Hybridization among sympatric species of Rhododendron (Ericaceae) in Turkey: morphological and molecular evidence

Rhododendron (Ericaceae) is a large genus in which barriers to hybridization are especially weak, but many species are maintained in sympatry. Hybridization among four species of Rhododendron subsect. Pontica, which occur in sympatry in Turkey, was investigated. Material of R. ponticum, R. smirnovii, R. ungernii, and R. caucasicum and their putative hybrids was collected from the wild. Based on morphology, chloroplast DNA and nuclear ribosomal DNA restriction fragment length polymorphism (RFLP) profiles, each accession was identified as a species or hybrid combination. Five of the six possible hybrid combinations among the four species were detected. Rhododendron ponticum × R. smirnovii was represented by a single individual and R. caucasicum × R. smirnovii by one small group of hybrid plants. The combinations R. ponticum × R. ungernii and R. ungernii × R. smirnovii showed evidence of frequent backcrossing, while R. ponticum × R. caucasicum appeared unusual in that an intermediate hybrid type was abundant, whereas hybrids with phenotypes approaching either parent were rare. Possible explanations of this latter situation are discussed. The results suggest that natural hybridization among Rhododendron species is common and that ecological factors are important in maintaining integrity when species occur in sympatry.     

Hybrid bridges to gene flow: a case study in milkweeds (Asclepias)

Natural hybridization occurs throughout areas of sympatry for the North American milkweeds Asclepias exaltata and A. syriaca (Asclepiadaceae), even though the formation of F1 hybrid seed is a rare event. For introgressive hybridization to proceed, F1 and advanced hybrids must be released from reproductive barriers and successfully mate with one or both parental species. I investigated the mating system of natural hybrids between A. exaltata and A. syriaca in three populations in Shenandoah National Park, Virginia. Allozyme data and a maximum-likelihood procedure were used to estimate the frequency of six genotypic classes (parentals, F1, F2, and backcrosses) of the hybridizing populations, the pollinia received by hybrid plants, and the paternal parents of seeds produced by hybrids. F1 hybrids, backcross A. syriaca, and parental A. syriaca individuals were common in three hybrid populations. Even though self-pollinations and interhybrid pollinations were common, F2 seed production and the occurrence of F2 individuals were rare in hybrid populations. Hybrid plants received more pollen from A. syriaca than A. exaltata, which resulted in the production of more backcross-A. syriaca seed than backcross-A. exaltata seed. Asclepias exaltata was rare in the hybrid populations, but A. exaltata pollinia were received by hybrids and this species sired between 15% and 36% of the seeds produced on hybrids. The potential for introgression with A. exaltata populations is lower because this species is unsuccessful as the maternal parent in interspecific and backcross hand-pollinations. The asymetry of hybridization with A. syriaca as the maternal parent is further supported by the incorporation of maternally inherited chloroplast DNA markers in hybrids. Hybrid milkweeds frequently backcross with both parental species and may be released from the reproductive barriers that limit the formation of F1 hybrids in natural populations. The direction of interspecific gene flow and introgression in milkweeds is influenced by the reproductive biology of hybrids, the constituency of the surrounding population, and failure of some crosses to produce seeds. Finally, introgressive hybridization remains an important evolutionary force even when the initial formation of F1 hybrids in natural populations is rare.     

Human disturbance causes the formation of a hybrid swarm between two naturally sympatric fish species

Most evidence for hybrid swarm formation stemming from anthropogenic habitat disturbance comes from the breakdown of reproductive isolation between incipient species, or introgression between allopatric species following secondary contact. Human impacts on hybridization between divergent species that naturally occur in sympatry have received considerably less attention. Theory predicts that reinforcement should act to preserve reproductive isolation under such circumstances, potentially making reproductive barriers resistant to human habitat alteration. Using 15 microsatellites, we examined hybridization between sympatric populations of alewife (Alosa pseudoharengus) and blueback herring (A. aestivalis) to test whether the frequency of hybridization and pattern of introgression have been impacted by the construction of a dam that isolated formerly anadromous populations of both species in a landlocked freshwater reservoir. The frequency of hybridization and pattern of introgression differed markedly between anadromous and landlocked populations. The rangewide frequency of hybridization among anadromous populations was generally 0–8%, whereas all landlocked individuals were hybrids. Although neutral introgression was observed among anadromous hybrids, directional introgression leading to increased prevalence of alewife genotypes was detected among landlocked hybrids. We demonstrate that habitat alteration can lead to hybrid swarm formation between divergent species that naturally occur sympatrically, and provide empirical evidence that reinforcement does not always sustain reproductive isolation under such circumstances.     

