Hybrid onion seed is produced using cytoplasmic-genic male sterility (CMS) systems. The most commonly used source of CMS in onion is conditioned by male-sterile (S) cytoplasm (Havey, 1993). For plants possessing S cytoplasm, a single nuclear locus (Ms) controls male-fertility restoration; a dominant allele at Ms conditions male fertility, whereas plants homozygous-recessive at Ms are male-sterile (Jones and Clarke, 1943). Plants possessing normal (N) cytoplasm are always male-fertile regardless of their genotype at Ms. Male-sterile inbred lines are seed-propagated by crossing male-sterile plants (S msms) with maintainer plants that possess N cytoplasm and the homozygous-recessive genotype at Ms (Jones and Davis, 1944). Therefore, a main goal of hybrid onion breeding programs is to identify superior maintainer (N msms) lines to seed-propagate male-sterile lines for hybrid development. The identification of maintainer lines can be expensive and time-consuming because of the biennial generation time of onion and the relatively high cost of using insects for crossing (Pike, 1986). To identify maintainers, male-sterile (S msms) plants are crossed with male-fertile (S Ms– or N–) plants. The male fertility of progenies from the male-sterile parent is then established. If all testcross progenies are male-fertile, the male parent was homozygous-dominant at Ms. If the progenies are all male-sterile, the male parent must be N msms. Finally, if the testcross progenies are segregating for male fertility, the male parent must be heterozygous at Ms. Plants that are identified as N msms or N Msms can be then used to develop maintainer lines for backcrossing to sources of S cytoplasm.
Numerous molecular markers distinguishing N and S cytoplasms of onion have been developed (Engelke et al., 2003; Havey, 1993, 1995; Kim et al., 2009; Sato, 1998). Identification of molecular markers tightly associated with the nuclear Ms locus would greatly aid the development of maintainer lines. These markers would allow breeders to identify plants carrying the recessive ms allele early during the first year and carry forward for testcrossing only those plants with a higher probability of possessing recessive allele(s) at Ms. We previously reported that the genomic region carrying a restriction fragment length polymorphism (RFLP) revealed by cDNA AOB272 is tightly linked (0.9 cM) to Ms and converted this marker to a polymerase chain reaction (PCR)-based polymorphism (Gökçe et al., 2002). Others have either converted our previously described markers to PCR-detectable polymorphisms (Bang et al., 2011) or developed new markers linked to Ms (Huo et al., 2012; Yang et al., 2012). Although these markers are useful for selection of maintainer lines after crossing (i.e., at maximum linkage disequilibrium), they may not be in linkage disequilibrium with Ms among plants from open-pollinated (OP) populations (Gökçe and Havey, 2002). Yang et al. (2012) identified two markers linked to the Ms locus that were consistent across a sample of inbreds and hybrids; however, they proposed that their markers will be useful to genotype plants from OP populations without actually testing their use. In this research, I identified SNPs that are in linkage disequilibrium with alleles at the Ms locus across OP and inbred populations of onion.
BangH.ChoD.Y.YooK.S.YoonM.K.PatilB.S.KimS.2011Development of simple PCR-based markers linked to the Ms locus, a restorer-of-fertility gene in onion (Allium cepa L.)Euphytica179439449
BarkO.H.HaveyM.J.1995Similarities and relationships among open-pollinated populations of the bulb onion as estimated by nuclear RFLPsTheor. Appl. Genet.90607614
DuangjitJ.BohanecB.ChanA.P.TownC.T.HaveyM.J.2013Transcriptome sequencing to produce SNP-based genetic maps of onionTheor. Appl. Genet.doi: 10.1007/s00122-013-2121-x
EngelkeT.TerefeD.TatliogluT.2003A PCR-based marker system monitoring CMS-(S), CMS-(T) and (N)-cytoplasm in the onion (Allium cepa L.)Theor. Appl. Genet.107162167
GökçeA.F.HaveyM.J.2002Linkage equilibrium among tightly linked RFLPs and the Ms locus in open-pollinated onion populationsJ. Amer. Soc. Hort. Sci.127944946
HaveyM.J.1995Cytoplasmic determinations using the polymerase chain reaction to aid in the extraction of maintainer lines from open-pollinated populations of onionTheor. Appl. Genet.90263268
HuoY.M.MiaoJ.LiuB.J.YangY.Y.ZhangY.H.WuX.2012The expression of pectin methylesterase in onion flower buds is associated with the dominant male-fertility restoration allelePlant Breed.131211216
JonesH.A.DavisG.N.1944Inbreeding and heterosis and their relation to the development of new varieties of onions. U.S. Dept. Agr. Tech. Bul. No. 874
KimS.LeeE.ChoD.Y.HanT.BangH.PatiB.S.AhnY.K.YoonM.2009Identification of a novel chimeric gene, orf725, and its use in development of a molecular marker for distinguishing among three cytoplasm types in onion (Allium cepa L.)Theor. Appl. Genet.118433441
PikeL.M.1986Onion breeding p. 357–394. In: Bassett M. (ed.). Breeding vegetable crops. AVI Publ. Roslyn NY
SatoY.1998PCR amplification of CMS-specific mitochondrial nucleotide sequences to identify cytoplasmic genotypes of onion (Allium cepa L.)Theor. Appl. Genet.96367370
YangY.HuoY.MiaoJ.LiuB.KongS.GaoL.LiuC.WangZ.TaharaY.KitanoH.WuX.2012Identification of two SCAR markers co-segregated with the dominant Ms and recessive ms alleles in onion (Allium cepa L.)Euphytica190267277