Molecular Marker Characterization of Commercially Used Cytoplasmic Male Sterilities in Onion

Hybrid-onion (Allium cepa) seed is produced using systems of cytoplasmic male sterility (CMS) and two different CMS systems have been genetically characterized. S cytoplasm was the first source of onion CMS identified in the 1920s, followed by T cytoplasm that was described in the 1960s. Numerous studies have documented polymorphisms in the organellar DNAs differentiating S and T cytoplasms from the normal male-fertile cytoplasm of onion. There may be additional source(s) of onion CMS that have been described as "T-like" and appear to be more similar to N and T cytoplasms than S cytoplasm. In this study, onion breeding lines from commercial entities were evaluated for molecular markers distinguishing sources of onion CMS. Our results reveal that bona fide T cytoplasm is rarely used commercially to produce hybrid-onion seed, and both S cytoplasm and "T-like" cytoplasm are widely used. We propose that this "T-like" cytoplasm be labeled as "R" cytoplasm because it may have originated from population(s) of 'Rijnsburger' onion in the Netherlands. The results of this study also help to clarify inconsistent reports regarding nuclear male-fertility restoration for different sources of onion CMS.