Gene flow from foreign provenances into local plant populations: Fitness consequences and implications for biodiversity restoration

Long-distance transplantation of seed material as done in restoration programs has raised concerns about the risks associated with the introduction of maladapted genotypes that may hybridize with neighboring native conspecifics and decrease local population fitness (outbreeding depression). We studied the consequences of gene flow from foreign provenances into local populations in the common grassland species Plantago lanceolata (Plantaginaceae). Three generations of intraspecific hybrids (F(1), F(2), and backcross to the local plants) were produced by controlled crossings between local plants and plants from geographically or environmentally distant populations. Their performance was compared to that of within-population crosses in a field experiment. Early growth in some interpopulation hybrids was significantly reduced, and this decrease in performance was higher in progeny of crosses with the local population from a different habitat than with geographically distant populations. At the end of the growing season, most fitness-related traits of the interpopulation hybrids were close to the average of their parents. Crosses with low-performing foreign parents therefore resulted in reduced fitness of the hybrids compared to the local plants and dilution of local adaptation. We conclude that the introduction of maladapted populations from distant or ecologically distinct environments might, at least temporarily, decrease the fitness of neighboring local plants.     

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.     

Multigenerational outbreeding effects in Chinook salmon (Oncorhynchus tshawytscha)

Outbreeding, mating between genetically divergent individuals, may result in negative fitness consequences for offspring via outbreeding depression. Outbreeding effects are of notable concern in salmonid research as outbreeding can have major implications for salmon aquaculture and conservation management. We therefore quantified outbreeding effects in two generations (F₁ hybrids and F₂ backcrossed hybrids) of Chinook salmon (Oncorhynchus tshawytscha) derived from captively-reared purebred lines that had been selectively bred for differential performance based on disease resistance and growth rate. Parental lines were crossed in 2009 to create purebred and reciprocal hybrid crosses (n = 53 families), and in 2010 parental and hybrid crosses were crossed to create purebred and backcrossed hybrid crosses (n = 66 families). Although we found significant genetic divergence between the parental lines (F_ST = 0.130), reciprocal F₁ hybrids showed no evidence of outbreeding depression (hybrid breakdown) or favorable heterosis for weight, length, condition or survival. The F₂ backcrossed hybrids showed no outbreeding depression for a suite of fitness related traits measured from egg to sexually mature adult life stages. Our study contributes to the current knowledge of outbreeding effects in salmonids and supports the need for more research to better comprehend the mechanisms driving outbreeding depression.     

Source population characteristics affect heterosis following genetic rescue of fragmented plant populations

Understanding the relative importance of heterosis and outbreeding depression over multiple generations is a key question in evolutionary biology and is essential for identifying appropriate genetic sources for population and ecosystem restoration. Here we use 2455 experimental crosses between 12 population pairs of the rare perennial plant Rutidosis leptorrhynchoides (Asteraceae) to investigate the multi-generational (F₁, F₂, F₃) fitness outcomes of inter-population hybridization. We detected no evidence of outbreeding depression, with inter-population hybrids and backcrosses showing either similar fitness or significant heterosis for fitness components across the three generations. Variation in heterosis among population pairs was best explained by characteristics of the foreign source or home population, and was greatest when the source population was large, with high genetic diversity and low inbreeding, and the home population was small and inbred. Our results indicate that the primary consideration for maximizing progeny fitness following population augmentation or restoration is the use of seed from large, genetically diverse populations.     

An ecological genetic delineation of local seed‐source provenance for ecological restoration

An increasingly important practical application of the analysis of spatial genetic structure within plant species is to help define the extent of local provenance seed collection zones that minimize negative impacts in ecological restoration programs. Here, we derive seed sourcing guidelines from a novel range‐wide assessment of spatial genetic structure of 24 populations of Banksia menziesii (Proteaceae), a widely distributed Western Australian tree of significance in local ecological restoration programs. An analysis of molecular variance (AMOVA) of 100 amplified fragment length polymorphism (AFLP) markers revealed significant genetic differentiation among populations (Φ_PT = 0.18). Pairwise population genetic dissimilarity was correlated with geographic distance, but not environmental distance derived from 15 climate variables, suggesting overall neutrality of these markers with regard to these climate variables. Nevertheless, Bayesian outlier analysis identified four markers potentially under selection, although these were not correlated with the climate variables. We calculated a global R‐statistic using analysis of similarities (ANOSIM) to test the statistical significance of population differentiation and to infer a threshold seed collection zone distance of ~60 km (all markers) and 100 km (outlier markers) when genetic distance was regressed against geographic distance. Population pairs separated by >60 km were, on average, twice as likely to be significantly genetically differentiated than population pairs separated by <60 km, suggesting that habitat‐matched sites within a 30‐km radius around a restoration site genetically defines a local provenance seed collection zone for B. menziesii. Our approach is a novel probability‐based practical solution for the delineation of a local seed collection zone to minimize negative genetic impacts in ecological restoration.     

