation. We discuss the consequences of these results both in terms of inbreeding and outbreeding in populations with genetic and spatial structuring, as well as for the recent resurgence of bed bug populations.     

Molecular and pedigree measures of relatedness provide similar estimates of inbreeding depression in a bottlenecked population

Individual‐based estimates of the degree of inbreeding or parental relatedness from pedigrees provide a critical starting point for studies of inbreeding depression, but in practice wild pedigrees are difficult to obtain. Because inbreeding increases the proportion of genomewide loci that are identical by descent, inbreeding variation within populations has the potential to generate observable correlations between heterozygosity measured using molecular markers and a variety of fitness related traits. Termed heterozygosity‐fitness correlations (HFCs), these correlations have been observed in a wide variety of taxa. The difficulty of obtaining wild pedigree data, however, means that empirical investigations of how pedigree inbreeding influences HFCs are rare. Here, we assess evidence for inbreeding depression in three life‐history traits (hatching and fledging success and juvenile survival) in an isolated population of Stewart Island robins using both pedigree‐ and molecular‐derived measures of relatedness. We found results from the two measures were highly correlated and supported evidence for significant but weak inbreeding depression. However, standardized effect sizes for inbreeding depression based on the pedigree‐based kin coefficients (k) were greater and had smaller standard errors than those based on molecular genetic measures of relatedness (RI), particularly for hatching and fledging success. Nevertheless, the results presented here support the use of molecular‐based measures of relatedness in bottlenecked populations when information regarding inbreeding depression is desired but pedigree data on relatedness are unavailable.     

Heterozygosity–fitness correlations and their relevance to studies on inbreeding depression in threatened species

The majority of reported multilocus heterozygosity–fitness correlations (HFCs) are from large, outbred populations, and their relevance to studies on inbreeding depression in threatened populations is often stressed. The results of such HFC studies conducted on outbred populations may be of limited application to threatened population management, however, as bottlenecked populations exhibit increased incidence of inbreeding, increased linkage disequilibrium, reduced genetic diversity and possible effects of historical inbreeding such as purging. These differences may affect both our ability to detect inbreeding depression in threatened species, and our interpretation of the underlying mechanisms for observed heterozygosity–fitness relationships. The study of HFCs in outbred populations is of interest in itself, but the results may not translate directly to threatened populations that have undergone severe bottlenecks.     

INBREEDING INFLUENCES WITHIN‐BROOD HETEROZYGOSITY‐FITNESS CORRELATIONS (HFCS) IN AN ISOLATED PASSERINE POPULATION

Molecular estimates of inbreeding may be made using genetic markers such as microsatellites, however the interpretation of resulting heterozygosity‐fitness correlations (HFCs) with respect to inbreeding depression is not straightforward. We investigated the relationship between pedigree‐determined inbreeding coefficients (f) and HFCs in a closely monitored, reintroduced population of Stewart Island robins (Petroica australis rakiura) on Ulva Island, New Zealand. Using a full sibling design, we focused on differences in juvenile survival associated specifically with individual sibling variation in standardized multilocus heterozygosity (SH) when expected f was identical. We found that within broods, siblings with higher SH at microsatellite loci experienced a higher probability of juvenile survival. This effect, however, was detected primarily within broods that experienced inbreeding or when inbreeding had occurred in their pedigree histories (i.e., at the parents’ level). Thus we show, for the first time in a wild population, that the strength of an HFC is partially dependent on the presence of inbreeding events in the recent pedigree history. Our results illustrate the importance of realized effects of inbreeding on genetic variation and fitness and the value of full‐sibling designs for the study of HFCs in the context of small, inbred populations.     

Heterozygosity and survival in blue tits (Cyanistes caeruleus): contrasting effects of presumably functional and neutral loci

The relationship between genetic diversity and fitness has important implications in evolutionary and conservation biology. This relationship has been widely investigated at the individual level in studies of heterozygosity-fitness correlations (HFC). General effects caused by inbreeding and/or local effects at single loci have been used as explanations of HFC, but the debate about the causes of HFC in open, natural populations is still ongoing. Study designs that control for variation in the inbreeding level of the individuals, and knowledge on the function and location of the markers used to measure heterozygosity, are fundamental to understand the causes of HFC. Here we investigated correlations between individual heterozygosity and estimates of survival at different life-history stages in an open population of blue tits (Cyanistes caeruleus). For survival at the embryo, nestling and fledgling stage, we used a full-sibling approach, i.e. we controlled for the level of inbreeding. We genotyped 1496 individuals with 79 microsatellites mapped across 25 chromosomes in the zebra finch (Taeniopygia guttata) that were classified either as potentially functional (58 loci) or as neutral (21 loci). We found different effects of standardized multilocus heterozygosity (SH): SH(functional) had a negative effect on the probability of hatching and local recruitment of females, whereas SH(neutral) had a positive effect on adult survival. The negative effects of functional loci are better explained by local effects, whereas the positive effects of neutral markers could reflect inbreeding effects in the population. Our results highlight the importance of considering the characteristics of the markers used in HFC studies and confirm the mixed effects of heterozygosity in different contexts (e.g. sex and life-history stage).     

