Morphological and molecular evidence of natural hybridization between two distantly related Rhododendron species from the Sino-Himalaya

Rhododendron (Ericaceae) is a large woody genus in which hybridization may play an important role in evolution and speciation, particularly in the Sino-Himalayan region, where many interfertile species often occur sympatrically. Natural hybridization between Rhododendron delavayi Franch. (=  R. arboreum ssp. delavayi ) and Rhododendron decorum Franch., which belong to different subsections of subgenus Hymenanthes, was investigated. Material of R. delavayi and R. decorum and their putative hybrids was collected from the wild. On the basis of morphology, chloroplast DNA, nuclear ribosomal DNA, and AFLP profiles, hybrids and parental species were identified. Hybridization occurred in both directions, but was asymmetrical, with R. delavayi as the major maternal parent in the hybrid zone. Most of the hybrids possessed intermediate phenotypes, and amongst the 15 hybrids detected were six F1s, two F2s, one first-generation backcross to R. delavayi , and two first-generation backcrosses to R. decorum . This indicates that, if Rhododendron underwent rapid radiation in this region, it did so in spite of permeable species barriers.  © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society , 2008, 156 , 119–129.     

Hybrids between Chorthippus brunneus and C. jacobsi (Orthoptera: Acrididae) do not show endogenous postzygotic isolation

Studies of prezygotic and postzygotic isolation in Drosophila have shown in general that species in sympatry tend to evolve prezygotic barriers earlier than do species in allopatry. However, postzygotic barriers tend to evolve at the same evolutionary rate in both sympatric and allopatric species. In contrast to these observations, the grasshoppers Chorthippus parallelus parallelus and C. p. erythropus show complete hybrid male sterility but only limited prezygotic isolation after an estimated 0.5 millions years of divergence. Like their congeners, C. brunneus and C. jacobsi form a hybrid zone where their ranges meet in northern Spain. However, the hybrid zone is mosaic and bimodal and, in contrast to the high levels of postzygotic isolation between C. parallelus subspecies, these two species showed no significant reduction in hybrid fitness in F₁ or backcross generations relative to the parental generations. The level of prezygotic isolation in laboratory tests was comparable to that between C. parallelus subspecies. These results suggest that endogenous postzygotic isolation does not play an important role in the reproductive isolation between C. brunneus and C. jacobsi , or in determining the structure of the hybrid zone. Exogenous postzygotic isolation may be present and should be tested in future studies.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 195–203.     

Species-specific song convergence in a moving hybrid zone between two passerines

Moving hybrid zones are receiving increasing attention. However, so far little is known about the proximate mechanisms underlying these movements. Signalling behaviour, by individuals engaged in interspecific sexual and aggressive interactions, may play a crucial role. In this study, we investigated song variation within a moving hybrid zone between two warblers, Hippolais polyglotta and H. icterina . In these species, song is involved in interspecific territoriality and, probably, in mixed pairings. We showed that allopatric populations of the two species are clearly acoustically differentiated. However, interspecific differences faded out in sympatry as a result of an overall pattern of convergence. Unexpectedly, the two species converged for different song parameters, namely temporal parameters for H. icterina and syntax for H. polyglotta . Hybridization and interspecific competition could explain convergence in H. icterina . Instead, in H. polyglotta we suggest that local adaptation to habitat and interspecific learning might contribute to convergence. We particularly stress that cross-species learning, by maintaining high levels of interspecific interactions, may influence the movement of the zone.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 80 , 507–517.     

Genome organization in diploid hybrid species of Argyranthemum (Asteraceae) in the Canary Islands

Two diploid species of hybrid origin, Argyranthemum lemsii and A. sundingii , have been described from different valleys in the Anaga peninsula, north-east Tenerife. They have previously been shown to originate from hybridization between the same parental species, the montane A. broussonetii and the coastal A. frutescens , A. broussonetii being the chloroplast donor in one valley and A. frutescens in the other. The specific status of the two hybrid species has been questioned. In this study we used karyotype analysis, FISH, and GISH to address the question of multiple diploid hybrid speciation. GISH did not discriminate clearly between the parental genomes, but differential labelling was observed in separate hybrid populations, indicating different chromosomal rearrangements in different valleys. Small karyotype differences and local loss of rDNA were also observed. Thus separate origins of the same hybrid combination in different valleys in Tenerife have been verified. Our results add some support to the recognition of two species of hybrid origin, but the case serves to illustrate some of the many problems connected with the species concept in plants.  © 2003 The Linnean Society of London, Botanical Journal of the Linnean Society , 2003, 141 , 491–501.     

