Natural hybridization origin of Rhododendron agastum (Ericaceae) in Yunnan, China: inferred from morphological and molecular evidence

Confusing boundaries of the Labrador tea species: dispersal history explains the lack of clear species structure

BACKGROUND AND AIMS

The Labrador teas (genus Rhododendron, subsection Ledum) are a complex of species widely distributed in the Northern Hemisphere. They occupy cold-resistant plant communities from highlands to forest understorey and wetland habitats almost circumboreally and they are especially abundant in Northeast Asia and northern North America, yet there are no clear species boundaries in this group. The genetic structure of species of subsect. Ledum from Eurasia and North America as well as the dispersal history of the group require clarification.

METHODS

The phylogeny and biogeography of subsect. Ledum of the genus Rhododendron were assessed using phylogenetic trees constructed based on the analysis of variation in chloroplast petB-petD, trnV-ndhC, trnH-psbA, K2R-K707, atpB oligo2 - rbcL oligo5 and nuclear (ITS1) markers of four Eurasian and one American species (65 populations, 408 individuals). The data were evaluated with maximum parsimony and Bayesian analysis. Molecular dating and reconstruction of ancestral areas were performed.

KEY RESULTS

Dense sampling revealed widespread presence of shared haplotypes and ribotypes among Ledum populations and species. Two American, three mixed and one Eurasian lineage diversified during the Neogene climate cooling and then rapidly dispersed during the Pleistocene. The ability to accumulate high genetic diversity and to preserve it across distribution ranges and generations prevented Ledum from lineage sorting. As a result, a species complex with a reserve of genetic variability appeared.

CONCLUSIONS

Although no clear phylogenetic inference can be obtained at present, the plastid genealogy is consistent with the nuclear genealogy and demonstrates the processes involved in speciation in the Ledum species complex.

Genome and GWAS analysis identified genes significantly related to phenotypic state of Rhododendron bark

As an important horticultural plant, Rhododendron is often used in urban greening and landscape design. However, factors such as the high rate of genetic recombination, frequent outcrossing in the wild, weak linkage disequilibrium, and the susceptibility of gene expression to environmental factors limit further exploration of functional genes related to important horticultural traits, and make the breeding of new varieties require a longer time. Therefore, we choose bark as the target trait which is not easily affected by environmental factors, but also has ornamental properties. Genome-wide association study (GWAS) of Rhododendron delavayi (30 samples), R. irroratum (30 samples) and their F1 generation R. agastum (200 samples) was conducted on the roughness of bark phenotypes. Finally, we obtained 2416.31 Gbp of clean data and identified 5 328 800 high-quality SNPs. According to the P-value and the degree of linkage disequilibrium of SNPs, we further identified 4 out of 11 candidate genes that affect bark roughness. The results of gene differential expression analysis further indicated that the expression levels of Rhdel02G0243600 and Rhdel08G0220700 in different bark phenotypes were significantly different. Our study identified functional genes that influence important horticultural traits of Rhododendron, and illustrated the powerful utility and great potential of GWAS in understanding and exploiting wild germplasm genetic resources of Rhododendron.

Ploidy variation in Rhododendron subsection Maddenia and its implications for conservation

Polyploidy, which is common in plants, can confound taxon recognition and hence conservation assessments. In the taxonomically complex genus Rhododendron, 25 % of the over 1,300 taxa are considered under threat and 27 % Near Threatened or Data Deficient, with their taxonomy needing to be resolved urgently. Although ploidy levels of Rhododendron taxa range from diploid (2x) to dodecaploid (12x) according to previous reports, the extent of polyploidy across the genus has not been examined. We first summarized the taxonomic distribution of polyploids in the genus based on the literature. Then as a case study, we estimated ploidy levels of 47 taxa in subsection Maddenia (subgenus Rhododendron, section Rhododendron) using flow cytometry, together with verification of meiotic chromosome counts for representative taxa. The summary of reported ploidy in Rhododendron indicates that polyploidy is most common in subgenera Pentanthera and Rhododendron. In subsection Maddenia, all examined taxa are diploids except for the R. maddenii complex that shows a high ploidy variation (2-8x, 12x). We investigated ploidy level of 12 taxa in subsection Maddenia for the first time, and estimated genome sizes of two Rhododendron species. Knowledge of ploidy levels will inform phylogenetic analysis of unresolved species complexes. Overall, our study of subsection Maddenia provides a model for examining multiple issues including taxonomic complexity, ploidy variation and geographic distribution in relation to biodiversity conservation.

