Disruption of mitochondrial function in interpopulation hybrids of Tigriopus californicus

Electron transport system (ETS) function in mitochondria is essential for the aerobic production of energy. Because ETS function requires extensive interactions between mitochondrial and nuclear gene products, coadaptation between mitochondrial and nuclear genomes may evolve within populations. Hybridization between allopatric populations may then expose functional incompatibilities between genomes that have not coevolved. The intertidal copepod Tigriopus californicus has high levels of nucleotide divergence among populations at mitochondrial loci and suffers F2 hybrid breakdown in interpopulation hybrids. We hypothesize that hybridization results in incompatibilities among subunits in ETS enzyme complexes and that these incompatibilities result in diminished mitochondrial function and fitness. To test this hypothesis, we measured fitness, mitochondrial function, and ETS enzyme activity in inbred recombinant hybrid lines of Tigriopus californicus. We found that (1) both fitness and mitochondrial function are reduced in hybrid lines, (2) only those ETS enzymes with both nuclear and mitochondrial subunits show a loss of activity in hybrid lines, and (3) positive relationships exist between ETS enzyme activity and mitochondrial function and between mitochondrial function and fitness. We also present evidence that hybrid lines harboring mitochondrial DNA (mtDNA) and mitochondrial RNA polymerase (mtRPOL) from the same parental source population have higher fitness than those with mtDNA and mtRPOL from different populations, suggesting that mitochondrial gene regulation may play a role in disruption of mitochondrial performance and fitness of hybrids. These results suggest that disruption of coadaptation between nuclear and mitochondrial genes contributes to the phenomenon of hybrid breakdown.     

Deleterious epistatic interactions between electron transport system protein-coding loci in the copepod Tigriopus californicus

The nature of epistatic interactions between genes encoding interacting proteins in hybrid organisms can have important implications for the evolution of postzygotic reproductive isolation and speciation. At this point very little is known about the fitness differences caused by specific closely interacting but evolutionarily divergent proteins in hybrids between populations or species. The intertidal copepod Tigriopus californicus provides an excellent model in which to study such interactions because the species range includes numerous genetically divergent populations that are still capable of being crossed in the laboratory. Here, the effect on fitness due to the interactions of three complex III proteins of the electron transport system in F2 hybrid copepods resulting from crosses of a pair of divergent populations is examined. Significant deviations from Mendelian inheritance are observed for each of the three genes in F2 hybrid adults but not in nauplii (larvae). The two-way interactions between these genes also have a significant impact upon the viability of these hybrid copepods. Dominance appears to play an important role in mediating the interactions between these loci as deviations are caused by heterozygote/homozygote deleterious interactions. These results suggest that the fitness consequences of the interactions of these three complex III-associated genes could influence reproductive isolation in this system.     

Environmental influences on epistatic interactions: viabilities of cytochrome c genotypes in interpopulation crosses

Abstract .The genetic incompatibilities that underlie F₂ hybrid breakdown and reproductive isolation between al-lopatric populations may be susceptible to environmental interactions. Here we show that epistatic interactions between cytochrome c ( CYC ) alleles and mitochondrial DNA (mtDNA) variation are dramatically influenced by environmental temperature in interpopulation hybrids of the copepod Tigriopus californicus . CYC is a nuclear-encoded gene that functionally interacts with electron transport system (ETS) complexes composed in part of mtDNA-encoded proteins. Previous studies have provided evidence for functional coadaptation between CYC and ETS complex IV (cytochrome c oxidase) and for cytoplasmic effects on the fitness of CYC genotype in copepod hybrids. In this study, selection on CYC genotype is shown to continue into advanced generation hybrids (F₂-F₈) increasing the likelihood that CYC itself is involved in the interaction (and not a linked factor). Relative viabilities varied markedly between copepods raised in two different temperature/light regimes. These results suggest that both intrinsic coadaptation and extrinsic selection will influence the outcome of natural hybridizations between populations. Furthermore, the results indicate that the fitness of particular hybrid genotypes depends on additional non-mtDNA encoded genes that interact with CYC.     

