The origin of a novel form of Senecio (Asteraceae) restricted to sand dunes in southern Sicily

The taxonomy of diploid Mediterranean Senecio sect. Senecio (Asteraceae) is complex, owing to a recent species radiation, high morphological plasticity and occasional interspecific hybridization. A study was conducted to resolve the origin of a novel form of Senecio restricted to sand dunes in southern Sicily, Italy. This has been described previously as morphologically intermediate to Senecio gallicus and Senecio glaucus ssp. coronopifolius, indicating a possible hybrid origin, or as a variant of Senecio leucanthemifolius. Plants of this form grown in a glasshouse were morphologically intermediate to S. glaucus and S. leucanthemifolius, but were also similar to some cultivated individuals of S. gallicus. No evidence for a hybrid origin was obtained from a survey of random amplified polymorphic DNA variation; instead the plants surveyed were most closely allied to Tunisian S. glaucus. They were also polymorphic for the same set of cpDNA haplotypes present in Tunisian S. glaucus. We conclude that the Sicilian Senecio is a variant form of North African S. glaucus ssp. coronopifolius, which most probably dispersed to sand dunes in southern Sicily in the relatively recent past. The presence of several cpDNA haplotypes in this material indicates that there have been multiple introductions of the species to Sicily.     

Repeat intercontinental dispersal and Pleistocene speciation in disjunct Mediterranean and desert Senecio (Asteraceae)

To explore the biogeographic history of Mediterranean/arid plant disjunctions, Old and New World Senecio sect. Senecio were analyzed phylogenetically using nuclear ribosomal DNA sequences (ITS). A clade corresponding to sect. Senecio was strongly supported. Area optimization indicated this clade to be of southern African origin. The Mediterranean and southern African floras were not distinguishable as sources of the main New World lineage, estimated to have become established during the middle Pliocene. Another previously suspected recent dispersal to the New World from the Mediterranean was confirmed for the recently recognized disjunction in S. mohavensis. The loss of suitable land connections by the Miocene means that both New World lineages must represent long-distance dispersal, providing the first evidence of repeat intercontinental dispersal in a Mediterranean group. In contrast, migration within Africa may have utilized an East African arid corridor. Recent dispersal to northern Africa is supported for S. flavus, which formed part of a distinct southern African lineage. Novel pappus modifications in both disjunct species may have enabled dispersal by birds. An estimated early Pliocene origin of sect. Senecio coincides with the appearance of summer-dry climate. However, diversification from 1.6 BP highlights the importance of Pleistocene climate fluctuations for speciation.     

Routes of origin of two recently evolved hybrid taxa: Senecio vulgaris var. hibernicus and York radiate groundsel (Asteraceae)

The possible pathways of origin of two recently arisen introgressant forms of Senecio vulgaris (i.e., var. hibernicus and York radiate groundsel) were investigated in experimental crosses between tetraploid S. vulgaris var. vulgaris and the normally diploid S. squalidus. Comparison of the morphology of synthesized hybrid progeny with established taxa, by discriminant function analysis, revealed that fertile hybrid offspring similar in morphology to S. vulgaris var. hibernicus and York radiate groundsel could be synthesized: (1) following formation of genomically stable diploid gametes by the triploid hybrid; (2) through the production of unreduced gametes by diploid S. squalidus; and (3) when a tetraploid form of S. squalidus acted as one of the parents. It was evident that hybrid offspring similar in morphology to the two introgressant taxa were more often produced in backcrosses to S. vulgaris than in segregating F2 or F3 generations (53% as opposed to 36%), and that fertile hybrid progeny were formed within two generations. Because hybridization between S. vulgaris and S. squalidus occurs regularly, although at very low frequency, in natural mixed populations in the British Isles, there is the potential for multiple origins to occur in the wild of both S. vulgaris var. hibernicus and York radiate groundsel.     

