Investigation Of Meiotic Homologous Recombination In Arabidopsis Thaliana And Oryza Sativa

Homologous recombination, including crossover and gene conversion, is indispensable in evolution. It’s an important way for genome variation. Currently, many studies have described the homologous recombination in different species, such as mouse, human, drosophila, yeast and Arabidopsis. However, these studies either covered only a part of the whole genome, or only had limited markers for the whole genome and mostly resulted in a low value. Also, it is hard to evaluate the gene conversion events accurately because the small size of these events makes them impossible to be detected.In this study, we use next-generation sequencing to investigate the crossover and gene conversion events after meiosis in Arabidopsis thaliana and Oryza Sativa, respectively. In A. thaliana, we sequenced 72 F2 individuals. Using 373,614 markers, we discovered an average of 7.65 crossover events per individual per meiosis. The recombination rate is 3.21cM/Mb, lower than in yeast but much higher than in the mammal. In the Arabidopsis genome, four cold spot regions for crossover were detected, mostly overlapping with the centromere region, while no hotspot regions were found. Furthermore, we found 6 gene conversion events in 2 Arabidopsis individuals with high-depth sequences, average 3 events per individual per meiosis. And all the gene conversion events only covered 1 marker, which indicated that the gene conversion events stepped over only a small region. All the results showed that gene conversion contributed less than crossover to the homologous recombination.We also sequenced 38 rice F2 individuals,24 of which were planted in normal growing condition, while the other 14 were treated in uninterrupted abiotic or biotic stress during the period of meiosis. Using 871,863 markers, we discovered an average of 33.8 crossover events per rice individual per meiosis (4.35cM/Mb). The normal individuals had no observably difference with the stressed ones. However, we found 4 stressed individuals had significantly more crossover events than the others by chi-test. This result illustrated that the stressed conditions could really influence the homologous recombination, but not all individuals were clearly affected. Similarly, we found 13 cold spot regions of the crossover events in the rice genome, most of which were centromere regions. Rice genome had 27 hotspot regions of crossover, which had not been found in the Arabidopsis genome. Furthermore, we found 2.47 gene conversion events per individual per meiosis in the rice genome. Both rice and Arabidopsis had few gene conversion events, which may be the feature of the plant genome.Moreover, we found the homologous recombination had a close relationship with the functional genes in the rice genome. The regions with high recombination rates always contain many stress response genes. And the density of the resistance genes had a positive correlation with the recombination rate. These results declared that the homologous recombination indeed had an important influence on the evolution of rice functional genes, especially the stress response genes.