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 [Formula: see text] diploid (2n = 2x = 8C + 8F) and F [Formula: see text] triploid (2n = 3x = 16F + 8C) hybrids. The genome structure of F [Formula: see text] hybrids was confirmed using genomic in situ hybridization (GISH). The analysis of bivalents in the pollen mother cells (PMCs) of the F [Formula: see text] triploid hybrid showed a significant shift in the localization of COs to the distal and interstitial regions. In F [Formula: see text] 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 [Formula: see text] 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 [Formula: see text] diploid hybrid (70%:30%) was the most similar to that of the A. cepa parent. F [Formula: see text] 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.