Evolutionary history of two evergreen Rhododendron species as revealed by chromosome-level genome assembly

Background

The genus Rhododendron (Ericaceae), a species-rich and widely distributed genus of woody plants, is distinguished for the beautiful and diverse flowers. Rhododendron delavayi Franch. and Rhododendron irroratum Franch., are highly attractive species widely distributed in south-west China and abundant new varieties have been selected from their genetic resources.

Methods

We constructed chromosome-scale genome assemblies for Rhododendron delavayi and Rhododendron irroratum. Phylogenetic and whole-genome duplication analyses were performed to elucidate the evolutionary history of Rhododendron. Further, different types of gene duplications were identified and their contributions to gene family expansion were investigated. Finally, comprehensive characterization and evolutionary analysis of R2R3-MYB and NBS-encoding genes were conducted to explore their evolutionary patterns.

Results

The phylogenetic analysis classified Rhododendron species into two sister clades, 'rhododendrons' and 'azaleas'. Whole-genome duplication (WGD) analysis unveiled only one WGD event that occurred in Rhododendron after the ancestral γ triplication. Gene duplication and gene family expansion analyses suggested that the younger tandem and proximal duplications contributed greatly to the expansion of gene families involved in secondary metabolite biosynthesis and stress response. The candidate R2R3-MYB genes likely regulating anthocyanin biosynthesis and stress tolerance in Rhododendron will facilitate the breeding for ornamental use. NBS-encoding genes had undergone significant expansion and experienced species-specific gain and loss events in Rhododendron plants.

Conclusions

The reference genomes presented here will provide important genetic resources for molecular breeding and genetic improvement of plants in this economically important Rhododendron genus.

Natural hybridization among three Rhododendron species (Ericaceae) revealed by morphological and genomic evidence

BACKGROUND

Natural hybridization can influence the adaptive response to selection and accelerate species diversification. Understanding the composition and structure of hybrid zones may elucidate patterns of hybridization processes that are important to the formation and maintenance of species, especially for taxa that have experienced rapidly adaptive radiation. Here, we used morphological traits, ddRAD-seq and plastid DNA sequence data to investigate the structure of a Rhododendron hybrid zone and uncover the hybridization patterns among three sympatric and closely related species.

RESULTS

Our results show that the hybrid zone is complex, where bi-directional hybridization takes place among the three sympatric parental species: R. spinuliferum, R. scabrifolium, and R. spiciferum. Hybrids between R. spinuliferum and R. spiciferum (R. ×duclouxii) comprise multiple hybrid classes and a high proportion of F₁ generation hybrids, while a novel hybrid taxon between R. spinuliferum and R. scabrifolium dominated the F₂ generation, but no backcross individuals were detected. The hybrid zone showed basically coincident patterns of population structure between genomic and morphological data.

CONCLUSIONS

Natural hybridization exists among the three Rhododendron species in the hybrid zone, although patterns of hybrid formation vary between hybrid taxa, which may result in different evolutionary outcomes. This study represents a unique opportunity to dissect the ecological and evolutionary mechanisms associated with adaptive radiation of Rhododendron species in a biodiversity hotspot.

Incongruent phylogenies and their implications for the study of diversification, taxonomy, and genome size evolution of Rhododendron

PREMISE

Classification of taxa depends on the quality of inferred phylogenies. Rhododendron, a highly species-rich genus (>1156 species) of woody plants, has a highly debated infrageneric classification, due to its huge diversity, homoplasy in key characters, and incongruence among data sets. We provide a broad coverage of representative species to resolve Rhododendron infrageneric phylogeny and highlight the areas of incongruence. We further investigate the effect of polyploidy and genome size evolution on diversification of Rhododendron.

METHODS

We generated two plastid and two nuclear loci for 260 Rhododendron species. We analyzed the loci separately as well as concatenated, utilizing both likelihood and Bayesian methods. We tested incongruence both among the data sets and with previous studies. We estimated genome sizes for 125 species through flow cytometry.