Population differentiation in the beetle Tribolium castaneum. II. Haldane'S rule and incipient speciation

The heterogametic sex tends to be rare, absent, sterile, or deformed in F1 hybrid crosses between species, a pattern called Haldane's rule (HR). The introgression of single genes or chromosomal regions from one drosophilid species into the genetic background of another have shown that HR is most often associated with fixed genetic differences in inter-specific crosses. However, because such introgression studies have involved species diverged several hundred thousand generations from a common ancestor, it is not clear whether HR attends the speciation process or results from the accumulation of epistatically acting genes postspeciation. We report the first evidence for HR prior to speciation in crosses between two populations of the red flour beetle, Tribolium castaneum, collected 931 km apart in Colombia and Ecuador. In this cross, HR is manifested as an increase in the proportion of deformed males compared to females and the expression of HR is temperature dependent. Neither population, when crossed to a geographically distant population from Japan, exhibits HR at any rearing temperature. Using joint-scaling analysis and additional data from backcrosses and F2's, we find that the hybrid incompatibilities and the emergence of HR are concurrent processes involving interactions between X-linked and autosomal genes. However, we also find many examples of incompatibilities manifest by F2 and backcross hybrids but not by F1 hybrids and most incompatibilities are not sex different in their effects, even when they involve both X-autosomal interactions and genotype-by-environment interactions. We infer that incipient speciation in flour beetles can occur with or without HR and that significant hybrid incompatibilities result from the accumulation of epistatically acting gene differences between populations without differentially affecting the heterogametic sex in F1 hybrids. The temperature dependence of the incompatibilities supports the inference that genotype-by-environment interactions and adaptation to different environments contribute to the genetic divergence important to postzygotic reproductive isolation.     

Analysis of ancestry heterozygosity suggests that hybrid incompatibilities in threespine stickleback are environment dependent

Hybrid incompatibilities occur when interactions between opposite ancestry alleles at different loci reduce the fitness of hybrids. Most work on incompatibilities has focused on those that are “intrinsic,” meaning they affect viability and sterility in the laboratory. Theory predicts that ecological selection can also underlie hybrid incompatibilities, but tests of this hypothesis using sequence data are scarce. In this article, we compiled genetic data for F₂ hybrid crosses between divergent populations of threespine stickleback fish (Gasterosteus aculeatus L.) that were born and raised in either the field (seminatural experimental ponds) or the laboratory (aquaria). Because selection against incompatibilities results in elevated ancestry heterozygosity, we tested the prediction that ancestry heterozygosity will be higher in pond-raised fish compared to those raised in aquaria. We found that ancestry heterozygosity was elevated by approximately 3% in crosses raised in ponds compared to those raised in aquaria. Additional analyses support a phenotypic basis for incompatibility and suggest that environment-specific single-locus heterozygote advantage is not the cause of selection on ancestry heterozygosity. Our study provides evidence that, in stickleback, a coarse—albeit indirect—signal of environment-dependent hybrid incompatibility is reliably detectable and suggests that extrinsic incompatibilities can evolve before intrinsic incompatibilities.

This study shows that hybrid incompatibilities between two independent pairs of hybridizing stickleback populations only appear under relevant ecological circumstances, implying that incompatibilities evolve before they can be detected in laboratory studies of speciation.     

The nature of interactions that contribute to postzygotic reproductive isolation in hybrid copepods

Deleterious interactions within the genome of hybrids can lower fitness and result in postzygotic reproductive isolation. Understanding the genetic basis of these deleterious interactions, known as Dobzhansky-Muller incompatibilities, is the subject of intense current study that seeks to elucidate the nature of these deleterious interactions. Hybrids from crosses of individuals from genetically divergent populations of the intertidal copepod Tigriopus californicus provide a useful model in which to study Dobzhansky-Muller incompatibilities. Studies of the basis of postzygotic reproductive isolation in this species have revealed a number of patterns. First, there is evidence for a breakdown in genomic coadaptation between mtDNA-encoded and nuclear-encoded proteins that can result in a reduction in hybrid fitness in some crosses. It appears from studies of the individual genes involved in these interactions that although this coadaptation could lead to asymmetries between crosses, patterns of genotypic viabilities are not often consistent with simple models of genomic coadaptation. Second, there is a large impact of environmental factors on these deleterious interactions suggesting that they are not strictly intrinsic in nature. Temperature in particular appears to play an important role in determining the nature of these interactions. Finally, deleterious interactions in these hybrid copepods appear to be complex in terms of the number of genetic factors that interact to lead to reductions in hybrid fitness. This complexity may stem from three or more factors that all interact to cause a single incompatibility or the same factor interacting with multiple other factors independently leading to multiple incompatibilities.     

