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4,480 results for “hybrid”
Data from: Death by sex in an Australian icon: a continent-wide survey reveals extensive hybridization between dingoes and domestic dogs
Hybridization between domesticated animals and their wild counterparts can disrupt adaptive gene combinations, reduce genetic diversity, extinguish wild populations and change ecosystem function. The dingo is a free-ranging dog that is an iconic apex predator and distributed throughout most of mainland Australia. Dingoes readily hybridize with domestic dogs, and in many Australian jurisdictions, distinct management strategies are dictated by hybrid status. Yet, the magnitude and spatial extent of domestic dog–dingo hybridization is poorly characterized. To address this, we performed a continent-wide analysis of hybridization throughout Australia based on 24 locus microsatellite DNA genotypes from 3637 free-ranging dogs. Although 46% of all free-ranging dogs were classified as pure dingoes, all regions exhibited some hybridization, and the magnitude varied substantially. The southeast of Australia was highly admixed, with 99% of animals being hybrids or feral domestic dogs, whereas only 13% of the animals from remote central Australia were hybrids. Almost all free-ranging dogs had some dingo ancestry, indicating that domestic dogs could have poor survivorship in nonurban Australian environments. Overall, wild pure dingoes remain the dominant predator over most of Australia, but the speed and extent to which hybridization has occurred in the approximately 220 years since the first introduction of domestic dogs indicate that the process may soon threaten the persistence of pure dingoes.
Data from: Can alternative mating tactics facilitate introgression across a hybrid zone by circumventing female choice?
Reproductive barriers and divergence in species' mate recognition systems underlie major models of speciation. However, hybridization between divergent species is common, and classic mechanisms to explain permeable reproductive barriers rarely consider how an individual may attain reproductive success. Alternative mating tactics exist in various forms across animal taxa. Such tactics may allow poorer quality individuals to gain mating opportunities and facilitate introgression either through asymmetrical positive selection, or by circumventing female choice altogether in areas of secondary contact. One such tactic is satellite behaviour in frogs, where silent males perch near advertisers in an attempt to intercept females. To test whether such satellite male tactics are context-dependent and favoured by hybrids, we genotyped and quantified the morphology of 80 male spring peeper (Pseudacris crucifer) individuals involved in caller-satellite associations from a secondary contact zone between two intraspecific mitochondrial lineages. Irrespective of population, satellite behaviour was best predicted by size but not body condition. Within the contact zone, pure individuals showed a significantly greater probability of being active callers, whereas hybrids of one lineage were more likely to adopt the satellite tactic. We suggest that satellite behaviour in P. crucifer promotes introgression, breaks-down reproductive isolating barriers, and contributes to asymmetrical introgression in this secondary contact zone. Alternative mating tactics may thus be an underexplored but important alternative to oft-discussed causes of genetic discordance found in hybrid zones.
Data from: Multifaceted, cross-generational costs of hybridization in sibling Drosophila species
Maladaptive hybridization, as determined by the pattern and intensity of selection against hybrid individuals, is an important factor contributing to the evolution of prezygotic reproductive isolation. To identify the consequences of hybridization between Drosophila pseudoobscura and D. persimilis, we estimated multiple fitness components for F1 hybrids and backcross progeny and used these to compare the relative fitness of parental species and their hybrids across two generations. We document many sources of intrinsic (developmental) and extrinsic (ecological) selection that dramatically increase the fitness costs of hybridization beyond the well-documented F1 male sterility in this model system. Our results indicate that the cost of hybridization accrues over multiple generations and reinforcement in this system is driven by selection against hybridization above and beyond the cost of hybrid male sterility; we estimate a fitness loss of >95% relative to the parental species across two generations of hybridization. Our findings demonstrate the importance of estimating hybridization costs using multiple fitness measures from multiple generations in an ecologically relevant context; so doing can reveal intense postzygotic selection against hybridization and thus, an enhanced role for reinforcement in the evolution of populations and diversification of species.
