Does hybridization with a widespread congener threaten the long-term persistence of the Eastern Alpine rare local endemic Knautia carinthiaca?

Asymmetric Introgression and Cryptic Natural Hybridization between Two Species of Teucrium Section Polium (Lamiaceae) on the Balkan Peninsula

In this work, we analyzed the morphology and genetic structure of Teucrium montanum, T. capitatum and their hybrid T. × rohlenae from three syntopic populations. A morphometric study showed that the parents and their hybrids exhibited continuous morphological variation, with the hybrid positioned exactly between the parents. Genetic analysis revealed that plants morphologically identified as T. × rohlenae are fertile hybrids that produce hybrid swarms dominated by later-generation hybrids. This suggests that introgression, rather than speciation, is the more likely outcome of hybridization between these plant species. The extent and direction of gene flow between the two species differed markedly between the three syntopic localities. At the Trilj locality, it was clearly unidirectional, with T. capitatum playing the dominant role. At the Sićevo locality, gene flow was slightly asymmetric, favoring the genetic background of T. capitatum, while at the Sliven site, it was completely asymmetric in the opposite direction. The extreme case of unidirectional gene flow was observed at the Trilj locality where plants morphologically identified as T. montanum could not be genetically distinguished from T. capitatum. This suggests that interspecific hybridization occurred long ago, leading to introgression and cryptic hybrids, blurring of species boundaries and generating evolutionary noise.

Comparison of plastid genomes and ITS of two sister species in Gentiana and a discussion on potential threats for the endangered species from hybridization

BACKGROUND

Gentiana rigescens Franchet is an endangered medicinal herb from the family Gentianaceae with medicinal values. Gentiana cephalantha Franchet is a sister species to G. rigescens possessing similar morphology and wider distribution. To explore the phylogeny of the two species and reveal potential hybridization, we adopted next-generation sequencing technology to acquire their complete chloroplast genomes from sympatric and allopatric distributions, as along with Sanger sequencing to produce the nrDNA ITS sequences.

RESULTS

The plastid genomes were highly similar between G. rigescens and G. cephalantha. The lengths of the genomes ranged from 146,795 to 147,001 bp in G. rigescens and from 146,856 to 147,016 bp in G. cephalantha. All genomes consisted of 116 genes, including 78 protein-coding genes, 30 tRNA genes, four rRNA genes and four pseudogenes. The total length of the ITS sequence was 626 bp, including six informative sites. Heterozygotes occurred intensively in individuals from sympatric distribution. Phylogenetic analysis was performed based on chloroplast genomes, coding sequences (CDS), hypervariable sequences (HVR), and nrDNA ITS. Analysis based on all the datasets showed that G. rigescens and G. cephalantha formed a monophyly. The two species were well separated in phylogenetic trees using ITS, except for potential hybrids, but were mixed based on plastid genomes. This study supports that G. rigescens and G. cephalantha are closely related, but independent species. However, hybridization was confirmed to occur frequently between G. rigescens and G. cephalantha in sympatric distribution owing to the lack of stable reproductive barriers. Asymmetric introgression, along with hybridization and backcrossing, may probably lead to genetic swamping and even extinction of G. rigescens.

CONCLUSION

G. rigescens and G. cephalantha are recently diverged species which might not have undergone stable post-zygotic isolation. Though plastid genome shows obvious advantage in exploring phylogenetic relationships of some complicated genera, the intrinsic phylogeny was not revealed because of matrilineal inheritance here; nuclear genomes or regions are hence crucial for uncovering the truth. As an endangered species, G. rigescens faces serious threats from both natural hybridization and human activities; therefore, a balance between conservation and utilization of the species is extremely critical in formulating conservation strategies.

The Use of Flow Cytometry for Estimating Genome Sizes and DNA Ploidy Levels in Plants

Flow cytometry has emerged as a uniquely flexible, accurate, and widely applicable technology for the analysis of plant cells. One of its most important applications centers on the measurement of nuclear DNA contents. This chapter describes the essential features of this measurement, outlining the overall methods and strategies, but going on to provide a wealth of technical details to ensure the most accurate and reproducible results. The chapter is aimed to be equally accessible to experienced plant cytometrists as well as those newly entering the field. Besides providing a step-by-step guide for estimating genome sizes and DNA-ploidy levels from fresh tissues, special attention is paid to the use of seeds and desiccated tissues for such purposes. Methodological aspects regarding field sampling, transport, and storage of plant material are also given in detail. Finally, troubleshooting information for the most common problems that may arise during the application of these methods is provided.