Geographical variation in postzygotic isolation and its genetic basis within and between two Mimulus species

The aim of this study is to investigate the evolution of intrinsic postzygotic isolation within and between populations of Mimulus guttatus and Mimulus nasutus. We made 17 intraspecific and interspecific crosses, across a wide geographical scale. We examined the seed germination success and pollen fertility of reciprocal F₁ and F₂ hybrids and their pure-species parents, and used biometrical genetic tests to distinguish among alternative models of inheritance. Hybrid seed inviability was sporadic in both interspecific and intraspecific crosses. For several crosses, Dobzhansky–Muller incompatibilities involving nuclear genes were implicated, while two interspecific crosses revealed evidence of cytonuclear interactions. Reduced hybrid pollen fertility was found to be greatly influenced by Dobzhansky–Muller incompatibilities in five out of six intraspecific crosses and nine out of 11 interspecific crosses. Cytonuclear incompatibilities reduced hybrid fitness in only one intraspecific and one interspecific cross. This study suggests that intrinsic postzygotic isolation is common in hybrids between these Mimulus species, yet the particular hybrid incompatibilities responsible for effecting this isolation differ among the populations tested. Hence, we conclude that they evolve and spread only at the local scale.     

Searching the genomes of inbred mouse strains for incompatibilities that reproductively isolate their wild relatives

Identification of the genes that underlie reproductive isolation provides important insights into the process of speciation. According to the Dobzhansky-Muller model, these genes suffer disrupted interactions in hybrids due to independent divergence in separate populations. In hybrid populations, natural selection acts to remove the deleterious heterospecific combinations that cause these functional disruptions. When selection is strong, this process can maintain multilocus associations, primarily between conspecific alleles, providing a signature that can be used to locate incompatibilities. We applied this logic to populations of house mice that were formed by hybridization involving two species that show partial reproductive isolation, Mus domesticus and Mus musculus. Using molecular markers likely to be informative about species ancestry, we scanned the genomes of 1) classical inbred strains and 2) recombinant inbred lines for pairs of loci that showed extreme linkage disequilibria. By using the same set of markers, we identified a list of locus pairs that displayed similar patterns in both scans. These genomic regions may contain genes that contribute to reproductive isolation between M. domesticus and M. musculus. This hypothesis can now be tested using laboratory crosses and surveys of introgression in the wild.     

ECOLOGICAL NOVELTY BY HYBRIDIZATION: EXPERIMENTAL EVIDENCE FOR INCREASED THERMAL TOLERANCE BY TRANSGRESSIVE SEGREGATION IN TIGRIOPUS CALIFORNICUS

Early generations of hybrids can express both genetic incompatibilities and phenotypic novelty. Insights into whether these conflicting interactions between intrinsic and extrinsic selection persist after a few generations of recombination require experimental studies. To address this question, we use interpopulation crosses and recombinant inbred lines (RILs) of the copepod Tigriopus californicus, and focus on two traits that are relevant for the diversification of this species: survivorship during development and tolerance to thermal stress. Experimental crosses between two population pairs show that most RILs between two heat‐tolerant populations show enhanced tolerance to temperatures that are lethal to the respective parentals, whereas RILs between a heat‐tolerant and a heat‐sensitive population are intermediate. Although interpopulation crosses are affected by intrinsic selection at early generational hybrids, most of the sampled F₉ RILs have recovered fitness to the level of their parentals. Together, these results suggest that a few generations of recombination allows for an independent segregation of the genes underlying thermal tolerance and cytonuclear incompatibilities, permitting certain recombinant lineages to survive in niches previously unused by parental taxa (i.e., warmer thermal environments) without incurring intrinsic selection.     

A fine-scale genetic analysis of hybrid incompatibilities in Drosophila

The sterility and inviability of species hybrids is thought to evolve by the accumulation of genes that cause generally recessive, incompatible epistatic interactions between species. Most analyses of the loci involved in such hybrid incompatibilities have suffered from low genetic resolution. Here I present a fine-resolution genetic screen that allows systematic counting, mapping, and characterizing of a large number of hybrid incompatibility loci in a model genetic system. Using small autosomal deletions from D. melanogaster and a hybrid rescue mutation from D. simulans, I measured the viability of hybrid males that are simultaneously hemizygous for a small region of the D. simulans autosomal genome and hemizygous for the D. melanogaster X chromosome. These hybrid males are exposed to the full effects of any recessive-recessive epistatic incompatibilities present in these regions. A screen of approximately 70% of the D. simulans autosomal genome reveals 20 hybrid-lethal and 20 hybrid-semilethal regions that are incompatible with the D. melanogaster X. In further crosses, I confirm the epistatic nature of hybrid lethality by showing that all of the incompatibilities are rescued when the D. melanogaster X is replaced with a D. simulans X. Combined with information from previous studies, these results show that the number of recessive incompatibilities is approximately eightfold larger than the number of dominant ones. Finally, I estimate that a total of approximately 191 hybrid-lethal incompatibilities separate D. melanogaster and D. simulans, indicating extensive functional divergence between these species' genomes.     

