The Evolutionary Establishment of Apomixis in Hybrids of the Ranunculus auricomus Complex: Developmental and Cytogenetic Studies

by Birthe Hilkka Barke
Cumulative thesis
Date of Examination:2019-06-19
Date of issue:2020-06-16
Advisor:Prof. Dr. Elvira Hörandl
Referee:Prof. Dr. Sigrid Hoyer-Fender
Referee:Prof. Dr. Christiane Gatz
Referee:Prof. Dr. Gregor Bucher
Referee:Prof. Dr. Bleidorn Christoph
Referee:Dr. Thomas Pd Teichmann
Persistent Address: http://dx.doi.org/10.53846/goediss-8040

 

 

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Abstract

English

In plant evolution, polyploidization and hybridization are important forces that both effectively contribute to developmental alterations. Asexual seed formation, known as apomixis, is mainly found in polyploid and/ or hybrid species but how these factors trigger the functional activation of apomictic reproduction remains elusive. The Ranunculus auricomus complex is worldwide distributed and consists almost exclusively of polyploid apomictic and a very small number of diploid and tetraploid sexual species. Due to the presence of sexual and apomictic species, this complex is an adequate model system to study the establishment and evolution of apomixis in natural plant populations. In this study, synthetically derived young diploid and polyploid Ranunculus hybrids, including their parental species were used for detailed microscopic investigations of their flow of male and female gametophyte development. In both, micro- and megasporogenesis multiple abnormalities were discovered. Female development in diploid F2 hybrids showed significantly enhanced frequencies of aposporous initial cell formation, which is associated with strong genomic dosage effects. These effects are considered to be the consequence of both F1 parent plants being aposporous. Analyses of seed formation revealed beside sexually formed seeds, several intermediate BIII seeds as well as a few completely apomictic ones. In agree with this, male development showed severe irregularities in meiotic cell division and in sporogenesis. Especially, allopolyploid plants performed significantly more abnormal than homoploids. The error frequency in diploid F2 Ranunculus hybrids was significantly higher as in the F1 or in the parental generation. Furthermore, meiosis in female plant organs was significantly more prone to severe alterations than in male. All these results indicate that hybridization, rather than polyploidization, is the apomixis-triggering factor in synthetic Ranunculus plants. In addition, the disturbed course of meiosis in micro- and megasporogenesis caused major implications in the female development, while on the male side only minor consequences were observed. These observations indicate strong, selective pressure acting on female development, whereas male gametophyte formation appears to be more or less unaffected. The switch to asexual seed formation in young, diploid Ranunculus hybrids is a rescue from imminent hybrid sterility, which is assumed to be caused by disturbed female meiosis.
Keywords: Ranunculus auricomus complex; Developmental Biology; Gametophytic Apomixis; Polyploidy; Hybrid
 
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