Competition for Chiasma Formation Between Identical and Homologous (But Not Identical) Chromosomes in Synthetic Autotetraploids of Arabidopsis thaliana

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University of Melbourne
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Polyploid organisms provide additional opportunities to study meiosis in a more complex context since more than two potential homologous chromosomes are available. When the chromosome complement of a diploid individual is duplicated, each chromosome is accompanied by one identical and two homologous chromosomes within the same nucleus. In this situation, a competition in pairing/synapsis/chiasma formation between identical and homologous (but not necessarily identical) chromosomes can occur. Several studies have been conducted in different species to address whether there are preferences in crossover formation between identical rather than homologous chromosomes. In this study, multivalent and chiasma frequencies were cytologically analyzed in synthetic autotetraploids of Arabidopsis thaliana including the accessions Col, Ler, and the Col/Ler hybrid. Fluorescence in situ hybridization was conducted to identify each chromosome at metaphase I. The new Col and Ler tetraploids showed high multivalent frequencies, exceeding the theoretical 66.66% expected on a simple random end-pairing model, thus indicating that there are more than two autonomous synaptic sites per chromosome despite their small size. However, a significant excess of bivalent pairs was found in the Col/Ler hybrid, mainly due to the contribution of chromosomes 2 and 3. The mean chiasma frequencies of the three artificial autotetraploids were about twofold the corresponding mean cell chiasma frequencies of their diploid counterparts. The relative contribution of each chromosome to the total chiasma frequency was similar in the three genotypes, with the exception of a lower contribution of chromosome 3 in the hybrid. Preferences for chiasma formation between identical and homologous chromosomes were analyzed in Col/Ler 4x, taking advantage of the cytological differences between the accessions: variations in the size of the 45S rDNA region on the short arm of chromosome 2 and changes in the size and localization of the 5S rDNA region in chromosome 3. We observed a different behavior of chromosomes 2 and 3, i.e., random chiasma formation between identical and homologous chromosomes 2, and preferences for chiasma formation between homologous chromosomes 3. Hence, our results reveal the existence of chromosome-specific mechanisms responsible for these preferences.
La poliploidía es un fenómeno que consiste en la presencia de más de dos juegos completos de cromosomas. Si los juegos cromosómicos provienen de la misma especie se habla de autopoliploidía, mientras que la alopoliploidía implica la presencia de juegos cromosómicos de al menos dos especies. En los autotetraploides, como consecuencia de la duplicación cromosómica, cada cromosoma tiene un cromosoma idéntico y dos cromosomas homólogos. El el emparejamiento cromosómico, a través de los fenómenos específicos que acontecen en esta división (apareamiento, sinapsis y recombinación), entre cromosomas idénticos y homólogos. Concretamente, en este estudio hemos analizado citogenéticamente la frecuencia de multivalentes y de quiasmas (resultantes del proceso de recombinación meiótica) en autotetraploides sintéticos de la planta modelo Arabidopsis thaliana, en las variedades Col y Ler y también en híbridos resultantes del cruce entre estas dos variedades. Los resultados del trabajo revelan la existencia de ciertas preferencias en la asociación de cromosomas homólogos frente a idénticos, pero sólo en algunos cromosomas del complemento. Este trabajo revela la existencia de preferencias en la asociación de cromosomas homólogos frente a idénticos en la meiosis de autotetraploides sintéticos de la planta modelo Arabidopsis thaliana. Estas preferencias varían en los distintos cromosomas del complemento.