The triparental triploid onion Allium × cornutum Clementi ex Visiani, 1842, possesses a sterile S-type of cytoplasm

Abstract

Triploid onion, Allium × cornutum Clementi ex Visiani, 1842 (2n = 3x = 24), a vegetatively reproduced garden crop, possess a complex triparental genome organization with three putative parental species, A. cepa L., A. pskemense B. Fedtsch., and A. roylei Stearn. Two of its most studied clones are the Croatian ‘Ljutika’ and the Indian ‘Pran’, which are genetically highly similar. Earlier studies have shown that ‘Pran’ possesses some molecular markers in the chloroplast DNA (cpDNA) identical to those of the unique male-sterile (S) cytoplasm, used for onion breeding. To find out whether ‘Ljutika’ also possesses a S-type of cytoplasm, we analyzed several cpDNA and mitochondrial (mtDNA) molecular markers. The PCR amplification and RFLP analysis of the chloroplast genes accD, atpF, petB and the mitochondrial gene cob, as well as the sequence analysis of the chloroplast matK and atpB-rbcL regions showed that ‘Ljutika’ possesses the male-sterile S-type of cytoplasm. The phylogenetic analysis of the matK and atpB-rbcL sequences of A. × cornutum, its parental species and other Allium species of the section Cepa showed that none of the analyzed species had the identical type of cpDNA as A. × cornutum. Results also suggested that A. pskemense can be excluded as a donor of the S-cytoplasm and a female parent, whereas cpDNA of A. roylei, although not identical to S-cytoplasm, possessed many polymorphisms of S-type. Fluorescent in situ hybridization, using fluorescently labelled parental genomic DNAs as probes in combination with fluorescently labelled 5S and 35S rDNAs enabled simultaneous visualization of the three genomes during meiosis and confirmed their homeologus intergenomic pairing.