Genome dosage and parent of origin effects in a ploidy series of Arabidopsis thaliana
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Polyploidy, the presence of more than two sets of chromosomes in an organism, plays major roles in plant evolution, development and function. Understanding the effects of polyploidization at the phenotypic and molecular levels has implications for both fundamental (evolutionary) and applied (crop science) perspectives. While the effects of hybridization and polyploidy in allopolyploid plants have been studied by the plant scientific community, the effects of polyploidy per se are still poorly understood. In this thesis, I have used genetically identical (isogenic) and hybrid Arabidopsis thaliana plants to study the effects of polyploidy in four different classes of plant: diploids, maternal excess triploids (containing two maternally-derived genome copies and one paternally-derived genome), paternal excess triploids (containing two paternally-derived genome copies and one maternally-derived genome), and tetraploids. The results show that despite being isogenic, these ploidy classes are strikingly different. Paternal excess triploids show greater growth rate and abiotic stress tolerance, together with transcriptomic differences, not only when compared with their diploid and tetraploid parents but also when compared to its genetically-identical maternal excess counterpart. Using this system, I demonstrate that heterosis (the ability of the offspring to display increased trait values compared with its parents) does not require genetic variation, but can instead be achieved epigenetically through unbalanced ploidy crosses. However, a genome-wide methylation analysis showed that CmCGG methylation is unlikely to be involved with these parent-of-origin effects. While investigating allele specific expression between reciprocal triploids, I also identified extensive imprinting-like effects in the triploids. Finally, using Arabidopsis mutants, I discovered that DNA demethylases are required for the triploid block (the post-fertilization abortion phenotype of diploid X tetraploid crosses) to occur. Altogether, this thesis demonstrates how cross direction and plant ploidy interact to produce phenotypic change in F1 offspring in the absence of genetic variation.
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