Maintenance of species boundaries despite rampant hybridization between three species of reef fishes (Hexagrammidae): implications for the role of selection

Hybridization and introgression are antagonistic to the process of speciation. If hybrids are viable and backcross, introgression will slow or prevent the evolution of reproductive isolation between populations. If species hybridize upon secondary contact, introgression will ultimately erode species boundaries. If hybridization is rampant and gene flow is high, the proportion of hybrids will approach values expected with random mating, and introgression will overcome species boundaries, homogenizing populations. Alternatively, if species are genetically distinct, the proportion of hybrids would be significantly less than expected with random mating, gene flow would be interrupted by selection against hybrids, and inviability or sterility of F ₁ hybrids or backcrosses would be expected. Here, we investigate a system with characteristics expected by both of these opposing scenarios. Hybridization occurs between three species of reef fishes in the genus Hexagrammos at unexpected high frequencies in a zone of distributional overlap. Backcrossed individuals are detected, indicating F ₁ hybrids are viable and capable of reproducing. Yet, these species are genetically distinct at multiple loci. To study this apparent paradox, we estimate the relative proportions of hybrids, patterns of symmetry, inviability, and cytonuclear disequilibria using one mitochondrial and two nuclear markers. We invoke selection against hybrids, at various life history stages, in the maintenance of species boundaries in this system.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 135–147.     

The role of salivary androgen-binding protein in reproductive isolation between two subspecies of house mouse: Mus musculus musculus and Mus musculus domesticus

Previous behavioural studies using inbred lines have suggested that the gene ( Abpa ) for the alpha subunit of salivary androgen-binding protein (ABP) plays a role in prezygotic isolation between house mouse Mus musculus subspecies. We tested this hypothesis in animals from wild allopatric (121 individuals from four samples) and parapatric (320 animals from 15 samples) populations sampled on the Czech–Bavarian transect across the hybrid zone between M. m. domesticus and M. m. musculus . The study did not reveal a consistent statistically significant trend of homosubspecific preferences in individual allopatric and parapatric populations. Nonetheless, the whole pattern of preference was skewed toward homosubspecific preference mostly on the M. m. musculus side of the hybrid zone. The pattern of homosubspecific preferences was stronger for the time spent sniffing than it was for the first choice of the signal (the ratio of homosubspecific vs. heterosubspecific preferences for both sexes was 6 : 2 in allopatric and 21 : 9 in parapatric populations, while the same rates were 4 : 4 and 16 : 14 for the first choice). To the extent that Y-maze tests reflect preference under wild conditions, we suggest that this slight preference may not in itself be sufficient to impede gene flow between the two subspecies and thus act as a reproductive barrier. ABP most probably participates in a complex system of subspecies-specific recognition in the hybrid zone, but the picture is far too complex at this time to allow a conclusive evaluation of the importance of this role.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 349–361.     

Direct and indirect analysis of the fitness of Chrysochus (Coleoptera: Chrysomelidae) hybrids

The chrysomelid beetles Chrysochus auratus and C. cobaltinus form a narrow hybrid zone in western North America. We used a combination of direct and indirect analyses to examine the fitness of Chrysochus hybrids. For the direct analyses, we compared the mating frequency, longevity, fecundity and fertility of hybrid females and parentals. For the indirect approach, we tested predictions of multilocus genotype frequencies at a focal site in the hybrid zone, based on the frequencies of mating combinations during the previous generation. Hybrid females produced fewer eggs than did parentals and the eggs they produced in the lab failed to hatch, in contrast to those of parental females. In addition, contrary to predictions that 15.8% of the individuals at the focal site would have multilocus genotypes other than those expected of parentals or F₁ individuals, we found no such genotypes at this site. This hybrid zone appears to be an example of a classic tension zone, with endogenous selection against hybrid individuals. We discuss the implications of low hybrid fitness for the evolution of premating barriers in this system, and argue that the integration of direct and indirect approaches is a powerful means of assessing the relative fitness of hybrids, particularly for species in which mate choices are easy to observe in the field.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 273–286.     