Data from: Parental and hybrid Daphnia from the D. longispina complex: long-term dynamics in genetic structure and significance of overwintering modes
In recent decades, hybridization has become a focus of attention because of its role in evolutionary processes. However, little is known about changes in genetic structure within and between parental species and hybrids over time. Here, we studied processes of genetic change in parental species and hybrids from the Daphnia longispina complex (Crustacea, Cladocera) over a period of six years across ten habitats. These cyclical parthenogens respond to fluctuating environments by switching from asexual to sexual reproduction. Importantly, sexually produced diapausing eggs, which resist extreme conditions such as low temperatures and serve as dispersal stages, are produced to a lower extent by hybrids. Long-term microsatellite data revealed clear differences between hybrids and parental species. In hybrids, clonal diversity values were lower, whereas heterozygosity and linkage disequilibrium values were higher compared to parental species. Clonal diversity of hybrids responded to the strength of the winter, with cold winters resulting in few genotypes in the following spring. In time windows when only asexual hybrid females survive, priority effects will favour the establishment of the hybrid offspring before hatchlings from parental diapause eggs can enter the community. The constant high levels of heterozygosity maintained by clonal reproduction in hybrids might lead to their successful establishment over time, when they are able to escape competition from both parental species. Although we found evidence that hybrids diversity depends on fluctuating environments, a direct link between hybrid abundance and the strength of winter was missing. Because of reduced adaptability in clonally reproducing hybrids, multiple factors must contribute to promoting their long-term success in fluctuating environments.
Data from: Hybridization promotes color polymorphism in the aposematic harlequin poison frog, Oophaga histrionica
Whether hybridization can be a mechanism that drives phenotypic diversity is a widely debated topic in evolutionary biology. In poison frogs (Dendrobatidae), assortative mating has been invoked to explain how new color morphs persist despite the expected homogenizing effects of natural selection. Here, we tested the complementary hypothesis that new morphs arise through hybridization between different color morphs. Specifically, we (1) reconstructed the phylogenetic relationships among the studied populations of a dart-poison frog to provide an evolutionary framework, (2) tested whether microsatellite allele frequencies of one putative hybrid population of the polymorphic frog O. histrionica are intermediate between O. histrionica and O. lehmanni, and (3) conducted mate-choice experiments to test whether putatively intermediate females prefer homotypic males over males from the other two populations. Our findings are compatible with a hybrid origin for the new morph and emphasize the possibility of hybridization as a mechanism generating variation in polymorphic species. Moreover, because coloration in poison frogs is aposematic and should be heavily constrained, our findings suggest that hybridization can produce phenotypic novelty even in systems where phenotypes are subject to strong stabilizing selection.
Data from: Empirical demonstration of hybrid chromosomal races in house mice
Western house mice (Mus musculus domesticus) and common shrews (Sorex araneus) are important models for study of chromosomal speciation. Both had ancestral karyotypes consisting of telocentric chromosomes, and each is subdivided into numerous chromosomal races many of which have resulted from fixation of new mutations (Robertsonian fusions and whole-arm reciprocal translocations). However, some chromosomal races in both species may alternatively have originated through hybridization, with particular homozygous recombinant products reaching fixation. Here, we demonstrate the process of generation of hybrid chromosomal races for the first time in either species using molecular markers. Analysis of centromeric microsatellite markers show that the Mid Valtellina (IMVA) and Upper Valtellina (IUVA) chromosomal races of the house mouse are recombinant products of hybridization of the Lower Valtellina (ILVA) and Poschiavo (CHPO) chromosomal races, supporting earlier theoretical analysis. IMVA and IUVA occupy a small area of the Italian Alps where ILVA makes contact with CHPO. IUVA and CHPO have previously been shown to be reproductively isolated in one village, emphasizing that hybrid chromosomal races in small mammals, as in plants, have the potential to be part of the speciation process.
Data from: Ecological differentiation, lack of hybrids involving diploids, and asymmetric gene flow between polyploids in narrow contact zones of Senecio carniolicus (syn. Jacobaea carniolica, Asteraceae)
Areas of immediate contact of different cytotypes offer a unique opportunity to study evolutionary dynamics within heteroploid species and to assess isolation mechanisms governing coexistence of cytotypes of different ploidy. The degree of reproductive isolation of cytotypes, i.e., the frequency of heteroploid crosses and subsequent formation of viable and (partly) fertile hybrids, plays a crucial role for the long-term integrity of lineages in contact zones. Here, we assessed fine-scale distribution, spatial clustering and ecological niches as well as patterns of gene-flow in parental and hybrid cytotypes in zones of immediate contact of di-, tetra- and hexaploid Senecio carniolicus (Asteraceae) in the Eastern Alps. Cytotypes were spatially separated also at the investigated micro-scale; the strongest spatial separation was observed for the fully interfertile tetra- and hexaploids. The three main cytotypes showed highly significant niche differences, which were, however, weaker than across their entire distribution ranges in the Eastern Alps. Individuals with intermediate ploidy levels were found neither in the diploid/tetraploid nor in the diploid/hexaploid contact zones indicating strong reproductive barriers. In contrast, pentaploid individuals were frequent in the tetraploid/hexaploid contact zone, albeit limited to a narrow strip in the immediate contact zone of their parental cytotypes. AFLP fingerprinting data revealed introgressive gene flow mediated by pentaploid hybrids from tetra- to hexaploid individuals, but not vice versa. The ecological niche of pentaploids differed significantly from that of tetraploids but not from hexaploids.