RESULTS

Our results suggest stronger support for larger subgenera; however, the smaller subgenera pose several problems; for example, R. tomentosum (former genus Ledum) occupies incongruent positions based on different DNA regions. The main shift to higher diversification in the genus occurs in the Himalayan/Southeast Asian clade of R. subg. Hymenanthes. We found that polyploidy occurs in almost all subgenera but most frequently within R. subg. Rhododendron sections Rhododendron and Schistanthe.

CONCLUSIONS

We endorse the recognition of five major clades at the subgeneric level, but a number of species cannot be confidently assigned to these clades due to incongruency. With regard to genome size evolution, results support previous reports that genome sizes of tropical plants are lower than those of colder and temperate regions and that genome downsizing promotes diversification.

Testing genome skimming for species discrimination in the large and taxonomically difficult genus Rhododendron

Standard plant DNA barcodes based on 2-3 plastid regions, and nrDNA ITS show variable levels of resolution, and fail to discriminate among species in many plant groups. Genome skimming to recover complete plastid genome sequences and nrDNA arrays has been proposed as a solution to address these resolution limitations. However, few studies have empirically tested what gains are achieved in practice. Of particular interest is whether adding substantially more plastid and nrDNA characters will lead to an increase in discriminatory power, or whether the resolution limitations of standard plant barcodes are fundamentally due to plastid genomes and nrDNA not tracking species boundaries. To address this, we used genome skimming to recover near-complete plastid genomes and nuclear ribosomal DNA from Rhododendron species and compared discrimination success with standard plant barcodes. We sampled 218 individuals representing 145 species of this species-rich and taxonomically difficult genus, focusing on the global biodiversity hotspots of the Himalaya-Hengduan Mountains. Only 33% of species were distinguished using ITS+matK+rbcL+trnH-psbA. In contrast, 55% of species were distinguished using plastid genome and nrDNA sequences. The vast majority of this increase is due to the additional plastid characters. Thus, despite previous studies showing an asymptote in discrimination success beyond 3-4 plastid regions, these results show that a demonstrable increase in discriminatory power is possible with extensive plastid genome data. However, despite these gains, many species remain unresolved, and these results also reinforce the need to access multiple unlinked nuclear loci to obtain transformative gains in species discrimination in plants.

Comparative population genetic analyses suggest hybrid origin of R hododendron pubicostatum, an endangered plant species with extremely small populations endemic to Yunnan, China

Gene flow between sympatric congeneric plants is thought to be very common and may pose serious threats to endangered species. In the present study, we evaluate the genetic diversity and divergence of three sympatric Rhododendron species in Jiaozi Mountain using newly developed microsatellites through the Illumina MiSeq sequencing approach. Genetic diversity of all three Rhododendron species studied was moderate in comparison to genetic parameters previously reported from species of this genus. Interestingly, genetic structure analysis of the three species identified a possible hybrid origin of the threatened Rh. pubicostatum. This sympatry should be considered a unimodal hybrid zone, since R h. pubicostatum is predominant here. Unimodal hybrid zones are uncommon in Rhododendron, despite the fact that hybridization frequently occurs in the genus. Issues pertaining to the conservation of R h. pubicostatum resulting from admixture of genetic material from its parental species are discussed.

Development of a nest-PCR for detection of Fasciola hepatica DNA in the intermediate snail host, Radix cucunorica, and the prevalence in northwestern China

Fasciolosis, a foodborne zoonotic disease, caused by Fasciola species which is considered an important problem for human health and livestock husbandry development. Snails are intermediate hosts of F. hepatica, the epidemiological surveillance of snails can evaluate the transmission risk of this disease in human and livestock. In this study, we developed a nest-polymerase chain reaction (nest-PCR) to detect the DNA of F. hepatica in Radix cucunorica, a prevalent intermediate host of this parasite in northwestern China. The nest-PCR was used to amplify a 208 bp fragment of the second internal transcribed spacer (ITS-2) of F. hepatica with two pairs of primers. The method was able to detect up to 0.16 fg genomic DNA in a 25 μL PCR reaction system even effected with high concentrations of snail DNA, and no cross reaction was observed from the genomic DNA of Paramphistomum cervi, Clonorchis sinensis, Orientobilharzia turkestanicum, Metorchis orientalis, Dicrocoelium chinensis. To evaluate the transmission risk of this disease, 409 snail samples collected from different areas of Gansu province were used to detect and analyze the transmission risk of F. hepatica in this area. Of 409 snail samples, the overall prevalence is 43.76%. The prevalence was 92.75% in Gannan Tibetan Autonomous Prefecture, while no snail was positive for F. hepatica in Linxia Hui Autonomous Prefecture. The nest-PCR was firstly used to detect the infection of F. hepatica in snail. It is a novel, useful and convenient method with high sensitivity and specificity. This study is the first report about the epidemiological surveillance of snail infection by F. hepatica in northwestern China, which will help to evaluate the transmission risk of F. hepatica in northwestern China.

