Spontaneous maize haploids occur in less than
0.1 % of progeny (Chase 1949), which is too low for
any commercial application. Anther or microspore
culture of maize gametophytes at an early developmental
stage has been successful in producing viable
and fertile haploids (401 Research Group 1975; Kuo
et al. 1978). However, the in vitro haploid induction
system has met with many obstacles, such as genotype
dependence, somaclonal variation, high cost and low
efficiency (Barret et al. 2004; Beckert 1994), that limit
its commercial application at large scale. The in vivo
haploid induction system based on the discovery of the
stock 6 haploid inducer line (Coe 1959) and the
subsequent improved inducer lines have become the
major tool in maize haploid production. Stock 6, when
used as a pollinator, produces *2.3 % haploid
progeny. The haploid genome is from the maternal
parent because of the chromosome elimination of the
paternal genome (Zhang et al. 2008).