GENETIC SIMILARITY IS HIGH BETWEEN INTERCONTINENTAL DISJUNCT SPECIES OF SENECIO (ASTERACEAE)

Senecio flavus (Decne.) Schultz-Bip. of the Saharo-Arabian and Namibian deserts and Senecio mohavensis Gray of the Mojave and Sonoran deserts form a pair of closely related disjunct annual taxa. The two species exhibit slight morphological differences and have different ploidy levels: all North American populations examined are tetraploid, while African and Asian populations are diploid. Senecio flavus comprises two morphologically and geographically distinct subspecies, subsp. flavus and subsp. breviflorus Kadereit. Electrophoretic examination of 13 enzymes revealed that the subsp. breviflorus has two additional isozymes relative to subsp. flavus, and that 5. mohavensis has four additional isozymes relative to subsp. breviflorus. Senecio flavus subsp. breviflorus and S. mohavensis are identical at 20 of the 21 (I = 0.952) remaining homologous loci. This value is much higher than the genetic identity values observed in other taxa with disjunct intercontinental distributions such as members of Agastache, Datisca, Liriodendron, and Styrax. This suggests that the Senecio disjunction may be of recent origin. Senecio flavus subsp. flavus from North Africa and Namibia has lower genetic identity values with S. mohavensis (0.857 and 0.805, respectively), implicating S. flavus subsp. breviflorus as the progenitor of 5. mohavensis. The complete lack of heterozygosity confirms that the plants are highly autogamous, and thus could have attained their disjunct distribution through long distance dispersal.     

Population decline despite high genetic diversity in the new allopolyploid species Senecio cambrensis (Asteraceae)

Senecio cambrensis (Welsh groundsel) is a new allohexaploid species, which originated in Wales, UK, in the early part of the 20th century following hybridization between the native tetraploid groundsel (Senecio vulgaris) and the introduced diploid Oxford ragwort (Senecio squalidus). A survey of the number of populations and flowering individuals per population of S. cambrensis in Wales was conducted at peak flowering time in June 2002, 2003 and 2004. The results show a dramatic decrease in both population number and population size of the species since the 1980s when the last population census was conducted. A survey of amplified fragment length polymorphism (AFLP) variation showed that this decline has occurred despite the fact that S. cambrensis contains a high level of genetic diversity with each individual screened possessing a unique multilocus phenotype. The level of variance within the species was similar to that found in one parent (S. vulgaris) and slightly greater than that among samples of the other parent (S. squalidus). Only a small proportion (5%) of AFLP diversity was partitioned among populations indicating a lack of population structure and possibly high levels of gene flow via seed dispersal in what is predominantly a selfing species. Senecio cambrensis showed closer similarity in AFLP phenotype to S. vulgaris than to S. squalidus. Possible causes of this and also the high level of AFLP diversity found in S. cambrensis are discussed. It is suggested that intergenomic recombination following occasional multivalent formation during meiosis in S. cambrensis is likely to be an important cause of both phenomena, although other causes are not ruled out.     

Recent, allopatric, homoploid hybrid speciation: the origin of Senecio squalidus (Asteraceae) in the British Isles from a hybrid zone on Mount Etna, Sicily

Homoploid hybrid speciation occurs through stabilization of a hybrid segregate (or segregates) isolated by premating and/or postmating barriers from parent taxa. Theory predicts that ecological and spatial isolation are of critical importance during homoploid hybrid speciation, and all confirmed homoploid hybrid species are ecologically isolated from their parents. Until recently, such species have been identified long after they originated, and consequently it has not been possible to determine the relative importance of spatial and ecological isolation during their origin. Here we present evidence for the recent origin (within the past 300 years) of a new homoploid hybrid species, Senecio squalidus (Asteraceae), in the British Isles, following long-distance dispersal of hybrid material from a hybrid zone between S. aethnensis and S. chrysanthemifolius on Mount Etna, Sicily, Italy. Historical records show that such hybrid material from Sicily was introduced to the Oxford Botanic Garden in Britain in the early part of the 18th century and that S. squalidus began to spread from there after approximately 90 years. A survey of randomly amplified polymorphic DNA/intersimple sequence repeats (RAPD/ISSR) marker variation demonstrated that S. squalidus is a diploid hybrid derivative of S. aethnensis and S. chrysanthemifolius that grow at high and low altitudes, respectively, on Mount Etna and that form a hybrid zone at intermediate altitudes. Senecio squalidus contained 11 of 13 RAPD/ISSR markers that were recorded at high frequency in S. chrysanthemifolius but were absent or occurred at low frequency in S. aethnensis, and 10 of 13 markers for which the reverse was true. Bayesian admixture analysis showed that all individuals of S. squalidus surveyed were of mixed ancestry with relatively high mean proportions of ancestry derived from both S. chrysanthemifolius and S. aethnensis (0.644 and 0.356, respectively). We argue that long-distance isolation of hybrid material from its parents on Mount Etna would have helped favor the origin and establishment of S. squalidus in the British Isles, regardless of whether the initial hybrid material introduced to Britain was preadapted to local conditions.     

