Four self-lines have been bred from normal maize hybrids (female) crossed with stock 6 (male), which generates haploids and double haploids.

Stock 6, first identified by Edward H. Coe in 1956 (Chang, Sci. Agric. 40:53-80, 1992), carried the R-nj gene with full expression of the R-nj phenotype. This gene can induce haploids at a frequency of 1-5%. Seeds having homozygous R-nj are white flint with purple color on the crown and plumule. The haploids planted can be doubled into pure lines under natural conditions in a frequency of 10%.

QI318xQI310 hybrid of normal maize was used as female, and was crossed with stock 6 (male). A total of 200 F1 ears or 17,600 F1 seeds were obtained. 493 haploids (2.8%) were identified among them by using kernel color screening technique. These 493 kernels were planted and 302 plants survived at seedling phase. 28 plants (9.3%) were doubled by pollen screening at maturity phase. 21 plants were self-pollinated and 15 self-ears were obtained. All of them were planted in ear-rows the next year and 4 good self-lines were selected. These lines have outstanding agronomic characters, good resistance to some diseases, and have been used in the production of hybrid maize. 

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