Data from: Genetic analysis of river, swamp and hybrid buffaloes of north-east India throw new light on phylogeography of water buffalo (Bubalus bubalis)
This study analysed buffaloes from north-east India and compared their nuclear and mitochondrial DNA variations with buffaloes of mainland India, China, Mediterranean and South-East Asia. Microsatellite genotypes of 338 buffaloes including 210 from six north-east Indian buffalo populations and three mainland Indian breeds were analysed to evaluate their genetic structure and evolutionary relationships. Phylogenetic analysis and multidimensional scaling plot of pairwise FST revealed the clustering of all swamp-type buffaloes of north-east India with Lower Assamese (significantly hybrid type) buffaloes in one plane and all the mainland river buffaloes in another plane while the upper Assamese buffaloes being distinct from both these clusters. Analysis of mtDNA D-loop region of 530-bp length was performed on 345 sequences belonging to 23 buffalo populations from various geographical regions to establish the phylogeography of Indian water buffalo. The swamp buffaloes of north-east India clustered with both the lineages of Chinese swamp buffalo. Multidimensional scaling display of pairwise FST derived from mitochondrial DNA data showed clustering of upper Assamese, Chilika and Mediterranean buffaloes distinctly from all the other Indian buffalo populations. Median-joining network analysis further confirmed the distinctness and ancestral nature of these buffaloes. The study revealed north-east region of India forming part of the wider hybrid zone of water buffalo that may probably extend from north-east India to South-East Asia.
Data from: Integrative taxonomy refutes a species hypothesis: the asymmetric hybrid origin of Arsapnia arapahoe (Plecoptera, Capniidae)
Molecular tools are commonly directed at refining taxonomies and the species that constitute their fundamental units. This has been especially insightful for groups for which species hypotheses are ambiguous and have largely been based on morphological differences between certain life stages or sexes, and has added importance when taxa are a focus of conservation efforts. Here we examine the taxonomic status of Arsapnia arapahoe, a winter stonefly in the family Capniidae that is a species of conservation concern because of its limited abundance and restricted range in northern Colorado, USA. Phylogenetic analyses of sequences of mitochondrial and nuclear genes of this and other capniid stoneflies from this region and elsewhere in western North America indicated extensive haplotype sharing, limited genetic differences, and a lack of reciprocal monophyly between A. arapahoe and the sympatric A. decepta, despite distinctive and consistent morphological differences in the sexual apparatus of males of both species. Analyses of autosomal and sex-linked single nucleotide polymorphisms detected using genotyping by sequencing, however, indicated that all individuals of A. arapahoe consisted of F1 hybrids between female A. decepta and males of another sympatric stonefly, Capnia gracilaria. Rather than constitute a self-sustaining evolutionary lineage, A. arapahoe appears to represent the product of nonintrogressive hybridization in the limited area of syntopy between two widely distributed taxa. This offers a cautionary tale for taxonomists and conservation biologists working on the less-studied components of the global fauna.
Data from: Genomic regions with a history of divergent selection affect fitness of hybrids between two butterfly species
Speciation is the process by which reproductively isolated lineages arise, and is one of the fundamental means by which the diversity of life increases. Whereas numerous studies have documented an association between ecological divergence and reproductive isolation, relatively little is known about the role of natural selection in genome divergence during the process of speciation. Here we use genome-wide DNA sequences and Bayesian models to test the hypothesis that loci under divergent selection between two butterfly species (Lycaeides idas and L. melissa) also affect fitness in an admixed population. Locus-specific measures of genetic differentiation between L. idas and L. melissa and genomic introgression in hybrids varied across the genome. The most differentiated genetic regions were characterized by elevated L. idas ancestry in the admixed population, which occurs in L. idas-like habitat, consistent with the hypothesis that local adaptation contributes to speciation. Moreover, locus-specific measures of genetic differentiation (a metric of divergent selection) were positively associated with extreme genomic introgression (a metric of hybrid fitness). Interestingly, concordance of differentiation and introgression was only partial. We discuss multiple, complementary explanations for this partial concordance.
