Reproductive strategies of alpine apomictic plants under different ecological conditions
by Christoph Carl-Friedrich Schinkel
Date of Examination:2019-03-26
Date of issue:2019-06-06
Advisor:Prof. Dr. Elvira Hörandl
Referee:Prof. Dr. Elvira Hörandl
Referee:Prof. Dr. Stefan Scheu
Referee:Prof. Dr. Christiane Gatz
Referee:PD Dr. Franz Hadacek
Referee:Prof. Dr. Holger Kreft
Referee:Prof. Dr. Mark Maraun
Persistent Address:
http://dx.doi.org/10.53846/goediss-7494
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Abstract
English
The origin of apomixis in natural populations is still not well understood. Apomictic plants tend to expand their distribution range more to higher latitudes and altitudes compared to their sexual progenitors, a phenomenon known as Geographical Parthenogenesis. Apomixis provides an increased colonizing ability due to uniparental reproduction, as no mating partners and pollinators are needed (Baker’s Law), but whether Geographical Parthenogenesis is directly related to the reproduction itself or rather a consequence of polyploidization was so far unknown. Polyploidy is one of the most important evolutionary processes in plants and thought to infer fitness advantages (higher vigor) that would enable plants to adjust better to more extreme climatic conditions. Indeed, occurrences of apomixis are often associated with colder climates. It is hypothesized that low temperatures have a direct effect on the origin of apomixis by triggering unreduced egg cell formation. Although it is assumed that apomixis is strictly connected to polyploidy, it has been shown that apomixis can also appear in diploids. This implies the possibility for polyploidization process via partial apomixis (BIII hybrids). Polyploids usually emerge from unreduced gametes which either fuse with reduced ones, resulting in triploid offspring (triploid bridge), or with other unreduced gametes, resulting in tetraploid embryos. Both pathways are largely unexplored regarding their frequencies and male versus female gamete contributions. In this thesis I focused on the complex causalities of Geographical Parthenogenesis in the al- pine diploid-autotetraploid system Ranunculus kuepferi among natural populations in the Alps. I conducted studies on the variation and distribution of reproductive modes, environ- mental influences and the genetic/epigenetic background, trying to disentangle some of the various syndetic issues in connection with polyploidy and apomixis, so that the presented outcome may contribute to a broader understanding of the dynamics and reasons behind this enigmatic phenomenon of evolution. Results showed that apomixis emerges spontaneously among random diploid individuals of R. kuepferi in geographically isolated populations. Although frequencies of apomixis in dip- loids are proportionally low, unreduced female gamete formation can lead to polyploidization if subsequent fertilization takes place. Resulting BIII hybrids were discovered in surprisingly large numbers and consisted of mostly triploid individuals. Absence of both, triploids arising from unreduced pollen and obligate sexual tetraploids, suggest a female triploid bridge as major pathway of polyploidization in R. kuepferi. Only one seed originated from biparental polyploidization. But sexual neopolyploids that develop in surrounding diploid populations are prone to fertilization with haploid pollen that would lower reproductive fit- ness due to endosperm imbalances and cause Minority Cytotype Exclusion. Coupling of un- reduced egg cell formation (apomeiosis) to parthenogenesis result in fully functional apomixis, which reliefs disadvantageous influence of putative F1 sterility and aids the establishment of polyploid complexes. Autopolyploidy resulting from whole genome duplication probably helped to establish respective complexes in colder climates, due to an increased physiological tolerance by adaptive features like dwarf growth. This would also explain significant ecological niche shifts towards lower temperatures among tetraploid populations. Additional low genetic divergence between di- and tetraploids indicate the involvement of epigenetic control mechanisms, which are known to play a crucial role in the response of plants to environmental conditions. Cytosine-methylation is heritable and supposedly has an important transgenerational effect on both spontaneous shifts in the mode of reproduction among same genotypes, as well as for rapid adaptations to environmental conditions. Apomixis may supported polyploid establishment and extension of occurrences to higher altitudes. Subsequent changes in the methylation pattern caused by environmental cold shocks then led to a separation between facultative and obligate tetraploid apomicts of R. kuepferi.
Keywords: R. kuepferi; Polyploidy; Apomixis; Geographical Parthenogenesis; 2n Gametes; Triploid Bridge; MS-AFLP; Flow Cytometry; FCSS