Interspecific hybridization in natural populations ofCyrtandra (Gesneriaceae) on the Hawaiian Islands: Evidence from RAPD markers

Interspecific hybridization among Hawaiian species ofCyrtandra (Gesneriaceae) was investigated using randomly amplified polymorphic DNA (RAPD) markers. Thirty-three different primers were used to investigate interspecific hybridization for 17 different putative hybrids based on morphological intermediacy and sympatry with putative parental species. RAPD data provided evidence for the hybrid origin of all putative hybrid taxa examined in this analysis. However, the patterns in the hybrid taxa were not found to be completely additive of the patterns found in the parental species. Markers missing in the hybrid taxa can be attributed to polymorphism in the populations of the parental species and the dominant nature of inheritance for RAPD markers. Unique markers found within hybrid taxa require further explanation but do not necessarily indicate that the taxa are not of hybrid origin. The implications suggest that these interspecific hybridization events had, and continue to have, an effect on the adaptive radiation and conservation biology ofCyrtandra.     

Detecting hybridization in mixed populations ofRhinanthus minor andRhinanthus angustifolius

Rhinanthus minor andRhinanthus angustifolius are known to hybridize in mixed populations in nature. These hybridization events can have important evolutionary consequences. The development and use of species-specific RAPD and ISSR markers allowed the detection of hybrid individuals not always distinguishable with morphological characters. Two mixed populations of different ages were studied. In a young mixed 2-year-old population, both individuals of the two parental species and F₁ hybrids were found using genetic analysis, showing that hybridization occurred rapidly. Flower morphology of F₁ hybrids was too variable to distinguish all these hybrids from the parental species. This morphological variability of F₁ hybrids was also confirmed in artificial crosses in the greenhouse. In an old and no longer mixed 30-year-old population, onlyR. angustifolius plants and a few genetically introgressed individuals close toR. angustifolius were present. Genetic markers showed traces of past hybridization and introgression. Unidirectional introgression ofR. minor intoR. angustifolius with the complete disappearance ofR. minor from this population was observed.     

Hybridization between Crocodylus acutus and Crocodylus moreletii in the Yucatan Peninsula: I. Evidence from mitochondrial DNA and morphology

The American crocodile (Crocodylus acutus) and the Morelet's crocodile (C. moreletii) are broadly sympatric in Belize and Mexico. The presence of morphologically anomalous individuals in the overlapping range area suggests possible hybridization between these species. Analysis of 477 base pairs of the mitochondrial tRNA(Pro)-tRNA(Phe)-Dloop region revealed the presence of pure C. acutus (N=43) and C. moreletii (N=56), as well as a high proportion of interspecific hybrids (N=17, 14.6%) in the Yucatan Peninsula, Mexico. Although all individuals could be assigned to one species or other based on phenotypic characters, some had been characterized as potential hybrids in the field by anomalous scale counts. The hybridization zone lies along the area of sympatry between C. acutus and C. moreletii investigated in this study, but extends further inland if hybrid localities from Belize are included. Hybridization in the Yucatan Peninsula is bidirectional, which indicates considerably more genetic contact between these species than previously recognized, and is probably more detrimental to the genetic integrity of smaller C. acutus populations. A more intensive study of the pattern of hybridization is warranted and supports continued classification of C. acutus as a critically threatened species in the Yucatan Peninsula.     

