Abstract:
Cassava production in Africa is compromised by
cassava brown streak disease (CBSD) and cassava mosaic
disease (CMD). To reduce costs and increase the precision
of resistance breeding, a QTL study was conducted to identify molecular markers linked to resistance against
these diseases. A bi-parental F1
mapping population was
developed from a cross between the Tanzanian farmer varieties,
Namikonga and Albert. A one-step genetic linkage
map comprising 943 SNP markers and 18 linkage groups
spanning 1776.2 cM was generated. Phenotypic data from
240 F1
progeny were obtained from two disease hotspots
in Tanzania, over two successive seasons, 2013 and 2014.
Two consistent QTLs linked to resistance to CBSD-induced
root necrosis were identified in Namikonga on chromosomes
II (qCBSDRNFc2Nm) and XI (qCBSDRNc11Nm)
and a putative QTL on chromosome XVIII (qCBSDRNc18Nm).
qCBSDRNFc2Nm was identified at Naliendele
in both seasons. The same QTL was also associated
with CBSD foliar resistance. qCBSDRNc11Nm was identified
at Chambezi in both seasons, and was characterized by
three peaks, spanning a distance of 253 kb. Twenty-seven
genes were identified within this region including two LRR
proteins and a signal recognition particle. In addition, two
highly significant CMD resistance QTL (qCMDc12.1A
and qCMDc12.2A) were detected in Albert, on chromosome
12. Both qCMDc12.1A and qCMDc12.2A lay within the range of markers reported earlier, defining the CMD2
locus. This is the first time that two loci have been identified
within the CMD2 QTL, and in germplasm of apparent
East African origin. Additional QTLs with minor effects on
CBSD and CMD resistance were also identified.
