Diversity among male-sterility-inducing and male-fertile cytoplasms of onion

Hybrid-onion (Allium cepa) seed is produced using systems of cytoplasmic-genic male sterility (CMS). Two different sources of CMS (S and T cytoplasms) have been genetically characterized. Testcrosses of N-cytoplasmic maintaining and restoring genotypes to S and T cytoplasmic lines demonstrated that different alleles, or loci, restore male fertility for these two male-sterile cytoplasms. Other sources of CMS have been used or reported in Europe, Japan and India, and their relationships to S and T cytoplasms are not clear. Restriction fragment length polymorphisms were identified in the organellar genomes among commercially used male-sterile cytoplasms from Holland, Japan and India, and were compared to S and T cytoplasms. Mitochondrial DNA diversity among 58 non-S-cytoplasmic open-pollinated onion populations was also assessed. All five putative CMS fines selected from the Indian population Nasik White Globe were identical to S cytoplasm for all polymorphisms in the chloroplast genome, and always possessed the same-sized mitochondrial fragments as S cytoplasm. T cytoplasm, the male-sterile cytoplasm used to produce the Dutch hybrid Hygro F-1, and two sources of CMS from Japan, were similar and showed numbers of mitochondrial polymorphisms similar to those observed among the 58 non-S-cytoplasmic open-pollinated populations. This research demonstrates that the same, or very similar, male-sterile cytoplasms have been independently isolated and exploited for hybrid-seed production in onion.