Maintenance of strong morphological differentiation despite ongoing natural hybridization between sympatric species of Lomatia (Proteaceae)

Background and Aims

When species cohesion is maintained despite ongoing natural hybridization, many questions are raised about the evolutionary processes operating in the species complex. This study examined the extensive natural hybridization between the Australian native shrubs Lomatia myricoides and L. silaifolia (Proteaceae). These species exhibit striking differences in morphology and ecological preferences, exceeding those found in most studies of hybridization to date.

Methods

Nuclear microsatellite markers (nSSRs), genotyping methods and morphometric analyses were used to uncover patterns of hybridization and the role of gene flow in morphological differentiation between sympatric species.

Key Results

The complexity of hybridization patterns differed markedly between sites, however, signals of introgression were present at all sites. One site provided evidence of a large hybrid swarm and the likely presence of multiple hybrid generations and backcrosses, another site a handful of early generational hybrids and a third site only traces of admixture from a past hybridization event. The presence of cryptic hybrids and a pattern of morphological bimodality amongst hybrids often disguised the extent of underlying genetic admixture.

Conclusions

Distinct parental habitats and phenotypes are expected to form barriers that contribute to the rapid reversion of hybrid populations to their parental character state, due to limited opportunities for hybrid/intermediate advantage. Furthermore, strong genomic filters may facilitate continued gene flow between species without the danger of assimilation. Stochastic fire events facilitate temporal phenological isolation between species and may partly explain the bi-directional and site-specific patterns of hybridization observed. Furthermore, the findings suggest that F₁ hybrids are rare, and backcrosses may occur rapidly following these initial hybridization events.     

Spatial and temporal genetic structure in a hybrid cordgrass invasion

Invasive hybrids and their spread dynamics pose unique opportunities to study evolutionary processes. Invasive hybrids of native Spartina foliosa and introduced S. alterniflora have expanded throughout San Francisco Bay intertidal habitats within the past 35 years by deliberate plantation and seeds floating on the tide. Our goals were to assess spatial and temporal scales of genetic structure in Spartina hybrid populations within the context of colonization history. We genotyped adult and seedling Spartina using 17 microsatellite loci and mapped their locations in three populations. All sampled seedlings were hybrids. Bayesian ordination analysis distinguished hybrid populations from parent species, clearly separated the population that originated by plantation from populations that originated naturally by seed and aligned most seedlings within each population. Population genetic structure estimated by analysis of molecular variance was substantial (F_ST=0.21). Temporal genetic structure among age classes varied highly between populations. At one population, the divergence between adults and 2004 seedlings was low (F_ST=0.02) whereas at another population this divergence was high (F_ST=0.26). This latter result was consistent with local recruitment of self-fertilized seed produced by only a few parental plants. We found fine-scale spatial genetic structure at distances less than ∼200 m, further supporting local seed and/or pollen dispersal. We posit a few self-fertile plants dominating local recruitment created substantial spatial genetic structure despite initial long-distance, human dispersal of hybrid Spartina through San Francisco Bay. Fine-scale genetic structure may more strongly develop when local recruits are dominated by the offspring of a few self-fertile plants.     

The Japanese knotweed invasion viewed as a vast unintentional hybridisation experiment

Chromosome counts of plants grown from open-pollinated seed from Japanese knotweed around the world have revealed the presence of extensive hybridisation with both native and other introduced taxa. These hybrids fit into three categories: inter- and intraspecific hybrids involving the taxa of Fallopia section Reynoutria (giant knotweeds), hybrids between Japanese knotweed and F. baldschuanica (Regel) Holub and hybrids between Japanese knotweed and the Australasian endemics of the genus Muehlenbeckia. In this minireview, the viability of the different classes of hybrid and the potential threats they pose are discussed in the context of recent examples of allopolyploid speciation, which generally involve hybridisation between a native and an alien species. Such wide hybridisations also challenge accepted taxonomic classifications. Japanese knotweed s.l. provides a fascinating example of the interplay between ploidy level, hybridisation and alien plant invasion. The octoploid (2n=88) Fallopia japonica var. japonica (Houtt.) Ronse Decraene is a single female clone throughout much of its adventive range, and provides an ideal system for investigating the potential for wide hybridisation.     