Does origin always matter? Evaluating the influence of nonlocal seed provenances for ecological restoration purposes in a widespread and outcrossing plant species

For restoration purposes, nature conservation generally enforces the use of local seed material based on the “local‐is‐best” (LIB) approach. However, in some cases recommendations to refrain from this approach have been made. Here we test if a common widespread species with no obvious signs of local adaptation may be a candidate species for abandoning LIB during restoration. Using 10 microsatellite markers we compared population genetic patterns of the generalist species Daucus carota in indigenous and formerly restored sites (nonlocal seed provenances). Gene diversity overall ranged between H_e = 0.67 and 0.86 and showed no significant differences between the two groups. Hierarchical AMOVA and principal component analysis revealed very high genetic population admixture and negligible differentiation between indigenous and restored sites (F_CT = 0.002). Moreover, differentiation between groups was caused by only one outlier population, where inbreeding effects are presumed. We therefore conclude that the introduction of nonlocal seed provenances in the course of landscape restoration did not jeopardize regional species persistence by contributing to inbreeding or outbreeding depressions, or any measurable adverse population genetic effect. On the basis of these results, we see no obvious objections to the current practice to use the 10‐fold cheaper, nonlocal seed material of D. carota for restoration projects.     

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.     

Interpopulation hybrid breakdown maps to the mitochondrial genome

Hybrid breakdown, or outbreeding depression, is the loss of fitness observed in crosses between genetically divergent populations. The role of maternally inherited mitochondrial genomes in hybrid breakdown has not been widely examined. Using laboratory crosses of the marine copepod Tigriopus californicus, we report that the low fitness of F(3) hybrids is completely restored in the offspring of maternal backcrosses, where parental mitochondrial and nuclear genomic combinations are reassembled. Paternal backcrosses, which result in mismatched mitochondrial and nuclear genomes, fail to restore hybrid fitness. These results suggest that fitness loss in T. californicus hybrids is completely attributable to nuclear-mitochondrial genomic interactions. Analyses of ATP synthetic capacity in isolated mitochondria from hybrid and backcross animals found that reduced ATP synthesis in hybrids was also largely restored in backcrosses, again with maternal backcrosses outperforming paternal backcrosses. The strong fitness consequences of nuclear-mitochondrial interactions have important, and often overlooked, implications for evolutionary and conservation biology.     

What is the fitness outcome of crossing unrelated fish populations? A meta-analysis and an evaluation of future research directions

Outbreeding has been shown to decrease fitness in a variety of species, including several species of fish. An understanding of the general outcomes following outbreeding is required in order to understand the consequences of conservation-related actions and hybridization in aquaculture. A meta-analysis was conducted on outbreeding studies in fishes using 670 comparisons between parent populations and their hybrid progeny. Five hundred and seventy-six comparisons involved first generation hybrids (F1), and a much smaller number (94) involved second generation hybrids (F2). The overall response to outbreeding in the F1 and F2 generations was positive and significant (F1: d _i+^* = 0.425 ± 0.121; F2: d _i+^* = 0.548 ± 0.295, where d _i+^* is the effect size of studies within generations); however, responses differed when studies were separated into groups describing the experimental environment, taxon, or trait. Findings may be biased by a few studies of large effect. Genetic distance explained little of the variance in effect sizes across studies. Results of the meta-analysis suggest that there is no reliable predictor for the effects of outbreeding in fishes, although inconsistencies in experimental design were noted across studies. Future research should include comparisons in both parental environments in order to detect the underlying mechanisms of outbreeding depression, and should focus on measurement of equivalent sets of fitness-related traits. Experimental design should permit estimation of genetic distances based on quantitative traits, which may in turn be meaningful predictors of the outcomes of outbreeding depression. Concerted and consistent research in this area will provide information of relevance to conservation, aquaculture and evolutionary studies.     