Flycatcher song in allopatry and sympatry--convergence, divergence and reinforcement

The theory of reinforcement predicts that natural selection against the production of unfit hybrids favours traits that increase assortative mating. Whether culturally inherited traits, such as bird song, can increase assortative mating by reinforcement is largely unknown. We compared songs of pied (Ficedula hypoleuca) and collared flycatchers (F. albicollis) from two hybrid zones of different ages with songs from allopatric populations. Previously, a character divergence in male plumage traits has been shown to reinforce premating isolation in sympatric flycatchers. In contrast, we find that the song of the pied flycatcher has converged towards that of the collared flycatcher (mixed singing). However, a corresponding divergence in the collared flycatcher shows that the species differences in song characters are maintained in sympatry. Genetic analyses suggest that mixed song is not caused by introgression from the collared flycatcher, but rather due to heterospecific copying. Circumstantial evidence suggests that mixed song may increase the rate of maladaptive hybridization. In the oldest hybrid zone where reinforcement on plumage traits is most pronounced, the frequency of mixed singing and hybridization is also lowest. Thus, we suggest that reinforcement has reduced the frequency of mixed singing in the pied flycatcher and caused a divergence in the song of the collared flycatcher. Whether a culturally inherited trait promotes or opposes speciation in sympatry may depend on its plasticity. The degree of plasticity may be genetically determined and accordingly under selection by reinforcement.     

Hybridization in Annual Plants: Patterns and DynamicsDuring a Four-Year Study in Mixed Rhinanthus Populations

Hybridization in annual plants is rare, but their short life cycle provides an excellent opportunity to study the dynamics of hybridization. Hybridization occurs between the annual hemiparasites Rhinanthus minor and Rhinanthus angustifolius (Orobanchaceae). Using flower morphology, Kwak (1980) found a prevalence of hybrids close to R. angustifolius in a single population. We aim to find whether this pattern is also found using genetic markers, whether it is generally occurring in mixed populations, and whether these populations are stable over time. We used species-specific genetic markers to determine the number of individuals in a range of hybrid classes in three mixed populations of different ages during four consecutive years. In the young population, F₁ hybrids were found in the first year and mostly hybrids between R. minor and these F₁s in the second year, but in the years after that, hybrids close to R. angustifolius became more abundant. We also found this in the two older populations, where hybrids close to R. angustifolius always occurred in higher frequencies than hybrids close to R. minor. Over time, R. angustifolius strongly increased in frequency in two populations. Patterns of marker presence and absence suggested that advanced-generation hybrids are mainly formed by backcrossing with one of the parents, predominantly R. angustifolius whenever its frequency in the population is higher than 15%. The dynamics of mixed populations depend on the ecological conditions that regulate the presence of the two parental species, and introgression into R. angustifolius seems prevalent.     

Hybridization between the endemic brown algae Carpophyllum maschalocarpum and Carpophyllum angustifolium (Fucales): Genetic and morphological evidence

Hybridization is an important evolutionary process, which can have significant effects on biodiversity. While hybridization is well known in plants, less is known about the prevalence of hybridization in other kingdoms. Hybridization in the field has been confirmed in brown algae in a few cases, mainly in the northern hemisphere genus Fucus. Putative hybrids have been found in the New Zealand endemic species Carpophyllum angustifolium and Carpophyllum maschalocarpum. We used nuclear‐encoded molecular data (ITS2) and morphometrics to confirm hybridization between C. angustifolium and C. maschalocarpum. Putative hybrid thalli were collected that had heterozygous ITS2, each copy corresponding to one of the parental species from that population. Morphological analysis also showed that the three classes (two parental species and hybrids) were easily distinguishable in these populations. It was found that the hybrids had an intermediate morphology to the parent species. Some individuals with C. angustifolium morphology had hybrid ITS2 ribotypes suggestive of backcrossing between the hybrids and C. angustifolium. Our data reveal another case of hybridization within the Fucales and suggests that further research on how these species remain separate is needed.     