Genetic characterization of hybridization between the New Zealand everlastings Helichrysum lanceolatum and Anaphalioides bellidioides (Asteraceae: Gnaphalieae)

Intergeneric hybridization between the endemic New Zealand species Helichrysum lanceolatum and Anaphalioides bellidioides (Asteraceae: Gnaphalieae) at two Banks Peninsula populations was investigated by DNA sequencing and amplified fragment length polymorphism fingerprinting. Crosses in both directions have contributed to the generation of the F1 hybrids, but in the study season hybridization was far more common with H. lanceolatum as the maternal parent. F1 hybrids have reduced, but considerable, fertility and seed collected from them includes back-crosses. Two hybrid plants sampled from the wild appear to be back-crosses to H. lanceolatum. Despite back-crossing, introgression was not demonstrated and is at most limited in scope. These results also corroborate important roles for small population size and rarity in the formation of hybrid lineages in New Zealand's everlastings.  © 2007 Landcare Research New Zealand Ltd. Journal compilation © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society , 2007, 154 , 89–98.     

Spartina anglica C. E. Hubbard: a natural model system for analysing early evolutionary changes that affect allopolyploid genomes

Spartina anglica arose during the end of the 19th century in England by hybridization between the indigenous Spartina maritima and the introduced East American Spartina alterniflora and following genome duplication of the hybrid ( S.  ×  townsendii ). This system allows investigations of the early evolutionary changes that accompany stabilization of a new allopolyploid species in natural populations. Various molecular data indicate that S. anglica has resulted from a unique parental genotype. This young species contains two distinctly divergent homoeologous genomes that have not undergone extensive change since their reunion. No burst of retroelements has been encountered in the F₁ hybrid or in the allopolyploid, suggesting a 'structural genomic stasis' rather than 'rapid genomic changes'. However, modifications of the methylation patterns in the genomes of S.  ×  townsendii and S. anglica indicate that in this system, epigenetic changes have followed both hybridization and polyploidization.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 82 , 475–484.     

Inferences of selection and migration in the Danish house mouse hybrid zone

We analysed the patterns of allele frequency change for ten diagnostic autosomal allozyme loci in the hybrid zone between the house mouse subspecies Mus musculus domesticus and M. m. musculus in central Jutland. After determining the general orientation of the clines of allele frequencies, we analysed the cline shapes along the direction of maximum gradient. Eight of the ten clines are best described by steep central steps with coincident positions and an average width of 8.9 km (support limits 7.6–12.4) flanked by tails of introgression, indicating the existence of a barrier to gene flow and only weak selection on the loci studied. We derived estimates of migration from linkage disequilibrium in the centre of the zone, and by applying isolation by distance methods to microsatellite data from some of these populations. These give concordant estimates of σ =  0.5–0.8 km generation     . The barrier to gene flow is of the order of 20 km (support limits 14–28), and could be explained by selection of a few per cent at 43–120 underdominant loci that reduces the mean fitness in the central populations to 0.45. Some of the clines appear symmetrical, whereas others are strongly asymmetrical, and two loci appear to have escaped the central barrier to gene flow, reflecting the differential action of selection on different parts of the genome. Asymmetry is always in the direction of more introgression into musculus , indicating either a general progression of domesticus into the musculus territory, possibly mediated by differential behaviour, or past movement of the hybrid zone in the opposite direction, impeded by potential geographical barriers to migration in domesticus territory.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 593–616.     

Morphological,genetic and behavioural analyses of a hybrid zone between the ground beetles Carabus lewisianus and C. albrechti (Coleoptera,Carabidae): asymmetrical introgression caused by movement of the zone?

