the alternative bulk (Mackay and Caligari 2000).
Further saturation of cassava genome with more
markers, most especially the SNP markers being
currently developed and validated may therefore lead
to detection of QTLs linked to EB in the three
populations as well as detection of more linked QTLs
in the remaining six populations.
Collard et al. (2005) stated that QTLs accounting for
more than 10 %of phenotypic variation (R2) are major
QTLs. In this study, seven of the nine QTLs to which
the markers are linked explained for 16 % or more of
the phenotypic variation indicating that the seven
markers are linked to major QTLs associated with EB
in cassava. As noted in earlier study by Gomez et al.
(2009), BSA also allowed identification of QTLs
linked to EB in cassava by allowing analysis in absence
of a linkage map and by reducing the effort required to
identify linkage of the markers with EB in this study.
Five SSR markers identified in this study were
found on the existing cassava genomic map (a genetic
linkage map of a cassava backcross based on a B1P2
family and SSR markers) (Akinbo 2008) and are in the
same region of QTLs identified by Okogbenin (2004)
in his study. Also, two of the nine markers (SSRY 250
and ESTs[SSRY 47]) are on linkage group 7 while
SSRY 63, SSRY 106 and SSRY 239 are on linkage
groups 8, 12 and 19, respectively. The remaining four
markers are yet to be placed on any existing cassava
genomic map. The different locations of the identified
QTLs (as indicated by the linkage groups on which the
associated markers are located) on the cassava genetic
map is indicative of the polygenic nature of the trait
studied as also observed by Okogbenin (2004).
Higher correlation coefficients were observed
between the genotypic and phenotypic data at bulk
opening stage than what was observed when the
candidate markers were tested among other genotypes
in each population. This indicates that there was more
noise among the remaining genotypes which lied
between the two extremes (contrasting bulks) in the
rank based on fresh root yield and other traits
associated with EB in each population. The fact that