The use of morphological or molecular markers could facilitate breeding for resistance to common root rot in barley. Evaluation of segregating material to this disease is labour intensive and influenced by environment. Selection of genetic markers linked to genes for resistance to common root rot could increase efficiency, particularly in early generations of breeding programs. Our objective was to determine associations among morphological and RAPD markers with common root rot reaction.
The susceptible check Brier, the moderately resistant cultivar Bowman, and four BC3F5 progeny lines derived from crosses between 45 marker stocks and Bowman (courtesy of J. Franckowiak, North Dakota State University) were evaluated for 3 years for disease reaction in a common root rot nursery. Lines were evaluated in randomized complete block design experiments. Thirty plants in each plot were rated for common root rot using a two category scale based on subcrown internode discoloration of greater than 50% or less than 50%.
The gs4 and o traits were found to be associated with susceptibility to common root rot. Both gs4 and o traits are expressed in the recessive genotype. They are closely linked and found on either side of the centromere of chromosome 6 in barley (Tsuchiya 1986). Orthogonal contrasts of the morphological marker lines versus Bowman revealed that the marker lines are more susceptible to common root rot than is Bowman (Table 1). These lines were also screened for RAPD markers using a procedure similar to Williams et al. (1990). A RAPD marker was found to be linked with the gs4 and o traits and associated with resistance to common root rot. Primer number 198 (GCA GGA CTG C) from the University of British Columbia collection was found to amplify a DNA fragment in the resistant parent Bowman, but not in the 4 progeny lines with the gs4 and o traits.
Confirmation of these associations was obtained by growing out F2-derived F3 families in a common root rot nursery. Chi-square analysis indicated morphological markers segregated in the expected 1:2:1 ratio (based on F3 identification) and the RAPD marker in a 3:1 ratio. The traditional method of identifying marker-QTL (quantitative trait locus) associations (Edwards et al. 1987) was used to determine associations between common root rot reaction and each genetic marker. For morphological markers this method compared the mean disease rating of lines homozygous dominant for each marker genotype to that of homozygous recessive genotypes. The RAPD marker is a dominant marker which does not allow homozygous dominant genotypes to be distinguished from heterozygous genotypes. For the RAPD marker the mean disease rating of lines that amplified a DNA fragment was compared to the mean disease rating of lines that did not amplify a DNA fragment. In each case the F test from the analysis of variance indicated a difference between the disease means and therefore an association between each morphological marker and the RAPD marker with common root rot reaction (Table 2).
Table 1. Mean common root rot disease reactions (%) for gs4-o progeny lines (9489, 9642, 9643, 9667), Bowman (Bwmn) and the susceptible check Brier.
Table 2. Mean common root rot disease reactions (%) of progeny lines homozygous dominant (MM) vs homozygous recessive (mm) for glossy sheath (gs4) and orange lemma (o) genotypes, and disease means for those lines amplifying a DNA fragment(M_) vs. those lines which did not amplify a fragment (mm).
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