Browsing by Subject "In vivo haploid induction"

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Dissection of the genetic architecture of stalk mechanical strength and in vivo haploid induction in maize
(2016) Hu, Haixiao; Melchinger, Albrecht E.
Stalk lodging causes yield losses in maize cultivation ranging from 5 to 20% annually worldwide and stalk mechanical strength is widely accepted as an indirect indicator for its measurement. QTL mapping can reveal the genetic basis of stalk strength and provide information about markers suitable for marker-assisted selection (MAS). Constantly increasing market demands urge maize geneticists and breeders not only to enhance the field performance of new hybrids, but also to improve the breeding process. During the last decade, advances in the double haploid (DH) technology based on in vivo haploid induction (HI) shifted the breeding paradigm and greatly accelerated the breeding process in maize. Further spread of DH technology urgently demands a simple but efficient way for developing new inducers, which could be achieved by introducing the mandatory QTL/gene(s) of HI to advanced breeding lines. Therefore, the main goal of my thesis was to dissect the genetic architecture of stalk strength and detect the mandatory genomic region(s) of HI using genome-wide molecular markers. Several methods have been developed and applied in the literature to evaluate stalk mechanical strength, among which the rind penetrometer resistance (RPR) is a simple, rapid and non-destructive measurement during data collection, whereas stalk bending strength (SBS) is more closely associated with stalk lodging in the field. According to common knowledge in the mechanics of materials, SBS is reflected by the maximum load exerted to breaking (Fmax), the breaking moment (Mmax) and the critical stress (σmax). Thus, to have a complete understanding of the genetic architecture of stalk strength in maize, RPR and SBS (measured by Fmax, Mmax and σmax) were used to characterize stalk strength in our study. Utilizing a segregating population with 216 recombinant inbred lines, our analysis showed that stalk strength traits, RPR and SBS, have high heritability, ranging from 0.75 to 0.91. Nine QTL and one epistatic interaction between QTL were detected for RPR. Two, three and two QTL were detected for Fmax, Mmax and σmax, respectively. All QTL showed minor effects and only one QTL on chromosome 10 had overlapping support intervals between RPR and SBS. Co-locations of QTL and high positive correlations between stalk strength traits and other stalk traits suggested presence of pleiotropism and a complex genetic architecture of stalk strength. Owing to lack of major QTL, MAS solely based on molecular markers was found to be less effective than classical phenotypic selection for stalk strength. However, for SBS we observed considerably higher proportions of genetic variance explained by a genomic selection approach than obtained in QTL mapping with cross validation. Therefore, genomic selection might be a promising tool to improve the efficiency of breeding for stalk strength. All QTL mapping studies conducted hitherto for unraveling the genetic architecture of HI rate detected a major QTL, termed qhir1, in bin 1.04. Dong et al. (2013) further narrowed down this QTL to a 243 kb region. Considering the complex genetic architecture of HI and genetic background noise possibly affecting fine mapping of qhir1, we attempted to validate these results with an alternative approach before embarking on map-based gene isolation. Utilizing 51 maize haploid inducers and 1,482 non-inducers collected worldwide, we were able to investigate the genetic diversity between inducers and non-inducers and detect genomic regions mandatory for HI. The genetic diversity analyses indicated that the inducer group was clearly separated from other germplasm groups and had high familial relatedness. Analyzing our data by a case-control association approach failed because the segregation of HI was heavily confounded with population structure. Moreover, selective sweep approaches commonly used in the literature that are designed for capturing selective sweeps in a long-term evolutionary context failed due to high familial relatedness among inducers. To solve this problem, we developed a novel genome scan approach to detect fixed segments among inducers. With this approach, we detected a segment, termed qhir12, 4.0 Mb in length, within the support interval of the qhir1. This segment was the longest genomic segment detected by our novel approach and was entirely absent in all non-inducers analyzed. However, qhir12 has no overlap with the fine mapping region of Dong et al. (2013), termed qhir11. This indicates that the genomic region harboring the mandatory gene of HI should be confirmed by further experiments to corroborate its existence and identify its location in the maize genome.