Admixture mapping of quantitative traits in Populus hybrid zones: power and limitations

Uncovering the genetic architecture of species differences is of central importance for understanding the origin and maintenance of biological diversity. Admixture mapping can be used to identify the number and effect sizes of genes that contribute to the divergence of ecologically important traits, even in taxa that are not amenable to laboratory crosses because of their long generation time or other limitations. Here, we apply admixture mapping to naturally occurring hybrids between two ecologically divergent Populus species. We map quantitative trait loci for eight leaf morphological traits using 77 mapped microsatellite markers from all 19 chromosomes of Populus. We apply multivariate linear regression analysis allowing the modeling of additive and non-additive gene action and identify several candidate genomic regions associated with leaf morphology using an information-theoretic approach. We perform simulation studies to assess the power and limitations of admixture mapping of quantitative traits in natural hybrid populations for a variety of genetic architectures and modes of gene action. Our results indicate that (1) admixture mapping has considerable power to identify the genetic architecture of species differences if sample sizes and marker densities are sufficiently high, (2) modeling of non-additive gene action can help to elucidate the discrepancy between genotype and phenotype sometimes seen in interspecific hybrids, and (3) the genetic architecture of leaf morphological traits in the studied Populus species involves complementary and overdominant gene action, providing the basis for rapid adaptation of these ecologically important forest trees.     

Seed bank dynamics govern persistence of Brassica hybrids in crop and natural habitats

Background and Aims Gene flow from crops to their wild relatives has the potential to alter population growth rates and demography of hybrid populations, especially when a new crop has been genetically modified (GM). This study introduces a comprehensive approach to assess this potential for altered population fitness, and uses a combination of demographic data in two habitat types and mathematical (matrix) models that include crop rotations and outcrossing between parental species.Methods Full life-cycle demographic rates, including seed bank survival, of non-GM Brassica rapa × B. napus F₁ hybrids and their parent species were estimated from experiments in both agricultural and semi-natural habitats. Altered fitness potential was modelled using periodic matrices including crop rotations and outcrossing between parent species.Key Results The demographic vital rates (i.e. for major stage transitions) of the hybrid population were intermediate between or lower than both parental species. The population growth rate (λ) of hybrids indicated decreases in both habitat types, and in a semi-natural habitat hybrids became extinct at two sites. Elasticity analyses indicated that seed bank survival was the greatest contributor to λ. In agricultural habitats, hybrid populations were projected to decline, but with persistence times up to 20 years. The seed bank survival rate was the main driver determining persistence. It was found that λ of the hybrids was largely determined by parental seed bank survival and subsequent replenishment of the hybrid population through outcrossing of B. rapa with B. napus.Conclusions Hybrid persistence was found to be highly dependent on the seed bank, suggesting that targeting hybrid seed survival could be an important management option in controlling hybrid persistence. For local risk mitigation, an increased focus on the wild parent is suggested. Management actions, such as control of B. rapa, could indirectly reduce hybrid populations by blocking hybrid replenishment.     

Eco-geographically divergent diploids, Caucasian clover (Trifolium ambiguum) and western clover (T. occidentale), retain most requirements for hybridization

Background and Aims

DNA sequence similarities and hybridization patterns in Trifolium (clovers) section Trifoliastrum suggest that rapid radiation from a common ancestral source led to this complex of diverse species distributed across Europe, western Asia and North Africa. Two of the most geographically and ecologically divergent of these species are the rhizomatous T. ambiguum from high altitudes in eastern Europe and western Asia and the stoloniferous T. occidentale from sea level in western Europe. Attempts were made to hybridize these species to ascertain whether, despite this separation, gene flow could be achieved, indicating the retention of the genetic factors necessary for hybridization.

