Outbreeding depression varies among cohorts of Ipomopsis aggregata planted in nature

Outbreeding depression in progeny fitness may arise from disruption of local adaptation, disruption of allelic coadaptation, or a combination of these "environmental" and "physiological" mechanisms. Thus the minimum spatial scale over which outbreeding depression arises should depend on the spatial scale of gene dispersal and (with an environmental mechanism) of change in selection regimes. We previously reported substantial outbreeding depression in lifetime fitness of progeny resulting from crosses among parents separated by 100 m in natural populations of the herbaceous plant Ipomopsis aggregata. In this paper we explore the effect of crossing distance on fitness in two additional experiments begun in 1987 and 1990. We planted seed progeny derived from partial diallel crossing designs in randomized blocks in maternal environments and scored emergence of seedlings, survival, and eventual flowering of individuals over the subsequent six to eight years. Nested within each diallel design were crossing distances of 1 m, 10 m, and 100 m. Compared to 1-m and 10-m progeny, 100-m progeny of the 1987 diallel suffered a significant reduction in seedling emergence, and both 1-m and 100-m progeny that survived to flower achieved lower lambda-values on average than 10-m progeny. Total outbreeding depression suffered by 100-m relative to 10-m progeny was approximately 10%, compared to approximately 30% in our earlier study of I. aggregata. Progeny of 10-m crosses also outperformed 1-m and 100-m progeny of the 1990 diallel by approximately 5%, but no difference among crossing distance treatments was significant. Thus, the magnitude of outbreeding depression in 100-m crosses varied among experiments. This is not surprising given likely spatial and temporal variation in gene flow and selection regimes, different population histories, and different parental and progeny environments. Characterizing outbreeding depression on the shortest spatial scales over which it is expressed, as well as its variation and causes, is worthwhile because it promises to shed light on the earliest stages of angiosperm speciation.     

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.     

Consequences of near and far between-population crosses for offspring fitness in a rare herb

Crosses between plants from different populations may result in heterosis or outbreeding depression. However, despite its importance for conservation, little is known about the spatial scale over which these effects may arise. To investigate the consequences of between-population crosses at two distinct spatial scales, we conducted reciprocal crosses between four populations from two regions in the rare perennial herb Aster amellus . We assessed seed set and offspring fitness in a common garden experiment. Overall, between-population crosses within regions (10 km) resulted in 8% lower seed set than within-population crosses, while between-region crosses (70 km) resulted in 17% higher seed set than within-population crosses. Moreover, offspring from between-population crosses produced 18% more flower heads than offspring from within-population crosses. We conclude that hybridisation between A. amellus plants from different populations did not lead to immediate outbreeding depression and, thus, could represent a valid conservation option to increase genetic diversity. Moreover, our results suggest that the distance between populations affects the outputs of between-population crosses and therefore needs to be taken into account when promoting gene flow between populations.     

CROSSING-DISTANCE EFFECTS IN DELPHINIUM NELSONII: OUTBREEDING AND INBREEDING DEPRESSION IN PROGENY FITNESS

Depending on its genetic causes, outbreeding depression in quantitative characters may occur first in the free-living F₁ generation produced by a wide cross. In 1981–1985, we generated F₁ progenies by hand-pollinating larkspurs (Delphinium nelsonii) with pollen from 1-m, 3-m, 10-m, or 30-m distances. From the spatial genetic structure indicated by previous electrophoretic and reciprocal transplantation studies, we estimate that these crosses range from being inbred (f ≈ 0.06) to outbred. We planted 594 seeds from 66 maternal sibships under natural conditions. As of 1992, there was strong evidence for both inbreeding depression and outbreeding depression. Progeny from intermediate crossing distances grew approximately twice as large as more inbred or outbred progeny in the first 5 yr after planting (P = 0.013, repeated measures ANOVA), and survived almost 1 yr longer on average (contrast of 3-m and 10-m treatments versus 1 m and 30 m; P = 0.028, ANOVA). Twenty maternal sibships produced flowering individuals; only four and two of these represented 1-m and 30-m crossing distances, respectively (P = 0.021, G-test). The cumulative fitness of intermediate distance sibships averaged about twice that of 1-m sibships, and five to eight times that of 30-m sibships (P = 0.017, ANOVA). Thus, even though progeny of 1-m crosses were inbred to a degree only about one-eighth that of selling, inbreeding depression approximated 50%, and outbreeding depression equaled or exceeded 50% for all fitness components.     

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.     