Population differentiation and hybrid success in Campanula americana: geography and genome size

Populations within a species may diverge through genetic drift and natural selection. Few studies report on population differentiation in autopolyploids where multiple gene copies and the ratio of cytoplasmic to nuclear genes differ from diploids and may influence divergence. In autotetraploid Campanula americana we created hybrids between populations that differed in geographic proximity and genome size. Differences in genome size (up to 6.5%) did not influence hybrid performance. In contrast, hybrid performance was strongly influenced by population proximity. F1 hybrids between distant populations performed poorly relative to their parents while hybrids between proximate populations outperformed their parents. Outbreeding depression was strongest for juvenile traits. The expression of outbreeding depression often differed between reciprocal hybrids indicating interactions between nuclear and cytoplasmic genes contribute to population differentiation. Because plants were grown under greenhouse conditions, the outbreeding depression was likely due to genetic (underdominance or loss of additive-by-additive epistasis) rather than ecological factors.     

Outbreeding Depression in Atlantic Salmon Revealed by Hypoxic Stress During Embryonic Development

The fitness consequences of outbreeding in wild populations are extremely variable. Heterosis and outbreeding depression can both be observed but the effect of environmental stressors on the occurrence of these phenomena is still poorly understood. We tested the influence of oxygen stress during embryonic development on consequences of outbreeding in wild populations of Atlantic salmon (Salmo salar). We used a common garden experiment and performed crosses within and between salmon populations to study performances of embryos reared under normal and hypoxic conditions. We detected both heterosis and outbreeding depression depending on traits but irrespective of divergence between parental populations. Nevertheless, outbreeding depression was observed almost exclusively under hypoxic conditions and prevailed over heterosis regarding survival during the whole embryonic development. Notably, the post-hatching survival of all between population crosses was approximately 15 % lower than the survival of within-population crosses under hypoxic conditions. Different hypoxia reaction norms for post-hatching survival, length and time to hatch were also noticed among within and between populations crosses further indicating outbreeding depression. These results demonstrate that consequences of outbreeding can dramatically vary depending on environmental conditions with outbreeding depression being possibly stronger under stressful conditions.     

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.     

OPTIMAL OUTCROSSING IN IPOMOPSIS AGGREGAT A: SEED SET AND OFFSPRING FITNEs

Restricted gene flow and localized selection should establish a correlation between physical proximity and genetic similarity in many plant populations. Given this situation, fitness may decline in crosses between nearby plants (inbreeding depression), and in crosses between more widely separated plants (“outbreeding depression”) mostly as a result of disruption of local adaptation. It follows that seed set and offspring fitness may be greatest in crosses over an intermediate “optimal outcrossing distance.” This prediction was supported for Ipomopsis aggregata, a long-lived herbaceous plant pollinated by hummingbirds. In six replicate pollination experiments, mean seed set per flower was higher with an outcrossing distance of 1–10 m than with selfing or outcrossing over 100 m. A similar pattern appeared in the performance of offspring from experimental crosses grown under natural conditions and censused for a seven-year period. Offspring from 10-m crosses had higher survival, greater chance of flowering, and earlier flowering than those from 1-m or 100-m crosses. As a result, 1-m and 100-m offspring achieved only 47% and 68%, respectively, of the lifetime fitness of 10-m offspring. Offspring fitness also declined with planting distance from the seed parent over a range of 1–30 m, so that adaptation to the maternal environment is a plausible mechanism for outbreeding depression. Censuses in a representative I. aggregata population indicated that the herbaceous vegetation changes over a range of 2–150 m, suggesting that there is spatial variation in selection regimes on a scale commensurate with the observed effects of outbreeding depression and planting distance. We discuss the possibility that differences in seed set might in part reflect maternal mate discrimination and emphasize the desirability of measuring offspring fitness under natural conditions in assessing outcrossing effects.     

