Recombinant inbred lines (RILs) produced without selection from singlecross hybrids have untapped 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 with difficult quantitative traits like tolerance to disease, pest and stress. The great advantage of RlLs is the ability to repeatedly test until characterization is definitive. When based 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. Once mapped, these major QTLs can be advanced by marker-assisted selection. When mapped quite accurately, QTLs can be defined to gene loci and become susceptible to excision and gene-cloning teehnologies. A series of II RILs have been produced in Hawaii and are available for study. The single-seed descent production of such inbreds without selection is rapid and routine. Most Hawaiian RIL sets involve one temperate and one tropical inbred parent. Three of these sets have been mapped with RFLPs and SSRs. This article has three sections dealing primarily with LTH research: (I) the production 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 to diseases 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 these QTLs have been mapped. Similar mono- and di-genic segregations were observed for many agronomic traits. It is concluded that there is a common misconception that quantitative traits are automatically governed by many genes. In contrast, RIL research is showing that such traits are often governed primarily by major QTLs with large effects, subject only to minor modification by environment, modifier loci and hybrid vigor. The uniformity of RILs, their ability to be repeatedly evaluated, and the absence of heterosis as a confounding 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 by such QTLs. MAS should gain wide acceptance where it can be shown to be economically competitive with present breeding methods.

English

0208|AGRIS 0201|AL-Maize Program|R01PROCE

Juan Carlos Mendieta

CIMMYT Publications Collection