Natural hybridization in Cardamine (Brassicaceae) in the Pyrenees: evidence from morphological and molecular data

While researching Cardamine (Brassicaceae) in the Pyrenees, putative hybrid plants were found at two natural sites. Pollen grain viability, AFLP, and multivariate morphometric analyses were performed in order to assess the plants' presumed hybrid origin, establishing that natural hybridization between the diploids C. crassifolia and C. amara ssp. pyrenaea had occurred. A new diploid nothospecies, C. × enriquei (2 n = 2x = 16), is described. Examination of 18 morphological characters showed the intermediacy of the hybrid between the parental taxa in most characters. AFLP analyses of C. amara ssp. amara , ssp. austriaca , ssp. olotensis and C. raphanifolia , demonstrated the close position of the hybrid to C. crassifolia , and revealed that the highest number of markers were shared with the parents. Polymorphism found in the AFLP pattern of the hybrid suggested recurrent origin, segregation and/or backcrosses, although assessment of pollen viability indicated high male sterility. The hybridization event reported here represents the second documented case between the C. pratensis group and C. amara . An account of the nomenclature of C. crassifolia is also presented, including lectotypification of relevant names.  © 2002 The Linnean Society of London, Botanical Journal of the Linnean Society , 2002, 139 , 275–294.     

A new Pyrenean hybrid Cirsium (Asteraceae) as revealed by morphological and molecular analyses

A new nothosubspecies Cirsium  ×  vivantii is described after a molecular and morphological characterization of eight individuals collected in two separate valleys of the French central Pyrenees. Both hypervariable Amplified Fragment Length Polymorphism (AFLP) markers and nuclear rDNA (ITS) and plastid ( trn L-F, TRNT-L) DNA sequences were analysed. The profiles of these hybrid samples were compared to those of 43 individuals belonging to their presumed parental taxa C. carniolicum ssp. rufescens and C. palustre . A total of 133 AFLP bands were scored from three primer-pair combinations. All 130 AFLP bands that amplified in the hybrid samples were present in either C. carniolicum ssp. rufescens , C. palustre or both taxa, supporting the participation of these plant genomes in the resulting hybrids. Several Additive Polymorphic Sites (APS) detected in the ITS sequences of the hybrid samples also confirmed their derived origins from ribotypes of the two parental taxa. The lack of exclusive AFLP markers and the nonconcerted evolution of rDNA polymorphisms towards either of the parental ribotypes indicated their recent origin. Plastid trn T-L sequences were used to identify C. palustre as the maternal parent of six of these hybrid individuals; either parent could have acted as the plastid genome donor for the other two individuals. The morphological study revealed that all hybrid individuals were morphologically intermediate between their parents showing largely lobed and less spiny basal leaves as in C. carniolicum ssp. rufescens and decurrent leaf bases and pinkish corollas as in C. palustre .  © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society , 2007, 154 , 421–434.     

AFLP fingerprinting suggests an allopolyploid origin of two members of the Leucanthemum vulgare aggregate (Compositae,Anthemideae) in central Europe

In order to clarify the origin of the wide‐spread tetraploid Leucanthemum ircutianum and the hexaploid L. adustum, both found in the alpine and dealpine parts of central Europe together with the diploid L. vulgare, we carried out a genetic analysis based on the distribution of amplified fragment length polymorphism (AFLP) fragments in populations of these three taxa, both in the northwestern Alps (Allgäu region, south Germany and adjacent parts of Austria) and the Franconian Jura (vicinity of Regensburg, Bavaria, Germany). Statistical analyses based on AFLP fragments (principal co‐ordinate analysis, PCoA; analysis of molecular variance, AMOVA; Bayesian analysis of population structure, BAPS) suggest that neither of the polyploids is of autopolyploid origin based on L. vulgare, but rather are allopolyploids with the second (maternal) diploid parent still unknown.     

A novel method to infer the origin of polyploids from Amplified Fragment Length Polymorphism data reveals that the alpine polyploid complex of Senecio carniolicus (Asteraceae) evolved mainly via autopolyploidy

