Organization of Genes Conferring Resistance to Anthracnose in Common Bean - Translational Genomics for Crop Breeding: Biotic Stress - page 160

segregation responses to C. lindemuthianum .No
segregation is interpreted as both genotypes shar-
ing the same resistance gene or locus. This inter-
pretation was used in classical studies reported
by Cardenas et al. (1964), Fouilloux (1976), and
Muhalet et al. (1981). For example, Fouilloux
(1976) investigated the resistance to two races
(gamma and delta 2) in the cvs. TO, TU, TV,
TX, TY, and TW. He analyzed the responses in
F 2 populations derived from crosses between the
four cvs. (TO, TU, TX, TY) and the suscep-
tible cv. Tenderette and from 3R:1S observed
ratios deduced that the four genotypes carry a
resistance gene. He also studied the responses in
F 2 populations derived from crosses among the
several resistant cultivars Silvert (carrying the
Are gene) and MY132 (carrying the Mexique 1
gene). From 15R:1S observed segregation ratios,
he concluded that cvs. TO, TU, TX, and TV carry
independent gene(s) to the Are and Mexique 1
genes. Finally, from observed segregation in the
F 2 population derived from the cross TO x TU,
he concluded that the cvs. TO and TU carried
different resistance genes, Mexique 2 and Mex-
ique 3 , respectively. Both genes conferred resis-
tance to the races gamma and delta 2. Recent evi-
dence indicates that cultivar TU has at least three
independent loci-conferred resistance to anthrac-
nose (Campa et al. 2009). Now the question is
which loci confer resistance against races used
by Fouilloux in cultivar TU. This question is
relevant because classical allelism tests consider
that cultivar TU carried only one resistance gene
( Mexique 3 or Co-5 ).
Classical analysis generally assumes that the
resistance to different races/isolates in a geno-
type is conferred by the same locus. Based on
this hypothesis, classical studies also assumed
that different resistance spectra in cultivars carry-
ing the same resistance gene are due to different
alleles residing at the same locus. For example,
resistance in cvs. MDRK and Kaboon was shown
to be controlled by the Co-1 gene (Melotto and
Kelly 2000). Since both genotypes differ in the
response to race 7, this difference was interpreted
as being conditioned by two different alleles at

the Co-1 locus: Co-1 in MDRK (susceptible to
race 7) and Co-1 2 in Kaboon (resistant to race
7). However, recent evidence suggests that resis-
tance to races 7 and 73 in Kaboon is controlled
by different loci, which mapped at the end of
linkage group Pv04 in the relative position of the
Co-3 locus (Campa et al. 2011). The confirma-
tion that the resistance to race 73 in MDRK is
controlled by a locus located in the Co-3 region is
very important because new Co -genes have been
described recently (Alzate-Marin et al. 2003;
Gon¸alves-Vidigal et al. 2009), based on the
results of allelism tests in which it was assumed
that resistance to race 73 in MDRK was condi-
tioned by the Co -1 gene on linkage group Pv01.
Allelism tests based on analysis of F2 seg-
regating populations have additional limitations:
plants can only be evaluated once against a single
race. A single evaluation can lead to errors due to
escapes. In addition, more complex segregation
patterns involving two or more loci with different
types of gene interaction are difficult to identify
correctly in F2 populations. For example, larger
populations are needed to differentiate between
the segregation ratios for a dominant gene and the
segregation for two genes where the resistance is
conferred by a combination of a dominant gene
and a recessive gene (expected ratio 13R:3S).
These limitations of F 2 populations can be solved
by using other types of segregating populations
such as recombinant inbred lines (RIL) or F 2:3
families where it is possible to repeatedly test
families or RILs against multiple races.

Linkage Analysis in Resistance
to Bean Anthracnose

A very important step in the characterization of
a gene is the knowledge of its relative position in
the genetic map of the species and its relation-
ship with other genes (epistasis). Linkage dise-
quilibrium between resistance genes and various
molecular markers has been identified (Kelly and
Vallejo 2004) and now provides opportunities
to map these genes. Currently, the availability
of numerous molecular markers, many of them