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Recombinant Inbred Lines in Tropical Maize improvementJmnes L. Brewbaker and Hyeon Gui MoonUniversity ofHawaii, 3190 Ivlaile Way, HOllolulu, HI 96822Rural Develop/nent Administration, Smvoll, 441-100, KoreaAbstractRecombinant inbred lines (RILs) produced without selection from singlecross hybrids haveuntapped potential to accelerate tropical maize improvement. Major QTLs (quantitative trait loci)with large effects can be identified easily in RILs, providing a basis for rapid genetic gains withdifficult quantitative traits like tolerance to disease, pest and stress. The great advantage of RILs isthe ability to repeatedly test until characterization is definitive. When based on elite inbreds, theRILs are also elite and easily worked.Molecular mapping technology is rapidly advancing, with SSRs (single-sequence repeats)becoming a powerful addition to the arsenal of markers such as RFLPs. Once n;Japped, these majorQTLs can be advanced by marker-assisted selection. When mapped quite accurately, QTLs can bedefined to gene loci and become susceptible to excision and gene-cloning technologies.·A series of I I RILs have been produced in Hawaii and are available for study. The single-seeddescent production of such inbreds without selection is rapid and routine. Most Hawaiian RIL setsinvolve one temperate and one tropical inbred parent. Three of these sets have been mapped withRFLPs and SSRs. This article has three sections dealing primarily with LTH research: (1) theproduction of recombinant inbred lines, (2) statistical interpretation of segregations, and (3)segregations observed among RILs.Major QTLs segregating monogenically in Hawaii's RILs controlled general resistance todiseases such as Southern rust and Common rust, Northern leaf blight and Southern leaf blight,Bacterial wilt and Bacterial leaf blight, Maize mosaic virus and Maize streak virus. Several of theseQTLs have been mapped. Similar mono- and di-genic segregations were observed for manyagronomic traits. It is concluded that there is a common misconception that quantitative traits areautomatically governed by many genes. In contrast, RIL research is showing that such traits are oftengoverned primarily by major QTLs with large effects, subject only to minor modification byenvironment. modifier loci and hybrid vigor.The uniformity of RILs, their ability to be repeatedly evaluated, and the absence of heterosis as aconfounding factor makes them excellent for identification of major QTLs. Marker-assisted selection(MAS) promises to be effective in accelerating breeding progress for quantitative traits affected bysuch QTLs. MAS should gain wide acceptance where it can be shown to be economically competitivewith present breeding methods.IntroductionRecombinant inbred lines (RILs) produced by single-seed descent without selection fromsinglecross hybrids have great untapped potential to accelerate tropical maize improvement. MajorQTLs (quantitative trait loci) with large effects can be identified easily in RILs, providing a basis forrapid genetic gains with difficult quantitative traits like tolerance to disease, pest and stress. Thegreat advantage of RILs is the ability to repeatedly test until characterization is definitive. Whenbased on elite inbreds, the RILs are also elite and easily worked.Molecular mapping technology is rapidly advancing, with SSRs (single-sequence repeats)becoming a powerful addition to the arsenal of markers such as RFLPs (restriction fragment lengthpolymorphisms). Once mapped, these major QTLs can be advanced by MAS (marker-assisted