We analysed the putative hybrid zone between the ground beetles Carabus lewisianus Breuning and C. albrechti Morawitz from the Kanto and Tanzawa Mountains in central Honshu, Japan, using morphological, genetic, and behavioural data. Canonical discriminant analysis of three external and five genital morphological characters revealed an apparent morphological gap, suggesting restricted sympatry in the contact zone. RFLP and molecular phylogenetic analyses of the mitochondrial gene ND5 revealed that haplotypes originating from C. lewisianus introgressed extensively into C. albrechti . Fitting tanh curves to morphological (external and genital) and genetic characters showed differential cline widths and centres, suggesting stronger selection for genital morphology with narrower cline widths, and extensive introgression of the nuclear genes responsible for external characters relative to those controlling genital morphology into the range of C. albrechti . Linkage disequilibria between the morphological characters did not differ from zero, suggesting sufficient recombination occurred in the zone but the effect of small sample sizes was not negligible. Mate choice trials demonstrated that males showed a weak but not significant preference for conspecific mates. We interpret these patterns as evidence for (1) partial reproductive isolation presumably by postmating processes; (2) selection against intermediate genital morphologies, and (3) independent responses of the characters to different evolutionary forces that may cause movement of genital clines and introgression of the mitochondrial gene and external morphology. The movement of the boundaries may have resulted in the particular species distribution patterns in the study area.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 86 , 79–94.     

Is the last glaciation the only relevant event for the present genetic population structure of the meadow brown butterfly Maniola jurtina (Lepidoptera: Nymphalidae)?

Phylogeographical studies are available for a considerable number of European species, but few analyses exist for temperate species with very large and fairly continuous populations that are also absent from Northern Europe. Therefore, we studied the butterfly Maniola jurtina as a model for this group. The species has two major genetic lineages (mean genetic distance between lineages: 0.033; F _CT: 0.052), most probably evolving in glacial differentiation centres in the western and eastern Mediterranean. The onset of this differentiation might have been the beginning of the last glacial stage maximum some 40 kyr bp . A hybrid zone between these two lineages exists in western Central Europe. No genetic substructures have been found within the two lineages ( F _SC: 0.017) and average genetic distances are very small. Therefore, it is highly probable that postglacial expansion was of the phalanx type. There is, at most, very limited differentiation at regional and local scales. However, the genetic diversity within populations is high (means: A : 2.68; H _E: 17.2%; P : 78%), as would be predicted for such a common species. Comparison of these results with a published allozyme analysis revealed a similar phylogeographical pattern, but lower genetic diversity in the latter. Morphological patterns of wings and genitalia show similar geographical patterns as allozyme data.   © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 85 , 419–431.     

Leaf morphometric variation in Quercus affinis and Q. laurina (Fagaceae), two hybridizing Mexican red oaks

Leaf variation was examined in two hybridizing Mexican red oaks, Quercus affinis and Q. laurina . Data of nine traits were obtained for ten randomly chosen leaves in each of 512 individuals from 16 populations sampled along a geographical gradient, including the distribution area of both species and a putative area of secondary contact and hybridization. A cluster analysis recognized two main groups of populations, which were congruent with the taxonomic assignment of the predominant morphological type within the populations and were thus labelled ' Q. affinis -like' and ' Q. laurina -like' population groups. A nested analysis of variance revealed that the largest proportion of the total variation was contained within populations, as among-tree variation (28–54%, depending on the trait), and as intra-individual variation (17–56%). However, differences between the two groups of populations (3–27%) and among populations within groups (5–21%) were also significant for the nine traits. A distinct pattern of change across populations was observed for each trait. Variation was particularly pronounced along the geographical gradient for petiole length and leaf-margin teeth number, possibly implying selection on these two traits. Results suggest that phenotypic plasticity, gene flow, hybridization and natural selection have shaped foliar variation in this oak complex. © 2005 The Linnean Society of London, Botanical Journal of the Linnean Society , 2005, 147 , 427–435.     

