Origin and maintenance of intrapopulation cytotype mixture

Polyploidy, i.e. the possession of more than two chromosome complements, is ubiquitous in angiosperms. Cytotypes of polyploid species may co-exist in close proximity resulting in cytotype mixture. The coexistence may either be of temporary nature, one cytotype eventually out-competing the other, or it may be permanent due to reproductive isolation of cytotypes, potentially leading to speciation. Hybrid zones consisting of polyploids and their lower-ploid progenitors enable us to identify and examine mechanisms responsible for the origin and maintenance of polyploids. Dependent on their mode of origin, primary and secondary hybrid zones can be differentiated. Primary hybrid zones form as direct consequence of the emergence of a polyploid within a diploid population, while secondary hybrid zones occur, when cytotypes regain contact after phases of allopatry, e.g., in different Pleistocene refugia. Persistent coexistence of polyploids with their diploid progenitors can be explained by a deviation from random mating due to, for instance, ecological sorting along some environmental gradient or incompatibilities between cytotypes and selection against inter- cytotype hybrids, which might lead to reproductive isolation. The proposed study system for investigating different aspects of cytotype mixture and its evolutionary consequences is the European high mountain plant Senecio carniolicus (Asteraceae). A recent study revealed three main cytotypes (diploid, tetraploid and hexaploid cytotypes) within the distribution area of S. carniolicus in the Eastern Alps, but also frequent cytotype-mixture within populations. Employing a wide array of interacting methodological approaches, the proposed research will pursue two complementary aims. First, origin and evolutionary relationships among different cytotypes will be investigated in both space and time using a phylogenetic and phylogeographic approach based on DNA sequence and AFLP fingerprint data, supplemented by molecular cytogenetic and genome size data. The following aspects will be addressed: (1) Origin of polyploids and cytotype mixture, (2) phylogeographic patterns within and across cytotypes, (3) chromosomal re- organisation accompanying polyploidisation. Second, mechanisms for maintenance of the cytotype mixture will be explored with respect to the potential role of several pre- and postzygotic isolation mechanisms. Microsite analysis (analysis of surrounding vegetation), phenological observations with reciprocal transplantations, cross-pollination and germination experiments will be employed to address the following aspects: (4) habitat segregation and ecological displacement, (5) flowering time differences and flowering time displacement, and (6) postzygotic isolation mechanisms.

Polyploidy, i.e., the multiplication of chromosome sets is recognized as a major force in plant speciation, but the factors and their interactions underlying the origin and maintenance of cytotype co-occurrence (cytotype mixture) remain insufficiently understood. Here, we address those questions in a complex system of cytotype mixture, the eastern Alpine Senecio carniolicus (Compositae) that comprises up to three co-occurring cytotypes (diploids, tetraploids, hexaploids). Applying a multitude of different methods (DNA sequencing, AFLP fingerprints, habitat analysis, transplantation and crossing experiments), the phylogenetic relationships among those cytotypes were resolved and isolating factors allowing the co-occurrence of cytotypes and their relevance for each cytotype were identified. Specifically, the major results are: (1) Diploids comprise two longitudinally separated lineages with the polyploids being autopolyploid derivatives of the eastern lineage. As these lineages are also morphologically differentiated, they warrant taxonomic recognition emphasizing the commonly underestimated relevance of autopolyploidy for lineage diversification and speciation. (2) Cytotypes show different, yet overlapping habitat requirements in line with the current understanding of ecological differentiation as one of the main factors fostering cytotype establishment and allowing cytotype co-occurrence. Furthermore, habitat displacement, i.e., the stronger differentiation of ecologically similar species in sympatry than in allopatry, reinforces ecological differentiation, thus widening the conditions for cytotype co-occurrence. (3) Post-pollination reproductive isolation is decoupled from habitat differentiation, as only crosses involving diploids show strongly reduced seed set and seedling survival, whereas crosses between tetraploids and hexaploids produce ample hybrid offspring. Differential dynamics at contact zones of different cytotypes as well as putatively higher adaptational potential of polyploids may lead to differing range dynamics caused by global change.