Methods

Three F₁ hybrids formed after embryo rescue were described, characterized by conventional and molecular cytogenetics, subjected to fertility tests and progeny generations were developed.

Results and Conclusions

Partially fertile hybrids between Trifolium ambiguum and T. occidentale were obtained for the first time. The F₁ hybrids produced seeds after open-pollination, and also produced triploid progeny in backcrosses to T. occidentale from the functioning of unreduced gametes in the hybrids. These plants were fertile and produced progeny with T. occidentale and with T. repens. Meiotic chromosome pairing in the F₁ showed six to eight bivalents per pollen mother cell, indicating pairing between the parental genomes. A chromosome-doubled form of one hybrid, produced using colchicine, showed some multivalents, indicative of interspecific chromosome pairing. The hybrid plants were robust and combined phenotypic characteristics of both species, having stolons, thick roots and a few rhizomes. Results show that despite separation by the entire breadth of Europe, the speciation process is incomplete, and these taxa have partially retained most of the genetic compatibilities needed for hybridization (possibly except for endosperm development, which was not tested). The fertile progeny populations could lead to new clover breeding strategies based on new hybrid forms.     

Reproductive isolation and hybridization in sympatric populations of three Dactylorhiza species (Orchidaceae) with different ploidy levels

Background and Aims

The potential for gene exchange between species with different ploidy levels has long been recognized, but only a few studies have tested this hypothesis in situ and most of them focused on not more than two co-occurring species. In this study, we examined hybridization patterns in two sites containing three species of the genus Dactylorhiza (diploid D. incarnata and D. fuchsii and their allotetraploid derivative D. praetermissa).

Methods

To compare the strength of reproductive barriers between diploid species, and between diploid and tetraploid species, crossing experiments were combined with morphometric and molecular analyses using amplified fragment length polymorphism markers, whereas flow cytometric analyses were used to verify the hybrid origin of putative hybrids.

Key Results

In both sites, extensive hybridization was observed, indicating that gene flow between species is possible within the investigated populations. Bayesian assignment analyses indicated that the majority of hybrids were F₁ hybrids, but in some cases triple hybrids (hybrids with three species as parents) were observed, suggesting secondary gene flow. Crossing experiments showed that only crosses between pure species yielded a high percentage of viable seeds. When hybrids were involved as either pollen-receptor or pollen-donor, almost no viable seeds were formed, indicating strong post-zygotic reproductive isolation and high sterility.

Conclusions

Strong post-mating reproductive barriers prevent local breakdown of species boundaries in Dactylorhiza despite frequent hybridization between parental species. However, the presence of triple hybrids indicates that in some cases hybridization may extend the F₁ generation.     

Unidirectional hybridization and reproductive barriers between two heterostylous primrose species in north-west Yunnan,China

Background and Aims

Heteromorphy in flowers has a profound effect on breeding patterns within a species, but little is known about how it affects reproductive barriers between species. The heterostylous genus Primula is very diverse in the Himalaya region, but hybrids there have been little researched. This study examines in detail a natural hybrid zone between P. beesiana and P. bulleyana.

Methods

Chloroplast sequencing, AFLP (amplified fragment length polymorphism) markers and morphological comparisons were employed to characterize putative hybrids in the field, using synthetic F₁s from hand pollination as controls. Pollinator visits to parent species and hybrids were observed in the field. Hand pollinations were conducted to compare pollen tube growth, seed production and seed viability for crosses involving different morphs, species and directions of crossing.