A discrete neuropeptide difference between two hybridizing grasshopper subspecies

Population divergence can be detected by the divergence of functional and neutral characters. Under some circumstances, it is desirable to have available a character that is discretely expressed in either of the diverging genomes, rather than the evaluation of qualitative variation of continuous characters. In the present study, we investigated mass peaks of peptide hormones in a model system of population divergence, the hybrid zone of two Chorthippus parallelus subspecies in the French–Spanish Pyrenees. Mass spectra from neuroendocrine tissues have previously been identified as species-specific and may have a sufficient resolution to vary at the subspecies level. For the first time, we succeeded in the detection of a subspecies-specific expression of neuropeptides collected from single individuals. Mass spectra sampled from populations across the C. parallelus grasshopper hybrid zone indicated neuropeptide identity between the sexes and within sample sites. The distribution of a single distinct but variable peptide signal, however, very closely followed the cline of the hybrid zone as derived from the mean variation in several continuous characters. The identity of this peptide in populations from the northern Pyrenees and central Europe supports a neuropeptide differentiation of preglacial origin. The observed differentiation in the peptide profile of the two subspecies demonstrates that a peptidomic approach may be a promising perspective to reconstruct reproductive isolation in an insect hybrid zone.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 541–548.     

Analysis of molecular differentiation in a hybrid zone between chromosomally distinct races of the common shrew Sorex araneus (Insectivora: Soricidae) suggests their common ancestry

During postglacial colonization, populations that diverged in different refugia produced a patchwork of genomes, often delimited with sharp hybrid zones. The outcome of hybridization following the secondary contact of two genetically distinct populations is hard to predict. In this context, the present study investigated the genetic structure of the hybrid zone between the Drnholec and Białowieża chromosome races of the common shrew ( Sorex araneus ) in Poland using biparentally inherited (seven autosomal microsatellites) and uniparentally inherited (Y-linked microsatellite and mtDNA) molecular markers. On the basis of diagnostic chromosomes, the Drnholec and Białowieża races were classified to different karyotypic groups, which were believed to have independent glacial histories. It was found that genetic differentiation between the Drnholec and Białowieża races was weak and nonsignificant with respect to all molecular markers. However, these results are in contrast with the chromosomal structure of this hybrid zone. The very sharp frequency clines of the diagnostic chromosomes strongly suggest that gene flow between the Drnholec and Białowieża races was reduced. Nonsignificant correlations between genetic differentiation and both the presence of an environmental barrier and geographical distance reveal that only differences in karyotypes might be a reason for limited gene exchange between the races. It is assumed that a lack of molecular differences between the Drnholec and Białowieża races results from a shared ancestral variation.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 89 , 79–90.     

Molecular evidence for repeated hybridization events involved in the origin of the genus × Crepidiastrixeris (Asteraceae) using RAPDs and ITS data

We investigated the hybrid origin of × Crepidiastrixeris denticulato-platyphylla using RAPDs and ITS sequence data. The putative parents Paraixeris denticulata and Crepidiastrum platyphyllum represent separate species, irrespective of geographical origin. The occurrence of species specific RAPD markers from P. denticulata and C. platyphyllum in × C. denticulato-platyphylla established unambiguously a hybrid origin between the two taxa. This was in line with the occurrence of a combination of morphological characters such as plant habit and floret numbers. The parent taxa differed from each other by 7 nucleotide substitutions and 2 indel events in the ITS region. The hybrids showed sequence additivity and most likely represent F₁ plants, with the exception of two plants which were of possible F₂ origin, possessing either the ITS sequences of one parent only, or one predominant ITS type. The hybrids occurred in two out of three localities where the parents occurred sympatrically. This fact, together with the short life-span of the plants, suggests that × C. denticulato-platyphylla exists as a result of repeated, frequent hybridization between the parent species.  © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 151 , 333–343.     

Pinus mugo (Pinaceae) in the Abruzzi Mountains: high morphological variation in isolated populations

Thirty-three individuals of Pinus mugo from the Abruzzian Apennines (Italy), the most isolated population of the species, were examined biometrically to verify the range of trait variation and compared with six other populations sampled in the north-eastern part of the species range. Ten needles and ten cones from each individual were measured and quantified on the basis of 31 traits. The interpopulation comparison revealed a close affinity of the Abruzzian population of P. mugo to three of the six other populations. The other three north-eastern populations, one from the Tatra Mountains and two from the Chornokhora Mountains, appeared to be different. This suggests that these populations originated from other Pleistocene centres of occurrence. The intrapopulation variation of the population from the Abruzzian Apennines is comparable with those of all six populations of P. mugo from the Sudety and Carpathian Mountains. These findings support the hypothesis that isolated but locally abundant plant populations do not necessarily show a reduction in variability resulting from isolation.  © 2005 The Linnean Society of London, Botanical Journal of the Linnean Society , 2005, 147 , 309–316.     

