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Genetic Relationships and Interspecific Hybridisations in the Genus Helleborus and Characterisation of the Causal Agent of Hellebore Leaf Spot Disease

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Genetic Relationships and Interspecific Hybridisations in the Genus Helleborus and Characterisation of the Causal Agent of Hellebore Leaf Spot Disease Genetic relationships and interspecific hybridisations in the genus Helleborus and characterisation of the causal agent of hellebore leaf spot disease von der Naturwissenschaftlichen Fakultät der Gottfried Wilhelm Leibniz Universität Hannover zur Erlangung des Grades Doktorin der Naturwissenschaften Dr. rer. nat. genehmigte Dissertation von M. Sc. Julia Meiners geboren am 20.03.1980 in Nienburg/Weser 2011 Referentin: Prof. Dr. Traud Winkelmann Korreferent: Prof. Dr. Thomas Debener Tag der Promotion: 18.02.2011 Abstract Abstract The genus Helleborus comprises 22 species, which are allocated to six Helleborus sections. Helleborus species are distributed in different parts of Europe and East Asia. They show differences with regard to leaf and flower morphology, especially flower colour, and in susceptibility to hellebore leaf spot disease (Coniothyrium hellebori). Breeding programs aiming at these traits require the inclusion of a broader spectrum of Helleborus species in addition to the most popular species H. niger (Christmas Rose) and H. x hybridus (Lenten Rose). As a prerequisite for interspecific hybridisations, the Helleborus plant material was characterised cytologically, via flow cytometry and DNA fingerprinting. Cytological analyses revealed the same chromosome number of 2n=32 for all analysed Helleborus species. Nuclear DNA contents of Helleborus species were estimated via flow cytometry and varied from 18.3 pg DNA/2C to 33.2 pg DNA/2C. Based on 1109 genome-wide distributed AFLP markers, genetic distances between species were calculated and a dendrogram was constructed to visualise genetic relationships. The phenogram reflected the taxonomic sub- division of the Helleborus genus into sections. As a next step, crossing barriers between Helleborus species were localised as predominantly postzygotic. Therefore, embryo rescue techniques via ovule culture were established to overcome these barriers. Ovules were isolated from the maternal plants five to seven weeks after pollination and then cultured in vitro. Overall, 217 hybrid offspring were successfully obtained, whereof 14 were derived from parental species belonging to different Helleborus sections. Thereby, larger genetic distances in hybrids between parental species belonging to different sections than in hybrids between species within the same section were overcome. In addition, the causal agent of the hellebore leaf spot disease was studied. A collection of 25 C. hellebori isolates was established from infected leaf material of different host species from various geographical locations. Their morphological characterisation by mycelial growth at different temperatures, conidial size and the induction of pycnidia revealed only marginal differences. All isolates were confirmed as causal agent of the disease by inoculation of H. niger plants. Using a molecular genetic approach based on RAPD markers, the genetic relationships were displayed in a phenogram, in which two C. hellebori groups were identified. A possible correlation of the isolates in these groups with the original Helleborus host species and morphological characteristics was found. This result indicates that more than one species may be associated with the disease. Keywords: Coniothyrium hellebori, embryo rescue, genetic diversity, Helleborus I Zusammenfassung Zusammenfassung Die Gattung Helleborus umfasst 22 Arten, die sechs Sektionen zugeordnet sind. Helleborus Arten sind natürlicherweise sowohl in verschiedenen Teilen Europas als auch in Ostasien verbreitet und unterscheiden sich hinsichtlich Blatt- und Blütenmorphologie, insbesondere Blütenfarbe, und weisen ein unterschiedliches Resistenzverhalten in Bezug auf den Erreger der Schwarzfleckenkrankheit (Coniothyrium hellebori) auf. Im Hinblick auf diese Merkmale ist die züchterische Weiterentwicklung der bekannten Helleborus Arten wie Helleborus niger (Christrose) und H. x hybridus (Lenzrose) nur durch Einbezug weiterer Arten möglich. Als Grundlage für die Durchführung von Artkreuzungen wurden verschiedene Helleborus Arten cytologisch, durchflusscytometrisch und molekulargenetisch mittels einer PCR- basierten DNA Fingerprinting-Methode charakterisiert. Für alle untersuchten Arten konnte eine gemeinsame Chromosomenzahl von 2n=32 ermittelt werden. Die DNA-Gehalte des Kerngenoms variierten zwischen den Arten von 18.3 pg DNA/2C bis 33.2 pg DNA/2C. Basierend auf 1109 genomweit verteilten AFLP Markern wurden genetische Distanzen ermittelt, und es wurde ein Dendrogramm erstellt, worin die Cluster der Helleborus Arten die Einteilung der Gattung in sechs Sektionen widerspiegeln. Für die Durchführung