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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 m 2 ). 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., (© 2024. The Author(s).)

Inbreeding adversely affects fitness traits in many plant and animal species, and the magnitude, stability and genetic basis of inbreeding depression (ID) will have short- and long-term evolutionary consequences. The effects of four degrees of inbreeding (selfing, f=50%; full- and half-sib matings, f=25 and 12.5%; and unrelated outcrosses, f=0%) on survival and growth of an island population of Eucalyptus globulus were studied at two sites for over 14 years. For selfs, ID in survival increased over time, reaching a maximum of 49% by age 14 years. However, their inbreeding depression for stem diameter remained relatively stable with age, and ranged from 28 to 36% across years and sites. ID for survival was markedly greater on the more productive site, possibly due to greater and earlier onset of inter-tree competition, but was similar on both sites for the diameter of survivors. The deleterious trait response to increasing inbreeding coefficients was linear for survival and diameter. Non-significant quadratic effects suggested that epistasis did not contribute considerably to the observed ID at the population level. Among- and within-family coefficients of variation for diameter increased with inbreeding degree, and the variance among the outcrossed families was significant only on the more productive site. The performance of self-families for diameter was highly stable between sites. This suggests that, for species with mixed mating systems, environmentally stable inbreeding effects in open-pollinated progenies may tend to mask the additive genotype-by-environment interaction for fitness traits and the adaptive response to the environment. [ABSTRACT FROM AUTHOR]

In fragmented populations, genetic drift and selection reduce genetic diversity, which in turn results in a loss of fitness or in a loss of evolvability. Genetic rescue, that is, controlled input of diversity from distant populations, may restore evolutionary potential, whereas outbreeding depression might counteract the positive effect of this strategy. We carried out self-pollination and crosses within and between populations in an experimental subdivided population of a selfing species, Triticum aestivum L., to estimate the magnitude of these two phenomena. Surprisingly, for a self-fertilizing species, we found significant inbreeding depression within each population for four of the six traits studied, indicating that mildly deleterious mutations were still segregating in these populations. The progeny of within- and between-population crosses was very similar, indicating low between-population heterosis and little outbreeding depression. We conclude that relatively large population effective sizes prevented fixation of a high genetic load and that local adaptation was limited in these recently diverged populations. The kinship coefficient estimated between the parents using 20 neutral markers was a poor predictor of the progeny phenotypic values, indicating that there was a weak link between neutral diversity and genes controlling fitness-related traits. These results show that when assessing the viability of natural populations and the need for genetic rescue, the use of neutral markers should be complemented with information about the presence of local adaptation in the subdivided population. [ABSTRACT FROM AUTHOR]

We investigated the variation and short-term evolution of the selfing rate and inbreeding depression (ID) across three generations within a cedar forest that was established from admixture ca 1860. The mean selfing rate was 9.5%, ranging from 0 to 48% among 20 seed trees (estimated from paternally inherited chloroplast DNA). We computed the probability of selfing for each seed and we investigated ID by comparing selfed and outcrossed seeds within progenies, thus avoiding maternal effects. In all progenies, the germination rate was high (88-100%) and seedling mortality was low (0-12%). The germination dynamics differed significantly between selfed and outcrossed seeds within progenies in the founder gene pool but not in the following generations. This transient effect of selfing could be attributed to epistatic interactions in the original admixture. Regarding the seedling growth traits, the ID was low but significant: 8 and 6% for height and diameter growth, respectively. These rates did not vary among generations, suggesting minor gene effects. At this early stage, outcrossed seedlings outcompeted their selfed relatives, but not necessarily other selfed seedlings from other progenies. Thus, purging these slightly deleterious genes may only occur through within-family selection. Processes that maintain a high level of genetic diversity for fitness-related traits among progenies also reduce the efficiency of purging this part of the genetic load. [ABSTRACT FROM AUTHOR]

Inbreeding is assumed to have negative effects on fitness, including the reduced ability to withstand immune challenges. We examined the immunological consequences of inbreeding in decorated crickets, Gryllodes sigillatus, by comparing lytic activity, phenoloxidase (PO) activity, and encapsulation ability of crickets from eight inbred lines with that of crickets from the outbred founder population. Surprisingly, crickets from inbred lines had a greater encapsulation ability compared with crickets from the outbred population. We suggest that because inbred crickets have reduced reproductive effort, they may, therefore, have the option of devoting more resources to this form of immunity than outbred individuals. We also found that both inbred and outbred females had higher immunity than males in PO activity and implant darkness. This result supports the hypothesis that females should devote more effort to somatic maintenance and immunity than males. PO activity and implant darkness were heritable in both males and females, but lytic activity was only heritable in females. Males and females differed in the heritability of, and genetic correlations among, immune traits, suggesting that differences in selective pressures on males and females may have resulted in a sexual conflict over optimal immune trait values. [ABSTRACT FROM AUTHOR]

How self-incompatibility systems are maintained in plant populations is still a debated issue. Theoretical models predict that self-incompatibility systems break down according to the intensity of inbreeding depression and number of S-alleles. Other studies have explored the function of asexual reproduction in the maintenance of self-incompatibility. However, the population genetics of partially asexual, self-incompatible populations are poorly understood and previous studies have failed to consider all possible effects of asexual reproduction or could only speculate on those effects. In this study, we investigated how partial asexuality may affect genetic diversity at the S-locus and fitness in small self-incompatible populations. A genetic model including an S-locus and a viability locus was developed to perform forward simulations of the evolution of populations of various sizes. Drift combined with partial asexuality produced a decrease in the number of alleles at the S-locus. In addition, an excess of heterozygotes was present in the population, causing an increase in mutation load. This heterozygote excess was enhanced by the self-incompatibility system in small populations. In addition, in highly asexual populations, individuals produced asexually had some fitness advantages over individuals produced sexually, because sexual reproduction produces homozygotes of the deleterious allele, contrary to asexual reproduction. Our results suggest that future research on the function of asexuality for the maintenance of self-incompatibility will need to (1) account for whole-genome fitness (mutation load generated by asexuality, self-incompatibility and drift) and (2) acknowledge that the maintenance of self-incompatibility may not be independent of the maintenance of sex itself. [ABSTRACT FROM AUTHOR]