Unexpected insularity effects in invasive plant mating systems: the case of Carpobrotus (Aizoaceae) taxa in the Mediterranean Basin

Although biological invasions are currently recognized as being especially severe on islands, little attention has been given to detecting parameters influencing this pattern. This study tested the common perceptions that uniparental reproductive modes are associated with invasive and/or island plant populations due to a lack of pollinator vectors and/or small initial population sizes, and that pollinator services and biparental seed production modes will subsequently be associated with adjacent mainland sites. Using controlled pollination and germination experiments on invasive Carpobrotus edulis and C.  aff. acinaciformis populations in both island and mainland habitats in south-east France, we found no evidence to support these hypotheses. All significant mean differences found between locations for reproductive indices describing uniparental reproductive modes were significantly smaller in island populations. In contrast, seedling sizes issuing from manual outcrossing and manual hybridization experiments, regardless of taxon, as well as C.  aff.  acinaciformis hybridization indices, were larger in island populations. The presence of significant, though limited, pollinator service was detected for all populations, and stressed the importance of such mutualisms. We suggest that invasive plant reproductive traits could be highly contextual, and that islands may have a potential role in the acceleration of the invasion process through the production of highly variable populations.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 85 , 65–79.     

Genital morphology and taxonomy of the water strider Aquarius remigis (Say) (Insecta, Hemiptera-Heteroptera: Gerridae)

The shape of the dorsal vesical plate was used to determine the extent of morphological differentiation among populations of the water strider Aquarius remigis (Say) (Hemiptera-Heteroptera: Gerridae), and between species assigned to the A. remigis species group. Populations were sampled throughout North America and Mesoamerica, and included paratypes of A. remigoides Gallant & Fairbairn, and material collected from the type localities of A. amplus (Drake & Harris) and A. nyctalis (Drake & Hottes). If taxonomy is to be inferred from the shape of the dorsal vesical plate, as suggested by previous authors, several taxonomic issues need to be reconsidered. First, there was one species distributed widely across North America and deep into Mexico that is very likely to represent A. remigis , meaning that populations from the western USA previously assigned to ' A. nyctalis ' should be synonymized with this species. Second, A. remigoides as currently defined also includes some populations of A. remigis , and is probably much less widely distributed in the south-eastern USA than previously thought. A hybridization zone in Pennsylvania between A. remigis and A. remigoides , as inferred from measurements of body size and allozyme electrophoresis, is not congruent with data from the male genitalia, which indicated a transition zone between the two forms in Virginia much further to the south. Finally, A. amplus was widespread in Mexico, reaching into Guatemala to the south and into Arizona to the north.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 90 , 381–398.     

Molecular and morphological characterization of Crataegus L. species (Rosaceae) in southern Syria

The systematics of the genus Crataegus (Rosaceae) have been considered problematic owing to the fact that hybridization, introgression, polyploidy and apomixis may occur in this genus. A study of the Crataegus species from the Arab mountains, Sweida Province, Syria, has been undertaken based on both plastid DNA sequences ( trn L- trn F, psb A- trn H) and morphological data. In the investigated region, three morphologically distinguishable Crataegus species: C. azarolus var. aronia L., C.  ×  sinaica Boiss. ssp. sinaica and C. monogyna var. monogyna Jacq. were investigated. Crataegus azarolus can be clearly distinguished morphologically from C. monogyna by the colour, size and structure of fruits, the number of pyrenes, the flowering and ripening time, the density of thorns, the tree shape and also the leaf shape. According to our morphological data, in Syria, C.  ×  sinaica is variable and could represent a hybrid of C. azarolus  ×  C. monogyna ; the cpDNA sequence analysis showed sequences corresponding to C. monogyna as the plausible mother of the hybrid.  © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society , 2007, 153 , 255–263.