Genetic Variability in Balkan Paleoendemic Resurrection Plants Ramonda serbica and R. nathaliae Across Their Range and in the Zone of Sympatry

The genus Ramonda includes three Paleoendemic and Tertiary relict species that survived in refugial habitats of the Balkan Peninsula (R. nathaliae and R. serbica) and the Iberian Peninsula (R. myconi). They are all "resurrection plants," a rare phenomenon among flowering plants in Europe. Ramonda myconi and R. nathaliae are diploids (2n = 2x = 48), while R. serbica is a hexaploid (2n = 6x = 144). The two Balkan species occur in sympatry in only two localities in eastern Serbia, where tetraploid potential hybrids (2n = 4x = 96) were found. This observation raised questions about the existence of gene flow between the two species and, more generally, about the evolutionary processes shaping their genetic diversity. To address this question, genetic markers (AFLP) and an estimate of genome size variation were used in a much larger sample and at a larger geographic scale than previously. The combination of AFLP markers and genome size results suggested ongoing processes of interspecific and interploidy hybridization in the two sites of sympatry. The data also showed that interspecific gene flow was strictly confined to sympatry. Elsewhere, both Ramonda species were characterized by low genetic diversity within populations and high population differentiation. This is consistent with the fact that the two species are highly fragmented into small and isolated populations, likely a consequence of their postglacial history. Within sympatry, enormous variability in cytotypes was observed, exceeding most reported cases of mixed ploidy in complex plant species (from 2x to >8x). The AFLP profiles of non-canonical ploidy levels indicated a diversity of origin pathways and that backcrosses probably occur between tetraploid interspecific hybrids and parental species. The question arises whether this diversity of cytotypes corresponds to a transient situation. If not, the question arises as to the genetic and ecological mechanisms that allow this diversity to be maintained over time.

Hybridization and cryptic speciation in the Iberian endemic plant genus Phalacrocarpum (Asteraceae-Anthemideae)

Understanding the role and impact of reticulation in phylogenetic inquiry has improved with extended use of high throughput sequencing data. Yet, due to the dynamism of genomes over evolutionary time, disentangling old hybridization events remains a serious challenge. Phalacrocarpum (DC.) Willk. is one of the 27 Iberian endemic plant genera, currently considered monotypic but including three subspecies. Its uncertain phylogenetic relationships within tribe Anthemideae (Asteraceae) point to an Early Miocene divergence from its sister group, and its persistent taxonomic instability has been proposed to be due to hybridization. We aim at understanding the evolutionary history of this genus using SNPs called from a genotyping-by-sequencing (GBS) analysis, Sanger sequences-from three plastid DNA regions (psbJ-petA, petB-petD, trnH-psbA) and the nuclear ribosomal ITS regions (cloned)-as well as leaf morphometric multivariate analysis. SNP data and Sanger sequences strongly support the unforeseen existence of a cryptic species in the eastern populations of P. oppositifolium subsp. anomalum. Broad molecular and morphometric patterns of variation found in conflictive populations from the Sanabria Valley region convincingly identify a recent previously undocumented hybrid zone. By contrast, evidence is less conclusive on relationships between subspecies hoffmannseggii, oppositifolium and a second conflictive group distributed along the Galician-Portuguese border (Orense massifs). Although genetic clustering analysis of SNP data suggests that the former subspecies was the maternal progenitor in hybridization events that gave rise to the other two groups, we found considerable uniqueness of ITS ribotypes and plastid haplotypes in them. This result, in the context of Pleistocene climatically-driven range shifts in NW Iberian Peninsula, can be due to periods of isolation, genetic bottlenecks and drift superimposed on old hybridization events. Our study confirms the idea that unravelling old hybridization events may be compromised by the suite of evolutionary processes accumulated subsequently, particularly in areas with a history of climatic instability.