Field and genetic studies testing optimal outcrossing in Agave schottii,a long-lived clonal plant

In thisstudy we combine field experiments, designed to test the predictions of optimal outcrossing theory in Agave schottii, with molecular genetic studies, using RAPD (random amplified polymorphic DNA), polymerase chain reaction to assess the underlying genetic hypothesis of optimal outcrossing theory. Initially, 48 females of A. schottii were hand-pollinated with pollen collected from 1 m, 10 m, 100 m, and 2500 m distances. Each female received all four distance treatments. Additionally, a subset of the focal females and their pollen donors were used in an analysis of genetic similarity across the four distances. Results of hand-pollinations showed that crosses of 1 m had significantly lower seed set than 10 m and 100 m crosses. Crosses of 2500 m had intermediate seed set. Combined relative fitness was significantly lower for 1 m crosses compared to 10 m crosses, while 100 m and 2500 m crosses were intermediate. Thus, A. schottii experiences inbreeding depression and a trend toward outbreeding depression. Genetic analyses showed a similar pattern: individuals 1 m apart had on average higher genetic similarity (proportion of bands shared) than individuals separated by greater distances, with a trend toward lower genetic similarity for plants located 2500 m distant. The observed spatial genetic patterns are likely maintained by the combined effects of clonal reproduction, clone longevity, limited seed dispersal and the substantial number of inbred progeny produced, counteracting distant allele transfer which tends to reduce population genetic structure. The correspondence between our ecological and genetic results indicates that RAPD markers are useful tools for assessing ecological phenomena.     

Consequences of inbreeding for offspring fitness and gender in Silene vulgaris,a gynodioecious plant

Abstract In gynodioecious plants, hermaphrodite and female plants co‐occur in the same population. In these systems gender typically depends on whether a maternally inherited cytoplasmic male sterility factor (CMS) is counteracted by nuclear restorer alleles. These restorer alleles are often genetically dominant. Although plants of the female morph are obligatorily outcrossing, hermaphrodites may self. This selfing increases homozygosity and may thus have two effects: (1) it may decrease fitness (i.e. result in inbreeding depression) and (ii) it may increase homozygosity of the nuclear restorer alleles and therefore increase the production of females. This, in turn, enhances outcrossing in the following generation. In order to test the latter hypothesis, experimental crosses were conducted using individuals derived from four natural populations of Silene vulgaris, a gynodioecious plant. Treatments included self‐fertilization of hermaphrodites, outcrossing of hermaphrodites and females using pollen derived from the same source population as the pollen recipients, and outcrossing hermaphrodites and females using pollen derived from different source populations. Offspring were scored for seed germination, survivorship to flowering and gender. The products of self‐fertilization had reduced survivorship at both life stages when compared with the offspring of outcrossed hermaphrodites or females. In one population the fitness of offspring produced by within‐population outcrossing of females was significantly less than the fitness of offspring produced by crossing females with hermaphrodites from other populations. Self‐fertilization of hermaphrodites produced a smaller proportion of hermaphroditic offspring than did outcrossing hermaphrodites. Outcrossing females within populations produced a smaller proportion of hermaphrodite offspring than did crossing females with hermaphrodites from other populations. These results are consistent with a cytonuclear system of sex determination with dominant nuclear restorers, and are discussed with regard to how the mating system and the genetics of sex determination interact to influence the evolution of inbreeding depression.     

Pollination by sexual deception promotes outcrossing and mate diversity in self‐compatible clonal orchids

The majority of flowering plants rely on animals as pollen vectors. Thus, plant mating systems and pollen dispersal are strongly influenced by pollinator behaviour. In Australian sexually deceptive orchids pollinated by male thynnine wasps, outcrossing and extensive pollen flow is predicted due to floral deception, which minimizes multiple flower visitations within patches, and the movement of pollinators under mate‐search rather than foraging behaviours. This hypothesis was tested using microsatellite markers to reconstruct and infer paternity in two clonal, self‐compatible orchids. Offspring from naturally pollinated Chiloglottis valida and C. aff. jeanesii were acquired through symbiotic culture of seeds collected over three seasons. In both species, outcrossing was extensive (t_m = 0.924–1.00) despite clone sizes up to 11 m wide. The median pollen flow distance based on paternity for both taxa combined was 14.5 m (n = 18, range 0–69 m), being larger than typically found by paternity analyses in other herbaceous plants. Unexpectedly for orchids, some capsules were sired by more than one father, with an average of 1.35 pollen donors per fruit. This is the first genetic confirmation of polyandry in orchid capsules. Further, we report a possible link between multiple paternity and increased seed fitness. Together, these results demonstrate that deceptive pollination by mate‐searching wasps enhances offspring fitness by promoting both outcrossing and within‐fruit paternal diversity.     

