The Power of Genomic situ Hybridization (GISH) in Interspecific Breeding of Bulb Onion ( L.) Resistant to Downy Mildew ( [Berk.] Casp.).

We exploited the advantages of genomic in situ hybridization (GISH) to monitor the introgression process at the chromosome level using a simple and robust molecular marker in the interspecific breeding of bulb onion ( L.) that is resistant to downy mildew. Downy mildew ( [Berk.] Casp.) is the most destructive fungal disease for bulb onions. With the application of genomic in situ hybridization (GISH) and previously developed DMR1 marker, homozygous introgression lines that are resistant to downy mildew were successfully produced in a rather short breeding time. Considering that the bulb onion is a biennial plant, it took seven years from the F₁ hybrid production to the creation of S₂BC homozygous lines that are resistant to downy mildew. Using GISH, it was shown that three progeny plants of S₂BC₂ possessed an homozygous fragment in the distal region of the long arm of chromosomes 3 in an genetic background. Previously, it was hypothesized that a lethal gene(s) was linked to the downy mildew resistance gene. With the molecular cytogenetic approach, we physically mapped more precisely the lethal gene(s) using the homozygous introgression lines that differed in the size of the fragments on chromosome 3.