New Insights into the Genetics ofin VivoInduction of Maternal Haploids, the Backbone of Doubled Haploid Technology in Maize

Abstract

Haploids and doubled haploid (DH) inbred lines have become an invaluable tool for maize genetic research and hybrid breeding, but the genetic basis of in vivo induction of maternal haploids is still unknown. This is the first study reporting comparative quantitative trait locus (QTL) analyses of this trait in maize. We determined haploid induction rates (HIR) in testcrosses of a total of 1061 progenies of four segregating populations involving two temperate haploid inducers, UH400 (HIR ¼ 8%) and CAUHOI (HIR ¼ 2%), one temperate and two tropical inbreds with HIR ¼ 0%, and up to three generations per population. Mean HIR of the populations ranged from 0.6 to 5.2% and strongly deviated from the midparent values. One QTL (qhir1) explaining up top ¼ 66% of the genetic variance was detected in bin 1.04 in the three populations involving a noninducer parent and the HIR-enhancing allele was contributed by UH400. Segregation ratios of loci in bin 1.04 were highly distorted against the UH400 allele in these three populations, suggesting that transmission failure of the inducer gamete and haploid induction ability are related phenomena. In the CAUHOI · UH400 population, seven QTL were identified on five chromosomes, with qhir8 on chromosome 9 havingp.20% in three generations of this cross. The large-effect QTL qhir1 and qhir8 will likely become fixed quickly during inducer development due to strong selection pressure applied for high HIR. Hence, marker-based pyramiding of small-effect and/or modifier QTL influencing qhir1 and qhir8 may help to further increase HIR in maize. We propose a conceptual genetic framework for inheritance of haploid induction ability, which is also applicable to other dichotomous traits requiring progeny testing, and discuss the implications of our results for haploid inducer development. IN VIVO induction of maternal haploids in maize has paved the way for large-scale production of doubled haploid (DH) inbred lines, which today form the backbone of the global hybrid maize industry. Traditionally, the maize plants' crossbreeding nature required recurrent self-pollinations for 6-10 generations to obtain sufficiently homozygous inbred lines (Hallauer et al. 2010). Application of DH technology reduces the time for inbred development by more than half compared to the traditional method and additionally provides several quantitative genetic, operational, logistical, and economic advantages (Nei 1963;Schmidt 2003;Melchinger et al. 2005;Seitz 2005;Smith et al. 2008;Chang and Coe 2009;Geiger 2009). By using haploid inducer genotypes as pollinators in crosses with source germplasm, ears obtained carry a proportion of seeds containing haploid embryos of maternal origin. Subsequent treatment of haploids with mitotic inhibitors facilitates chromosome duplication resulting in diploid and completely homozygous inbred lines (see Prigge and Melchinger 2012 for a detailed description of DH production).Modern maize inducers have haploid induction rates (HIR) of about 8% on average (e.g., Röber et al. 2005;...

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