Despite its evolutionary and ecological relevance, the mode of polyploid origin has been notoriously difficult to be reconstructed from molecular data. Here, we present a method to identify the putative parents of polyploids and thus to infer the mode of their origin (auto‐ vs. allopolyploidy) from Amplified Fragment Length Polymorphism (AFLP) data. To this end, we use Cohen's d of distances between in silico polyploids, generated within a priori defined scenarios of origin from a priori delimited putative parental entities (e.g. taxa, genetic lineages), and natural polyploids. Simulations show that the discriminatory power of the proposed method increases mainly with increasing divergence between the lower‐ploid putative ancestors and less so with increasing delay of polyploidization relative to the time of divergence. We apply the new method to the Senecio carniolicus aggregate, distributed in the European Alps and comprising two diploid, one tetraploid and one hexaploid species. In the eastern part of its distribution, the S. carniolicus aggregate was inferred to comprise an autopolyploid series, whereas for western populations of the tetraploid species, an allopolyploid origin involving the two diploid species was the most likely scenario. Although this suggests that the tetraploid species has two independent origins, other evidence (ribotype distribution, morphology) is consistent with the hypothesis of an autopolyploid origin with subsequent introgression by the second diploid species. Altogether, identifying the best among alternative scenarios using Cohen's d can be straightforward, but particular scenarios, such as allopolyploid origin vs. autopolyploid origin with subsequent introgression, remain difficult to be distinguished.     

Recent hybrid origin and invasion of the British Isles by a self-incompatible species, Oxford ragwort (Senecio squalidus L., Asteraceae)

Senecio squalidus is a diploid hybrid species which originated in the British Isles following the introduction of material collected from a hybrid zone on Mount Etna, Sicily, approximately 300 years ago. Introduced hybrid material was cultivated in the Oxford Botanic Garden and gave rise to the stabilized diploid hybrid species, which later spread throughout much of the UK and into some parts of Ireland. Unusually for an invasive species, S. squalidus has a strong system of sporophytic self-incompatibility (SSI) that may have become modified as a result of its recent hybrid origin and spread. First, S. squalidus contains relatively few S alleles (between 2 and 6 S alleles within individual UK populations) compared to other species with SSI (estimates average ~17 S alleles per population). This most probably reflects the population bottleneck experienced by introduced hybrid material. Second, dominance relationships among S. squalidus S alleles are more extensive than those reported in other species with SSI. Third, although pseudo-self-compatibility occurs sporadically in S. squalidus, it is not widespread, indicating that SSI is maintained in the species despite potential mate availability restrictions imposed by low numbers of S alleles. Surveys of other forms of genetic diversity in S. squalidus show that allozyme variation is reduced relative to that within the progenitor species, but Randomly Amplified Polymorphic DNA variation is relatively high. Both types of genetic variation show little or no pattern of isolation-by-distance between populations in keeping with the recent range expansion of the species. During its spread in the British Isles, S. squalidus has hybridized with the native self-compatible (SC) tetraploid species, S. vulgaris, which has led to the origin of three new SC hybrid taxa: a radiate form of S. vulgaris (var. hibernicus), a tetrapoid hybrid species (S. eboracensis) and an allohexaploid (S. cambrensis).     

Taxonomìc analysis of Senecio pauciflorus and S. indecorus (Asteraceae)

Senecio pauciflorus and S. indecorus (= Packera pauciflora and P. indecora ) are autogamous, high polyploid species with similar geographic ranges in North America. Their east-west distribution pattern including a disjunction in California is unusual within the aureoid Senecio complex and may provide insights into the origins of the aureoid complex. Taxonomic treatment of the two taxa has varied in the past due to the similarity and variability of the taxa. Results of morphological analyses of 192 herbarium specimens show that the taxa are best treated as two distinct species and that infraspecifìc variation is not related to geographical distribution. Separation of taxa remains difficult. California specimens are the most difficult to classify and may represent the ancestral group. Based on morphology and chromosome number it is suggested that the 5. pauciflorus-S. indecorus group may have originated from the arctic-alpine aureoid group including S. cymbalaria .     

The Genetics of Evolutionary Change in Senecio vulgaris L.: A QTL Mapping Approach

Abstract: The cosmopolitan weed Senecio vulgaris var. vulgaris is likely to have originated from the non-weedy S. vulgaris ssp. denticulatus from which it differs by showing no seed dormancy, by completing its life cycle from germination to seed formation much faster and by lacking ray florets. An F2 generation of 120 individuals obtained through selfing of one hybrid individual between var. vulgaris and ssp. denticulatus was used to construct a linkage map based on RAPD polymorphisms and the presence or absence of ray florets. This linkage map was used for a QTL analysis of 12 characters distinguishing the two taxa. For seven of these 12 characters, three significant QTLs could be found. One of these QTLs controls the speed of development, height of plants, leaf number, number of lateral branches and number of outer involucral bracts. A second QTL, located in the same linkage group, coincides with the ray floret locus and controls the number of disc florets. Plant height and leaf number are controlled by a third QTL in a different linkage group. Considering earlier evidence on the genetics of seed dormancy, it is argued that probably only three chromosomal regions, or even genetic loci, control seed dormancy, speed of development and presence or absence of ray florets as the ecologically most important differences between var. vulgaris and ssp. denticulatus. These findings have important implications for the genetics of evolutionary change and speciation.     