Inbreeding and outbreeding depression in Stylidium hispidum: implications for mixing seed sources for ecological restoration

The benefits of composite rather than local seed provenances for ecological restoration have recently been argued, largely on the basis of maximizing evolutionary potential. However, these arguments have downplayed the potentially negative consequences of outbreeding depression once mixed provenances interbreed. In this study, we compared intraspecific F1 hybrid performance and molecular marker differentiation among four populations of Stylidium hispidum, a species endemic to Southwestern Australia. Multivariate ordination of 134 AFLP markers analyzed genetic structure and detected two clusters of paired sites that diverged significantly for marker variation along a latitudinal boundary. To test for outbreeding depression and to determine the consequences of molecular population divergence for hybrid fitness, we conducted controlled pollinations and studied germination and survival for three cross categories (within‐population crosses, short‐ and long‐distance F1 hybrids) for paired sites distributed within and between the two genetically differentiated regions. We found evidence of outbreeding depression in long‐distance hybrids (111–124 km), and inbreeding depression among progeny of within‐population crosses, relative to short‐distance (3–10 km) hybrids, suggesting an intermediate optimal outcrossing distance in this species. These results are discussed in light of the evolutionary consequences of mixing seed sources for biodiversity restoration.     

Evaluating the role of inbreeding depression in heterozygosity‐fitness correlations: how useful are tests for identity disequilibrium?

Heterozygosity‐fitness correlations (HFCs) have been observed for several decades, but their causes are often elusive. Tests for identity disequilibrium (ID, correlated heterozygosity between loci) are commonly used to determine if inbreeding depression is a possible cause of HFCs. We used computer simulations to determine how often ID is detected when HFCs are caused by inbreeding depression. We also used ID in conjunction with HFCs to estimate the proportion of variation (r²) in fitness explained by the individual inbreeding coefficient (F). ID was not detected in a large proportion of populations with statistically significant HFCs (sample size = 120 individuals) unless the variance of F was high (σ²(F) ≥ 0.005) or many loci were used (100 microsatellites or 1000 SNPs). For example, with 25 microsatellites, ID was not detected in 49% of populations when HFCs were caused by six lethal equivalents and σ²(F) was typical of vertebrate populations (σ²(F) ≈ 0.002). Estimates of r² between survival and F based on ID and HFCs were imprecise unless ID was strong and highly statistically significant (P ≈ 0.01). These results suggest that failing to detect ID in HFC studies should not be taken as evidence that inbreeding depression is absent. The number of markers necessary to simultaneously detect HFC and ID depends strongly on σ²(F). Thus the mating system and demography of populations, which influence σ²(F), should be considered when designing HFC studies. ID should be used in conjunction with HFCs to estimate the correlation between fitness and F, because HFCs alone reveal little about the strength of inbreeding depression.     

Negative effects of individual heterozygosity on reproductive success in a wild bird population

The evolutionary consequences of individual genetic diversity are frequently studied by assessing heterozygosity–fitness correlations (HFCs). The prevalence of positive and negative HFCs and the predominance of general versus local effects in wild populations are far from understood, partly because comprehensive studies testing for both inbreeding and outbreeding depression are lacking. We studied a genetically diverse population of blue tits in southern Germany using a genome‐wide set of 87 microsatellites to investigate the relationship between proxies of reproductive success and measures of multilocus and single‐locus individual heterozygosity (MLH and SLH). We used complimentary measures of MLH and partitioned markers into functional categories according to their position in the blue tit genome. HFCs based on MLH were consistently negative for functional loci, whereas correlations were rather inconsistent for loci found in nonfunctional areas of the genome. Clutch size was the only reproductive variable showing a general effect. We found evidence for local effects for three measures of reproductive success: arrival date at the breeding site, the probability of breeding at the study site and male reproductive success. For these, we observed consistent, and relatively strong, negative effects at one functional locus. Remarkably, this marker had a similar effect in another blue tit population from Austria (~400 km to the east). We suggest that a genetic local effect on timing of arrival might be responsible for most negative HFCs detected, with carry‐over effects on other reproductive traits. This effect could reflect individual differences in the distance between overwintering areas and breeding sites.     

