RESUMO
Maize rough dwarf disease (MRDD) is caused by viruses in the Fijivirus genus in the family Reoviridae. MRDD resistance can be improved by a combination of marker-assisted selection (MAS) and conventional breeding strategies. In our previous study, we fine-mapped a major QTL qMrdd8 and developed the functional Indel marker IDP25K. In the present study, qMrdd8 from the donor parent X178 was introgressed into elite inbred lines derived from the three corn heterotic groups using multi-generation backcrossing and MAS. Recipient lines included Huangzao4, Chang7-2, Ye478, Zheng58, Zhonghuang68, B73, and Ji846. Markers used for foreground selection included IDRQ4, IDRQ47, IDP25K, and IDP27K. Background selection was carried out in the BC3 or BC4 using 107 SSR markers to select lines with the highest rate of recovery of the particular recurrent parent genome. Plants from BC4F2 and BC3F2 that carried the shortest qMrdd8 interval from X178 and those with the highest rate of recovery of the recurrent parent genome were then selected to create converted homozygous inbred lines. In 2017, seven converted inbred lines and five hybrids exhibited enhanced resistance to MRDD, while other agronomic traits were not affected under nonpathogenic stress conditions. Thus, the MRDD resistance allele at the qMrdd8 locus, or IDP25K, should be valuable for maize breeding programs in China.
RESUMO
KEY MESSAGE: A QTL qMrdd8 that confers resistance to MRDD was fine mapped into an interval of 347 kb; one SNP and two InDels identified in the interval were significantly associated with resistance to MRDD. Maize rough dwarf disease (MRDD) is highly prevalent in the summer maize-growing areas in China, and leads to significant yield losses in maize (Zea mays L.). In this study, the quantitative trait locus (QTL) qMrdd8, which confers resistance to MRDD, was fine mapped. Initially, qMrdd8 was consistently identified in the interval between the simple sequence repeat markers umc1617 and phi121 in three F2 sub-populations derived from a cross between the resistant recombinant inbred line NL203 and the susceptible line B73. Subsequently, qMrdd8 was fine mapped into an interval of 347 kb defined by the markers IDRQ2 and IDRQ20 using a recombinant-derived progeny test strategy. Based on single nucleotide polymorphism (SNP) genotypes identified using the MaizeSNP50 BeadChip, a long haplotype including qMrdd8 was identified in four resistant inbred lines. One SNP, the 2549-bp insertion/deletion polymorphism (InDel) InDel25, and the 2761-bp InDel27, which all were significantly associated with resistance to MRDD in a set of 226 maize inbred lines (P < 0.05), were detected within qMrdd8. Furthermore, two candidate genes, CG1 and CG2, were detected in the interval using RNA sequencing (RNA-Seq), and InDel25 was localized within the candidate gene CG1. In conclusion, the fine mapping of qMrdd8 will be helpful in cloning the resistance gene, and the three polymorphic markers identified in this study could be used to improve MRDD resistance via a marker-assisted selection approach.