von Artkreuzungen wurden mittels blütenbiologischer Untersuchungen Kreuzungsbarrieren zwischen den Helleborus Arten als vorwiegend postzygotisch identifiziert. Aus diesem Grund wurde ein Embryo Rescue Verfahren entwickelt, bei dem Samenanlagen fünf bis sieben Wochen nach einer Bestäubung von der Mutterpflanze isoliert und in vitro kultiviert wurden. Damit wurden insgesamt 217 interspezifische Hybriden gewonnen, von denen 14 aus Kreuzungen zwischen Arten stammen, die unterschiedlichen Helleborus Sektionen zugeordnet sind. Dabei wurden größere genetische Distanzen zwischen den elterlichen Arten überwunden als bei Hybriden zwischen Arten der gleichen Sektion. Neben der Pflanze Helleborus wurde der Fokus auf den Erreger der Schwarzfleckenkrankheit gelegt. Anhand einer Sammlung von 25 C. hellebori Isolaten deckten morphologische Vergleiche der Sporengröße, der Induktion von Pyknidien und des radialen Mycelwachstums nur marginale Unterschiede zwischen den Isolaten auf. Im Rahmen eines Pathogenitätstests an H. niger Pflanzen wurden die Isolate als Erreger der Krankheit identifiziert. Weiterhin wurden die C. hellebori Isolate molekulargenetisch mittels RAPD Markern untersucht, wobei die Isolate in zwei Cluster aufspalteten. Dabei war ein Zusammenhang mit der Helleborus Art der Wirtspflanze und morphologischen Parametern zu erkennen. Möglicherweise handelt es sich bei den zwei Gruppen um zwei Arten oder Unterarten des Erregers. Schlagwörter: Coniothyrium hellebori, Embryo Rescue, genetische Diversität, Helleborus II Publications Publications derived from this thesis Journal research papers (peer-reviewed) Meiners, J., Debener, T., Schweizer, G., Winkelmann, T. (2011). Analysis of the taxonomic subdivision within the genus Helleborus by nuclear DNA content and genome- wide DNA markers. Scientia Horticulturae 128: 38–47 Meiners, J., Winkelmann, T. (2011). Morphological and genetic analyses of hellebore leaf spot disease isolates (Coniothyrium hellebori) from different geographic origins. Journal of Phytopathology, submitted Miscellaneous research papers Meiners, J., Winkelmann, T. (2010). Ovule culture of Helleborus species. Acta Horticulturae 855: 195–200 Meiners, J., Debener, T., Schweizer, G., Winkelmann, T. (2010). Nuclear DNA content and genetic relationships based on AFLP data in the genus Helleborus. Acta Horticulturae, submitted Conference contributions Talks Meiners, J., Winkelmann, T. (2010). „Embryo Rescue“ bei Helleborus Arten. 46. Gartenbauwissenschaftliche Tagung, BHGL-Schriftenreihe Band 27, Stuttgart-Hohenheim, Germany Meiners, J., Winkelmann, T. (2010). Charakterisierung des Erregers der Schwarzfleckenkrankheit mit dem Ziel der Resistenzzüchtung bei Helleborus. Tagung der GPZ-AG 18 (Zierpflanzen), Kornwestheim, Germany Meiners, J., Oenings, P., Winkelmann, T. (2010). Entwicklung resistenter, homogener und ertragreicher Sorten von Helleborus spec.. BLE Innovationstage 2010, Berlin, Germany Posters Meiners, J., Gerlach, W., Winkelmann, T. (2009). Morphologische und molekulargenetische Charakterisierung verschiedener Herkünfte von Coniothyrium hellebori. 45. Gartenbauwissenschaftliche Tagung, BHGL-Schriftenreihe Band 26, Berlin, Germany Meiners, J., Winkelmann, T. (2009). Ovule culture of Helleborus species. 23rd Eucarpia Symposium-Section Ornamentals, Leiden, The Netherlands Meiners, J., Debener, T., Schweizer, G., Winkelmann, T. (2010). Genetic relationships based on AFLP data in the genus Helleborus. GPZ Haupttagung 2010, Freising, Germany Meiners, J., Debener, T., Schweizer, G., Winkelmann, T. (2010). Nuclear DNA content and genetic relationships based on AFLP data in the genus Helleborus. 28th International Horticultural Congress Lisboa 2010, Lisbon, Portugal III Contents Contents Abstract ...................................................................................................................................... I Zusammenfassung ................................................................................................................... II Publications derived from this thesis ................................................................................... III Contents .................................................................................................................................. IV Figures ................................................................................................................................... VII Tables ........................................................................................................................................ X Abbreviations ....................................................................................................................... XII 1 General foreword ................................................................................................................. 1 1.1 The genus Helleborus ...................................................................................................
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