Genetic differentiation in the Bladder campions, Silene vulgaris and S. uniflora (Caryophyllaceae), in Sweden

Allozyme variation was studied in Swedish populations of Silene vulgaris (a widespread weed), S. uniflora ssp. uniflora (restricted to coastal habitats) and S. uniflora ssp. petraea (endemic to Sweden and confined to open limestone habitats). The taxa are diploid, gynodioecious, perennial herbs and showed high levels of within-taxon and within-population gene diversity at four polymorphic loci. Within-taxon diversity was highest (H_tax=0.52) in S. vulgaris and lowest (H_tax=0.36) in S. uniflora ssp. uniflora. The weedy S. vulgaris has more alleles than either of the other two taxa and 5 out of a total of 27 alleles are unique to S. vulgaris. Most of the gene diversity within each of the taxa is accounted for by within-population diversity. The between-population component of diversity is 10% in S. vulgaris , and 24% and 5%, respectively, in S. uniflora ssp. uniflora and ssp. petraea. Hybrids may occur between S. vulgaris and S. uniflora , but introgression is limited by the species' ecology. Neither allozyme nor distributional data support the suggestion that ssp. petraea is a recent hybrid between S. vulgaris and S. uniflora ssp. uniflora , although an older hybrid origin for ssp. petraea is possible. Patterns of allele frequency variation suggest that there has been some historical gene flow between taxa, outside their present areas of sympatry. It is likely that the two subspecies of S. uniflora , which occur in naturally open habitats, were able to colonize Sweden during the Late Glacial or early post-glacial, whereas S. vulgaris followed the spread of agriculture into Sweden.     

Recurrent origin of peripheral,coastal (sub)species in Mediterranean Senecio (Asteraceae)

Background: It is argued that coastal endemic taxa may evolve in parallel at the periphery of the distributional range of a widespread species.

Aims: We tested this hypothesis for the origins of three peripheral, coastal isolates of Senecio, S. glaucus ssp. glaucus (Israel), S. g. ssp. coronopifolius p.p. (Sicily), and S. hesperidium (Morocco), from widespread S. glaucus ssp. coronopifolius. We also determined the relative roles of selection vs. genetic drift in shaping phenotypic divergence in ssp. glaucus and S. hesperidium, using Lande’s test of neutral morphological change.

Methods: We surveyed morphological and/or allozyme variation in the three peripheral isolates and mainly inland populations of S. g. ssp. coronopifolius.

Results: Genetic data supported independent origins of the coastal taxa from nearby populations of ssp. coronopifolius. These descendant and ancestral populations showed pronounced morphological but weak genetic differentiation. Phenotypic similarities between ssp. glaucus (Israel) and S. hesperidium (Morocco) in plant height and floral traits may have resulted from parallel divergent selection from ssp. coronopifolius, though drift remains an alternative cause in S. hesperidium.

Conclusions: Our results indicate parallel ecotype formation and (sub)speciation in Senecio in which primarily selective vs. neutral determinants promoted the recurrent origin of coastal types in, respectively, Israel and Morocco.     

Possible causes of morphological variation in an endemic Moroccan groundsel (Senecio leucanthemifolius var. casablancae): evidence from chloroplast DNA and random amplified polymorphic DNA markers

Genetic variation was assessed in Senecio leucanthemifolius var. casablancae (Compositae), a Moroccan Atlantic coast endemic, in order to examine possible causes of atypical leaf morphology in three populations south of the known range. Evidence for introgression from S. glaucus ssp. coronopifolius and/or divergence was investigated with molecular markers. Both random amplified polymorphic DNA (RAPD) and chloroplast (cp) DNA restriction fragment length polymorphism (RFLP) differentiated the species well. Some evidence that hybridization may have occurred between the two species was provided by cpDNA markers. However, biparentally inherited RAPD markers failed to provide any support for the hypothesis that intermediate leaf morphologies in atypical populations arose through hybridization. Consequently, they are most likely to have arisen via divergence caused by drift and/or selection. Genetic distances among populations of S. leucanthemifolius were significant in all but one case. Isolation by distance was indicated by a significant positive correlation between genetic and geographical distances (r = 0.68, P = 0.01, Mantel test). These results suggest that long-distance achene dispersal is rare, despite the presence of a well-developed pappus. The observed loss of pappus at achene maturity may explain this unexpected result. Due to the morphological distinction of var. casablancae from other varieties of S. leucanthemifolius, we suggest elevation to species rank and treatment of the atypical material at infraspecific rank.     

