The Control of the Crossover Localization in Allium

Meiotic crossovers/chiasmata are not randomly distributed and strictly controlled. The mechanisms behind crossover (CO) patterning remain largely unknown. In Allium cepa, as in the vast majority of plants and animals, COs predominantly occur in the distal 2/3 of the chromosome arm, while in Allium fistulosum they are strictly localized in the proximal region. We investigated the factors that may contribute to the pattern of COs in A. cepa, A. fistulosum and their F-1 diploid (2n = 2x = 8C + 8F) and F-1 triploid (2n = 3x = 16F + 8C) hybrids. The genome structure of F-1 hybrids was confirmed using genomic in situ hybridization (GISH). The analysis of bivalents in the pollen mother cells (PMCs) of the F-1 triploid hybrid showed a significant shift in the localization of COs to the distal and interstitial regions. In F-1 diploid hybrid, the COs localization was predominantly the same as that of the A. cepa parent. We found no differences in the assembly and disassembly of ASY1 and ZYP1 in PMCs between A. cepa and A. fistulosum, while F-1 diploid hybrid showed a delay in chromosome pairing and a partial absence of synapsis in paired chromosomes. Immunolabeling of MLH1 (class I COs) and MUS81 (class II COs) proteins showed a significant difference in the class I/II CO ratio between A. fistulosum (50%:50%) and A. cepa (73%:27%). The MLH1:MUS81 ratio at the homeologous synapsis of F-1 diploid hybrid (70%:30%) was the most similar to that of the A. cepa parent. F-1 triploid hybrid at the A. fistulosum homologous synapsis showed a significant increase in MLH1:MUS81 ratio (60%:40%) compared to the A. fistulosum parent. The results suggest possible genetic control of CO localization. Other factors affecting the distribution of COs are discussed.

Automated summary

Meiotic crossovers/chiasmata are not randomly distributed and are strictly controlled. The distribution of crossovers in Allium cepa and Allium fistulosum is determined by the parental locations. The localization of crossovers in F-1 hybrids is genetically controlled, and other factors also affect crossover distribution.