Genetic variation that is expressed only under specific environmental conditions can contribute to additional adverse effects of inbreeding if environmental conditions change. We present a proteomic characterization of a conditional lethal found in an inbred line of Drosophila melanogaster. The lethal effect is apparent as a large increase in early mortality at the restrictive temperature (29 °C) as opposed to normal survival at the permissive temperature (20 °C). The increased mortality in response to the restrictive temperature is probably caused by a single recessive major locus. A quantitative trait locus (QTL) region segregating variation affecting the lethal effect has been identified, allowing for a separation of primary/causal effects and secondary consequences in the proteome expression patterns observed. In this study, the proteomic response to the restrictive temperature in the lethal-line (L-line) was compared with the response in an inbred-control-line (IC-line) and an outbred-control-line (OC-line). Quantitative protein changes were detected using isobaric tags for relative and absolute quantitation (iTRAQ) two-dimensional liquid chromatography-tandem mass spectrometry. In all, 45 proteins were found to be significantly differently regulated in response to the restrictive temperature in the L-line as compared with the IC-line. No proteins were significantly differently regulated between the IC-line and the OC-line, verifying that differential protein regulation was specific to a genetic defect in the L-line. Proteins associated with oxidative phosphorylation and mitochondria were significantly overrepresented within the list of differentially expressed proteins. Proteins related to muscle contraction were also found to be differentially expressed in the L-line in response to the restrictive temperature, supporting phenotypic observations of moribund muscle hyper-contraction. [ABSTRACT FROM AUTHOR]

We have little understanding of how environmental conditions affect the expression of the genetic load for lifespan and adult mortality rates, or how this environmental dependence affect tests of models for the evolution of senescence. We use the seed-feeding beetle, Callosobruchus maculatus, as a model to explore how the inbreeding load (L) affecting adult lifespan varies with rearing conditions (diet and temperature), and how rearing conditions affect tests of the mutation accumulation model of senescence. When reared under benign conditions, there was a large sex difference in inbreeding depression (δ) and the inbreeding load (L=0.51–0.86 lethal equivalents per gamete for females L=∼0 for males). This sex difference in L was dependent on temperature, but not on rearing host or heat shock. At both high and low temperatures (relative to intermediate temperature) L increased for males, and L converged for the sexes at low temperature (L=0.26–0.53 for both sexes). Correlations were small for L between pairs of temperatures, indicating that the genes responsible for the inbreeding load differed between temperatures. In contrast to predictions of the mutation accumulation model of senescence, the age-specific inbreeding load for the adult mortality rate (Lu(t)) did not increase with age in any rearing environment. The genetic load underlying lifespan and adult mortality rates, and large sex differences in the genetic load, is highly dependent on environmental conditions. Estimating the genetic load in benign laboratory environments may be insufficient to predict the genetics underlying lifespan variation in nature where environmental variation is the norm.Heredity (2009) 103, 62–72; doi:10.1038/hdy.2009.31; published online 1 April 2009 [ABSTRACT FROM AUTHOR]

Inbreeding depression is a key factor in the maintenance of separate sexes in plants through selection for the avoidance of self-fertilization. However, very little is known about the levels of inbreeding depression in dioecious species, obviously because it is difficult to self-fertilize males or females. We overcame this problem by clonally propagating males from lineages in a dioecious metapopulation of the European annual plant Mercurialis annua, feminizing some of them and crossing the feminized with the unfeminized clones. Using this method, we compared the fitness of selfed vs outcrossed progeny under field conditions in Spain, where this species grows naturally. Multiplicative inbreeding depression (based on seed germination, early and late survival, seed mass and pollen viability) ranged from −0.69 to 0.82, with a mean close to zero. We consider possible explanations for both the low mean and high variance in inbreeding depression in M. annua, and we discuss the implications of our results for the maintenance of dioecy over hermaphroditism.Heredity (2009) 102, 600–608; doi:10.1038/hdy.2009.21; published online 18 March 2009 [ABSTRACT FROM AUTHOR]

Mixed mating systems are somewhat of an enigma as most models predict that organisms should either inbreed when inbreeding depression is low, or outbreed when inbreeding depression is high. Many wasps mix routine inbreeding with a little random mating. This random mating is most common when all local sibmating opportunities are exhausted and dispersal is the only way males can further increase their fitness. The males of the pollinating fig wasp, Platyscapa awekei, are slightly different in that they disperse before all sibmating opportunities have been exhausted. To see if this is a response to inbreeding depression we quantify inbreeding depression by comparing females' life time reproductive success to their heterozygosity at multiple microsatellite loci. We find that a female wasp's heterozygosity is an accurate predictor of her inbreeding coefficient and that P. awekei females actually seem to suffer from outbreeding depression and possibly from a little inbreeding depression. Male dispersal is thus not a means to effect the optimal mating system, but more likely a mechanism to reduce competition among brothers. The number of mature offspring a female produces depends on her own heterozygosity and not on that of the offspring, and may be determined by egg and gall quality.Heredity (2009) 102, 349–356; doi:10.1038/hdy.2009.2; published online 18 February 2009 [ABSTRACT FROM AUTHOR]