Elucidative studies on the generic concept of Senecio (Asteraceae)

VINCENT, P. L. D. & GETLIFFE, F. M., 1992. Elucidative studies on the generic concept of Senecio (Asteraceae) . This paper presents the results of studies of the generic concept of Senecio sensu stricto. The sample of taxa studied consisted of 93 Natal senecios (including seven varieties and two forms), five Cape heterochromous senecios and nine non-southern African senecios, including the type of the genus, S. vulgaris L. Also included in the study were six species from taxonomically closely related genera in the tribe Senecioneae and one Senecio of uncertain taxonomic position. The phenotype of these taxa was investigated with respect to a large number (122) of morphological and micromorphological characters. Six characters were selected as being taxonomically important with respect to elucidating the generic concept of Senecio sensu stricto. The generic concept of Senecio has been provisionally re-circumscribed and the generic status of each of the senecios and non-senecios studied has been tested according to this concept of Senecio sensu stricto sensu Vincent. In the light of this concept of Senecio , the following species are recommended for exclusion from Senecio sensu stricto sensu Vincent: S. cissampelinus, S. transvaalensis, S. syringifolius and S. hockii. The following species are considered to be peripheral to Senecio sensu stricto sensu Vincent: S. tanacetopsis, S. seminiveus, S. medley-woodii, S. tamoides, S. helminthioides, S. barbertonicus, S. brevilorus, S. viminalis, S. radicans and S.fulgens. Before any taxonomic changes are made to the current composition of Senecio , the concept of Senecio sensu stricto sensu Vincent, is being tested on a worldwide sample of the genus.     

The ancestry of Hordeum depressum (Poaceae, Triticeae)

Based on morphological, cytological, and molecular data, three diploid Hordeum species have by various authors been suggested to may have participated in the ancestry of the tetraploid H. depressum . In order to test the different hypotheses we undertook hybridizations between the proposed taxa, H. brachyantherum ssp. californicum, H. intercedens , and H. pusillum . Autoploids and amphiploids were achieved by treatment of young shoots with colchicine. These were then hybridized with H. depressum and the resulting hybrid plants were tested for fertility. The results strongly indicate that H. depressum arose as a consequence of hybridization between H. brachyantherum ssp. californicum and H. intercedens , probably with H. brachyantherum ssp. californicum as the female parent.     

Sex chromosome mosaicism and hybrid speciation among tiger swallowtail butterflies

Hybrid speciation, or the formation of a daughter species due to interbreeding between two parental species, is a potentially important means of diversification, because it generates new forms from existing variation. However, factors responsible for the origin and maintenance of hybrid species are largely unknown. Here we show that the North American butterfly Papilio appalachiensis is a hybrid species, with genomic admixture from Papilio glaucus and Papilio canadensis. Papilio appalachiensis has a mosaic phenotype, which is hypothesized to be the result of combining sex-linked traits from P. glaucus and P. canadensis. We show that P. appalachiensis' Z-linked genes associated with a cooler thermal habitat were inherited from P. canadensis, whereas its W-linked mimicry and mitochondrial DNA were inherited from P. glaucus. Furthermore, genome-wide AFLP markers showed nearly equal contributions from each parental species in the origin of P. appalachiensis, indicating that it formed from a burst of hybridization between the parental species, with little subsequent backcrossing. However, analyses of genetic differentiation, clustering, and polymorphism based on molecular data also showed that P. appalachiensis is genetically distinct from both parental species. Population genetic simulations revealed P. appalachiensis to be much younger than the parental species, with unidirectional gene flow from P. glaucus and P. canadensis into P. appalachiensis. Finally, phylogenetic analyses, combined with ancestral state reconstruction, showed that the two traits that define P. appalachiensis' mosaic phenotype, obligatory pupal diapause and mimicry, evolved uniquely in P. canadensis and P. glaucus, respectively, and were then recombined through hybridization to form P. appalachiensis. These results suggest that natural selection and sex-linked traits may have played an important role in the origin and maintenance of P. appalachiensis as a hybrid species. In particular, ecological barriers associated with a steep thermal cline appear to maintain the distinct, mosaic genome of P. appalachiensis despite contact and occasional hybridization with both parental species.