Molecular markers associated with male-sterile cytoplasms and male-fertility restorer locus in onion (Allium cepa): a review

Purpose: CMS hybrid seed production systems are employed effectively for onion. Cytoplasmic male sterility (CMS) has long been used to economically produce hybrids that harness growth vigor through heterosis. Three types of onion CMS (CMS-S, CMS-R, and CMS-T) have been used in hybrid onion breeding. Findings: The sources of onion CMS, their maintainer plants and fertility restorer lines can be distinguished by markers, saving times spent on crop establishment and avoiding the complex of phenotypic screening. Numerous molecular markers especially PCR markers associated with male-sterile (CMS-S and CMS-T) cytoplasms, male-fertile normal (N) cytoplasm and nuclear-male- fertility restorers (Ms locus) were developed. These simple PCR markers are valuable tools for the marker-assisted selection of segregating individuals in onion F۱ hybrid breeding programs. The present review reveals practical utility and functional effectiveness in the MAS of male-sterile cytoplasm types with nuclear-fertility-restorer locus. Limitations: the environment effect especially temperature may cause male-sterility instability and deviations in segregation ratios for male sterility. Also, maximum exploitation of molecular markers linked to Ms/ms and CMS-S, -T genes aid in the recovery of male-sterile traits requires to a perfect linkage disequilibrium that must be investigated further. Directions for future research: Furthermore, this information could highly have paved the way for hybrid onion development by applicating of the molecular findings to identify onion male sterility, maintainer and male fertility restorer lines.