Genetic admixture despite ecological segregation in a North African sparrow hybrid zone (Aves,Passeriformes, Passer domesticus × Passer hispaniolensis)

Under different environmental conditions, hybridization between the same species might result in different patterns of genetic admixture. Particularly, species pairs with large distribution ranges and long evolutionary history may have experienced several independent hybridization events over time in different zones of overlap. In birds, the diverse hybrid populations of the house sparrow (Passer domesticus) and the Spanish sparrow (Passer hispaniolensis) provide a striking example. Throughout their range of sympatry, these two species do not regularly interbreed; however, a stabilized hybrid form (Passer italiae) exists on the Italian Peninsula and on several Mediterranean islands. The spatial distribution pattern on the Eurasian continent strongly contrasts the situation in North Africa, where house sparrows and Spanish sparrows occur in close vicinity of phenotypically intermediate populations across a broad mosaic hybrid zone. In this study, we investigate patterns of divergence and admixture among the two parental species, stabilized and nonstabilized hybrid populations in Italy and Algeria based on a mitochondrial marker, a sex chromosomal marker, and 12 microsatellite loci. In Algeria, despite strong spatial and temporal separation of urban early‐breeding house sparrows and hybrids and rural late‐breeding Spanish sparrows, we found strong genetic admixture of mitochondrial and nuclear markers across all study populations and phenotypes. That pattern of admixture in the North African hybrid zone is strikingly different from i) the Iberian area of sympatry where we observed only weak asymmetrical introgression of Spanish sparrow nuclear alleles into local house sparrow populations and ii) the very homogenous Italian sparrow population where the mitogenome of one parent (P. domesticus) and the Z‐chromosomal marker of the other parent (P. hispaniolensis) are fixed. The North African sparrow hybrids provide a further example of enhanced hybridization along with recent urbanization and anthropogenic land‐use changes in a mosaic landscape.     

Interspecific hybridization in Rorippa (Brassicaceae): Patterns and processes

The current knowledge on natural hybridization between European lowland Rorippa species is reviewed. Morphological, cytological and molecular markers provide substantial evidence that four Rorippa species (R. amphibia, R. austriaca, R. palustris and R. sylvestris) are involved in hybridization processes. The main factors promoting initial hybridization events between Rorippa species in Europe are (1) natural and anthropogenic habitat disturbance, (2) the invasion of non‐native R. austriaca into western central Europe and (3) the self‐incompatibility system of R. amphibia, R. austriaca and R. sylvestris coupled with clonal growth. Outcomes of hybridization between European Rorippa species show a broad range from incidental hybrids through the formation of hybrid swarms to the evolution of new hybrid taxa. Rorippa × armoracioides is certainly the most successful Rorippa hybrid. It results from hybridization between R. austriaca and R. sylvestris and is known from many localities in the native range of R. austriaca and in regions invaded by R. austriaca. Within this system hybrid zones of different age provide the rare opportunity to analyse different stages of hybrid formation and hybrid speciation in natural populations.     

A second spontaneous hybrid in the genusLeucaena (Leguminosae, Mimosoideae)

The detection of hybridity inLeucaena is discussed in relation to: (i) traditional criteria, (ii) molecular criteria and (iii) models to predict hybrid leaf morphology. Morphological, geographical and molecular evidence for the occurrence of interspecific hybrids betweenL. leucocephala andL. diversifolia in south-central Mexico, northern Guatemala, Jamaica, Dominican Republic, the Philippines and Papua New Guinea is presented. Predicted mean hybrid leaf trait values calculated from parent material are compared with data from putative hybrids and shown to be similar. The origin of these hybrids is discussed and shown to be the result of artificial sympatry resulting from indigenous, and recent exotic, domestication of the parent species. The hybrid is described asL. ×spontanea.     

Genetic monitoring of two decades of hybridization between allis shad (Alosa alosa) and twaite shad (Alosa fallax)