Crop-to-wild hybridization in cherries-Empirical evidence from Prunus fruticosa

Crop cultivation can lead to genetic swamping of indigenous species and thus pose a serious threat for biodiversity. The rare Eurasian tetraploid shrub Prunus fruticosa (ground cherry) is suspected of hybridizing with cultivated allochthonous tetraploid P. cerasus and autochthonous diploid P. avium. Three Prunus taxa (447 individuals of P. fruticosa, 43 of P. cerasus and 73 of P. avium) and their hybrids (198 individuals) were evaluated using analysis of absolute genome size/ploidy level and multivariate morphometrics. Flow cytometry revealed considerable differentiation in absolute genome size at the tetraploid level (average 2C of P. fruticosa = 1.30 pg, average 2C of P. cerasus = 1.42 pg, i.e., a 9.2% difference). The combination of methods used allowed us to ascertain the frequency of hybrids occurring under natural conditions in Central Europe. The morphological evaluation of leaves was based upon distance-based morphometrics supplemented by elliptic Fourier analysis. The results provided substantial evidence for ongoing hybridization (hybrids occurred in 39.5% of P. fruticosa populations). We detected homoploid introgressive hybridization with alien P. cerasus at the tetraploid level. We also found previously overlooked but frequent triploid hybrids resulting from heteroploid hybridization with indigenous P. avium, which, however, probably represent only the F1 generation. Although both hybrids differ in ploidy, they cannot be distinguished using morphometrics. Hybrids are frequent and may endanger wild populations of genuine P. fruticosa via direct niche competition or, alternatively or in addition, via introgression at the homoploid level (i.e., genetic swamping). The cultivation of cherries thus substantially threatens the existence of genuine P. fruticosa.

Recovering the Genetic Identity of an Extinct-in-the-Wild Species: The Puzzling Case of the Alagoas Curassow

The conservation of many endangered taxa relies on hybrid identification, and when hybrids become morphologically indistinguishable from the parental species, the use of molecular markers can assign individual admixture levels. Here, we present the puzzling case of the extinct in the wild Alagoas Curassow (Pauxi mitu), whose captive population descends from only three individuals. Hybridization with the Razor-billed Curassow (P. tuberosa) began more than eight generations ago, and admixture uncertainty affects the whole population. We applied an analysis framework that combined morphological diagnostic traits, Bayesian clustering analyses using 14 microsatellite loci, and mtDNA haplotypes to assess the ancestry of all individuals that were alive from 2008 to 2012. Simulated data revealed that our microsatellites could accurately assign an individual a hybrid origin until the second backcross generation, which permitted us to identify a pure group among the older, but still reproductive animals. No wild species has ever survived such a severe bottleneck, followed by hybridization, and studying the recovery capability of the selected pure Alagoas Curassow group might provide valuable insights into biological conservation theory.

Patterns of rapid diversification in heteroploid Knautia sect. Trichera (Caprifoliaceae, Dipsacoideae), one of the most intricate taxa of the European flora

BACKGROUND

Polyploidy is one of the most important evolutionary pathways in flowering plants and has significantly contributed to their diversification and radiation. Due to the prevalence of reticulate evolution spanning three ploidy levels, Knautia is considered one of the taxonomically most intricate groups in the European flora. On the basis of ITS and plastid DNA sequences as well as AFLP fingerprints obtained from 381 populations of almost all species of the genus we asked the following questions. (1) Where and when did the initial diversification in Knautia take place, and how did it proceed further? (2) Did Knautia undergo a similarly recent (Pliocene/Pleistocene) rapid radiation as other genera with similar ecology and overlapping distribution? (3) Did polyploids evolve within the previously recognised diploid groups or rather from hybridisation between groups?

RESULTS

The diversification of Knautia was centred in the Eastern Mediterranean. According to our genetic data, the genus originated in the Early Miocene and started to diversify in the Middle Miocene, whereas the onset of radiation of sect. Trichera was in central parts of the Balkan Peninsula, roughly 4 Ma. Extensive spread out of the Balkans started in the Pleistocene about 1.5 Ma. Diversification of sect. Trichera was strongly fostered by polyploidisation, which occurred independently many times. Tetraploids are observed in almost all evolutionary lineages whereas hexaploids are rarer and restricted to a few phylogenetic groups. Whether polyploids originated via autopolyploidy or allopolyploidy is unclear due to the weak genetic separation among species. In spite of the complexity of sect. Trichera, we present nine AFLP-characterised informal species groups, which coincide only partly with former traditional groups.

CONCLUSIONS

Knautia sect. Trichera is a prime example for rapid diversification, mostly taking place during Pliocene and Pleistocene. Numerous cycles of habitat fragmentation and subsequent reconnections likely promoted hybridisation and polyploidisation. Extensive haplotype sharing and unresolved phylogenetic relationships suggest that these processes occurred rapidly and extensively. Thus, the dynamic polyploid evolution, the lack of crossing barriers within ploidy levels supported by conserved floral morphology, the highly variable leaf morphology and unstable indumentum composition prevent establishing a well-founded taxonomic framework.

The impact of hybridization on long-term persistence of polyploid Dactylorhiza species

PREMISE OF THE STUDY

Hybridization may pose severe threats to the long-term survival of the parental taxa through introgression and the formation of hybrid swarms. However, when the resulting hybrids show strong male and female sterility, backcrossing and introgression are unlikely to occur, but the parental species may suffer from reduced male and female fitness.