An understanding of inbreeding and inbreeding depression are important in evolutionary biology, conservation genetics, and animal breeding. A new method was developed to detect departures from the classical model of inbreeding; in particular, it investigated differences between the effects of inbreeding in recent generations from that in the more distant past. The method was applied in a long-term selection experiment on first-litter size in mice. The total pedigree included 74 630 animals with ∼30 000 phenotypic records. The experiment comprised several different lines. The highest inbreeding coefficients (F) within a line ranged from 0.22 to 0.64, and the average effective population size (Ne) was 58.1. The analysis divided F into two parts, corresponding to the inbreeding occurring in recent generations (‘new’) and that which preceded it (‘old’). The analysis was repeated for different definitions of ‘old’ and ‘new’, depending on length of the ‘new’ period. In 15 of these tests, ‘new’ inbreeding was estimated to cause greater depression than ‘old’. The estimated depression ranged from −11.53 to −0.79 for the ‘new’ inbreeding and from −5.22 to 15.51 for ‘old’. The difference was significant, the ‘new’ period included at least 25 generations of inbreeding. Since there were only small differences in Ne between lines, and near constant Ne within lines, the effect of ‘new’ and ‘old’ cannot be attributed to the effects of ‘fast’ versus ‘slow’ inbreeding. It was concluded that this departure from the classical model, which predicts no distinction between this ‘old and ‘new’ inbreeding, must implicate natural selection and purging in influencing the magnitude of depression.Heredity (2007) 99, 81–88; doi:10.1038/sj.hdy.6800968; published online 23 May 2007 [ABSTRACT FROM AUTHOR]

We use regression models to investigate the effects of inbreeding in 119 zoo populations, encompassing 88 species of mammals, birds, reptiles and amphibians. Meta-analyses show that inbreeding depression for neonatal survival was significant across the 119 populations although the severity of inbreeding depression appears to vary among taxa. However, few predictors of a population's response to inbreeding are found reliable. The models are most likely to detect inbreeding depression in large populations, that is, in populations in which their statistical power is maximised. Purging was found to be significant in 14 populations and a significant trend of purging was found across populations. The change in inbreeding depression due to purging averaged across the 119 populations is <1%, however, suggesting that the fitness benefits of purging are rarely appreciable. The study re-emphasises the necessity to avoid inbreeding in captive breeding programmes and shows that purging cannot be relied upon to remove deleterious alleles from zoo populations.Heredity (2007) 98, 172–182. doi:10.1038/sj.hdy.6800923; published online 20 December 2006 [ABSTRACT FROM AUTHOR]

An important question emerging from theoretical studies of mating system evolution is whether the fitness of a randomly extracted, fully inbred genotype will exceed the mean of outbred individuals. We introduce two statistics (I1 and I2) related to the probability of extracting a high line. I1 and I2 can be estimated from the family structured experimental designs typically used to estimate inbreeding depression (ID). Maximum likelihood procedures are developed from an explicit genetic model. These yield parameter estimates and provide the likelihoods necessary to test hypotheses, for example, whether population-level ID is nonzero. Finally, we describe a new publicly available computer program titled ‘IDG’ (Inbreeding Depression Genetics) to execute these procedures.Heredity (2006) 97, 346–354. doi:10.1038/sj.hdy.6800879; published online 9 August 2006 [ABSTRACT FROM AUTHOR]

Mixed-mating animals self-fertilize a proportion of their offspring. Outcrossing rate may covary with the ecological and historical factors affecting the population. Theory predicts that outcrossing is favored when inbreeding depression is high and when individual heterozygosity is important. Self-fertilization is predicted to be favored when costs of male function, or mate finding are high, for example, when empty patches are colonized by few individuals. In this study, we assessed primary (after hatching) and secondary (after juvenile mortality) outcrossing rates of two mixed-mating snail populations. Our purpose was to assess the variation in mating-system parameters and estimate significance of inbreeding depression for secondary outcrossing rate (the realized outcrossing rate of parents that produce the next generation). Secondary outcrossing rate was higher than the primary outcrossing rate in one of the two populations, suggesting considerable inbreeding depression. In the other study population, secondary outcrossing rates were found to increase when initially low, or decrease when initially high, depending on the family. Moderate outcrossing rates were found to be more stable. Parental inbreeding coefficients were close to zero in both populations. Outcrossing rate was much more variable among families in the population with the lower average outcrossing rate, suggesting that individuals differed considerably in their mating system. Our results add to recent studies suggesting that populations of mixed-mating animals may differ in their mating system parameters and expression of inbreeding depression.Heredity (2006) 97, 275–282. doi:10.1038/sj.hdy.6800851; published online 24 May 2006 [ABSTRACT FROM AUTHOR]

Various methods have been proposed to estimate inbreeding depression and to assess its consequences for natural populations. As an alternative to controlled crosses, the use of molecular markers has allowed direct investigation of inbreeding depression in natural populations, but usually suffers from low statistical power. Here, we investigated the effect of inbreeding depression on survival in two populations of the rare species Brassica insularis, using both controlled crosses and a marker-based approach. We compare the respective merits of the two approaches for studying inbreeding depression. We also use information from the molecular markers to dissect in detail patterns of inbreeding depression in this species. A posteriori, we find that combining the approaches was not necessary to obtain simple point estimates of inbreeding depression. However, using molecular markers may give insight into the genetic basis of inbreeding depression, such as the occurrence of epistatic interactions among deleterious alleles or purging.Heredity (2006) 97, 304–311. doi:10.1038/sj.hdy.6800870; published online 19 July 2006 [ABSTRACT FROM AUTHOR]