Key Results

Molecular data revealed all hybrid derivatives examined to be backcrosses of first or later generations towards P. bulleyana: all had the chloroplast DNA (cpDNA) of this species. Some individuals had morphological traits suggesting they were hybrids, but they were genetically similar to P. bulleyana; they might have been advanced generation backcrosses. Viable F₁s could not be produced with P. bulleyana pollen on P. beesiana females, irrespective of the flower morphs used. Within-morph crosses for each species had very low (<10 %) seed viability, whereas crosses between pin P. bulleyana (female) and pin P. beesiana had a higher seed viability of 30 %. Thus genetic incompatibility mechanisms back up mechanical barriers to within-morph crosses in each species, but are not the same between the two species. The two species share their main pollinators, and pollinators were observed to fly between P. bulleyana and hybrids, suggesting that pollinator behaviour may not be an important isolating factor.

Conclusions

Hybridization is strongly asymmetric, with P. bulleyana the only possible mother and all detected hybrids being backcrosses in this direction. Partial ecological isolation and inhibition of heterospecific pollen, and possibly complete barriers to F₁ formation on P. beesiana, may be enough to make F₁ formation very rare in these species. Therefore, with no F₁ detected, this hybrid zone may have a finite life span as successive generations become more similar to P. bulleyana.     

Asymmetric introgression between Magnolia stellata and M. salicifolia at a site where the two species grow sympatrically

In order to understand the ongoing evolutionary relationships between species, it is important to elucidate patterns of natural hybridization. In the zone where two species are sympatrically distributed, we examined 274 individuals of Magnolia stellata, Magnolia salicifolia, and their putative hybrids by means of 16 nuclear and three chloroplast microsatellite markers. Hybrid classes of individuals were estimated by admixture analyses. Morphological traits were also investigated for 64 of the 274 individuals. Admixture analyses revealed that 66 of the 274 individuals were classified as hybrids, comprising 17 F₁ and 19 F₂ individuals, 27 backcrosses to M. salicifolia, and 3 individuals of unknown origin. Morphological data from the 64 individuals agreed well with their genetic admixture rates. Spatial locations of F₁ and F₂ hybrids at the study site were intermediate between the two purebred species, indicating that the site preferences of hybrids are intermediate. The occurrences of F₂ and backcross hybrids indicate that F₁ hybrids are fertile. The chloroplast DNA haplotypes of all F₁ hybrids corresponded to those detected in M. salicifolia, so that maternal parents of the F₁ hybrids were all M. salicifolia. Furthermore, no hybrid individuals derived from a backcross to M. stellata were detected. These results suggest that the direction of hybridization and the subsequent introgression have been quite asymmetric and that the introgression occurred from M. stellata into M. salicifolia.     

Spontaneous hybrids between native and exotic Rubus in the Western United States produce offspring both by apomixis and by sexual recombination

Facultative asexual reproduction is a trait commonly found in invasive species. With a combination of sexual and asexual reproductive modes, such species may adapt to new environments via sexual recombination during range expansion, while at the same time having the benefits of asexuality such as the maintenance of fitness effects that depend upon heterozygosity. In the Western United States, native species of Rubus (Rosaceae) reproduce sexually whereas exotic naturalized Rubus species reproduce by pseudogamous apomixis. We hypothesized that new asexual lineages of Rubus could arise from hybridization in this range. To detect hybridization between native and exotic Rubus, we genotyped 579 individuals collected across California, Oregon and Washington with eight nuclear microsatellites and two chloroplast markers. Principal Coordinate Analysis and Bayesian clustering revealed a limited amount of hybridization of the native R. ursinus with the exotic R. armeniacus and R. pensilvanicus, as well as cultivated varieties. Genetic distances between these hybrids and their offspring indicated that both R. ursinus × R. armeniacus and R. ursinus × R. pensilvanicus produced a mix of apomictic and sexual seeds, with sexual seeds being more viable. Although neither of these hybrid types is currently considered invasive, they model the early stages of evolution of new invasive lineages, given the potential for fixed heterosis and the generation of novel genotypes. The hybrids also retain the ability to increase their fitness via sexual recombination and natural selection. Mixed reproductive systems such as those described here may be an important step in the evolution of asexual invasive species.     