Habitat alteration has been implicated in driving hybridization between the sympatric migratory shads Alosa alosa and Alosa fallax. Morphological and molecular evidence is consistent with hybridization across the overlapping range of these species, but the temporal extent of hybrid occurrence and genetic consequences for populations have not been explored. Using eight nuclear microsatellite loci and samples collected between 1989 and 2008 in the Solway Firth (UK), we genetically identified hybrids, studied temporal changes in their frequency, and explored changes in allele frequencies of parental populations. These molecular data confirmed the hybrid status of individuals identified using morphology (number of rakers on the outer gill arch), and enabled separation of hybrids from purebred individuals. Mitochondrial cytochrome-b sequencing revealed the presence of two haplogroups, each predominantly occurring in one species. Heterospecific haplotypes were found in 22.3 and 12.8% of A. alosa and A. fallax individuals, respectively, consistent with backcrossing and suggesting that hybrids are fertile. On average, microsatellite-identified hybrids comprised 12.7% of all samples, but when individuals with cytonuclear discordance were also considered introgressed on average 25.4% of individuals were of hybrid ancestry. Overall, allelic richness remained largely unchanged within species, but there were declines in the inbreeding coefficient (F _IS) of both species and episodes of significant temporal allelic frequency change. Hybrids sampled between 2004 and 2008 showed no evidence of lower fecundity relative to purebred individuals. Together, results suggest that hybridization between shad species in northern Europe is prevalent, and has been ongoing over at least two decades. The challenge is now to understand the extent to which observed patterns are linked to immigration from other populations, and the mechanisms that have prevented species collapse despite apparent hybrid fertility and longstanding introgression of neutral markers.     

Asymmetric gene introgression in two closely related Orchis species: evidence from morphometric and genetic analyses

ABSTRACT: BACKGROUND: In food-deceptive orchids of the genera Anacamptis, Neotinea and Orchis floral isolation has been shown to be weak, whereas late-acting reproductive barriers are mostly strong, often restricting hybridization to the F1 generation. Only in a few species hybridization extends beyond the F1 generation, giving rise to hybrid swarms. However, little is known about the abundance of later-generation hybrids and what factors drive their occurrence in hybrid populations. In this study, molecular analyses were combined with detailed morphological measurements in a hybrid population of two closely related Orchis species (Orchis militaris and O. purpurea) to investigate the hypothesis that the abundance of later-generation hybrids is driven by changes in floral characters after hybridization that exert selective pressures that in turn affect hybridization. RESULTS: Both the molecular and morphological data point to extensive genetic and morphological homogenization and asymmetric introgression. Estimating genomic clines from the multi-locus genotype data and testing for deviation from neutrality revealed that 30 out of 113 (27%) AFLP markers significantly deviated from neutral expectations. Plants with large floral displays or plant with flowers that resembled more O. purpurea had higher female fitness than plants with small floral displays or plants with flowers resembling more O. militaris, suggesting that directional selection may have contributed to the observed patterns of introgression. CONCLUSIONS: These results indicate that in closely related orchid species hybridization and gene introgression may be partly driven by selection for floral traits of one of the parental types. However, because some pure individuals were still present in the studied population, the parental species appeared to be sufficiently isolated to survive the challenge of sympatry.     

Sympatry leads to reduced body condition in chickadees that occasionally hybridize

Both abiotic and biotic drivers influence species distributions. Abiotic drivers such as climate have received considerable attention, even though biotic drivers such as hybridization often interact with abiotic drivers. We sought to explore the (1) costs of co‐occurrence for ecologically similar species that hybridize and (2) associations between ecological factors and condition to understand how abiotic and biotic factors influence species distributions. For two closely related and ecologically similar songbirds, black‐capped and mountain chickadees, we characterized body condition, as a proxy for fitness, using a 1358‐individual range‐wide dataset. We compared body condition in sympatry and allopatry with several abiotic and biotic factors using species‐specific generalized linear mixed models. We generated genomic data for a subset of 217 individuals to determine the extent of hybridization‐driven admixture in our dataset. Within this data subset, we found that ~11% of the chickadees had hybrid ancestry, and all hybrid individuals had typical black‐capped chickadee plumage. In the full dataset, we found that birds of both species, independent of demographic and abiotic factors, had significantly lower body condition when occurring in sympatry than birds in allopatry. This could be driven by either the inclusion of cryptic, likely poor condition, hybrids in our full dataset, competitive interactions in sympatry, or range edge effects. We are currently unable to discriminate between these mechanisms. Our findings have implications for mountain chickadees in particular, which will encounter more black‐capped chickadees as black‐capped chickadee ranges shift upslope and could lead to local declines in mountain chickadee populations.     