Haplodiploidy is one of the most widespread mechanisms of sex determination in animals. In many Hymenoptera, including all hitherto investigated social species, diploid individuals, which are heterozygous at the sex locus, develop as females, whereas haploid, hemizygous individuals develop as males (single-locus complementary sex determination, sl-CSD). Inbreeding leads to homozygosity at the sex locus, resulting in the production of diploid males, which are usually sterile and constitute a considerable fitness cost. Nevertheless, regular inbreeding without diploid male production is known from several solitary wasps, suggesting that in these species sex is not determined by sl-CSD but alternative mechanisms. Here, we examine sex determination in an ant with regular inbreeding, Cardiocondyla obscurior. The almost complete absence of diploid males after 10 generations of brother–sister mating in the laboratory documents for the first time the absence of sl-CSD and CSD with two or a few unlinked sex loci in a species of social Hymenoptera. Queens, which mated with a brother, appeared to decrease the number of males in their brood, as expected from the relatedness relationships under inbreeding. In contrast, some colonies began to show signs of an inbreeding depression after several generations of sib-mating, such as shortened queen life span, higher brood mortality, and a shift to more male-biased sex ratios in some colonies, presumably due to lower insemination capability of sperm.Heredity (2006) 97, 75–80. doi:10.1038/sj.hdy.6800846; published online 17 May 2006 [ABSTRACT FROM AUTHOR]

We evaluated the effects of inbreeding on traits related to territorial dominance and tested whether the magnitude of inbreeding depression (ID) was modified by social environment in Coho salmon. Evaluation of behaviour in paired contests between juvenile salmon with different inbreeding (low, LI=9.5%; medium, MI=29.6%), did not show significant differences between their capacities for establishing territorial dominance (mean aggressiveness score, LI=20.0±22; MI=16.7±23 or for feeding attempts, LI=18.3±12; MI=21.1±12). However, fish with low inbreeding (LI) showed almost twice the aggressive pursuit of fish with medium inbreeding (MI), and had a higher specific growth rate (SGR) in culture (SGRMI=1.83±0.58; SGRLI=2.20±0.67). Additionally, we found evidence that the magnitude of ID was modified by social environment: (1) Masking: In small groups of fish (N=20), large dominant fish of MI, cultivated with small subordinate fish of LI, showed the same SGR as dominant fish of LI cultivated with small subordinate fish of MI. (2) Magnifying: A significant effect of ID on juvenile survival was detected only in high-density competitive environments. Thus, the number of lethal equivalents was 2.70 at high-density, and only 0.24 in a low-density environment. Our results show that differences in size associated with territorial dominance may mask deleterious effects of inbreeding under certain conditions, and support the concept that intraspecific competition usually magnifies the deleterious effects of inbreeding.Heredity (2005) 95, 449–456. doi:10.1038/sj.hdy.6800741; published online 28 September 2005 [ABSTRACT FROM AUTHOR]

Understanding the consequences of inbreeding has important implications for a wide variety of topics in population biology. Although it is often stated in the literature that the deleterious effects of inbreeding (inbreeding depression) are expected to be more pronounced under stressful than benign conditions, this issue remains unresolved and controversial. We review the current literature on the relationship between the magnitude of inbreeding depression and environmental stress and calculate haploid lethal equivalents expressed under relatively benign and stressful conditions based on data from 34 studies. Inbreeding depression increases under stress in 76% of cases, although this increase is only significant in 48% of the studies considered. Estimates of lethal equivalents are significantly greater under stressful (mean=1.45, median=1.02) than relatively benign (mean=0.85, median=0.61) conditions. This amounts to an approximately 69% increase in inbreeding depression in a stressful vs a benign environment. However, we find strong lineage effects to be ubiquitous among studies that examine inbreeding depression in multiple environments, and a prevalence of conditionally expressed deleterious effects within lineages that are uncorrelated across environments. These results have important implications for both evolutionary and conservation biology.Heredity (2005) 95, 235–242. doi:10.1038/sj.hdy.6800721; published online 3 August 2005 [ABSTRACT FROM AUTHOR]

Under several assumptions such as infinite population size with unlinked loci at linkage equilibrium (LD) under mutation–selection (M–S) balance, the rate (U), and the average effects (dominance and selection coefficients \[hmacr] and \[smacr]) of deleterious genomic mutations (DGM) can be estimated by the Deng–Lynch method in some natural populations. However, all natural populations are finite in size and many of them are not large enough to be considered as approximately infinite. In the absence of an analytical estimation approach to characterize DGM in finite populations, we test the robustness and applicability of the Deng–Lynch method in finite populations with computer simulations. The results indicate that the estimation obtained by the Deng–Lynch method in finite populations with LD is generally robust when population size is greater than 400. With constant mutation effects, in outcrossing populations, the estimates Û and ĥ are unbiased or only slightly upwardly biased, and ŝ is unbiased for most cases. In highly selfing populations, Û and ĥ are upwardly biased, Û is no more than 1.5U and ĥ is less than 1.1 h, and ŝ is either unbiased or slightly downwardly biased. With variable mutation effects, Û ranges from 0.56 to 0.72U, and &smacrcrc; ranges from 1.4 to 1.8\[smacr]. Generally speaking, with the same finite population size, the estimation in outcrossing populations is better than in highly selfing populations. Given that even the order of the magnitude of the parameters of DMG (U in particular) is controversial, our investigation here may provide a basis for using the Deng–Lynch method to characterize DGM in finite populations of size greater than 400 in the presence of LD.Heredity (2005) 95, 59–68. doi:10.1038/sj.hdy.6800689 Published online 4 May 2005 [ABSTRACT FROM AUTHOR]

Nuclear and cytoplasmic genomes can coevolve antagonistically or harmoniously to affect fitness. One commonly used test for nuclear-cytoplasmic coadaptation relies on the breakup of coadapted gene complexes by introgression, potentially resulting in an increased frequency of nuclear alleles in deleterious interaction with an alien cytoplasm. We investigated the phenotypic effect of such genes on female reproduction in outbred and inbred introgressed lines of the haplodiploid mite Tetranychus urticae. Introgression changed female lifetime fecundity and increased male production, in ways suggesting a control of fecundity by nuclear genes. Conversely introgression reduced the fertilization rate, possibly due to sperm-egg incompatibility or maternal effects. The intensity of inbreeding depression expressed as a reduction in fecundity was more severe in introgressed females than in nonintrogressed ones, giving evidence for recessive interacting alleles contributing to residual nucleo-cytoplasmic incompatibility. Overall, our data suggest recessive negative interactions between nuclear and cytoplasmic genes. This study is the first report of a contribution of nuclear polymorphism within a population to deleterious interactions with an alien cytoplasmic genome.Heredity (2004) 93, 551-558. doi:10.1038/sj.hdy.6800552 Published online 25 August 2004 [ABSTRACT FROM AUTHOR]

