Abstract
Key messageQTL consistent across seasons were detected for resistance to cassava brown streak disease induced root necrosis and foliar symptoms. The CMD2 locus was detected in an East African landrace, and comprised two QTL.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.
Highlights
Introductioncassava mosaic disease (CMD) is widely distributed across the African continent and Indian sub-continent (Alabi et al 2011; Hillocks 1997), whereas cassava brown streak disease (CBSD) was initially restricted to low-altitude areas of East Africa along the Indian Ocean (Hillocks and Jennings 2003; Jennings 2003), but later reported from high-altitude areas (>1000 meters above sea level) (Ntawuruhunga and Legg 2007)
Cassava (Manihot esculenta Crantz) (2n = 36), a starchy root crop, is widely grown in many parts of sub-Saharan Africa, with the major producers being Nigeria, Democratic Republic of Congo, Angola, Ghana, Mozambique, Uganda,1 3 Vol.:(0123456789)Theor Appl Genet (2017) 130:2069–2090 and Tanzania (FAOSTAT 2015)
Two quantitative trait locus (QTL) and a third putative QTL associated with cassava brown streak disease (CBSD) root necrosis resistance were identified in the Tanzanian cassava variety Namikonga
Summary
CMD is widely distributed across the African continent and Indian sub-continent (Alabi et al 2011; Hillocks 1997), whereas CBSD was initially restricted to low-altitude areas of East Africa along the Indian Ocean (Hillocks and Jennings 2003; Jennings 2003), but later reported from high-altitude areas (>1000 meters above sea level) (Ntawuruhunga and Legg 2007) These areas include those surrounding Lake Victoria in northwestern Tanzania, western Kenya and central Uganda (Alicai et al 2007; Ntawuruhunga and Legg 2007), Burundi (Bigirimana et al 2011), and some areas of the Democratic Republic of Congo (Mulimbi et al 2012). Further spread of CBSD towards West Africa, the largest cassava-producing region in Africa, is projected (Legg et al 2014; Patil et al 2014)
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