Interspecific crossing is a promising approach for introgression of valuable traits to develop cultivars with improved characteristics. Allium fistulosum L. possesses numerous pest resistances that are lacking in the bulb onion (Allium cepa L.), including resistance to Stemphylium leaf blight (SLB). Advanced generations were produced by selfing and backcrossing to bulb onions of interspecific hybrids between A. cepa and A. fistulosum that showed resistance to SLB. Molecular classification of the cytoplasm established that all generations possessed normal (N) male−fertile cytoplasm of bulb onions. Genomic in situ hybridization (GISH) was used to study the chromosomal composition of the advanced generations and showed that most plants were allotetraploids possessing the complete diploid sets of both parental species. Because artificial doubling of chromosomes of the interspecific hybrids was not used, spontaneous polyploidization likely resulted from restitution gametes or somatic doubling. Recombinant chromosomes between A. cepa and A. fistulosum were identified, revealing that introgression of disease resistances to bulb onion should be possible.

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.

Meiotic crossovers/chiasmata are not randomly distributed and strictly controlled. The mechanisms behind COs patterning remain largely unknown. In many species COs predominantly occur in the distal 2/3 of the chromosome arm and suppression of COs is observed in the proximal regions. The exceptions are some species, among which Allium fistulosum has strictly localized COs in the proximal region. The ability to manipulate the COs localization can be useful for onion breeding. We investigated the factors that may contribute to the pattern of COs in two closely related onion species A. cepa and A. fistulosum, which differ significantly in the localization of chiasmata, and their F1 diploid and triploid hybrids in pollen mother cells. We demonstrate a significant shift in the COs localization to the distal and interstitial region in F1 triploid hybrid, which has a complete diploid set of A. fistulosum chromosomes and haploid set of A. cepa chromosomes. This observation points to a possible genetic control of COs distribution. We did not find the differences in the assembly and disassembly of ASY1 and ZYP1 between A. cepa and A. fistulosum while the difference between parental species and their hybrids was observed. In diploid F1 hybrids at pachytene the chromosome pairing delay marked by ASY1 was revealed. Immunolocalization of MLH1, a marker for class I of the interference-dependent COs, and MUS81, a marker for class II of the interference-free COs, and mlh1/mus81 gene expression profiling in different stages of meiosis showed a spatiotemporal asymmetry among A. cepa, A. fistulosum and their diploid and triploid F1 hybrids. Our results support the hypothesis of genetic control of CO distribution as one of the players that affect meiotic recombination and exchange of genomic material.