The inheritance of host plant resistance and its effect on the relative infection efficiency of Magnaporthe grisea in rice cultivars

Abstract

The inheritance of host plant resistance and its effect on the relative infection efficiency for leaf blast was studied in the crosses 'IR36'/'CO39' (partially resistant × highly susceptible) and 'IR36'/'IR64' (both partially resistant). On the natural scale, gene action appeared multiplicative. After log transformation, additive effects described most of the genetic variation in the cross 'IR36'/'CO39', while additive and dominance effects were about equal in magnitude in the cross 'IR36'/'IR64'. Dominance was towards increased resistance. No transgressive segregation occurred in the cross 'IR36'/'CO39'. The number of genes that reduce lesion number was estimated to be zero in 'CO39' and five or more in 'IR36'. The cross 'IR36'/'IR64' showed transgressive segregation in both directions, and 'IR36' and 'IR64' each contain at least one gene that is not present in the other cultivar. The heritabilities (narrow sense) in the F2 were low (range 0.06-0.16), while narrow sense heritabilities based on F3 lines were much higher (range 0.41-0.68). Lesion numbers in F3 lines were reasonably correlated with those in F5 progenies derived from the same F2 plant (r was±0.6 in both crosses). Partial resistance can be effectively improved by selecting the most resistant plants from the most resistant F3 lines.

Response to selection for high and low partial resistance to leaf blast in F2 populations of three rice crosses

“…The distribution of the F3 lines of these crosses indicates oligo-or polygenic control of the low number of lesions in IR36 and IR64 (Fig . 2), which is in agreement with the result of an earlier study of these cultivars (Roumen, 1994), and which supports the broadly held view that PR to leaf blast usually is under polygenic control (Ezuka, 1979) . The fact that the proportion of the F3 lines with a phenotype as the more resistant parent in the crosses IR36/CO39 and IR64/CO39 was higher than the proportion with a phenotype as the more susceptible parent, may be explained by a progressively smaller (absolute) effect of each additional gene when more genes are already present (Roumen, 1994) .…”

“…However, the efficiency of the selection was too low to merit practical use of the applied selection criteria by breeders, mainly because they are too time consuming . The realized heritabilities for the number of lesions in the crosses IR36/CO39 and IR36/IR64 are slightly higher than those calculated for these crosses in an earlier study (Roumen, 1994) and are similar to those obtained by Wang et al . (1989) .…”

“…At least 350 F2 plants of each cross were grown as described previously (Roumen, 1994), together with 60 plants of each parent . All F2 plants of one cross were derived from a single Fi plant .…”

Is heterosis an artefact governed by the choice of scale?

“…When at least one of these assumptions is violated, the most common reaction is to search for a data transformation, which will meet all assumptions simultaneously, at least approximately. This approach has been used frequently in studies of heterosis and dominance, some of which were published in this journal (Boiteux et al, 2004;Baker et al, 2003;Tefera & Peat, 1997;Roumen, 1994). Alternatively, one may take recourse to a generalized linear model (GLM) analysis (McCullagh & Nelder, 1989), which transforms the linear predictor rather than the data.…”

Mapping QTLs for field resistance to the rice blast pathogen and evaluating their individual and combined utility in improved varieties

“…resistance is characterized by lesions that are typically spindle-shaped but may be fewer in number, reduced in size, slower to develop, or shorter-lived than those produced in the absence of partial resistance. The net effect is a reduced inoculum potential and a lower probability of a blast epidemic (Marchetti 1983a, b;Wang et al 1989;Katsar 1993;Roumen 1994;Correa-Victoria and Zeigler 1995). Partial resistance is more difficult than complete resistance for breeders to utilize because it is quantitatively inherited, usually oligo-or poly-genic, and sensitive to environmental factors such as temperature, leaf wetness duration, N-fertilization, soil type and water stress (Ou 1985;Roumen 1994).…”