This study compares sexually and asexually produced fruit set, seed production, biomass, germination, and seedling size in Carpobrotus acinaciformis and C. edulis following controlled pollination experiments in order to evaluate the potential role of reproductive traits with respect to the invasive potential of these taxa. C. edulis is slightly agamospermic, completely self-fertile, slightly preferentially self-compatible, experiences no inbreeding depression, and has low hybrid vigour. In contrast, C. acinaciformis does not have reliable agamospermy, is only slightly self-fertile and self-compatible, experiences a slight inbreeding depression, and has a strong hybrid vigour. Both taxa have relatively low, although significantly different germination frequencies, and insignificantly different seedling sizes. Owing to the high performance in hybridisation as compared to all other controlled pollinations in C. acinaciformis, as well as a large amount of previously demonstrated introgression, we refer to the population studied on the island of Bagaud (France) as C. affine acinaciformis. We conclude that both C. edulis and C. affine acinaciformis should be considered as harmful invasive plants in the Mediterranean Basin, the former because of the flexibility of its mating system and high seed production, and the latter because of its strong clonality, high hybrid vigour, and potential for continued introgression from C. edulis genes. These differences require different control strategies, while the avoidance of sympatry is a distinct priority. [ABSTRACT FROM AUTHOR]

Despite the importance of Narcissus to ornamental horticulture, there have been no population genetic studies of wild species, many of which have narrow distributions. Here, we measure selfing rates and levels of genetic diversity at allozyme loci in six populations of Narcissus longispathus, a self-compatible daffodil endemic to a few mountain ranges in southeastern Spain. The populations were distributed among four distinct river valleys encompassing two main watersheds in the Sierra de Cazorla mountains. Selfing rates averaged 0.37 (range 0.23-0.46), resulting in significant inbreeding coefficients for the progeny (f = 0.324). In contrast, estimates of inbreeding in parental genotypes were not significantly different from zero (f = 0.001), indicating that few selfed offspring survive to maturity because of inbreeding depression. Species-wide estimates of genetic diversity for the six populations were Ps = 0.38, Hes = 0.119 and As = 1.27 with significant genetic differentiation among populations θ = 0.15. The observed patterns of genetic differentiation among populations are likely influenced by the mating system, and a combination of local topography, watershed affinities and gene flow. [ABSTRACT FROM AUTHOR]

After an inbreeding event, lifespan can be curtailed through the expression of deleterious alleles. This will impact on both mortality patterns and interactions with the environment as visualised in reaction norms. We have established the effects of inbreeding on the temperature dependence of lifespan and on mortality patterns in Drosophila melanogaster. Four inbred lines displaying severely decreased lifespan and five outbred controls were assessed for male adult survival at three temperatures. As expected, all inbred lines showed a shorter lifespan than noninbred lines. The mechanisms behind this, however, appeared to be very diverse. Two inbred lines showed a significantly decreased temperature dependence of lifespan compared to the control lines. Analysis of variance on the mortality parameters over all lines showed that inbreeding changes the age-independent mortality but not the age-dependent mortality, whereas temperature does the opposite. This suggests that gene-by-environment interaction caused by inbreeding is the result of changes in the processes of lifespan determination. Importantly, for the two other inbred lines, a particular temperature regime triggered the expression of conditional lethal alleles. Mortality was concentrated in short lethal phases early in adult life. These conditionally expressed lethal alleles affecting lifespan demonstrate line specificity for inbreeding depression and will help ageing studies as such alleles may serve as candidate genes for ageing processes and age-related pathologies in humans.Heredity (2004) 92, 275-281. doi:10.1038/sj.hdy.6800412 Published online 17 December 2003 [ABSTRACT FROM AUTHOR]

Bud pollinations allowed me to examine the effects of homozygosity at loci in the area of suppressed recombination around the S-locus in Solanum carolinense, whose S-alleles show surprisingly low diversification rates. The total number of seeds produced was lower for incompatible than compatible pollinations, revealing that self-incompatibility was only somewhat overcome by bud pollination. However, low seed set in incompatible crosses was not due solely to the incompatibility response; crosses between distinct plants sharing the same alleles at the S-locus led to dramatically high seed abortion, nearly equal to that found upon selfing. An excess of heterozygotes in the surviving progeny supports the supposition that these high abortion rates are due to sheltered load, that is, previously unexpressed load accumulated due to enforced heterozygosity and recombination suppression around the S-locus. Of the seven alleles examined in total, two showed evidence of severe load and five did not. The magnitude of load was consistent with terminal branch length in some, but not all, cases.Heredity (2004) 92, 335-342. doi:10.1038/sj.hdy.6800425 Published online 4 February 2004 [ABSTRACT FROM AUTHOR]

Is this short review we explore the genetic threats facing declining populations, focusing in particular on empirical studies and the emerging questions they raise. At face value, the two primary threats are slow erosion of genetic variability by drift and short-term lowering of fitness owing to inbreeding depression, of which the latter appears the more potent force. However, the picture is not this simple. Populations that have passed through a severe bottleneck can show a markedly reduced ability to respond to change, particularly in the face of novel challenges. At the same time, several recent studies reveal subtle ways in which species are able to retain more useful genetic variability than they ‘should’, for example by enhanced reproductive success among the most outbred individuals in a population. Such findings call into question the validity of simple models based on random mating, and emphasize the need for more empirical data aimed at elucidating precisely what happens in natural populations. [ABSTRACT FROM AUTHOR]