Homoploid hybridization of plants in the Hengduan mountains region

The Hengduan Mountains Region (HMR) is a major global biodiversity hotspot. Complex tectonic and historical climatic conditions created opportunities for natural interspecific hybridization. Likewise, anthropogenic disturbance potentially raises the frequency of hybridization. Among species studies to date, the frequency of homoploid hybridization appears in the HMR. Of nine taxa in which natural hybridization has been detected, three groups are involved in homoploid hybrid speciation, and species pairs from the remaining six genera suggest that continuous gene flow occurs in hybrid zones. Reproductive isolation may greatly affect the dynamic and architecture of hybrid zones in the HMR. Asymmetrical hybridization and introgression can primarily be attributed to both prezygotic and postzygotic barriers. The frequent observation of such asymmetry may imply that reproductive barrier contributes to maintaining species boundaries in the alpine region. Ecological isolations with environmental disturbance may promote breeding barriers between parental species and hybrids. Hybrid zones may be an important phase for homoploid hybrid speciation. Hybrid zones potentially provided abundant genetic resources for the diversification of the HMR flora. The ecological and molecular mechanisms of control and mediation for natural hybridization will help biologists to understand the formation of biodiversity in the HMR. More researches from ecological and molecular aspects were required in future studies.

Incomplete reproductive isolation between Rhododendron taxa enables hybrid formation and persistence

The evolutionary consequences of hybridization ultimately depend on the magnitude of reproductive isolation between hybrids and their parents. We evaluated the relative contributions of pre- and post-zygotic barriers to reproduction for hybrid formation, hybrid persistence and potential for reproductive isolation of hybrids formed between two Rhododendron species, R. spiciferum and R. spinuliferum. Our study established that incomplete reproductive isolation promotes hybrid formation and persistence and delays hybrid speciation. All pre-zygotic barriers to reproduction leading to hybrid formation are incomplete: parental species have overlapping flowering; they share the same pollinators; reciprocal assessments of pollen tube germination and growth do not differ among parents. The absence of post-zygotic barriers between parental taxa indicates that the persistence of hybrids is likely. Reproductive isolation was incomplete between hybrids and parents in all cases studied, although asymmetric differences in reproductive fitness were prevalent and possibly explain the genetic structure of natural hybrid swarms where hybridization is known to be bidirectional but asymmetric. Introgression, rather than speciation, is a probable evolutionary outcome of hybridization between the two Rhododendron taxa. Our study provides insights into understanding the evolutionary implications of natural hybridization in woody plants.

Asymmetrical natural hybridization varies among hybrid swarms between two diploid Rhododendron species

Background and Aims

The extent to which hybridization leads to gene flow between plant species depends on the structure of hybrid populations. However, if this varies between locations, species barriers might prove permeable in some locations but not in others. To assess possible variation in hybrid population structure, the magnitude and direction of natural hybridization between two Chinese endemic species, Rhododendron spiciferum and Rhododendron spinuliferum , were evaluated.

Methods

Thirteen nuclear microsatellite markers were employed to characterize 566 individuals collected from 15 non-allopatric populations and nine allopatric parental populations. Chloroplast DNA (cpDNA) sequences were obtained from a subset of samples. Genetic structure and direction of gene flow was determined using a combination of STRUCTURE and NEWHYBRIDS analysis.

Key Results

Nuclear analysis revealed that parental taxa formed two genetically distinct clusters and hybrids shared the genetic background of both parents and did not form a separate genetic lineage. Overall, hybrid swarms were dominated by early- and later-generation hybrids, with a significantly higher proportion of hybrids (59·6 %) possessing >50 % R. spiciferum-like nuclear germplasm. The cpDNA analysis further indicated that a significantly greater proportion of hybrids (61·1 %) possessed the R. spiciferum cpDNA haplotype.