GENETIC ARCHITECTURE AND POSTZYGOTIC REPRODUCTIVE ISOLATION: EVOLUTION OF BATESON–DOBZHANSKY–MULLER INCOMPATIBILITIES IN A POLYGENIC MODEL

The Bateson–Dobzhansky–Muller model predicts that postzygotic isolation evolves due to the accumulation of incompatible epistatic interactions, but few studies have quantified the relationship between genetic architecture and patterns of reproductive divergence. We examined how the direction and magnitude of epistatic interactions in a polygenic trait under stabilizing selection influenced the evolution of hybrid incompatibilities. We found that populations evolving independently under stabilizing selection experienced suites of compensatory allelic changes that resulted in genetic divergence between populations despite the maintenance of a stable, high‐fitness phenotype. A small number of loci were then incompatible with multiple alleles in the genetic background of the hybrid and the identity of these incompatibility loci changed over the evolution of the populations. For F₁ hybrids, reduced fitness evolved in a window of intermediate strengths of epistatic interactions, but F₂ and backcross hybrids evolved reduced fitness across weak and moderate strengths of epistasis due to segregation variance. Strong epistatic interactions constrained the allelic divergence of parental populations and prevented the development of reproductive isolation. Because many traits with varying genetic architectures must be under stabilizing selection, our results indicate that polygenetic drift is a plausible hypothesis for the evolution of postzygotic reproductive isolation.     

Temperature stress increases hybrid incompatibilities in the parasitic wasp genus Nasonia

Hybrid incompatibilities, measured as mortality and sterility, are caused by the disruption of gene interactions. They are important post-zygotic isolation barriers to species hybridization, and much effort is put into the discovery of the genes underlying these incompatibilities. In hybridization studies of the haplodiploid parasitic wasp genus Nasonia, genic incompatibilities have been shown to affect mortality and sterility. The genomic regions associated with mortality have been found to depend on the cytotype of the hybrids and thus suggest cytonuclear incompatibilities. As environmental conditions can affect gene expression and gene interaction, we here investigate the effect of developmental temperature on sterility and mortality in Nasonia hybrids. Results show that extreme temperatures strongly affect both hybrid sterility (mainly spermatogenic failure) and mortality. Molecular mapping revealed that extreme temperatures increase transmission ratio distortion of parental alleles at incompatible loci, and thus, cryptic incompatible loci surface under temperature stress that remain undiscovered under standard temperatures. Our results underline the sensitivity of hybrid incompatibilities to environmental factors and the effects of unstable epistasis.     

Complex deleterious interactions associated with malic enzyme may contribute to reproductive isolation in the copepod Tigriopus californicus

Dobzhansky-Muller incompatibilities can result from the interactions of more than a single pair of interacting genes and there are several different models of how such complex interactions can be structured. Previous empirical work has identified complex conspecific epistasis as a form of complex interaction that has contributed to postzygotic reproductive isolation between taxa, but other forms of complexity are also possible. Here, I probe the genetic basis of reproductive isolation in crosses of the intertidal copepod Tigriopus californicus by looking at the impact of markers in genes encoding metabolic enzymes in F(2) hybrids. The region of the genome associated with the locus ME2 is shown to have strong, repeatable impacts on the fitness of hybrids in crosses and epistatic interactions with another chromosomal region marked by the GOT2 locus in one set of crosses. In a cross between one of these populations and a third population, these two regions do not appear to interact despite the continuation of a large effect of the ME2 region itself in both crosses. The combined results suggest that the ME2 chromosomal region is involved in incompatibilities with several unique partners. If these deleterious interactions all stem from the same factor in this region, that would suggest a different form of complexity from complex conspecific epistasis, namely, multiple independent deleterious interactions stemming from the same factor. Confirmation of this idea will require more fine-scale mapping of the interactions of the ME2 region of the genome.     

