For Hybrid seed production of onion which system is used?

For Hybrid seed production of onion CMS system is used.
Cytoplasmic male sterility (CMS), a condition under which a plant is unable to produce functional pollen, is widespread among higher plants.
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 which was described in the 1960s.