Conclusions

Gene flow between R. spiciferum and R. spinuliferum was found to be bidirectional in 14 of the 15 hybrid swarms and asymmetrical in six hybrid swarms. Asymmetrical gene flow was evident for only nuclear DNA (nDNA) in two populations, for only cpDNA in three populations, and for both nDNA and cpDNA in one population. Collectively, the variation in genetic structure found among the 15 hybrid swarms suggests that introgression rather than hybrid speciation is a more likely outcome of hybridization between these hybridizing taxa.

Why is population information crucial for taxonomy? A case study involving a hybrid swarm and related varieties

Hybridization has become a focal topic in evolutionary biology, and many taxonomists are aware that the process occurs more frequently than previously assumed. Nonetheless many species and varieties are still described without explicitly considering the possibility of hybridization, especially in countries that have relatively short scientific histories, but which often possess the highest species diversities. Furthermore, new taxa are often described based only on herbarium specimens, not taking into account information from wild populations, significantly decreasing the potential to detect morphologies arising from hybridization at this crucial descriptive stage. We used morphological data from a hybrid swarm involving two Rhododendron species to showcase possible character combinations in intermediates. Certain characters used to distinguish taxa were more variable within the same individual than between species, emphasizing the importance of population information for an adequate choice of characters. Most described varieties of the two species fell within the spectrum of hybrid morphology, suggesting that these taxa would be unlikely to have merited formal description if contemporary standards had been employed. In all investigated cases the hybrid nature of described varieties seems to have been detectable with adequate morphological data alone, if populations had been assessed. A post hoc assessment of taxa is often complicated, especially if certain types of information are not provided. To avoid accumulation of such invalid taxa, careful scrutiny should be employed for new descriptions. Hybrids (not hybrid species) described as taxa obscure valuable information about natural processes and impact negatively on further research that depends on taxonomic data.

Strong reproductive isolation despite occasional hybridization between a widely distributed and a narrow endemic Rhododendron species

Reproductive isolation (RI) plays an important role for speciation, but assessing reproductive barriers at all life-cycle stages remains challenging. In plants, most studies addressing the topic have been focusing on herbs with short generation times. The present study attempted to quantify several reproductive barriers between a hybridizing species pair of long-lived woody rhododendrons. Consistent with findings of previous studies, pre-zygotic reproductive barriers contributed more to total RI than post-zygotic reproductive barriers. Especially in the more widespread species geographic isolation was an important barrier, and pollinator constancy contributed exceptionally to RI in both species. Additionally to strong pre-zygotic reproductive barriers, post-zygotic reproductive barriers were considerable, and had asymmetric tendencies favoring one of the species as maternal parent. Overall, despite occasional hybridization, the present study provides evidence for strong RI between R. cyanocarpum and R. delavayi.

In the wild hybridization of annual Datura species as unveiled by morphological and molecular comparisons

BACKGROUND

The present work aimed to verify whether intermediate variants were natural crosses between Datura species (D. stramonium forms and D. ferox). Their existence has been long ago insinuated but has not been studied using morphological features and molecular tools. The variants differed in stem coloring, upper bearing forks, and fruit characters.

RESULTS

Principal Components Analysis of 11 morphological characteristics showed that D. ferox and D. stramonium (forms stramonium and tatula) were quite different and the putative hybrids were intermittent. The D. ferox × D. stramonium f. tatula was closer to the latter of its parents. Sequencing analysis revealed identical amplified trnL intron in all variants and a 100% homology with D. stramonium accession number EU580984.1 suggested that this plastid cannot discern Datura variants. However, genomic analysis with URP markers indicated that the hybrids had >60% genetic makeup similarity with both parents suggesting that the intermediate variants were putative inter-specific hybrids. Moreover, the dendrogram stemmed from cluster analysis of the fingerprint profile of variants placed D. stramonium and D. ferox in different branches indicating their genetic differentiation from each other as well as from their hybrids.

CONCLUSIONS

The findings suggest that the natural hybridization of annual Datura species occurs. Extrapolating, this hybridization could be the first step for speciation. More possibly, it can alter population composition, its weediness and adaptability to local conditions.

