Marker-assisted purification of maize parental lines and hybridization crosses for downy mildew resistance

Improved host resistance in maize against downy mildew provides the efficient control especially in infested areas. Molecular markers are powerful tools to locate and pyramid resistance genes through marker-assisted selection (MAS) breeding. The study was conducted to apply simple sequence repeats (SSR) marker technology to purify outstanding corn parental lines and introgress downy mildew resistance (DMR). Parental lines include two susceptible A-lines and breeding lines with differential DMR i.e., 1D, 1E, 2FG, and 2 HJ-lines. DMR is derived from two resistant yellow maize lines P345 and Nei9008. The likely resistance gene of P345 has been mapped. An initial hybrid-cross between Nei9008 and A-line has also been produced. SSR-assisted line purification was conducted in two succeeding generations involving plant-genotyping and selection. Using pre-screened SSR markers representing each maize chromosome, P345 showed averages of 89.5% homozygosity, while 100% for Nei9008. Among the recipient parental lines, A, D, E, F and H showed 100% homozygosity for all SSR loci screened, and G and J-line showing 97.2% and 75% homozygosity, repectively. Hybridization was performed between homozygous plants of DMR and recipient parent lines. While, hybridity testing with SSRs was done on (Nei9008 × A)-cross wherein 16.7% (3/18) plants were false hybrids. The SSRs were homozygous for maternal alleles in these plants confirming escape from cross-pollination within an ear. The false hybrids were roughed and backcrossing/selfing was conducted on genuine hybrid plants. A total of 54 new hybrids and 11 BC1F2s were produced. Especially for F345, final generation of individual plant SSR genotyping and selection/purification is being set-up. Hybridity testing of new FIs and eventual MAS to pyramid resistance genes for DMR will be conducted.