The Power of Genomic in situ Hybridization (GISH) in Interspecific Breeding of Bulb Onion (Allium cepa L.) Resistant to Downy Mildew (Peronospora destructor [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 (Allium cepa L.) that is resistant to downy mildew. Downy mildew (Peronospora destructor [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 F(1) hybrid production to the creation of S(2)BC(2) homozygous lines that are resistant to downy mildew. Using GISH, it was shown that three progeny plants of S(2)BC(2) possessed an A. roylei homozygous fragment in the distal region of the long arm of chromosomes 3 in an A. cepa 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 A. roylei fragments on chromosome 3.