Pollen limitation in the endangered Chinese endemic species Sinocalycanthus chinensis

Pollen limitation negatively impacts endangered and endemic plants with small fragmented populations, such as Sinocalycanthus chinensis, an endangered plant endemic to China. In this study, we analyzed the pollen limitation of the S. chinensis Damingshan (DMS) population in 2006, 2009, and 2010, and crossed plants with mates separated by different distances, both within and between populations. The DMS population exhibited strong pollen limitation in fruit set, seed set, and seeds per fruit in 2006, 2009, and 2010. The average accumulated pollen limitation (for fruit set times seeds per fruit) was 0.510 ± 0.180. Progeny crossed with pollen from intermediate neighboring plants within the same population (separated by 30–50 m from pollen recipients) had the lowest fitness. No optimal outcrossing distance was found within the DMS population. Progeny from crosses with the Shunxiwu (SXW) and Daleishan (DLS) populations performed relatively better, while those from crosses with Qingliangfeng (QLF) and Longxushan (LXS) populations performed worse. Compared with average reproductive success, outbreeding depression was found in progeny from crosses with the LXS and QLF populations. Reproductive success from pure self‐pollination indicated S. chinensis is self‐compatible. Geitonogamous selfing increased reproductive success. Based on geitonogamous selfing, the proportion of selfed offspring was relatively high. These results provide basic references for the conservation of this species.     

LIFETIME ESTIMATES OF BIPARENTAL INBREEDING DEPRESSION IN THE SELF-INCOMPATIBLE ANNUAL PLANT RAPHANUS SATIVUS

Studies of inbreeding depression in plant populations have focused primarily on comparisons of selfing versus outcrossing in self-compatible species. Here we examine the effect of five naturally occurring levels of inbreeding (f ranging from 0 to 0.25 by pedigree) on components of lifetime fitness in a field population of the self-incompatible annual, Raphanus sativus. Pre- and postgermination survival and reproductive success were examined for offspring resulting from compatible cross-pollinations. Multiple linear regression of inbreeding level on rates of fruit and seed abortion as well as seed weight and total seed weight per fruit were not significant. Inbreeding level was not found to affect seed germination, offspring survival in the field, date of first flowering, or plant biomass (dry weight minus fruit). The effect of inbreeding on seedling viability in the greenhouse and viability to flowering was significant but small and inconsistently correlated with inbreeding level. Maternal fecundity, however, a measure of seed yield, was reduced almost 60% in offspring from full-sib crosses (f = 0.25) relative to offspring resulting from experimental outcross pollinations (f = 0). Water availability, a form of physiological stress, affected plant biomass but did not affect maternal fecundity, nor did it interact with inbreeding level to influence these characters. The delayed expression of strong inbreeding depression suggests that highly deleterious recessive alleles were not a primary cause of fitness loss with inbreeding. Highly deleterious recessives may have been purged by bottlenecks in population size associated with the introduction of Raphanus and its recent range expansions. In general, reductions in total relative fitness of greater than 50% associated with full-sib crosses should be sufficient to prohibit the evolution of self-compatibility via transmission advantage in Raphanus.     

Effects of crossing distance on fitness components in the carnivorous plant Pinguicula moranensis (Lentibulariaceae)

Plant species vary in the pollination distance at which negative fitness effects are expressed, and it has been proposed that optimal mating should occur at a distance large enough to diminish the risks of inbreeding, but short enough to prevent outbreeding depression. In a natural population of Pinguicula moranensis we assessed the fitness of plants (seeds per fruit) and their progeny (germination and seedling survival) as a function of pollination distance by hand-pollinating with donors from five distances: 0 m (self-fertilization), 1, 4 and 460 m (within-population) and 9,900 m (between-population) under in situ and ex situ conditions. We found that average values for fitness components were consistently lower in the in situ experiment than in the ex situ experiment. Under both conditions, the self-fertilization treatment had the lowest values. Inbreeding depression values were high (0.94 to 0.98), corresponding with the values expected for an outcrossing species subjected to self-fertilization. Overall, no evidence of outbreeding depression was detected. Considering that our experiment was restricted to one single donor population and one recipient population, our results indicate that despite the within-population patchy distribution of individuals, biparental inbreeding could occur at low rates, due in part to short seed dispersal distance. However, pollen movement of at least 1 m is apparently sufficient to reverse negative effects.     

Genomic compatibility occurs over a wide range of parental genetic similarity in an outcrossing plant

The theory of inbreeding and outbreeding suggests that there is a hump-shaped relationship between the genetic similarity of sexually reproducing parents and the performance of their offspring. Inbreeding depression occurs when genetic similarity is high, whereas hybrid breakdown is expected when genetic similarity is low. Between these extremes, the effect of genetic similarity on fitness is unclear. We studied the shape of this relationship by crossing 65 target genotypes of the clonal, self-incompatible Ranunculus reptans with partner genotypes spanning a broad scale of genetic similarity, ranging from crosses within populations to between-population crosses and hybridisation with a closely related species. Offspring were raised in outdoor tubs. Results revealed a quadratic relationship between parental genetic distance and offspring performance, with the clonal component of fitness more strongly hump-shaped than the sexual component. Optimal genetic similarity encompassed a broad range of within-population and between-population crosses. This pattern of genomic compatibility has important implications for the evolution of mating systems and mate choice.     

