ARS | Publication request: The Dominant Ms Allele in Onion Shows Reduced Penetrance

Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 10, 2009
Publication Date: February 12, 2010
Citation: Melgar, S., De Biologia, E., Havey, M.J. 2010. The Dominant Ms Allele in Onion Shows Reduced Penetrance. Journal of the American Society for Horticultural Science. 135:49-52.

Interpretive Summary: Cytoplasmic male sterility (CMS) is used widely to produce hybrid-onion seed. CMS in onion is controlled by the interaction of the cytoplasm [male-sterile (S) or normal (N) male-fertile] and one nuclear male-fertility-restoration locus (Ms). Scoring of genotypes at Ms requires a minimum of 4 years and is complicated by the uniform expression of male-fertility restoration in plants with S cytoplasm. We identified two N-cytoplasmic families, one that was homozygous dominant and the other segregating at Ms. Plants from each of these two families were individually testcrossed to male-sterile onions. Nuclear restoration of male fertility in testcross progenies was evaluated in the field over 4 years. For male plants homozygous dominant at Ms, we expected 100% male-fertility restoration, but observed mean values between 46% and 100%. For plants segregating at Ms, we again observed lower than expected frequencies of male-fertility restoration. These results demonstrate that the dominant Ms allele shows reduced penetrance, meaning that the dominant allele may be present but not expressed, complicating the development of inbred lines for hybrid production. Therefore, scoring of male-fertility restoration must be done over environments. The genetic distance between Ms and a tightly linked molecular marker at AOB272 was estimated at 4.1 cM, slightly larger than previously reported. These results will be important for onion breeders in the public and private sectors working to develop inbred lines for the production of hybrid onion cultivars.

Technical Abstract: The most commonly used source of cytoplasmic male sterility in onion is controlled by the interaction of the cytoplasm [male-sterile (S) or normal (N) male-fertile] and one nuclear male-fertility-restoration locus (Ms). Scoring of genotypes at Ms is generally done by testcrossing male-fertile to male-sterile (S msms) plants, followed by scoring of testcross progenies for male-fertility restoration. We identified two N-cytoplasmic families, one that was homozygous dominant and the other segregating at Ms. Plants from each of these two families were individually testcrossed to male-sterile onions. Nuclear restoration of male fertility in testcross progenies was evaluated in the field over 4 years. For male plants homozygous dominant at Ms, we expected 100% male-fertility restoration, but observed mean values between 46% and 100%. For plants segregating at Ms, we again observed lower than expected frequencies of male-fertility restoration. These results demonstrate that the dominant Ms allele shows reduced penetrance, requiring the scoring of male-fertility restoration over environments. The genetic distance between Ms and a tightly linked molecular marker at AOB272 was estimated at 4.1 cM, slightly larger than previously reported.