Data from: A phylogeographic investigation of the hybrid origin of a species of swordtail fish from Mexico
Hybrid speciation may contribute significantly to generating biodiversity, but only a few well-documented examples for it exist so far that do not involve polyploidization as a mechanism. The swordtail fish, Xiphophorus clemenciae, shows common hall-marks of a hybrid origin and still overlaps in its current geographic distribution with its putative ancestral species (X. helleri and X. maculatus). Xiphophorus clemenciae provides an ideal system for investigating the possible continued genetic interactions between a hybrid and its parental species. Here we use microsatellite and mitochondrial markers to investigate the population structure of these species of swordtails and search for signs of recent hybridization. Individuals were sampled from 21 localities across the known range of X. clemenciae – the Isthmus of Tehuantepec (IT) Mexico, and several environmental parameters that might represent barriers to dispersal were recorded. The hybridization event that gave rise to X. clemenciae appears to be rather ancient and a single origin is likely. We find negligible evidence for ongoing hybridization and introgression between the putative ancestral species, since they now occupy distinct ecological niches, and a common haplotype is shared by most populations of X. clemenciae. The population structure within these species shows an isolation-by-distance (IBD) pattern and genetic differentiation between most populations is significant and high. We infer that tectonic evolution in the Isthmus has greatly restricted gene flow between the southern and central IT populations of X. clemenciae and X. helleri and provide preliminary information to aid in conservation management of this geographically restricted hybrid species, X. clemenciae.
Data from: Haldane's rule revisited: do hybrid females have a shorter lifespan? Survival of hybrids in a recent contact zone between two large gull species
Haldane's rule predicts that particularly high fitness reduction should affect the heterogametic sex of interspecific hybrids. Despite the fact that hybridization is widespread in birds, survival of hybrid individuals is rarely addressed in studies of avian hybrid zones, possibly because of methodological constraints. Here, having applied capture-mark-recapture models to an extensive, 19-year-long dataset on individually marked birds, we estimate annual survival rates of hybrid individuals in the hybrid zone between herring (Larus argentatus) and Caspian (L. cachinnans) gulls. In both parental species, males have a slightly higher survival rate than females (model-weighted mean ± SE: herring gull males 0.88 ± 0.01, females 0.87 ± 0.01, Caspian gull males 0.88 ± 0.01, females 0.87 ± 0.01). Hybrid males do not survive for a shorter time than non-hybrid ones (0.88 ± 0.01), whereas hybrid females have the lowest survival rate among all groups of individuals (0.83 ± 0.03). This translates to a shorter adult (reproductive) lifespan (on average by 1.7-1.8 years, i.e. ca 25%) compared to non-hybrid females. We conclude that, in line with Haldane's rule, the lower survival rate of female hybrids may contribute to selection against hybrids in this hybrid zone.
Data from: The extent and meaning of hybridization and introgression between Siberian spruce (Picea obovata) and Norway spruce (Picea abies): cryptic refugia as stepping stones to the west?
Boreal species were repeatedly exposed to ice ages and went through cycles of contraction and expansion while sister species alternated periods of contact and isolation. The resulting genetic structure is consequently complex, and demographic inferences are intrinsically challenging. The range of Norway spruce (Picea abies) and Siberian spruce (Picea obovata) covers most of northern Eurasia; yet their geographical limits and histories remain poorly understood. To delineate the hybrid zone between the two species and reconstruct their joint demographic history, we analysed variation at nuclear SSR and mitochondrial DNA in 102 and 88 populations, respectively. The dynamics of the hybrid zone was analysed with approximate Bayesian computation (ABC) followed by posterior predictive structure plot reconstruction and the presence of barriers across the range tested with estimated effective migration surfaces. To estimate the divergence time between the two species, nuclear sequences from two well-separated populations of each species were analysed with ABC. Two main barriers divide the range of the two species: one corresponds to the hybrid zone between them, and the other separates the southern and northern domains of Norway spruce. The hybrid zone is centred on the Urals, but the genetic impact of Siberian spruce extends further west. The joint distribution of mitochondrial and nuclear variation indicates an introgression of mitochondrial DNA from Norway spruce into Siberian spruce. Overall, our data reveal a demographic history where the two species interacted frequently and where migrants originating from the Urals and the West Siberian Plain recolonized northern Russia and Scandinavia using scattered refugial populations of Norway spruce as stepping stones towards the west.