Hybridization and introgression between the exotic Siberian elm, Ulmus pumila, and the native Field elm, U. minor, in Italy

In response to the first Dutch elm disease (DED) pandemic, Siberian elm, Ulmus pumila, was planted to replace the native elm, U. minor, in Italy. The potential for hybridization between these two species is high and repeated hybridization could result in the genetic swamping of the native species and facilitate the evolution of invasiveness in the introduced species. We used genetic markers to examine the extent of hybridization between these two species and to determine the pattern of introgression. We quantified and compared the level of genetic diversity between the hybrids and the two parental species. Hybrids between U. pumila and U. minor were common. The pattern of introgression was not as strongly biased towards U. pumila as was previously observed for hybrids between U. rubra and U. pumila in the United States. The levels of heterozygosity were similar between U. minor and the hybrids and both groups had higher levels of heterozygosity relative to U. pumila. The programs Structure and NewHybrids indicated the presence of first- (F₁) and second- generation (F₂) hybrids and of backcrosses in the hybrid population. The presence of healthy DED resistant U. minor individuals combined with the self-compatibility of U. minor could help explain the presence of F₂ individuals in Italy. The presence of F₂ individuals, where most of the variability present in the hybrids will be released, could facilitate rapid evolution and the potential evolution of invasiveness of U. pumila in Italy.     

Barriers to interspecific hybridization between Juglans nigra L. and J. regia L species

Juglans nigra and Juglans regia are phylogenetically divergent species. Despite the economic interest in Juglans?×?intermedia (J. nigra?×? J. regia), walnut hybridization is rare under natural conditions and still difficult using controlled pollination. Here, we evaluated some reproductive mechanisms that may prevent successful natural hybridization. The study of flowering phenology of 11 J. nigra and 50 J. regia trees growing in a plantation provided information regarding the opportunity for interspecific crosses. Variation in flower size, pollen quality of putative donors, and variation in seed yield and rate of hybrid production among putative maternal trees were examined. DNA fingerprinting and parentage analyses based on nine microsatellites permitted the identification of hybrids and hybridogenic parent. Our data indicated that overlap occurred between the staminate flowering of protogynous J. regia and the beginning of pistillate flowering of protogynous J. nigra. Differences in floral size were computed between walnut species. Only three hybrids among 422 offspring of eleven J. nigra progenies were identified. Interspecific hybridization involving pollination of one early-flowering-protogynous J. nigra by three protogynous J. regia trees was detected. The correct development of J. regia male gametophytes, high pollen viability (86.5 %), and germination (57.6 %) ruled out the possibility that low pollen quality contributed to depressed hybrid production. Our findings indicated that these two species tended to remain reproductively isolated. The substantial disjunction in flowering time and additional prezygotic barriers such as differences in floral size and conspecific pollen advance may affect interspecific gene flow between J. regia and J. nigra.     

Interspecific gene flow in a multispecies oak hybrid zone in the Sierra Tarahumara of Mexico

Background and Aims

Interspecific gene flow can occur in many combinations among species within the genus Quercus, but simultaneous hybridization among more than two species has been rarely analysed. The present study addresses the genetic structure and morphological variation in a triple hybrid zone formed by Q. hypoleucoides, Q. scytophylla and Q. sideroxyla in north-western Mexico.

Methods

A total of 247 trees from ten reference and 13 presumed intermediate populations were characterized using leaf shape variation and geometric morphometrics, and seven nuclear microsatellites as genetic markers. Discriminant function analysis was performed for leaf shape variation, and estimates of genetic diversity and structure, and individual Bayesian genetic assignments were obtained.

