In Vivo Haploid Production in Crop Plants: Methods and Challenges

Doubled haploids offer a rapid method of producing homozygous lines for accelerated breeding of varieties and hybrids necessary to address the food demands of the next 2–3 decades. Further, doubled haploids are invaluable in basic genetic research such as gene discovery, genetic mapping, and genome sequencing, especially in perennial crops. Haploids are produced mainly through in vitro culture of anthers, microspores, or ovules. Some inter-specific, inter-generic crosses and crosses of different ploidy of the same species or different species also consistently give haploids through selective elimination of a set of chromosomes coming from one of the parents during zygotic divisions. Haploid inducer (HI) lines that yield haploids upon crossing have been commercially employed in breeding of maize, barley, and wheat. In recent years, novel ways of artificially synthesizing HI lines have been devised based on basic studies with Arabidopsis. The role of centromeric histone protein coding gene CENH3 , which plays critical role in centromere specification and chromosome segregation, has been exploited to engineer HI lines in Arabidopsis and maize. Further, the discovery of the gene encoding pollen-specific phospholipase A ( MTL / NLD / ZmPLA1 ) responsible for inducing haploids in maize has opened an additional way of constructing HI lines in crop species. We present here a brief overview of various in vivo haploid production methods and discuss the challenges in practical use of doubled haploid technology.