Data from: Genetic structure and hybridization in the species group of Ficus auriculata: can closely related sympatric Ficus species retain their genetic identity while sharing pollinators?
Obligate mutualistic nursery pollination systems between insects and plants have led to substantial co-diversification involving at least some parallel cladogenesis, as documented in Yucca, Ficus and Phyllanthaceae. In such systems pollinators are generally species specific thus limiting hybridization and introgression among interfertile host species. Nevertheless, in the three systems, cases of one insect pollinating several plant species are reported. In most cases host plants sharing pollinators are allopatric. However in the case of the species group of Ficus auriculata, forms may co-occur over large parts of their range. We show here that the species group of F. auriculata is constituted by four well-defined genetic entities that share pollinators. We detected hybrids in nature mainly when both parental forms were growing nearby. Controlled crosses showed that F1 offspring could be successfully backcrossed. Hence, despite sharing pollinators and despite hybrid viability, the different forms have preserved their genetic and morphological identity. We propose that ecological differentiation among forms, coupled with limited overlap of reproductive season has facilitated the maintenance of these inter-fertile forms. As such, establishment of pollinator host specificity may not be a prerequisite for sympatric diversification in Ficus.
Data from: Complex phylogeography in Rhinoclemmys melanosterna: conflicting mitochondrial and nuclear evidence suggests past hybridization (Testudines: Geoemydidae)
We examined differentiation within the Colombian wood turtle Rhinoclemmys melanosterna, and among R. melanosterna and the closely allied species R. diademata, R. funerea and R. punctularia, based on 1060 base pairs of the mitochondrial cyt b gene. We also assessed the phylogenetic relationships among these species using 2050 bp of mtDNA (partial cyt b, 12S and 16S genes) and 3620 bp of nuclear DNA (partial Rag 1, Rag 2, C-mos, R35 and ODC genes). There is considerable phylogeographic structuring within R. melanosterna, with seven distinct clades distributed across the species' range. These clades correspond to some extent with previously described differences in the dorsal pattern of head coloration. Individual and combined analyses of mitochondrial and nuclear DNA indicated contradictory relationships among R. melanosterna, R. diademata, R. funerea and R. punctularia. Mitochondrial DNA sequences revealed R. melanosterna to be non-monophyletic with respect to R. diademata, R. funerea and R. punctularia. In contrast, R. melanosterna constituted a well-supported monophyletic clade using nuclear DNA. This conflict between mitochondrial and nuclear data suggests past gene flow among the allopatrically and parapatrically distributed species R. melanosterna, R. diademata, R. funerea and R. punctularia. Compared to the other Rhinoclemmys species, the taxa under study are weakly differentiated. To assess their taxonomic status, further research is warranted using additional nuclear markers and additional samples of R. diademata, R. funerea and R. punctularia. For the time being, a continued classification of R. melanosterna, R. diademata, R. funerea and R. punctularia as distinct species is justified owing to their allopatric and parapatric distributions, and to conserve the established usage of names that is based on characteristic and distinct phenotypes associated with each species.
Data from: Strong selection against hybrids maintains a narrow contact zone between morphologically cryptic lineages in a rainforest lizard
Phenotypically cryptic lineages comprise an important yet understudied part of biodiversity; in particular, we have much to learn about how these lineages are formed and maintained. To better understand the evolutionary significance of such lineages, we studied a hybrid zone between two morphologically-cryptic phylogeographic lineages in the rainforest lizard, Lampropholis coggeri. Analyzing a multilocus genetic dataset through cline inference, individual-based methods and population measures of disequilibrium and using simulations to explore our genetic results in context of theoretical expectations, we inferred the processes maintaining this hybrid zone. We find that these lineages meet in a hybrid zone that is narrow (~400 m) relative to inferred dispersal rate. Further, the hybrid zone exhibits substantial genetic disequilibrium and sharply coincident and largely concordant clines. Based on our knowledge about the region's biogeography, the species' natural history, and our simulation results, we suggest that strong selection against hybrids structures this system. As all clines show a relatively narrow range of introgression, we posit that this hybrid zone might not yet be in equilibrium. Nonetheless, our results clearly show that phylogeographic lineages can evolve substantial reproductive isolation without concomitant morphological diversification, suggesting that such lineages can constitute a significant component of evolutionary diversity.