Key Results

Reference populations formed three completely distinct groups according to discriminant function analysis based on the morphological data, and showed low, but significant, genetic differentiation. Populations from the zone of contact contained individuals morphologically intermediate between pairs of species in different combinations, or even among the three species. The Bayesian admixture analysis found that three main genetic clusters best fitted the data, with good correspondence of reference populations of each species to one of the genetic clusters, but various degrees of admixture evidenced in populations from the contact area.

Conclusions

The three oak species have formed a complex hybrid zone that is geographically structured as a mosaic, and comprising a wide range of genotypes, including hybrids between different species pairs, backcrosses and probable triple hybrids.     

Chromosome pairing affinities in interspecific hybrids reflect phylogenetic distances among lady's slipper orchids (Paphiopedilum)

Background and Aims

Lady''s slipper orchids (Paphiopedilum) are of high value in floriculture, and interspecific hybridization has long been used for breeding improved cultivars; however, information regarding the genome affinities of species and chromosome pairing behaviour of the hybrids remains almost unknown. The present work analyses the meiotic behaviour of interspecific hybrids by genomic in situ hybridization and cytologically evaluates the genomic relationships among parental species.

Methods

Eight interspecific F₁ hybrids of Paphiopedilum species in various subgenera or sections were investigated in this study. The chromosome behaviour in meiosis of these interspecific hybrids was analysed and subjected to genomic in situ hybridization and fluorescent in situ hybridization.

Key Results

Genomic in situ hybridization was demonstrated as an efficient method to differentiate between Paphiopedilum genomes and to visualize the chromosome pairing affinities in interspecific F₁ hybrids, clarifying the phylogenetic distances among these species. Comparatively regular chromosome pairing observed in the hybrids of P. delenatii × P. bellatulum, P. delenatii × P. rothschildianum and P. rothschildianum × P. bellatulum suggested high genomic affinities and close relationships between parents of each hybrid. In contrast, irregular chromosome associations, such as univalents, trivalents and quadrivalents occurred frequently in the hybrids derived from distant parents with divergent karyotypes, such as P. delenatii × P. callosum, P. delenatii × P. glaucophyllum, P. rothschildianum × P. micranthum and P. rothschildianum × P. moquetteanum. The existence of multivalents and autosyndesis demonstrated by genomic in situ hybridization in this study indicates that some micro-rearrangements and other structural alterations may also play a part in differentiating Paphiopedilum species at chromosomal level, demonstrated as different chromosome pairing affinities in interspecific hybrids.

Conclusions

The results indicate that genome homology and the interaction of genetic factors, but not chromosome number nor karyotype similarity, determine the chromosome pairing behaviour in Paphiopedilum hybrids.     

Genome size and base composition variation in natural and experimental Narcissus (Amaryllidaceae) hybrids

Background and Aims

Although there is evidence that both allopolyploid and homoploid hybridization lead to rapid genomic changes, much less is known about hybrids from parents with different basic numbers without further chromosome doubling. Two natural hybrids, Narcissus × alentejanus (2n = 19) and N. × perezlarae (2n = 29), originated by one progenitor (N. cavanillesii, 2n = 28) and two others (N. serotinus, 2n = 10 and N. miniatus, 2n = 30, respectively) allow us to study how DNA content and composition varies in such hybrids.

Methods

Flow cytometry measurements with two staining techniques, PI and DAPI, were used to estimate 2C values and base composition (AT/GC ratio) in 390 samples from 54 wild populations of the two natural hybrids and their parental species. In addition, 20 synthetic F₁ hybrid individuals were also studied for comparison.

Key Results

Natural hybrids presented 2C values intermediate between those found in their parental species, although intra-population variance was very high in both hybrids, particularly for PI. Genome size estimated from DAPI was higher in synthetic hybrids than in hybrids from natural populations. In addition, differences for PI 2C values were detected between synthetic reciprocal crosses, attributable to maternal effects, as well as between natural hybrids and those synthetic F₁ hybrids in which N. cavanillesii acted as a mother.