Effects of crossing distance on offspring fitness and developmental stability in Dalechampia scandens (Euphorbiaceae)

Crosses between genetically close and distant populations of Dalechampia scandens (Euphorbiaceae) were made to test whether the responses of various fitness components and measurements of developmental stability were affected by the outcrossing distance (level of outbreeding). Two fecundity traits, seed set and seed mass, decreased consistently with increasing level of outbreeding, and hybrids between the most divergent populations were sterile. Effects of the genetic distance between parental populations on viability traits, survival and vigor at 1 month of age, were highly idiosyncratic. Hybrids of one long-distance combination had no reduction in survival and vigor, while both traits were greatly reduced in the other long-distance combination. The expression of outbreeding depression on fecundity traits differed between reciprocal crosses in some hybrids but not others; thus, hybrid breakdown may have been due to cytoplasmic-by-nuclear gene interactions, reduced endosperm formation, or an interaction between progeny and maternal genotype. None of the measures of developmental stability had a consistent relationship with either genetic distance between parental populations or seedling vigor. These results suggest that fecundity and viability traits may be differentially affected by hybridization, probably due to differences in genetic architecture among populations. This study also confirms that developmental stability, as measured by the level of fluctuating asymmetry, is not a reliable index of genetic stress.     

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.     

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

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

Nonlocal transplantation and outbreeding depression in the subshrub Lotus scoparius (Fabaceae)

The genetic background of transplants used to create or augment wild populations may affect the long-term success of restored populations. If seed sources are from differently adapted populations, then the relative performance of progeny from crosses among populations may decrease with an increase in genetic differences of parents and in the differences of parental environments to the transplant location. We evaluated the potential for such outbreeding depression by hybridizing individuals from six different populations of Lotus scoparius var. scoparius and L. s. var. brevialatus. We used allozyme data to calculate genetic distances between source populations, and compiled climatic data and measured soil traits to estimate environmental distances between source populations. We found significant outbreeding depression following controlled crosses. In the greenhouse, the success of crosses (seeds/flower × seedlings/seed) decreased with increasing genetic distance between populations revealing genetically based outbreeding depression unrelated to local adaptation. After outplanting to one native site (in situ common garden), field cumulative fitness of progeny (survival × fruit production) decreased significantly with mean environmental distance of the parental populations to the transplant site, but not with genetic distance between the crossed populations. This result is consistent with a disruption of local adaptation. At the second, ecologically contrasting common garden, where low survival reduced statistical power, field cumulative fitness (survival × progeny height) did not decrease significantly with either environmental distance or genetic distance. Overall, intervariety crosses were 40 and 50% as fit (seeds/flower × seedlings/seed × survival × fruits at the first garden or × height at the second) as intravariety crosses. These results suggest that the cumulative outbreeding depression was caused by a combination of genetically based ecological differences among populations and other genomic coadaptation. We conclude that mixing genetically differentiated seed sources of Lotus scoparius may significantly lower the fitness of augmented or restored populations. Genetic and environmental similarities of source populations relative to the transplant site should be considered when choosing source materials, a practice recommended by recent seed transfer policies. Geographic separation was not a good surrogate for either of these measures.     

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.     

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.     

Effects of self-pollination and outcrossing with cultivated plants in small natural populations of American ginseng, Panax quinquefolius (Araliaceae)

For rare plants, self-pollination and inbreeding can increase in small populations, while unusual levels of outcrossing can occur through restoration efforts. To study both inbreeding and outcrossing, we performed experimental pollinations using Panax quinquefolius (American ginseng), a wild-harvested plant with a mixed mating system. For inbreeding, plants were either cross-pollinated within the population or self-pollinated, which resulted in a higher proportion of seeds from self-pollinated flowers. For outcrossing, wild plants were either cross-pollinated within the population or with cultivated plants from West Virginia or Wisconsin. Offspring of all crosses were followed for 4 yr. Two-yr-old seedlings from self-pollination had 45% smaller leaf areas and 33% smaller heights relative to those from cross-pollination. Leaf area is a positive predictor of longer-term survival in wild populations. Our results suggest inbreeding depression, which is unexpected in this self-fertile species. Seedlings from crosses with cultivated plants had 127% greater leaf area and 165% greater root biomass relative to outcrosses within the population. The accelerated growth suggests genetic differences between wild and cultivated populations, but outbreeding depression may not appear until later generations. Assessment of the ultimate fitness consequences of introducing cultivated genotypes requires monitoring over longer time periods.     

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.