Genetic rescue of small inbred populations: meta‐analysis reveals large and consistent benefits of gene flow

Many species have fragmented distribution with small isolated populations suffering inbreeding depression and/or reduced ability to evolve. Without gene flow from another population within the species (genetic rescue), these populations are likely to be extirpated. However, there have been only ~ 20 published cases of such outcrossing for conservation purposes, probably a very low proportion of populations that would potentially benefit. As one impediment to genetic rescues is the lack of an overview of the magnitude and consistency of genetic rescue effects in wild species, I carried out a meta‐analysis. Outcrossing of inbred populations resulted in beneficial effects in 92.9% of 156 cases screened as having a low risk of outbreeding depression. The median increase in composite fitness (combined fecundity and survival) following outcrossing was 148% in stressful environments and 45% in benign ones. Fitness benefits also increased significantly with maternal ΔF (reduction in inbreeding coefficient due to gene flow) and for naturally outbreeding versus inbreeding species. However, benefits did not differ significantly among invertebrates, vertebrates and plants. Evolutionary potential for fitness characters in inbred populations also benefited from gene flow. There are no scientific impediments to the widespread use of outcrossing to genetically rescue inbred populations of naturally outbreeding species, provided potential crosses have a low risk of outbreeding depression. I provide revised guidelines for the management of genetic rescue attempts.     

Outbreeding depression and breeding system evolution in small,remnant populations of <Emphasis Type="Italic">Primula vulgaris</Emphasis>: consequences for genetic rescue

Many species suffer from anthropogenic habitat fragmentation. The resulting small and isolated populations are more prone to extinction due to, amongst others, genetic erosion, inbreeding depression and Allee-effects. Genetic rescue can help mitigate such problems, but might result in outbreeding depression. We evaluated offspring fitness after selfing and outcrossing within and among three very small and isolated remnant populations of the heterostylous plant Primula vulgaris. We used greenhouse-grown offspring from these populations to test several fitness components. One population was fixed for the pin-morph, and was outcrossed with another population in the field to obtain seeds. Genetic diversity of parent and offspring populations was studied using microsatellites. Morph and population-specific heterosis, inbreeding and outbreeding depression were observed for fruit and seed set, seed weight and cumulative fitness. Highest fitness was observed in the field-outcrossed F1-population, which also showed outbreeding depression following subsequent between-population (back)crossing. Despite outbreeding depression, fitness was still relatively high. Inbreeding coefficients indicated that the offspring were more inbred than their parent populations. Offspring heterozygosity and inbreeding coefficients correlated with observed fitness. One population is evolving homostyly, showing a thrum morph with an elongated style and high autonomous fruit and seed set. This has important implications for conservation strategies such as genetic rescue, as the mating system will be altered by the introduction of homostyles.     

Negative heterozygosity-fitness correlations observed with microsatellites located in functional areas of the genome

Genome-wide heterozygosity inferred from neutral markers such as microsatellites is often expected to (i) reflect individual inbreeding and (ii) covary positively with fitness, generating positive heterozygosity-fitness correlations (HFCs). The often forgotten other end of the inbreeding-outbreeding continuum is outbreeding depression: past a certain degree of heterozygosity, heterozygotes tend to have lower fitness than homozygotes. Outbreeding depression arises from the breakup of co-adapted gene complexes and/or the introgression of nonlocally adapted genes. Provided that a correlation in heterozygosity exists across loci, outbreeding depression will be reflected in negative HFCs. In this issue, Olano-Marin et al. (2011a) describe negative heterozygosity-fitness correlations (HFCs) in blue tits Cyanistes caeruleus (Fig. 1), whereby heterozygosity has a significant, negative effect on female hatching success and recruitment. This study, together with a similar study by the same authors published in Evolution (Olano-Marin et al. 2011b), forms an original contribution in two respects. First, in the same population, positive and negative HFCs were recorded, revealing both inbreeding and outbreeding depression depending on the trait studied (whereby both processes were reliant on unknown, and possibly different, sets of coding loci). Second, a large number of microsatellite markers were split into two functional groups: microsatellite markers were either designed using zebra finch expressed sequence tags (ESTs) or derived using traditional cloning methods and presumed to be neutral. Contrasting large classes of loci and their varying levels of polymorphism, rather than looking for one locus that would stand out among tens of randomly selected markers, pave the way for a more elegant and powerful approach to explore how HFCs vary across traits and among regions of the genome. [Figure: see text].