Data from: Patterns of mating and generation of diversity in a Geum hybrid swarm
To understand the evolutionary consequences of hybridization between the outcrossing plant Geum rivale and the selfer G. urbanum we tested the predictions of two simple models which assume either A). low or B). high pollen fitness in hybrids. Model A predicts only four genotypic classes (G. rivale, G. rivale backcross (BCR), F1 and G. urbanum) and asymmetric introgression from inbreeding to outbreeding species. Model B predicts additional genotypic classes and potential generation of novel inbreeding lines in the hybrid swarm. AFLP analysis of adults revealed only the four genotypes predicted by model A. However, microsatellite analysis of parent-progeny arrays demonstrated production of selfed offspring by F1 and BCR maternal parents and contribution of these genotypes to outcross pollen pools, as predicted by model B. Moreover AFLP and morphological analysis showed that the offspring generation comprised genotypes and phenotypes covering the entire spectrum of variation between the two parental species, in line with model B. A common garden experiment indicated no systematic reduction in fitness of offspring derived from hybrid parents. The genetic structure of the adults in the Geum hybrid swarm cannot be explained by restricted mating patterns but may result from ecological selection acting on a diverse offspring population.
Data from: Arabidopsis hybrid speciation processes
The genus Arabidopsis provides a unique opportunity to study fundamental biological questions in plant sciences utilizing the diploid model species A. thaliana and A. lyrata. However, only a few studies have focused on introgression and hybrid speciation in Arabidopsis, although polyploidy is a common phenomenon within this genus. More recently, there is growing evidence of significant gene flow between the various Arabidopsis species. So far, we know A. suecica and A. kamchatica as fully stabilized allopolyploid species. Both species evolved during Pleistocene glaciation and deglaciation cycles in Fennoscandinavia and the amphi-Beringian region, respectively. These hybrid studies were conducted either on a phylogeographic scale, or hybridization was reconstructed experimentally in the laboratory. With our study we focus at a regional and populational level. Our research area is located in the foothills of the Eastern Austrian Alps, where two Arabidopsis species, A. arenosa and A. lyrata ssp. petraea, are sympatrically distributed. Our hypothesis of genetic introgression, migration, and adaptation to the changing environment during the Pleistocene has been confirmed: We observed significant, mainly unidirectional gene flow between the two species, which has given rise to the tetraploid A. lyrata. This cytotype was able to escape from the narrow ecological niche occupied by diploid A. lyrata ssp. petraea on limestone outcrops by migrating northward into siliceous areas, leaving behind a trail of genetic differentiation.
Data from: Para-allopatry in hybridizing fire-bellied toads (Bombina bombina and B. variegata): inference from transcriptome-wide coalescence analyses
Ancient origins, profound ecological divergence, and extensive hybridization make the fire-bellied toads Bombina bombina and B. variegata (Anura: Bombinatoridae) an intriguing test case of ecological speciation. Previous modeling has proposed that the narrow Bombina hybrid zones represent strong barriers to neutral introgression. We test this prediction by inferring the rate of gene exchange between pure populations on either side of the intensively studied Kraków transect. We developed a method to extract high confidence sets of orthologous genes from de novo transcriptome assemblies, fitted a range of divergence models to these data and assessed their relative support with analytic likelihood calculations. There was clear evidence for postdivergence gene flow, but, as expected, no perceptible signal of recent introgression via the nearby hybrid zone. The analysis of two additional Bombina taxa (B. v. scabra and B. orientalis) validated our parameter estimates against a larger set of prior expectations. Despite substantial cumulative introgression over millions of years, adaptive divergence of the hybridizing taxa is essentially unaffected by their lack of reproductive isolation. Extended distribution ranges also buffer them against small-scale environmental perturbations that have been shown to reverse the speciation process in other, more recent ecotypes.
Data from: 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, and hybrid swarm fitness always equalled or exceeded that of the midparent. For a second pair, controlled crosses showed F2 hybrid breakdown, but elevated fitness in backcrosses, and hybrid swarm fitness deviated both above and below that of the parentals. Nevertheless, individual swarm replicates exhibited divergent 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 generally 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, suggesting 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.
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