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Doubled haploid (DH) technology is an essential component in producing inbred lines for a competitive maize ( Zea mays  L.) breeding program. The  R1 - navajo  ( R1 - nj ) gene provides phenotypic marker that insures only variable reliability for seed selection of haploid embryos. Therefore, in the present study we outline a complex protocol for early stage genome size determination that integrates the phenotypic screening with the flow cytometry of nuclei from root tips and with the use of DNA isolated from seedlings for molecular marker-based genotyping. In a representative experiment with three genotypes, only 59% of the color marker pre-selected seeds were confirmed to be haploid by cytometric analysis of nuclei isolated from root tips. As a novel tool we have identified the UMC1152 SSR marker being polymorphic between the haploid inducer line (K405) and the K4390 hybrid as parents to screen seedlings pre-selected with the  R1 - navajo  marker. Using this molecular marker, alleles characteristic for the inducer K405 line could not be detected in 83% of seedlings previously selected as haploid candidate. Seedlings identified as haploids were exposed to 0.06% colchicine solution for rediploidization. This procedure resulted in doubled haploids with 3% frequency relative to the initial population as it was quantified by the number of mature maize plants with fertile tassel. The described complex approach can support safer identification of haploids at early seedling stage in a hybrid population derived from crossing with a haploid inducer line.

Improved reliability in production of maize inbred lines by the combination of the R1-navajo marker with flow cytometry or microsatellite genotyping