Project description:Changes in genome structure and gene expression have been documented in both resynthesized and natural allopolyploids that contain two or more divergent genomes. The underlying mechanisms for rapid and stochastic changes in gene expression are unknown. Arabidopsis suecica is a natural allotetraploid derived from the extant species A. thaliana and A. arenosa. Here we report that reduced DNA methylation in met1-RNAi A. suecica lines altered the expression of ~200 genes encoding transposons, centromeric and heterochromatic RNAs, and predicted proteins. Reduced DNA methylation occurred frequently in promoter regions of the upregulated genes but not of the repressed genes and led to increased mobility of En/Spm-like transposons in met1-RNAi A. suecica lines. Compared to A. arenosa centromeres, A. thaliana centromeres were hypermethylated, which correlates with higher levels of small RNA accumulation in A. thaliana centromeres than that in A. arenosa centromeres. Derepression of the genes examined was primarily derived from A. thaliana subgenome, and A. arenosa genes are less affected by methylation defects. Moreover, non-CG (CC) methylation in the promoter region of A. thaliana At2g23810 was maintained in resynthesized allotetraploids, and the methylation spread within the promoter region in natural A. suecica, leading to silencing of At2g23810, which was demethylated and reactivated in met1-RNAi A. suecica lines. We suggest that a subset of A. thaliana and A. arenosa genes are differentially methylated in natural allopolyploids, and some A. thaliana genes including centromeres are subjected to transcriptional repression and genome-specific RNA-mediated DNA methylation in Arabidopsis allopolyploids. Keywords: gene expression in reduced DNA methylation lines in Arabidopsis allotetraploids
Project description:The Microrchidia (MORC) family of ATPases are required for transposable element (TE) silencing and heterochromatin condensation in plants and animals, and C. elegans MORC-1 has been shown to topologically entrap and condense DNA. In Arabidopsis thaliana, mutation of MORCs has been shown to reactivate silent methylated genes and transposons and to decondense heterochromatic chromocenters, despite only minor changes in the maintenance of DNA methylation. Here we provide the first evidence localizing Arabidopsis MORC proteins to specific regions of chromatin and find that MORC4 and MORC7 are closely co-localized with sites of RNA directed DNA methylation (RdDM). We further show that MORC7, when tethered to DNA by an artificial zinc finger, can facilitate the establishment of RdDM. Finally, we show that MORCs are required for the efficient RdDM mediated establishment of DNA methylation and silencing of a newly integrated FWA transgene, even though morc mutations have no effect on the maintenance of preexisting methylation at the endogenous FWA gene. We propose that MORCs function as a molecular tether in RdDM complexes to reinforce RdDM activity for methylation establishment. These findings have implications for MORC protein function in a variety of other eukaryotic organisms.
Project description:Polyploidy is a major force shaping eukaryote evolution but poses challenges for meiotic chromosome segregation. As a result, first-generation polyploids often suffer from more meiotic errors and lower fertility than established wild polyploid populations. How established polyploids adapt their meiotic behaviour to ensure genome stability and accurate chromosome segregation remains an active research question. We present here a cytological description of meiosis in the model allopolyploid species Arabidopsis suecica (2n = 4x = 26). In large part meiosis in A. suecica is diploid-like, with normal synaptic progression and no evidence of synaptic partner exchanges. Some abnormalities were seen at low frequency, including univalents at metaphase I, anaphase bridges and aneuploidy at metaphase II; however, we saw no evidence of crossover formation occurring between non-homologous chromosomes. The crossover number in A. suecica is similar to the combined number reported from its diploid parents Arabidopsis thaliana (2n = 2x = 10) and Arabidopsis arenosa (2n = 2x = 16), with an average of approximately 1.75 crossovers per chromosome pair. This contrasts with naturally evolved autotetraploid A. arenosa, where accurate chromosome segregation is achieved by restricting crossovers to approximately 1 per chromosome pair. Although an autotetraploid donor is hypothesized to have contributed the A. arenosa subgenome to A. suecica, A. suecica harbours diploid A. arenosa variants of key meiotic genes. These multiple lines of evidence suggest that meiosis in the recently evolved allopolyploid A. suecica is essentially diploid like, with meiotic adaptation following a very different trajectory to that described for autotetraploid A. arenosa.