The genomic and ecological context of hybridization affects the probability that symmetrical incompatibilities drive hybrid speciation

Despite examples of homoploid hybrid species, theoretical work describing when, where, and how we expect homoploid hybrid speciation to occur remains relatively rare. Here, I explore the probability of homoploid hybrid speciation due to “symmetrical incompatibilities” under different selective and genetic scenarios. Through simulation, I test how genetic architecture and selection acting on traits that do not themselves generate incompatibilities interact to affect the probability that hybrids evolve symmetrical incompatibilities with their parent species. Unsurprisingly, selection against admixture at “adaptive” loci that are linked to loci that generate incompatibilities tends to reduce the probability of evolving symmetrical incompatibilities. By contrast, selection that favors admixed genotypes at adaptive loci can promote the evolution of symmetrical incompatibilities. The magnitude of these outcomes is affected by the strength of selection, aspects of genetic architecture such as linkage relationships and the linear arrangement of loci along a chromosome, and the amount of hybridization following the formation of a hybrid zone. These results highlight how understanding the nature of selection, aspects of the genetics of traits affecting fitness, and the strength of reproductive isolation between hybridizing taxa can all be used to inform when we expect to observe homoploid hybrid speciation due to symmetrical incompatibilities.     

Indirect Evolution of Hybrid Lethality Due to Linkage with Selected Locus in Mimulus guttatus

Most species are superbly and intricately adapted to the environments in which they live. Adaptive evolution by natural selection is the primary force shaping biological diversity. Differences between closely related species in ecologically selected characters such as habitat preference, reproductive timing, courtship behavior, or pollinator attraction may prevent interbreeding in nature, causing reproductive isolation. But does ecological adaptation cause reproductive incompatibilities such as hybrid sterility or lethality? Although several genes causing hybrid incompatibilities have been identified, there is intense debate over whether the genes that contribute to ecological adaptations also cause hybrid incompatibilities. Thirty years ago, a genetic study of local adaptation to copper mine soils in the wildflower Mimulus guttatus identified a locus that appeared to cause copper tolerance and hybrid lethality in crosses to other populations. But do copper tolerance and hybrid lethality have the same molecular genetic basis? Here we show, using high-resolution genome mapping, that copper tolerance and hybrid lethality are not caused by the same gene but are in fact separately controlled by two tightly linked loci. We further show that selection on the copper tolerance locus indirectly caused the hybrid incompatibility allele to go to high frequency in the copper mine population because of hitchhiking. Our results provide a new twist on Darwin''s original supposition that hybrid incompatibilities evolve as an incidental by-product of ordinary adaptation to the environment.     

Variable patterns of introgression in two sculpin hybrid zones suggest that genomic isolation differs among populations

Theory predicts that reproductive isolation may be due to intrinsic genetic incompatibilities or extrinsic ecological factors. Therefore, an understanding of the genetic basis of isolation may require analyses of evolutionary processes in situ to include environmental factors. Here we study genetic isolation between populations of sculpins ( Cottus ) at 168 microsatellites. Genomic clines were fit using 480 individuals sampled across independent natural hybrid zones that have formed between one invading species and two separate populations of a resident species. Our analysis tests for deviations from neutral patterns of introgression at individual loci based on expectations given genome-wide admixture. Roughly 51% of the loci analysed displayed significant deviations. An overall deficit of interspecific heterozygotes in 26% and 21% of the loci suggests that widespread underdominance drives genomic isolation. At the same time, selection promotes introgression of almost 30% of the markers, which implies that hybridization may increase the fitness of admixed individuals. Cases of overdominance or epistatic interactions were relatively rare. Despite the similarity of the two hybrid zones in their overall genomic composition, patterns observed at individual loci show little correlation between zones and many fit different genotypic models of fitness. At this point, it remains difficult to determine whether these results are due to differences in external selection pressures or cryptic genetic differentiation of distinct parental populations. In the future, data from mapped genetic markers and on variation of ecological factors will provide additional insights into the contribution of these factors to variation in the evolutionary consequences of hybridization.     