Conclusions

Our results suggest that natural hybrid populations are composed of a mixture of markedly different hybrid genotypes produced either by structural chromosome changes, consistent with classic cytogenetic studies in Narcissus, or by transposon-mediated events.     

Meloidogyne partityla on Pecan Isozyme Phenotypes and Other Host

Meloidogyne sp. from five pecan (Carya illinoensis) orchards in Texas were distinctive in host range and iszoyme profiles from common species of Meloidogyne but were morphologically congruent with Meloidogyne partityla Kleynhans, a species previously known only in South Africa. In addition to pecan, species of walnut (Juglans hindsii and J. regia) and hickory (C. ovata) also were hosts. No reproduction was observed on 15 other plant species from nine families, including several common hosts of other Meloidogyne spp. Three esterase phenotypes and two malate dehydrogenase phenotypes of M. partityla were identified by polyacrylamide gel electrophoresis. Each of these isozyme phenotypes was distinct from those of the more common species M. arenaria, M. hapla, M. incognita, and M. javanica.     

The influence of pre- and post-zygotic barriers on interspecific Corymbia hybridization

Background and Aims

Corymbia species from different sections hybridize readily, with some of increasing economic importance to plantation forestry. This study explores the locations of reproductive barriers between interspecific Corymbia hybrids and investigates the reproductive success of a wide taxonomic range of C. torelliana hybrid crosses.

Methods

Pollen, pistil and embryo development were investigated for four C. torelliana crosses (×C. torelliana, ×C. citriodora subsp. citriodora, ×C. tessellaris and ×C. intermedia) using fluorescent and standard microscopy to identify the locations of interspecific reproductive isolating barriers. Corymbia torelliana was also crossed with 16 taxa, representing six of the seven Corymbia sections, both Corymbia subgenera and one species each from the related genera, Angophora and Eucalyptus. All crosses were assessed for capsule and seed yields.

Key Results

Interspecific C. torelliana hybridization was controlled by pre-zygotic reproductive isolating barriers inhibiting pollen adhesion to the stigma, pollen germination, pollen tube growth in the style and pollen tube penetration of the micropyle. Corymbia torelliana (subgenus Blakella, sect. Torellianae) was successfully hybridized with Corymbia species from subgenus Blakella, particularly C. citriodora subsp. citriodora, C. citriodora subsp. variegata, C. henryi (sect. Maculatae) and C. tessellaris (sect. Abbreviatae), and subgenus Corymbia, particularly C. clarksoniana and C. erythrophloia (sect. Septentrionales). Attempted intergeneric hybrids between C. torelliana and either Angophora floribunda or Eucalyptus pellita were unsuccessful.

Conclusions

Corymbia hybrids were formed between species from different sections and subgenera, but not with species from the related genera Angophora or Eucalyptus. Reproductive isolation between the interspecific Corymbia hybrid crosses was controlled by early- and late-acting pre-zygotic isolating barriers, with reproductive success generally decreasing with increasing taxonomic distance between parent species. These findings support the monophyly of Corymbia and the close relationships of infrageneric clades. The hybridizing propensity of Corymbia species provides opportunities for breeding but suggests risks of environmental gene flow.     

Little or no gene flow despite F1 hybrids at two interspecific contact zones

Hybridization can create the selective force that promotes assortative mating but hybridization can also select for increased hybrid fitness. Gene flow resulting from hybridization can increase genetic diversity but also reduce distinctiveness. Thus the formation of hybrids has important implications for long‐term species coexistence. This study compares the interaction between the tree wētā Hemideina thoracica and its two neighboring species; H. crassidens and H. trewicki. We examined the ratio of parent and hybrid forms in natural areas of sympatry. Individuals with intermediate phenotype were confirmed as first generation hybrids using nine independent genetic markers. Evidence of gene flow from successful hybridization was sought from the distribution of morphological and genetic characters. Both species pairs appear to be largely retaining their own identity where they live in sympatry, each with a distinct karyotype. Hemideina thoracica and H. trewicki are probably reproductively isolated, with sterile F₁ hybrids. This species pair shows evidence of niche differences with adult size and timing of maturity differing where Hemideina thoracica is sympatric with H. trewicki. In contrast, evidence of a low level of introgression was detected in phenotypes and genotypes where H. thoracica and H. crassidens are sympatric. We found no evidence of size divergence although color traits in combination with hind tibia spines reliably distinguish the two species. This species pair show a bimodal hybrid zone in the absence of assortative mating and possible sexual exclusion by H. thoracica males in the formation of F₁ hybrids.     