METHODS

We assessed the impact of hybridization on the long-term persistence of two food-deceptive orchids in the genus Dactylorhiza (the common Dactylorhiza maculata and the rare D. sphagnicola). The extent of hybridization was investigated using both molecular markers and morphometric measurements. To determine the strength of postmating reproductive isolation, hand pollinations were conducted between pure and hybrid individuals. Finally, fruit set and seed viability of open-pollinated plants were determined in sympatric and allopatric populations to investigate the impact of hybridization on the reproductive output of the pure parental species.

KEY RESULTS

Our results showed that postmating reproductive isolation was weak and that hybridization occurred frequently within the studied sympatric population. Although hybrids were characterized by very low female fitness, mainly because of strongly reduced seed viability, backcrossing appeared to occur and was asymmetric toward the rare D. sphagnicola. Fruit set and seed viability of open-pollinated plants were also significantly lower in the sympatric population than in the allopatric populations, indicating that hybridization and ongoing introgression incurred fitness costs in the pure parental species.

CONCLUSIONS

Overall, our results suggest that extensive hybridization can affect the long-term viability of the parental species through the combined effect of introgression following interspecific hybrid fertilization and reduced fitness of the parental species.

Heteroploid Knautia drymeia includes K. gussonei and cannot be separated into diagnosable subspecies

PREMISE OF THE STUDY

Knautia drymeia is a morphologically variable, diploid and tetraploid temperate forest understory species distributed in southeastern Europe and adjacent areas. The species is an excellent system to explore the influence of polypoidy on taxonomic delineations, the role of hybridization among genetically distant populations in polyploid evolution, and the impact of glacial refugia on the evolution of polyploids.

METHODS

Amplified fragment length polymorphism fingerprinting and multivariate analyses of morphological characters were performed on 57 populations spanning the distribution area of K. drymeia. K-means clustering, comparison of in-silico tetraploids and observed tetraploids, and a phylogeographic analysis using relaxed random walks were used to explore the genetic structure within the diploids, to infer the origin of the tetraploids and to reconstruct range expansion through time. Further, we contrasted the morphology and genetic groups with current taxonomy and evaluated the status of the tetraploid Apennine endemic K. gussonei and the intraspecific taxa of K. drymeia.

KEY RESULTS

The genetic structure was strongly geographically correlated and yielded four genetic groups; K. gussonei was inseparable from K. drymeia. Distributions of diploid lineages are suggestive of glacial refugia in the northwesternmost and southeastern Balkan Peninsula. Polyploids originated at least two times, as autopolyploids and probably additionally also as allopolyploids. Morphological divergence corresponded with neither genetic groups nor current taxonomy.

CONCLUSIONS

Genetic and morphometric data confirmed neither divergence of K. gussonei nor recognition of subspecies within K. drymeia. We therefore propose treating K. drymeia as a morphologically and genetically variable species without infraspecific taxa.

Does hybridization with a widespread congener threaten the long-term persistence of the Eastern Alpine rare local endemic Knautia carinthiaca?

Interspecific hybridization, especially when regularly followed by backcrossing (i.e., introgressive hybridization), conveys a substantial risk for many endangered organisms. This is particularly true for narrow endemics occurring within distributional ranges of widespread congeners. An excellent example is provided by the plant genus Knautia (Caprifoliaceae): Locally endemic K. carinthiaca is reported from two isolated populations in southern Austria situated within an area predominantly occupied by widespread K. arvensis. While K. carinthiaca usually inhabits low-competition communities on rocky outcrops, K. arvensis occurs mainly in dry to mesic managed grasslands, yet both species can coexist in marginal environments and were suspected to hybridize. Flow cytometry revealed that diploid K. carinthiaca only occurs at its locus classicus, whereas the second locality is inhabited by the morphologically similar but tetraploid K. norica. In the, therefore, single population of K. carinthiaca, flow cytometry and AFLP fingerprinting showed signs of introgressive hybridization with diploid K. arvensis. Hybridization patterns were also reflected in intermediate habitat preferences and morphology of the hybrids. Environmental barriers to gene flow seem to prevent genetic erosion of K. carinthiaca individuals from the core ecological niches, restricting most introgressed individuals to peripheral habitats. Efficient conservation of K. carinthiaca will require strict protection of its habitat and ban on forest clear cuts in a buffer zone to prevent invasion of K. arvensis. We demonstrate the large potential of multidisciplinary approaches combining molecular, cytometric, and ecological tools for a reliable inventory and threat assessment of rare species.