In this study we examined the relative effects of inbreeding and outbreeding on offspring fitness over two generations in Anchusa crispa Viv., a rare species which on Corsica (France) occurs in small populations composed of patches of few individuals. Self- and outcross-pollinations were carried out in a single population and F1 progeny grown to flowering. Plants grown from selfing and outcrossing were then selfed or outcrossed to produce F2 individuals in four combinations of pollination treatments over two generations, i.e. self + self, self + outcross, outcross + self and outcross + outcross. In the F1 generation, selfed progeny had a significantly greater number of cymes per plant than outcrossed progeny (P=0.006). Plants from two generations of selfing had fewer seeds per fruit (P=0.06) but a significantly greater survival rate (P < 0.001) and a greater number of cymes (P=0.06) than those from two generations of outcrossing. Selfed F2 from outcrossed F1 had a significantly greater number of cymes (P < 0.01) than outcrossed F2 from outcrossed F1. In the comparison between selfed and outcrossed F2 from selfed F1 the former had a significantly greater survival rate (P < 0.001), but the latter significantly more flowers per cyme (P < 0.05). Together, these results indicate that outbreeding depression may occur on a very local scale within populations of this rare endemic species, with important implications for the conservation of rare plants. [ABSTRACT FROM AUTHOR]

A combination of field experiments and allozyme studies was used to test whether two spring-ephemeral plants growing in eastern North America, Trillium erectum and T. grandiflorum (Liliaceae), exhibited an optimal outcrossing distance for fruit and seed production. Furthermore, the spatial genetic structure of the Trillium populations was examined in light of the outcrossing results. In field experiments, recipient plants were pollinated with either self pollen or with pollen from donors growing 1 m, 10 m, 100 m, and 1500 m away. These distances represented crosses between individuals growing within the same population (1 m, 10 m, and 100 m crosses) and between individuals growing in different populations (1500 m crosses). Self-pollinated T. erectum and T. grandiflorum produced 71% and 89% fewer seeds, respectively, than all other outcrossing treatments. However, there were no significant differences among outcrossing treatments for fruit or seed production. As neither T. erectum nor T. grandiflorum exhibited an optimal outcrossing distance for fruit or seed production, it was predicted that populations of the two would not demonstrate strong spatial genetic structure or isolation by distance. The allozyme results only partially supported the patterns revealed in the outcrossing treatments. Populations of T. erectum and T. grandiflorum showed moderate spatial genetic differentiation based on F-statistics, and only T. grandiflorum exhibited significant isolation by distance based on spatial autocorrelation analyses. The lack of optimal outcrossing distances and the patterns of allozyme variation in T. erectum and T. grandiflorum populations may be attributed to a number of factors, including active seed dispersal by ants, rare long-distance gene-flow events, post-pollination and post-fertilization selection, and/or the severity of inbreeding depression. [ABSTRACT FROM AUTHOR]

We established three closed lines of N=10 for the guppy Poecilia reticulata, to evaluate the relationships among temporal changes in allele frequency, genetic variation and inbreeding depression for a fitness-related trait in small populations. Genetic variation at the allozyme loci, expressed by the proportion of polymorphic loci, number of alleles per locus and heterozygosity, decreased somewhat in two closed lines but it increased in one closed line over six generations. Effective population size (Ne) at each generation was estimated from the standardized variance in the allele frequencies. The average Ne was 24.4, 10.3 and 10.0 in the three closed lines. The inbreeding coefficient calculated from the Ne increased to 0.186, 0.321 and 0.414, respectively. As an index of the amount of inbreeding depression, changes in salinity tolerance were examined, because this trait is strongly sensitive to inbreeding depression and decreases linearly with an increase in inbreeding coefficient. The mean value of the salinity tolerance significantly decreased to 82.5%, 71.7% and 67.6% in the three closed lines during the six generations, suggesting inbreeding depression for salinity tolerance. Although a significant correlation was not observed between the amount of inbreeding depression and the genetic variation, the amount of inbreeding depression correlated with the inbreeding coefficient calculated from Ne. The regression line indicated an 8.4% decrease in the mean per 10% increase in the inbreeding coefficient and was similar to that obtained directly from full-sib matings. These results indicate that the temporal changes in the allele frequencies can provide an estimation of the amount of inbreeding depression during successive generations in small populations. [ABSTRACT FROM AUTHOR]

I used parallel family-structured crossing designs to investigate the relative performance of self and outcross progeny in selfing and predominantly outcrossing populations of the annual plant Arenaria uniflora. The selfer population experienced much lower inbreeding depression (δ=0.05 ± 0.02 SE) than the outcrossers (δ=0.19 ± 0.02 SE). The negative association between genetic load and selfing rate suggests that purgable partially recessive alleles are the primary source of inbreeding depression, as does its late expression in both populations. Inbreeding depression in the selfer population, which naturally consists of highly inbred lines, was used to calculate the mean dominance (h=0.33) and incidence rate (U=0.30) of deleterious mutations. In the outcrosser population, significant variation among individuals in the expression of inbreeding depression may reflect lineage-specific differences in inbreeding history or, more probably, random variation in mutational load. The low (<<0.5) inbreeding depression of outcrossers suggests that the maintenance of a mixed mating system in some A. uniflora populations and the evolution of nearly cleistogamous self-pollination in others may reflect local pollinator-mediated selection for selfing rather than the constant 3:2 genetic advantage invoked by many models. [ABSTRACT FROM AUTHOR]

