Genetic linkage mapping of field resistance to cassava brown streak disease in cassava (Manihot esculenta Crantz) landraces from Tanzania
Kulembeka, Heneriko Philbert Kayogoro
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Cassava brown streak disease (CBSD) is an important biotic constraint to cassava production in cassava growing areas of east, central and southern Africa. A study was initiated to study the genetics of CBSD resistance and search for molecular markers associated with disease resistance. Two resistant and two susceptible cassava parents were selected based on their flowering ability, genetic distance and phenotypic reaction to CBSD. They were crossed in a half diallel to produce six families and F1 progeny plus parents were screened for their reaction to CBSD and evaluated for yield traits at two locations. Results indicated that general combining ability (GCA) was higher than specific combining ability (SCA) for CBSD root necrosis, CBSD severity in the shoot at three, six and nine months after planting (MAP), cassava mosaic disease (CMD) severity at three, six and nine MAP and harvest index indicating the importance of additive gene action in controlling CBSD resistance. Results indicated that CBSD resistance was quantitatively controlled by polygenic genes. Higher SCA than GCA was detected for number of roots per plant, fresh root weight per plant and fresh shoot weight per plant indicating the importance of non-additive gene effects in controlling yield traits. The Namikonga x Albert cross was used to generate a segregating mapping population comprising of 190 F1‟s which was planted and screened for CBSD reaction at two locations and genotyped using 605 SSR markers. A total of 220 polymorphic SSR markers, tested on 60 F1 and 51 S1, were finally used for linkage analysis and four genetic linkage maps were produced. The maps produced included integrated (male and female), Namikonga (female), Albert (male) and Namikonga-S1maps which spanned a distance of 995.46cM, 779.48 cM, 854.26 cM and 524.58 cM consisting of 174, 131,123 and 124 mapped SSR markers respectively. Mean map distance between markers ranged from 4.23 cM in Namikonga-S1 to 6.94 cM in Albert map while average map length per linkage group was 43.23 cM, 45.85 cM, 47.46 cM and 30.86 cM for integrated, Namikonga, Albert and Namikonga-S1 maps respectively. The linkage maps were used for QTL analysis and three putative QTL were detected on the integrated map on linkage groups C4, C16 and C18 linked to NS945, ESSRY105 and two flanking markers SSRY4 and ESSRY96 respectively. One QTL linked to NS667 was detected on Namikonga map on linkage group N4 while four putative QTL were detected on the Namikonga-S1 map on linkage groups S1 (at Chambezi and Naliendele in 2008), S8 and S12a at Chambezi 2008 and on S17 at Naliendele 2008. These QTL were respectively found to be linked to SSRY295, NS347 and SSRY4. These findings will have an implication in breeding for CBSD resistance. The observed relative importance of additive gene effects will mean that a mass recurrent selection will be the most efficient approach for CBSD resistance breeding in cassava. Markers associated with CBSD resistance will greatly improve selection efficiency for CBSD resistance in which selection for resistant genotypes can efficiently be done at seedling stage.
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