Genetic mapping of a dominant gene conferring resistance to cassava mosaic disease

Abstract

Cassava mosaic disease (CMD) is the most-important disease of cassava ( Manihot esculenta) in Africa, and is a potential threat to Latin American (LA) cassava production. Although this viral disease is still unknown in LA, its vector - the whitefly - has recently been found. The disease is best controlled through host-plant resistance, which was first found in third backcross derivatives of an interspecific cross between cassava and Manihot glaziovii, and is thought to be polygenic. Recently, high levels of resistance were also found in several Nigerian cassava landraces. Classical genetic analysis and molecular genetic-mapping of the landraces showed that a major dominant gene confers this resistance. Bulk segregant analysis (BSA) was used to quickly identify a simple sequence repeat (SSR) marker linked to the CMD-resistance gene. The marker, SSRY28, is located on linkage group R of the male-parent-derived molecular genetic map. The gene, designated as CMD2, is flanked by the SSR and RFLP marker GY1 at 9 and 8 cM, respectively. To our knowledge, this is the first report of qualitative virus resistance in cassava, and of molecular markers that tag CMD resistance in cassava. We discuss the use of markers linked to CMD2 for marker-assisted breeding of CMD resistance in Latin America and for increasing the cost-effectiveness of resistance breeding in Africa.

Mapping New Genetic Markers Associated with CMD Resistance in Cassava (Manihot esculenta Crantz) Using Simple Sequence Repeat Markers

“…The SSR markers found to be associated with the CMD resistance viz. SSRY28 from previous studies (Akano et al, 2002;Moreno et al, 2004;Lokko et al, 2005) and NS158 and SSRY106 from the present study are on the same linkage group covering a distance of 20.9 cM. Lokko et al (2004) identified seven different SSR markers associated with resistance to CMD in population derived from TMS30572 and TME117 and established the contribution of both the parents towards the resistance to CMD.…”

“…However, Lokko et al (2005) identified two more DNA markers SSRY106 and E-ACC/M-CTC (an AFLP marker) having strong association with resistance to CMD. By considering the results of the present study and the studies of Akano et al (2002) and Lokko et al (2005), it is very clear that the genomic regions around the marker viz. SSRY28 harbours one of the major gene conferring resistance to CMD irrespective of resistance sources.…”

“…The level of resistance to CMD resistance across F 1 seedlings of CO2 x MNga-1 was done six months after planting based on the scoring system adopted by Hahn et al (1980b) and Akano et al (2002). The level of resistance was determined based on the 1-5 scores established as per the severity of the symptoms developed, Score 1-Unaffected shoots, no symptoms in leaves; Score 2-Mild chlorosis, mild distortions at bases of most leaves, while the remaining parts of the leaves and leaflets appear green and healthy; Score 3-Pronounced mosaic pattern on most leaves, narrowing and distortion of the lower one-third of the leaflets; Score 4-Severe mosaic distortion of two thirds of most leaves and general reduction of leaf size and stunting of shoots and Score 5-Very severe mosaic symptoms on all leaves, distortion, twisting, mis-shapen and severe leaf reductions of most leaves accompanied by severe stunting of plants.…”

“…Similar attempts to detect the DNA markers associated with resistance to CMD led to identification of SSRY28 by two independent groups involving the Nigerian landraces viz. TME3 (Akano et al, 2002) and TME7 (Lokko et al, 2005). Further saturation of TME3 resistance source with more number of SSR markers and RAPD markers, two more tightly linked markers viz., NS158 and RME-1 were identified (Moreno Tomkins, & Fregene, 2004).…”

“…The studies conducted by Hahn, Howland, and Terry (1980a) ;Hahn, Terry, and Leuschner (1980b) to establish the genetics of resistance to CMD indicated the possibility of several genes responsible for resistance. Akano, Dixon, Mba, Barrera, and Fregene (2002) reported that the resistance could be due to a major dominant gene (CMD2). Given the importance of CMD2 to cassava production, and the limited knowledge on host plant resistance to geminiviruses, an effort was initiated to clone the gene.…”