Abstract
Anthracnose (Colletotrichum lindemuthianum), Angular leaf spot (Pseudocercospora griseola) and Pythium root rot are important pathogens affecting common bean production in the tropics. A promising strategy to manage these diseases consists of combining several resistance (R) genes into one cultivar. The aim of the study was to determine genetic linkage between gene pairs, Co-42/Phg-2, on bean-chromosome Pv08 and Co-5/“P.ult” on-chromosome Pv07, to increase the efficiency of dual selection of resistance genes for major bean diseases, with molecular markers. The level of recombination was determined by tracking molecular markers for both BC3F6 and F2 generations. Recombination fraction r, among gene pairs, the likelihood of linkage, L(r), and logarithm of odds (LOD) scores were computed using the statistical relationship of likelihood which assumes a binomial distribution. The SCAR marker pair SAB3/PYAA19 for the gene pair Co-5/“P.ult” exhibited moderate linkage (r = 32 cM with a high LOD score of 9.2) for BC3F6 population, but relatively stronger linkage for the F2 population (r = 21 cM with a high LOD score of 18.7). However, the linkage among SCAR marker pair SH18/SN02, for the gene pair Co-42/Phg-2 was incomplete for BC3F6 population (r = 47 cM with a low LOD score of 0.16) as well as F2 population (r = 44 cM with a low LOD score of 0.7). Generally, the weak or incomplete genetic linkage between marker pairs studied showed that all the four genes mentioned earlier have to be tagged with a corresponding linked marker during selection. The approaches used in this study will contribute to two loci linkage mapping techniques in segregating plant populations.
Highlights
Diseases are critical production constraints for common beans in tropical countries, causing total crop failure when several pathogens attack susceptible bean genotypes under favorable conditions (Schwartz and Galvez, 1980; Mahuku et al, 2011)
This study aimed at estimating recombination frequencies and genetic linkage between gene pairs, Co-42/Phg-2 on bean chromosome Pv08 and Co-5/“P.ult” on chromosome Pv07
A BC3F5 population previously developed at CIAT of Kawanda (Uganda) to combine six disease resistance genes of common bean, namely Co-42, Co-5, Phg-2, “P.ult”, I and bc-3 (Figure 1) formed the genetic material for this study
Summary
Diseases are critical production constraints for common beans in tropical countries, causing total crop failure when several pathogens attack susceptible bean genotypes under favorable conditions (Schwartz and Galvez, 1980; Mahuku et al, 2011). Management of diseases at farm level in short and long term is practical, through genetic control by incorporating resistant alleles for different pathogens into selected cultivars through. 1996; Malav et al, 2016). Marker assisted breeding strategies for pyramiding genes requires information on marker co-transmissions (Mahuku et al., 2011; http://www.extension.org/pages/32465/gene-pyramidingusing-molecularmarkers#.ViTJovlViko). Molecular markers and genetic maps accelerate identification of desirable homozygotes without need for progeny tests (Pathania et al, 2014)
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