Long-term experimental hybrid swarms between nearly incompatible Tigriopus californicus populations: persistent fitness problems and assimilation by the superior population

For the intertidal copepod Tigriopus californicus, outbreeding depression for a variety of fitness measures is typically observed in early-generation interpopulation hybrids. We examined both controlled crosses and long-term, freely mating experimental hybrid swarms composed of individuals from Baja California (Mexico) populations Playa Altamira and Punta Morro. In controlled crosses, F1 and F2 hybrids showed large and significant declines in hatching numbers compared to parentals, while reciprocal backcrosses produced no offspring at all. For long-term studies, four treatment groups were initiated: 100%PA, 100%PM, 50%PA: 50%PM, and 80%PA: 20%PM. Replicates were surveyed at 3-month intervals for morphometric, census and fitness measures. The PA and 80PA:20PM treatments had initial fitness below the PM treatment, and went extinct within the first 12 months of the experiment. The 50:50 treatment had fitness below the PM parent at the 3- and 6-month time points, recovered to equivalent or superior fitness from months 9 to 18, and dropped again below PM at month 21. Limited genotyping of diagnostic microsatellites was consistent with PM alleles going nearly to fixation in hybrid replicates and male morphological data were concordant with a shift toward PM values. Results were strikingly different from a recent study of a different pair of populations showing extensive introgression and superior fitness in hybrid populations. This demonstrates how long-term consequences of population mixing depend on the relative fitness and level of compatibility between hybridizing populations.     

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.     

The fitness effects of outcrossing in Calylophus serrulatus, a permanent translocation heterozygote

Small and relatively isolated populations that occupy fragmented habitat are at risk of local extinction. However, fitness consequences of fragmentation related to mating distance, such as inbreeding depression following increased self- and near-neighbor mating, may not follow standard expectations in species with specialized genetic systems. We investigated the effect of mating distance on progeny fitness in Calylophus serrulatus, a primarily autogamous, permanent translocation heterozygote that is restricted to prairie fragments in the North American tallgrass prairie region. We pollinated flowers by hand in the field with pollen sampled at various distances from the maternal parent within and between three populations in southeastern Minnesota. We raised the progeny in a greenhouse and measured fitness-related characters. Because their genetic system prevents loss of heterozygosity throughout much of the genome, regardless of inbreeding, permanent translocation heterozygotes are not expected to exhibit inbreeding depression. Consistent with this expectation, in no case did progeny of self matings suffer significantly reduced mean fitness compared to progeny from crosses between plants. Crosses between plants in the two closely situated (2 km) populations yielded progeny with fitness intermediate to their parents, but crosses between each of those populations and the more distant (20 km) population yielded progeny with reduced fitness, suggesting outbreeding depression at this largest spatial scale. Similarly, fitness of self-pollinated progeny and progeny from "near" crosses (<2 m) within populations tended to be higher than "mid" (10-25 m) and "far" (>35 m) cross-progeny fitness. Under the current conditions of fragmentation, it seems likely that the distant matings that produce outbreeding depression are rare. It appears that mean fitness in this species is maintained in the context of severe fragmentation of its populations, largely because of its genetic system.     

Does provenance matter? Fitness is not determined by genetic distance or the scale of pollen dispersal in Grevillea sphacelata (Proteaceae)

Understanding how the scale of pollen transfer determines the outcome of matings is important evolutionarily and a key issue in restoration ecology. We tested the effects of pollen transfer distance for the self‐incompatible shrub Grevillea sphacelata using (1) open pollination and transfer among (2) near neighbours, (3) neighbouring subpopulations and (4) populations separated by c. 4 km. We used AFLP markers to test for evidence of genetic differentiation within and among populations. Patterns of seed initiation suggest that open pollinated flowers were pollen limited, although in one subpopulation open seed set was greater than that achieved with pollen from near neighbours or other subpopulations. We detected no other effects of pollen source on seed initiation or seed and seedling development. In contrast, our genetic survey revealed significant spatial autocorrelation to 5 m, moderate differentiation of populations separated by up to 4 km and significant isolation by distance > 16 km. Our data suggest that, although dispersal of pollen may typically be localized, gene flow prevents localized adaptation or co‐adaptation and we detected no effects of inbreeding depression. In a restoration context, our results imply that movement of seed between populations separated by 4 km will not have detrimental consequences, despite significant differentiation at neutral genetic markers, and may be beneficial in maintaining genetic diversity and evolutionary potential. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2013, 173 , 290–302.     