Patterns of hybridization and introgression between invasive Ulmus pumila (Ulmaceae) and native U. rubra

Natural hybridization between introduced species and their native congeners occurs frequently and can create serious conservation concerns. Ulmus pumila (Siberian elm) is an introduced Asian elm species that has naturalized in the United States and is now considered invasive in 41 states. Red elm (U. rubra), a native to the eastern United States, often occurs in sympatry with Siberian elm, and the two species are thought to hybridize. Here, we genetically characterized reference populations of the two elm species to identify species-specific microsatellite alleles. These markers were used to classify individuals in putative hybrid zones as parental species or hybrids, assess the extent of hybridization, and track patterns of introgression. We identified nine U. rubra, 32 U. pumila, and 51 hybrid individuals in our hybrid zones. Of the 51 hybrids, 35 were classified as first-generation hybrids and 16 as backcrosses. The majority of the backcrosses (88%) were introgressed toward U. pumila. Our classification of genotypes was consistent whether we used manual classification, principal coordinate analyses or Bayesian clustering. We observed greater genetic diversity and new combination of alleles in the hybrids. Our study indicates widespread hybridization between U. pumila and U. rubra and an asymmetric pattern of introgression toward U. pumila.     

Hybridization and evolution in Picradeniopsis (Compositae)

The predominantly allopatric species of the genusPicradeniopsis, P. oppositifolia andP. woodhousei, are distinct in morphological, in phenolic and terpeniod chemical, and in cytological aspects (n = 24 andn = 12, respectively). In an area of sympatry in northeastern New Mexico, interbreeding occurs frequently with the production of morphologically intermediate hybrids. Morphological and phenolic chemical data from 191 plants in 40 isolated parental populations and from 91 plants in four hybrid populations are of limited value in determining the nature of this hybridization, but meiotic configurations of 12_II and 12_I and low pollen viabilities of 1–11% in the hybrids indicate that they are all of the F₁ generation (with one possible backcross). The absence of observed introgression, and therefore the absence of gene flow between the two taxa, strengthens the case for taxonomic recognition of two species in the genus. Comparisons of the morphology, phenolic and terpenoid chemistry, and cytology of parents and F₁ hybrids suggest that the tetraploid,P. oppositifolia, has arisen by allopolyploidy from a cross betweenP. woodhousei and an unknown diploid species. An extrapolated morphological and chemical reconstruction of this putative diploid parent is advanced.     

A direct regional scale estimate of transgene movement from genetically modified oilseed rape to its wild progenitors

One of the major environmental concerns over genetically modified (GM) crops relates to transgene movement into wild relatives. The pattern of hybridization ultimately affects the scale and rapidity of ecological change and the feasibility of containment. A new procedure for quantifying hybrid formation over large areas is described. Remote sensing was used to identify possible sites of sympatry between Brassica napus and its progenitor species across 15 000 km2 of south-east England in 1998. Two sympatric populations with B. rapa and one with B. oleracea were found over the entire survey area. Every newly recruited plant in these populations in 1999 was screened for hybrid status using flow cytometry and molecular analyses. One hybrid was observed from the 505 plants screened in the B. rapa populations but none of the nine B. oleracea recruits were hybrids. Measures to minimize gene flow are suggested, and a procedure for the post-release evaluation and containment of GM cultivars is proposed.     

Hybridization and reproductive isolation among syntopic populations of the topminnows Fundulus notatus and F. olivaceus

Fundulus notatus and Fundulus olivaceus are two closely related topminnow species that exhibit similar ecological niches and broad, largely overlapping, North American ranges extending throughout much of the Mississippi River drainage as well as the coastal drainages of the Gulf of Mexico. Previous studies have suggested that these two species are reproductively compatible despite cytogenetic differences and will hybridize when syntopic. We used nuclear and mtDNA loci to assess levels of hybridization and test for introgression in syntopic populations of these two species in four drainages in southern Illinois. Although hybridization was detected in all syntopic populations, an assessment of the proportion of hybrid individuals indicated a deficiency of hybrids relative to expectations under random mating. We determined that, although mtDNA introgression was prevalent and extended beyond the zones of contact, evidence of nuclear introgression was limited to the zone of sympatry.