The effect of physiological stress on the magnitude of inbreeding depression in plants has been the subject of few studies and is currently controversial because of contradictory results. We measured the inbreeding depression at three drought stress levels, precisely defined by a preliminary physiological experiment. We also tested the hypothesis that more highly self-compatible populations exhibit reduced inbreeding depression due to purging of deleterious mutations. The study was conducted on two populations of the annual and allogamous plant Crepis sancta collected from the French Mediterranean region and two other populations from marginal areas with various self-incompatibility levels. Drought stress did not increase inbreeding depression in terms of plant mortality but significantly increased the inbreeding depression for the date of first flowering, number of heads per plant and relative growth rate. The most self-fertile marginal population showed an absence of inbreeding depression in most of the measured traits indicating that purging could have taken place in this population. The three others populations showed relatively low and similar estimates of inbreeding depression (δ ≈ 0.35). The relatively low values obtained compared to the results found in allogamous plants suggests that the absence of competition for C. sancta in our experiment probably underestimated the effects of inbreeding in natural populations where competition occurs. [ABSTRACT FROM AUTHOR]

This study examined the genetic contribution of single male immigrants to small, inbred laboratory populations of Drosophila melanogaster. Genetic contribution was assessed by measuring the relative frequency of immigrant marker alleles in the first and second generations after immigration, while controlling for any selection effects at the marker locus, and for the experience of male immigrants. When immigrants were outbred, the mean frequency of the immigrant allele was significantly higher than its initial frequency, in both the first and second generations after immigration. There was no significant change in allele frequency for populations receiving inbred immigrants. The increase in allele frequency for outbred immigrants was attributed to an initial outbred vigour fitness advantage of immigrant males over resident males experiencing inbreeding depression. Hybrid vigour of immigrant progeny and the rare-male effect did not have a statistically significant role in the fitness advantage of the immigrant allele. The results suggest that inbreeding may have a considerable impact on the contribution of immigrants to the genetic diversity of populations. [ABSTRACT FROM AUTHOR]

We use Wright’s distribution of equilibrium allele frequency to demonstrate that hybrids between populations interconnected by low to moderate levels of migration can have large positive heterosis, especially if the populations are small in size. Beneficial alleles neither fix in all populations nor equilibrate at the same frequency. Instead, populations reach a mutation–selection–drift–migration balance with sufficient among-population variance that some partially recessive, deleterious mutations can be masked upon crossbreeding. This heterosis is greatest with intermediate mutation rates, intermediate selection coefficients, low migration rates and recessive alleles. Hybrid vigour should not be taken as evidence for the complete isolation of populations. Moreover, we show that heterosis in crosses between populations has a different genetic basis than inbreeding depression within populations and is much more likely to result from alleles of intermediate effect. [ABSTRACT FROM AUTHOR]

Theory predicts that, at equilibrium, the magnitude of inbreeding depression caused by recessive alleles should decrease as the level of self-fertilization increases, facilitating the evolution of even higher selfing rates. Data are presented on mating systems and inbreeding depression in multiple populations of two annual species in the genus Linanthus (Polemoniaceae). Outcrossing rates were found to be less than 0.075 in populations of a small-flowered species, L. bicolor, and ranged from 0.104 to 0.300 in populations of a larger-flowered species, L. jepsonii. Inbreeding depression for both species was low for all life history stages, with cumulative inbreeding depression less than 0.23 in all populations. Significant inbreeding depression for pollen fertility was found in L. jepsonii populations and may contribute substantially to total inbreeding depression. A competitive treatment had no significant effect on the expression of inbreeding depression for above-ground dry mass. In accordance with theoretical expectations, the magnitude of inbreeding depression was lower in L. bicolor, the more highly selfing species. The magnitude of inbreeding depression in L. jepsonii, although greater than that of L. bicolor, is not sufficient to prevent evolution of increased selfing rates in this species, according to some theoretical models of mating system evolution. [ABSTRACT FROM AUTHOR]

The spatial genetic structure of a plant population provides a potential record of past gene flow and mating. We used hierarchical F-statistics and spatial autocorrelation to characterize spatial genetic differentiation of allozymes in adult Delphinium nuttallianum plants within and among six natural populations separated from one another by up to 3 km. Previous direct estimates suggested that gene flow is highly localized, averaging < 10 m. Earlier studies of seed-set, pollen-tube growth and progeny fitness suggested that partial reproductive isolation exists between plants growing too close together (<3 m) and too far apart (>100 m). Thus we anticipated substantial genetic differentiation on scales of a few to hundreds of metres. However, we detected little differentiation among the six populations, among replicate study plots within populations, or among subsections of study plots, except at the smallest scale of cm to m. These results suggest that relatively rare long-distance pollen movement has gone undetected and that postpollination selection may further modify genetic structure during the life cycle. Lack of differentiation is not at odds with the observation of partial reproductive isolation, because some loci may respond to spatial variation in selection without this response being evident at marker loci. [ABSTRACT FROM AUTHOR]

Despite its practical application in conservation biology and evolutionary theory, the cost of inbreeding in natural populations of plants and animals remains to a large degree unknown. In this review we have gathered estimates of inbreeding depression (δ) from the literature for wild species monitored in the field. We have also corrected estimates of δ by dividing by F (coefficient of inbreeding), to take into account the influence that the variation in F will have on δ. Our data set includes seven bird species, nine mammal species, four species of poikilotherms (snakes, fish and snails) and 15 plant species. In total we obtained 169 estimates of inbreeding depression for 137 traits; 81 of those estimates included estimates of F. We compared our mammalian data (limited to those traits related to juvenile mortality) to the estimates for captive zoo species published by Ralls et al. (1988) to determine if, as predicted from the literature, natural estimates of inbreeding depression are higher than captive estimates. The mean δ ± SE (significantly different from zero and not corrected for F ) for homeotherms was 0.509 ± 0.081; for poikilotherms, 0.201 ± 0.039; and for plants, 0.331 ± 0.038. Levels of inbreeding depression this high in magnitude will be biologically important under natural conditions. When we limited our data set to mortality traits for mammals and corrected for F=0.25 (as is the case for the Ralls et al. data set), we found a significant difference between the two data sets; wild estimates had a substantially higher mean cost of inbreeding at F = 0.25: 2.155 (captive species: 0.314). Of the 169 estimates of δ, 90 were significantly different from zero, indicating that inbred wild species measured under natural conditions frequently exhibit moderate to high levels of inbreeding depression in fitness traits. [ABSTRACT FROM AUTHOR]

