Abstract:
Coffee is known to be one of the most important beverages in the world with a current
estimated value of US$10 billion. It is ranked as the second largest traded commodity
after petroleum. Coffea arabica is better known for its excellent cup quality but suffers
from a narrow genetic base due to its history on domestication and susceptibility to coffee
diseases and pests. /Coffea Arabica/ production in Africa is majorly constrained by
Coffee Berry Disease (CBD) caused by Colletotrichum kahawae. Transfer of desired
genes from related wild diploid Coffea species into the cultivated allotetraploid C.
arabica has been known to confer better traits such as pest/disease resistance to the
cultivated coffee varieties. Recently, conscious efforts have begun to integrate
DNA/molecular marker based technologies, which have provided impetus, dependability
and direction to the efforts on coffee genetic improvement. This study was aimed at
establishing genetic diversity and microsatellite markers that co-segregate with resistance
to CBD in an F
2
population derived from two coffee cultivars; Rume Sudan (resistant)
and SL 28 (susceptible). Phenotypic studies using Colletotrichum kahawae inoculum to
screen F
2
population was carried out in order to understand the segregation of Coffee
Berry Disease resistance as well as their association with SSR markers. Effect of
genotypes on mean infection was highly significant (P≤0.0001) and this was due to
segregation in the F
2
population. Upon separation of means, the F
2
genotypes were
clustered into two classes; 33 resistant and 16 susceptible genotypes. F
2
population
phenotypically segregated in a 3:1 ratio for resistant and susceptible plants respectively.
Among the 12 simple sequence repeats (SSR) markers tested, six markers were
polymorphic but only two markers, M 24 and Sat 227 discriminated between the parents,
F
1
and the F
2
population. These two SSR markers showed a “goodness-of-fit” to the
expected Mendelian segregation ratio (1:2:1) for single gene effect (d.f= 1.0, P<0.05) in
the chi-square (χ
2
) analyses. The F
2
plants showed that resistance to Colletotrichum
kahawae were putatively linked to two alleles of SSR markers, M-24 (∼210 bp) and Sat227
(∼200
bp). The two SSR loci were putatively associated to CBD resistance gene in
Rume Sudan. Due to SSR co-dominance nature, the F
2
genotypes were delineated into homozygous resistant, heterozygous resistant and homozygous susceptible. This diversity
among the F
2
genotypes was clearly seen in a dendogram produced by Artemis 5.0
software. Therefore, there was a correlation between phenotypic data and molecular data
with regard to resistance to Colletotrichum kahawae. The findings of this study could be
useful in CBD molecular analysis of segregating generations, breeding lines and varieties
having Rume Sudan as one of the parents.