Epistatic and cytonuclear interactions govern outbreeding depression in the autotetraploid Campanulastrum americanum

The consequences of combining divergent genomes among populations of a diploid species often involve F1 hybrid vigor followed by hybrid breakdown in later recombinant generations. As many as 70% of plant species are thought to have polyploid origins; yet little is known about the genetic architecture of divergence in polyploids and how it may differ from diploid species. We investigated the genetic architecture of population divergence using controlled crosses among five populations of the autotetraploid herb, Campanulastrum americanum. Plants were reciprocally hybridized to produce F1, F2, and F1-backcross generations that were grown with parental types in a greenhouse and measured for performance. In contrast to diploid expectations, most F1 hybrids lacked heterosis and instead showed strong outbreeding depression for early life traits. Recombinant hybrid generations often showed a recovery of performance to levels approximating, or at times even exceeding, the parental values. This pattern was also evident for an index of cumulative fitness. Analyses of line means indicated nonadditive gene action, especially forms of digenic epistasis, often influenced hybrid performance. However, standard diploid genetic models were not adequate for describing the underlying genetic architecture in a number of cases. Differences between reciprocal hybrids indicated that cytoplasmic and/or cytonuclear interactions also contributed to divergence. An enhanced role of epistasis in population differentiation may be the norm in polyploids, which have more gene copies. This study, the first of its kind on a natural autotetraploid, suggests that gene duplication may cause polyploid populations to diverge in a fundamentally different way than diploids.     

Genetic Delineation of Local Provenance in Persoonia longifolia: Implications for Seed Sourcing for Ecological Restoration

Restoration of diverse native plant communities typically requires the collection of large amounts of seed. Thus, practitioners often struggle to find adequate supplies near project sites and need to know from how far they can collect without compromising restoration success—how far does local provenance extend? We addressed this issue by assessing genetic variation within, and differentiation among, 12 potential seed source populations of Persoonia longifolia, a key component of the jarrah forest of Western Australia. An analysis of molecular variance of 66 polymorphic amplified fragment length polymorphism markers partitioned 92% of the total genetic variation within populations and 8% among populations, indicating relatively weak but statistically significant population genetic differentiation. Ordination of these genetic data showed marked west/east and north/south gradients. Pairwise population genetic dissimilarity was correlated with both geographic distance and environmental distance derived from five climate variables. However, partial Mantel tests showed that the relationship between genetic and geographic distance was not independent of environmental distance, suggesting a non‐neutral signature in these markers. Bayesian outlier analysis identified two markers, and spatial analysis method tests identified highly significant associations between these two markers and three environmental variables. Frequency differences at these markers across populations suggested the possibility of climatically adapted provenances. The global significance value from analyses of similarities for these two markers correlated to a general provenance distance of 47 km, in contrast to a threshold of 60 km for the complete dataset. Guidelines for seed sourcing that consider these population genetic data should lead to more effective ecological restoration with this species.     

Fitness and morphological outcomes of many generations of hybridization in the copepod Tigriopus californicus

Hybridization between genetically divergent populations is an important evolutionary process, with an outcome that is difficult to predict. We used controlled crosses and freely mating hybrid swarms, followed for up to 30 generations, to examine the morphological and fitness consequences of interpopulation hybridization in the copepod Tigriopus californicus. Patterns of fitness in two generations of controlled crosses were partly predictive of long‐term trajectories in hybrid swarms. For one pair of populations, controlled crosses revealed neutral or beneficial effects of hybridization after the F1 generation, and hybrid swarm fitness almost always equalled or exceeded that of the midparent. For a second pair, controlled crosses showed F2 hybrid breakdown, but increased fitness in backcrosses, and hybrid swarm fitness deviated both above and below that of the parentals. Nevertheless, individual swarm replicates exhibited different fitness trajectories over time that were not related in a simple manner to their hybrid genetic composition, and fixation of fitter hybrid phenotypes was not observed. Hybridization did not increase overall morphological variation, and underlying genetic changes may have been masked by phenotypic plasticity. Nevertheless, one type of hybrid swarm exhibited a repeatable pattern of transgressively large eggsacs, indicating a positive effect of hybridization on individual fecundity. Additionally, both parental and hybrid swarms exhibited common phenotypic trends over time, indicating common selective pressures in the laboratory environment. Our results suggest that, in a system where much work has focused on F2 hybrid breakdown, the long‐term fitness consequences of interpopulation hybridization are surprisingly benign.