Morphological and molecular evidence for natural hybridization in sympatric population of Roscoea humeana and R. cautleoides (Zingiberaceae)

The sympatric occurrence of some species in Roscoea is very common, but little information is available on natural hybridization. However, some intermediate individuals were found on the sympatric population of Roscoea humeana and R. cautleoides at Ganhaizi population in northwestern Yunnan Province, China. We suspected that these intermediate individuals were the hybrids of R. humeana and R. cautleoides from the previous evidence, but could not confirm them. In this study, morphometric analysis was followed by examination of HAT-RAPD polymorphisms to determine the occurrence of natural hybridization between sympatric R. humeana and R. cautleoides. The results showed that most morphological characters of the putative hybrids were found to be intermediate between those of R. humeana and R. cautleoides. Meanwhile, molecular analysis confirmed that the morphological intermediates were derived from hybridization between the two species. From the analysis of the NewHybrids, the hybridization individuals were mainly F₁s. These results indicated that interspecific hybridization between R. humeana and R. cautleoides indeed occurred in sympatric population.

Genetic variation of Croton stellatopilosus Ohba based on non-coding DNA sequences of ITS, trnK and trnL-F regions

Croton stellatopilosus Ohba (Plau-noi), a well-known Thai medicinal plant, was investigated for its genetic variation by analyzing three DNA regions, one nuclear internal transcribed spacer (ITS) region and two chloroplast trnL-F intergenic spacer and trnK intron regions. The results of ITS sequencing from 30 leaf samples showed that there were two major genotypes of C. stellatopilosus which were designated as STEL Type A and B. In addition, various nucleotide additive sequences which had presumably arisen from these two groups were also found. These so-called "putative hybrids", interestingly, displayed trnK intron sequences identical to the STEL Type B but different from the Type A. For the trnL-F region, all the 30 samples showed identical sequences. Thus, it was suggested that in the hybridization of C. stellatopilosus, the Type A genotype acts as paternal parent whereas the Type B genotype acts as maternal parent. In addition, all C. stellatopilosus samples were analyzed for their plaunotol content using TLC densitometry. We found that the Type A genotype, hybrid group and Type B genotype had plaunotol content in the ranges 0.209-0.492, 0.319-0.896 and 0.442-1.000% (w/w) dry weight, respectively. The results indicated that there is a correlation between the plaunotol contents and non-coding DNA sequences of ITS, trnK and trnL-F regions of C. stellatopilosus.

Unusual patterns of hybridization involving a narrow endemic Rhododendron species (Ericaceae) in Yunnan, China

PREMISE OF THE STUDY

One potential threat to rare species is genetic swamping caused by hybridization, but few studies have quantified this threat. Rhododendron cyanocarpum is a narrow endemic species that occurs sympatrically with potentially interfertile congeners throughout its range within Yunnan, China. We searched the entire distribution of R. cyanocarpum for hybrids and examined the patterns of hybridization to assess potential threat from hybridization. •

METHODS

In a comprehensive field survey, we detected only one instance of hybridization involving R. cyanocarpum, with R. delavayi, at Huadianba near Dali. Material of both species and putative hybrids was examined using morphology, chloroplast DNA, nuclear ribosomal DNA, and Bayesian analysis of AFLP profiles. •

KEY RESULTS

Of 10 putative hybrids, two were F(1)(')s and at least seven were F(2)(')s. Four backcrosses to R. delavayi were detected among material with R. delavayi-like morphology within the hybrid zone. Backcrosses to R. cyanocarpum were not detected. Therefore F(2)(')s outnumbered all other classes within the hybrid zone, a situation not previously confirmed for plants and extremely rare generally. Hybridization was asymmetrical, with R. delavayi as the maternal parent in all but one of the hybrids detected. •

CONCLUSIONS

Although natural hybridization is common in Rhododendron, it is rare in R. cyanocarpum and is apparently not accompanied by backcrossing toward R. cyanocarpum. Hence, there is no immediate risk of genetic swamping, unless habitat disturbance increases and changes the patterns of hybridization. Our study is the first to report a plant hybrid zone dominated by F(2) hybrids. This pattern might contribute to species barrier maintenance.