Floral scent in natural hybrids of Ipomopsis (Polemoniaceae) and their parental species

Background and Aims

Floral traits, such as floral volatiles, can contribute to pre-zygotic reproductive isolation by promoting species-specific pollinator foraging. When hybrid zones form, floral traits could also influence post-zygotic isolation. This study examined floral volatiles in parental species and natural hybrids in order to explore potential scent mediation of pre-zygotic and post-zygotic isolation.

Methods

Floral bouquets were analysed for the sister species Ipomopsis aggregata and I. tenuituba and their natural hybrids at two contact sites differing in both hybridization rate and temporal foraging pattern of hawkmoth pollinators. Floral volatiles were quantified in diurnal and nocturnal scent samples using gas chromatography–mass spectrometry.

Key Results

The bouquets of parental species and hybrids showed qualitative overlap. All flowers emitted similar sets of monoterpenoid, sesquiterpenoid, aliphatic and benzenoid compounds, but separated into groups defined by multivariate analysis of quantitative emissions. The parental species differed most strikingly in the nitrogenous compound indole, which was found almost exclusively in nocturnal bouquets of I. tenuituba. Natural hybrid bouquets were highly variable, and showed emission rates of several compounds that appeared transgressive. However, indole emission rates were intermediate in the hybrids compared with rates in the parents. Volatile bouquets at the contact site with lower hybridization did not show greater species specificity in overall scent emission, but I. tenuituba presented a stronger indole signal during peak hawkmoth activity at that site.

Conclusions

The two species of Ipomopsis differed in patterns of floral bouquets, with indole emitted in nocturnal I. tenuituba, but not in I. aggregata. Natural hybrid bouquets were not consistently intermediate between the parents, although hybrids were intermediate in indole emission. The indole signal could potentially serve as a hawkmoth attractant that mediates reproductive isolation both before and after hybrid formation.     

Identification of interspecific hybrids among domesticated apple and its wild relatives

Potential interspecific hybrids are usually identified in natural populations by their proximity to interbreeding species or their intermediate phenotypes; hybridization can then be confirmed by comparing the genetic makeup of putative hybrids to pure species. In contrast, detecting interspecific hybridization and misclassifications in ex situ collections can be difficult because fine-scale geographic locations and species-specific phenotypic data are generally unavailable. Thus, there is little a priori information available to suggest which individuals might be hybrids. Instead, hybrids or misclassified individuals must be identified based on molecular data via population assignment and admixture detection programs. We have applied a variety of population assignment and admixture detection programs to over 400 samples of four closely related Malus species held in the US Department of Agriculture?CAgricultural Research Service National Plant Germplasm System that were genotyped at 19 simple sequence repeat loci. Our findings indicate that over 10?% of the samples of the wild species Malus sieversii and Malus orientalis and nearly 20?% of the samples of the wild species Malus sylvestris may be admixed or misclassified. The percentage of admixed or misclassified samples of the domesticated species, Malus × domestica, was much lower, at <5?%. These findings provide an illustration of how to detect hybridization and misclassification in ex situ collections using molecular data and, ultimately, should help to maximize the utility of the collections. In addition, the presence of wild-collected samples that show admixture with domesticated apple suggests that gene flow may be occurring from the crop into natural populations of the wild species.