No 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.

Several molecular marker systems have been developed for assessing genetic diversity in crop germplasm collections. A trade-off often exists between the number of loci that can feasibly be sampled by a marker system and the amount of information provided by each locus. We compared the usefulness of two marker systems for revealing genetic diversity and population structure in cassava (Manihot esculenta Crantz): simple sequence repeats (SSRs) and diversity array technology (DArT) markers. DArTs survey many more loci per reaction than do SSRs; however, as bi-allelic, dominant markers, DArTs provide less polymorphism information per locus. Genetic differentiation was assessed in a randomly selected set of 436 cassava accessions, consisting of 155 African and 281 Latin American accessions. A genome-wide set of 36 SSR markers and a DArT array of approximately 1000 polymorphic clones were used to assess genetic diversity and differentiation. Cluster analyses were performed using principal coordinate analysis (PCoA). Results were compared with a priori expectations of genetic differentiation based on previous genetic analyses. Analyses of the two datasets generated broadly similar clustering patterns. However, SSRs revealed greater differentiation than DArTs, and more effectively recovered patterns of genetic differentiation observed in previous analyses (differentiation between Latin American and African accessions, and some geographical differentiation within each of these groups). These results suggest that SSR markers, while low throughput in comparison with DArTs, are relatively better at detecting genetic differentiation in cassava germplasm collections. Nonetheless, DArTs will likely prove useful in ‘orphan crop’ species, where alternative molecular markers have not been developed.

No abstract

El desarrollo del mapa genético de yuca ha facilitado las investigaciones en la búsqueda de genes asociados con resistencias a plagas que disminuyen producción y calidad. Las herramientas moleculares como los marcadores facilitan la identificación de los genes que dominan la resistencia a plagas como el caso del ácaro verde (AVY) Mononychellus tanajoa en yuca (Manihot esculenta). Inicialmente se identificaron los individuos de poblaciones retrocruces (BC1) progenie de las cuatro familias denominadas CW. De estos BC1 se seleccionaron individuos resistentes y susceptibles teniendo en cuenta la escala de daño reportada por el programa de Entomología del Centro Internacional de Agricultura Tropical. Se evaluaron 500 cebadores microsatélites mediante análisis de grupos segregantes (BSA), cinco de ellos expresaron polimorfismo y mostraron diferencias significativas entre los parentales, los grupos contrastantes y tres cebadores SSRY 11 SSRY 346 y NS 1099. Al realizar los análisis estadísticos correspondientes se encontró que los cebadores NS 1099 y NS 346 presentaron asociación entre estos y los individuos de las familias CW 67 CW 66 y CW 67 respectivamente. Las familias CW67 Y CW66 presentaron la mejor asociación con los marcadores y éstos presentaron mayor asociación con la característica de resistencia al daño causado por M. tanajoa en la población segregante (cebador NS1099 familias CW66 0.56 CW67 0.61 y NS346 familia CW67 0.61). Con base en estos resultados se puede sugerir la utilización de microsatélites en análisis posteriores en otras poblaciones para lograr la identificación de algunas regiones cromosómicas que confieren la resistencia al ácaro verde de la yuca.