Natural hybridization between Rhododendron delavayi and R. cyanocarpum (Ericaceae), from morphological, molecular and reproductive evidence

The natural hybridization that occurs between two sympatric species of Rhododendron subgenus Hymenanthes in Yunnan, China, was investigated. In field observations, it was noted that the putative hybrids between R. delavayi Franch. and R. cyanocarpum (Franch.) Franch. ex W.W. Sm. had intermediate morphologies. On the basis of morphology, chloroplast DNA (trnL-rpl32) and nuclear DNA (waxy), hybrids and parental species were identified. Hybridization occurred in both directions, but was asymmetrical, with R. delavayi as the major maternal parent. Reciprocal hand pollination treatments showed that either species, as pollen donor or pollen receiver, could produce fruits. It was noted that fruit set varied among treatments. The same pollinators (bumblebees) were shared in both parental species. From these results, we conclude that individuals with intermediate morphologies are indeed of hybrid origin from natural hybridization between R. cyanocarpum and R. delavayi. Furthermore, we presume the hybridization at the study site could have been initiated by habitat disturbance in the 1950s, and we may hence witness the early stages of hybrid swarm formation.

Asymmetric hybridization in Rhododendron agastum: a hybrid taxon comprising mainly F1s in Yunnan, China

BACKGROUND AND AIMS

Rhododendron (Ericaceae) is a large woody genus in which hybridization is thought to play an important role in evolution and speciation, particularly in the Sino-Himalaya region where many interfertile species often occur sympatrically. Rhododendron agastum, a putative hybrid species, occurs in China, western Yunnan Province, in mixed populations with R. irroratum and R. delavayi.

METHODS

Material of these taxa from two sites 400 km apart (ZhuJianYuan, ZJY and HuaDianBa, HDB) was examined using cpDNA and internal transcribed spacer (ITS) sequences, and amplified fragment length polymorphism (AFLP) loci, to test the possibility that R. agastum was in fact a hybrid between two of the other species. Chloroplast trnL-F and trnS-trnG sequences together distinguished R. irroratum, R. delavayi and some material of R. decorum, which is also considered a putative parent of R. agastum.

KEY RESULTS

All 14 R. agastum plants from the HDB site had the delavayi cpDNA haplotype, whereas at the ZJY site 17 R. agastum plants had this haplotype and four had the R. irroratum haplotype. R. irroratum and R. delavayi are distinguished by five unequivocal point mutations in their ITS sequences; every R. agastum accession had an additive pattern (double peaks) at each of these sites. Data from AFLP loci were acquired for between ten and 21 plants of each taxon from each site, and were analysed using a Bayesian approach implemented by the program NewHybrids. The program confirmed the identity of all accessions of R. delavayi, and all R. irroratum except one, which was probably a backcross. All R. agastum from HDB and 19 of 21 from ZJY were classified as F1 hybrids; the other two could not be assigned a class.

CONCLUSIONS

Rhododendron agastum represents populations of hybrids between R. irroratum and R. delavayi, which comprise mostly or only F1s, at the two sites examined. The sites differ in that at HDB there was no detected variation in cpDNA type or hybrid class, whereas at ZJY there was variation in both.

Reticulate hybridization of Alpinia (Zingiberaceae) in Taiwan

Reticulate hybridization is a complicated and creative mechanism in plant evolution that can cause interference in phylogenetic studies. Based on observations of intermediate morphology, low pollen fertility, and overlapping distributions of putative parent species, Yang and Wang (Proceedings of the cross-strait symposium on floristic diversity and conservation. National Museum of Natural Science, Taichung, Taiwan, pp 183-197, 1998) first proposed reticulate hybridization of Alpinia in Taiwan. In the present study, molecular tools were used to explore relationships between four parental species and their homoploidy hybrids, and the impact of hybridization on phylogeny reconstruction. Based on DNA markers, maternal heritance of the chloroplast genome, and additivity of nuclear ribosomal internal transcribed spacer, the present results provide strong support for the hybridization hypothesis. Co-existence of parental ribotypes within hybrids revealed that these hybridization events were current, while reciprocal and introgressive hybridization were inferred from chloroplast DNA data. Furthermore, iterative hybridizations involving more than two parental species may occur in notorious hybrid zones. Ecological, phenological, and physiological evidence provides insight into why such frequent hybridization occurs in Taiwanese Alpinia. In the phylogenetic tree of the Zerumbet clade reconstructed in this study, the chloroplast sequences from one hybrid species were not grouped into a subclade, implying instability caused by hybridization. Failure to find morphological apomorphies and biogeographical patterns in this clade was likely partially due to reticulate hybridization.