Marker‐Assisted Introgression of Resistance to Cassava Mosaic Disease into Latin American Germplasm for the Genetic Improvement of Cassava in Africa

Abstract

The gene pools for breeding cassava (Manihot esculenta Crantz) in Africa currently contain only a fraction of the existing genetic variation found in Latin America where the crop originates. Our research aimed to broaden the genetic base in Africa by introducing Latin American (LA) germplasm. The first set of introductions comprised sexual seeds that led to the evaluation of 20,032 seedlings in Nigeria between 1990 and 1994. A second set comprised in vitro cultures, where the dominant CMD2 gene for cassava mosaic disease (CMD) resistance was introgressed into LA germplasm through marker‐assisted selection (MAS). Through MAS 156 genotypes were preselected for the gene and evaluated in Nigeria between 2004 and 2006. Initial results from the first set of introductions indicated that LA germplasm was highly susceptible to CMD, minimizing its usefulness in African cassava‐breeding programs. In the second set of introductions from LA, introgression of the CMD2 gene resulted in high CMD resistance under African field conditions. Now at advanced stages in the African breeding program, 14 genotypes combining CMD resistance and high yield are being evaluated. Marker‐assisted introgression of CMD resistance into LA germplasm has improved the potential value of LA germplasm for Africa and enhanced the prospect of elite LA genotypes being released as improved varieties in Africa.

Molecular-assisted selection for resistance to cassava mosaic disease in Manihot esculenta Crantz

“…In another study, Okogbenin et al (2007) made a first introduction of the germplasm from Latin America in Africa with materials from crosses between elite parental from breeding programs and a second introduction of material that was selected based on MAS (two microsatellites and six SCARs) and a subsequent evaluation of genotypes under natural conditions in a field that was infested with CMD in Nigeria. In the first set of entries, there was a devastating effect of the disease, infecting 100 % of the plants one month after planting.…”

“…In fact, the introduction of the CMD2 gene into various germplasm seems to be the most effective strategy in CMD control, as the incorporation of the CMD1 gene into exotic germplasm and subsequent field testing in Africa indicated that the incorporation of CMD1 has not been effective because this is a gene of minor quantitative effect (Fregene et al, 2006;Okogbenin et al, 2007). Moreover, the use of the dominant CMD2 gene has been successful in transferring gene resistance genotypes originally from Latin America (Okogbenin et al, 2007).…”

“…Moreover, the use of the dominant CMD2 gene has been successful in transferring gene resistance genotypes originally from Latin America (Okogbenin et al, 2007).…”

“…In addition, because the disease is controlled by just a few dominant effect genes, it is possible to implement marker-assisted selection (MAS) aimed at the indirect selection of resistant genotypes (Okogbenin et al, 2007).…”

“…Subsequently, other markers were identified flanking the CMD2 gene (Akano et al, 2002;Lokko et al, 2005;Fregene et al, 2006;Okogbenin et al, 2007). Faced with the prospect of this disease arriving and taking hold in Brazil, there is a pressing need to direct efforts towards the preventive identification of resistance sources, similar to what is being done in Africa where CMD is an endemic disease that causes significant economic loss.…”