Finding Hybrid Incompatibilities Using Genome Sequences from Hybrid Populations

Natural hybrid zones offer a powerful framework for understanding the genetic basis of speciation in progress because ongoing hybridization continually creates unfavorable gene combinations. Evidence indicates that postzygotic reproductive isolation is often caused by epistatic interactions between mutations in different genes that evolved independently of one another (hybrid incompatibilities). We examined the potential to detect epistatic selection against incompatibilities from genome sequence data using the site frequency spectrum (SFS) of polymorphisms by conducting individual-based simulations in SLiM. We found that the genome-wide SFS in hybrid populations assumes a diagnostic shape, with the continual input of fixed differences between source populations via migration inducing a mass at intermediate allele frequency. Epistatic selection locally distorts the SFS as non-incompatibility alleles rise in frequency in a manner analogous to a selective sweep. Building on these results, we present a statistical method to identify genomic regions containing incompatibility loci that locates departures in the local SFS compared with the genome-wide SFS. Cross-validation studies demonstrate that our method detects recessive and codominant incompatibilities across a range of scenarios varying in the strength of epistatic selection, migration rate, and hybrid zone age. Our approach takes advantage of whole genome sequence data, does not require knowledge of demographic history, and can be applied to any pair of nascent species that forms a hybrid zone.     

Chromosomal basis of viability differences in Tigriopus californicus interpopulation hybrids

Crosses between populations of Tigriopus californicus result in backcross and F2 hybrid breakdown for a variety of fitness related measures. The magnitude of this hybrid breakdown is correlated with evolutionary divergence. We assessed the chromosomal basis of viability differences in nonrecombinant backcross hybrids using markers mapped to individual chromosomes. To assess effects of evolutionary divergence we crossed one population to three different populations: two distantly related (approximately 18% mitochondrial COI sequence divergence) and one closely related (approximately 1% mitochondrial COI sequence divergence). We found that all three interpopulation crosses resulted in significant deviations from expected Mendelian ratios at a majority of the loci studied. In all but one case, deviations were due to a deficit of parental homozygotes. This pattern implies that populations of T. californicus carry a significant genetic load, and that a combination of beneficial dominance and deleterious homozygote-heterozygote interactions significantly affects hybrid viability. Pairwise tests of linkage disequilibrium detected relatively few significant interactions. For the two divergent crosses, effects of individual chromosomes were highly concordant. These two crosses also showed higher heterozygote excess in females than males across the vast majority of chromosomes.     

Dominance, epistasis and the genetics of postzygotic isolation

The sterility and inviability of species hybrids can be explained by between-locus "Dobzhansky-Muller" incompatibilities: alleles that are fit on their "normal" genetic backgrounds sometimes lower fitness when brought together in hybrids. We present a model of two-locus incompatibilities that distinguishes among three types of hybrid interactions: those between heterozygous loci (H(0)), those between a heterozygous and a homozygous (or hemizygous) locus (H(1)), and those between homozygous loci (H(2)). We predict the relative fitnesses of hybrid genotypes by calculating the expected numbers of each type of incompatibility. We use this model to study Haldane's rule and the large effect of X chromosomes on postzygotic isolation. We show that the severity of H(0) vs. H(1) incompatibilities is key to understanding Haldane's rule, while the severity of H(1) vs. H(2) incompatibilities must also be considered to explain large X effects. Large X effects are not inevitable in backcross analyses but rather-like Haldane's rule-may often reflect the recessivity of alleles causing postzygotic isolation. We also consider incompatibilities involving the Y (or W) chromosome and maternal effects. Such incompatibilities are common in Drosophila species crosses, and their consequences in male- vs. female-heterogametic taxa may explain the pattern of exceptions to Haldane's rule.     

Lifetime fitness in two generations of Ipomopsis hybrids

Various models purporting to explain natural hybrid zones make different assumptions about the fitness of hybrids. One class of models assumes that hybrids have intrinsically low fitness due to genetic incompatibilities, whereas other models allow hybrid fitness to vary across natural environments. We used the intrinsic rate of increase to assess lifetime fitness of hybrids between two species of montane plants Ipomopsis aggregata and Ipomopsis tenuituba planted as seed into multiple field environments. Because fitness is predicted to depend upon genetic composition of the hybrids, we included F1 hybrids, F2 hybrids, and backcrosses in our field tests. The F2 hybrids had female fitness as high, or higher, than expected under an additive model of fitness. These results run counter to any model of hybrid zone dynamics that relies solely on intrinsic nuclear genetic incompatibilities. Instead, we found that selection was environmentally dependent. In this hybrid zone, cytoplasmic effects and genotype-by-environment interactions appear more important in lowering hybrid fitness than do intrinsic genomic incompatibilities between nuclear genes.