Cytotype Associations, Ecological Divergence and Genetic Variation in the Apomictic Complex Paspalum intermedium Munro Ex Morong (Poaceae)

Abstract

Polyploidization (whole genome duplication – WGD) is a recurrent process in plants and provides greater potential for diversification. Neopolyploids in natural populations should go under substantial structural changes in their genetics, reproductive mode (e.g. apomixis – asexual reproduction via seeds), and ecological preferences to ensure their successful establishment. Apomixis in plants provides reproductive assurance, and superior colonizing abilities respect to sexuals, but it also constrains genetic variation and clonal plants are expected to have restricted adaptive capabilities. These complex rearrangement processes and adaptations in polyploid complexes are well reflected by their genetic variation. However, there is a lack of non-model systems that exhibit successful changes with pronounced reflection for studies. Paspalum intermedium is a grass species with diverging genetic systems (diploidy vs. autopolyploidy, allogamy vs. autogamy and sexuality vs. apomixis) with substantial ecological differentiation between cytotypes occurring in allopatry, sympatry and parapatry, hence provides an ideal platform to study polyploidization, apomixis and their ecological and genetic importance in plant evolution. Therefore, in this thesis, I used P. intermedium as a model system to recognize the causality of biogeographic patterns, adaptation and ecological flexibility of cytotypes, to study variations in the expression of sexuality and apomixis, to analyze developmental competition between reproductive modes, and their effects on reproductive fitness, and to study genetic variation and its significance in polyploid complexes. I used chromosome counts, flow cytometry, and embryological analyses to characterize within-species genetic systems diversity. Environmental niche modelling was performed to evaluate intraspecific ecological attributes and to assess correlations among ploidy, and ecological conditions ruling species’ population dynamics, range expansion, adaptation and evolutionary history. Proportions of sexuality and apomixis in situ were analyzed against local climatic conditions to study the influence of environmental factors on reproductive modes. Total seed set and germinability analyses were used to estimate the reproductive fitness. Analysis of genetic markers AFLPs was used to assess the genetic variation between and within cytotypes and within and among populations. To get insights into the genetic structure variation depending on the reproductive mode and how it explains the niche variation between cytotypes, the results were compared with the geographical distribution patterns and different ecological preferences of the cytotypes. My results show that the two dominant cytotypes of P. intermedium are non-randomly distributed along local and regional geographical scales and displayed niche differentiation. Polyploidy and contrasting reproductive traits between cytotypes have promoted shifts in niche optima, and increased ecological tolerance and niche divergence. Ecologically specialized diploids maintain cytotype stability in core areas by displacing tetraploids, while broader ecological preferences and a shift from sexuality to apomixis favored polyploid colonization in peripheral areas promoting range expansion. The expression of sex and apomixis in tetraploid populations shows high variation both within and among populations. Even though ovule and seed analyses show apomictic development has higher competitive ability, fitness of apomictic individuals is depleted compared to sexual individuals and populations, indicating asexuality suffering higher seed abortion. Environmental modulation of reproduction was evident at population level where sex increased with higher mean diurnal range (MDR) while apomixis decreased. Thus, a Tug of War situation was identified between factors intrinsic to apomixis and environmental stressors promoting sex, suggesting a crucial role of local ecological conditions in sexual expression and adaptation of apomictic populations. Population structure analyses show that apomictic autotetraploids are of multiple independent origin. Although diploids show higher genetic variation, within and among population genetic variation equally make up the observed variation in all cytotypes. All individuals fall into three genetic clusters with substantial genetic admixture, and geographical distribution of genetic variation is in accordance with niche differentiation. The contact zone of the two cytotypes is primary in origin where tetraploids may frequently occur in mix ploidy populations. Polyploidization in P. interemedium is a recurring phenomenon and the newly arisen polyploids successfully establish themselves by acquiring enough genetic variation that allows them to adapt to new environments. Genetic variation analysis points to a slight deviation from the known General Purpose Ghenotype and the Frozen Niche Variation concepts as there is neither a common genotype nor are the diploids occupying a part of diploid sexuals’ niche.