An abnormal female producing only female progeny was found in Lymantria dispar in Hokkaido, Japan, in July 1996. Similarly, its progeny produced only females. Egg hatch rates were near 50% in all-female matrilines. Therefore, a certain cytoplasmic factor was thought to kill males in eggs differentially, resulting in only female hosts. In the next generation, the field population was estimated to contain 9.1% abnormal females. Severe inbreeding depression was also observed in egg hatch rates during confirmation of maternal inheritance. The cost of inbreeding was estimated at 0.395, which is one of the highest in insects. Inbreeding avoidance by their host has been cited as one of the advantages of a male-killing factor, but we suggest that this is not applicable in this moth. [ABSTRACT FROM AUTHOR]

The magnitude of inbreeding depression can influence many aspects of a population’s ecology and evolution, including the nature of selection acting on the mating system and the chances that the population will go extinct during periods of small population size. If inbreeding depression is caused primarily by recessive mutations of large effect on fitness, such as lethals or steriles, then it is expected to be purged rapidly by selection with moderate amounts of inbreeding. In contrast, inbreeding depression primarily caused by many genes with mild effects on fitness will not be rapidly purged with inbreeding, so it should be a more resilient barrier to the evolution of self-fertilization and a more significant threat to the survival of endangered species. Here I show that recessive male-sterility alleles at individual loci are common in a primarily outcrossing population of the plant Mimulus guttatus. Despite the high frequency of these major mutations, most of the inbreeding depression for male fertility and cumulative measures of lifetime fitness results from more mildly deleterious alleles. Male-sterility alleles contribute to 31% of the inbreeding depression for the fraction of viable pollen grains, and to 26% of the inbreeding depression for total fitness. These results suggest that most of the inbreeding depression for male fertility in this population would not be purged, in the short term, with moderate inbreeding. [ABSTRACT FROM AUTHOR]

We present data on late-acting inbreeding depression in pollen performance, siring success and seed production in Echium vulgare. Pollen viability and rate of pollen-tube growth were both lower for pollen from plants derived from selfing than for pollen from plants derived from outcrossing. Pollen tube numbers within the styles did not differ for pollen from plants derived from selfing or outcrossing. A pollination experiment with two mixtures of pollen from plants derived from selfing or outcrossing, revealed a significant decline of 55% in siring success for pollen from plants derived from selfing. A second experiment with a complete diallel design revealed inbreeding depression for both siring success of the offspring (32.8%) and a decline in seed production of the offspring (34.8%–40.6%). In addition, results indicated a heritable component for seed number per flower. Offspring fitness, measured as seed production and siring ability, can be severely affected by late-acting inbreeding depression. Inbreeding depression values for male and female functions were not correlated. Both functions must therefore be considered when calculating inbreeding depression. [ABSTRACT FROM AUTHOR]

Recent studies have found moderate to high levels of selfing in plants despite high inbreeding depression. Because both factors influence the evolution and persistence of rare plants, we conducted glasshouse and field studies of pollination and inbreeding in Silene douglasii var. oraria, a perennial tetraploid endemic to coastal prairies. We detected: (i) variation in reproduction or inbreeding depression among life stages, years and maternal families; (ii) partial selfing yet higher relative fitness in outcrossed than selfed progeny; (iii) differing values of selfing and inbreeding depression using population means vs. matched maternal families. Fruit and seed production varied significantly with pollination treatment and year in flowers manipulated in situ during three seasons of growth. Hand-pollinations providing pollen in excess of ovule production in 1996 yielded more seeds than marked, open-pollinated flowers, implying pollen limitation of seed production. However, among-year differences in reproductive success after open-pollination (i.e. values equivalent to autogamy, selfing or outcrossing) suggest that pollination levels also vary temporally. In pollinations matched by maternal family, selfing yielded significantly fewer seeds than outcrossing. Fitness differences between inbred and outbred progeny were significant (P < 0.05) for seed production, percentage germination, and biomass or fecundity, but not for survival. Maternal family data gave selfing rates intermediate between obligate outcrossers and predominant selfers (S = 0.34–0.51), but population-wide means gave unusually high values (S = 1.1–1.6). Cumulative inbreeding depression was 76% for maternal families, and 70–85% using population means; in all cases, inbreeding depression values were high in early and late life stages, and lowest for survival. Thus far, reproductive assurance offers the most cogent explanation for the coexistence of moderate selfing and high inbreeding depression in this strongly protandrous Silene once thought to be highly outcrossing. This possibility merits further study in other rare plants with mixed-mating systems, where inbreeding depression and pollinator scarcity may both compromise population persistence and raise the threshold below which selfing is favoured by evolution. [ABSTRACT FROM AUTHOR]

Detailed studies suggest that the level of inbreeding depression may vary between populations. In a study of Scots pine from Finland, the level of inbreeding depression was much lower in northern than in southern populations. We have examined theoretically whether population genetic factors, such as the level of selfing, intensity of selection against heterozygotes or homozygotes, level of mutation, a bottleneck, finite population size, or the level of polyembryony could account for this difference. Higher selfing or stronger selection against heterozygotes in the north, both at biologically reasonable levels, appear to produce changes consistent with the observed differences and we consider these to be the most likely explanations. In addition, the differences could have accumulated by these mechanisms over the age of the northern population, ≈100 generations. Finally, the differences generated by these factors could still be maintained in the face of reasonable levels of gene flow from the south. Such a comprehensive theoretical investigation of this example has given some general insight into the potential influence of these evolutionary factors on the level of inbreeding depression and provides an approach that could be used to understand similar phenomena in other examples. [ABSTRACT FROM AUTHOR]