Control of meiotic recombination during the diploidization of autopolyploids in arabidopsis
2021
Polyploids are organisms whose genomes consist of more than two sets of chromosomes. Following polyploidy, one of the most important drawbacks is the impact of chromosome doubling on meiosis. Strategies to control pairing preferences must be developed, otherwise, the coexistence of more than two copies of the same (autopolyploids) or similar (allopolyploids) chromosome sets may lead to multivalent formation and subsequent fertility problems. The process by which polyploids return to a diploid-like state is known as cytological diploidization.
The genus Arabidopsis constitutes a great model system for the analysis of both long- and short-term consequences of polyploidy. In this thesis, several approaches have been conducted to shed light on the meiotic process in autopolyploids of Arabidopsis thaliana.
Firstly, the study was focused on the cytogenetic analysis of meiosis in several synthetic, established, and natural autotetraploids. The results revealed that different genetic backgrounds show different responses to polyploidy in terms of both chiasma formation and chromosome associations at metaphase I. However, it was not possible to establish a clear correlation between chiasma and bivalent frequencies. It is worth to mention that the shortest chromosomes displayed higher bivalent rates, suggesting different diploidization speeds depending on chromosome size and/or morphology. In addition, gene expression analyses revealed that the genetic background, rather than genome duplication, might have a prevalent influence in determining transcription levels of genes involved in meiotic recombination.
Secondly, we analyzed the impact of intraspecific differences in the cytological diploidization during meiosis. The characterization of a synthetic autotetraploid hybrid (obtained after crossing the accessions Col and Ler) revealed that heterozygosity could play an important role during diploidization, since the hybrid showed a higher bivalent frequency than the synthetic autotetraploids of the parental accessions. Furthermore, the assessment of preferences in chiasma formation between homologous or identical chromosomes highlighted that different chromosomes present dissimilar association preferences.
Thirdly, the consequences of polyploidy in mutants defective in meiotic recombination have been analyzed. The analyses revealed that mutants defective for several ZMM proteins, essential for the formation of most chiasmata in Arabidopsis, have similar mean cell chiasma frequencies at the diploid level, whereas they can show differences at the tetraploid level. Interestingly, the autotetraploid fancm (defective for a protein with an antirecombinase function) presented an unexpected behavior. This mutant displayed an increased chiasma frequency at the diploid level, whereas in a tetraploid situation, it showed lower levels of quadrivalents, a significant decrease in the mean number of chiasmata per cell, and a significant increase in the frequency of trivalents and univalents compared to those of the control. In contrast, we found that in the absence of either the axis-related protein ASY1 or CAND1 (essential for ubiquitination), the duplication of the genome increases the capacity to form chiasmata. Besides, the analysis of class I (interference-sensitive)crossovers revealed that polyploidy produces alterations in the proportion of these crossovers with respect to interference-insensitive crossovers.
In summary, in this thesis, we shed some light on the understanding about the changes in meiotic recombination that emerge as a consequence of chromosome doubling. The results obtained show that diploidization is an extremely complex process in which the genetic background and the degree of heterozygosity have a great influence. In addition, we demonstrate that polyploidy can be a very useful tool to understand the functionality of proteins involved in meiotic recombination.
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