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Molecular markers for
introgression of useful traits from wild Manihot relatives of
cassava: marker-assisted selection of disease and root quality traits
M.
Fregene, N. Morante, T. Sanchez, J. Marin, C. Ospina, E. Barrera, J.
Gutierrez, J. Guerrero, A. Bellotti, L. Santos, A. Alzate, S. Moreno, H.
Ceballos
Molecular markers associated with genes and quantitative trait loci (QTL)
controlling genes of agronomic interest provide a powerful means of
increasing heritability in the early stages of cassava breeding and for efficient
introgression of genes from wild relatives. Wild
Manihot
germplasm has many useful genes for the cultivated
M. esculenta.
Several accessions of
M.esculenta
sub spp. flabelifolia,
M. esculenta
sub spp. peruviana,
and M. tristis
collected in Brazil have high protein content, between 10 to 18% in dry
weight, and dry matter content. High resistance to whiteflies
and cassava green mites has been found in accessions of
M.esculenta
sub spp. flabelifolia.
Dramatically delayed physiological post-harvest deterioration (PPD) has
been identified
in an interspecific
hybrid between cassava and
Manihot walkerae.
An advanced backcross QTL scheme to introgress high protein is in its
fifth
year and delayed PPD from wild relatives into cassava in its third at CIAT.
Molecular marker-assisted selection (MAS) for resistance to the cassava
mosaic disease (CMD) has also been used to transfer resistance to the
disease into neotropical cassava gene pools.
Accessions of wild relatives of cassava with high protein or having
delayed post-harvest deterioration were crossed to elite cassava parental
lines and the trait evaluated. First backcross generations (BC1)
were generated and established in the
field.
Results reveal some wild genotypes such as OW 231-3, 280-2, OW132-2 and
OW284-1 have good general combining ability for root protein content.
Amino acid profile
revealed very high arginine, about half the total amino acids, and low
methionine and lysine in the roots, but high in leaves. A BC1
cross for high protein content is being evaluated this year. Evaluating
interspecifi
c hybrid CW429-1 revealed genes for delayed PPD transferred into cassava
from a wild relative. Results of mean PPD values at 5 days after harvest
ranged from 0, in CW 429-1 and MBRA 337, to 44.85%, in CM523-7. A BC1 half-sib
family for delayed PPD was established in the
field
and evaluated. A set of 500 simple sequence repeat markers (SSR) have been
employed to evaluate the parental genotypes of BC1 families
and polymorphic markers are currently being analysed in the progeny.
For MAS of CMD resistance, 18 F1 progenies
of TME-3, a local Nigeria cassava variety having the
CMD2
resistance gene, were crossed extensively to elite cassava parents of the
four neotropical cassava gene pools. A total of 1100 genotypes from 31 F1 crosses
between CIAT elite cassava parents and CMD-resistant parents were
successfully established in vitro, analysed with markers and 503 resistant
genotypes identified.
Evaluations of all 503 genotypes at 6 and 9 months after planting in
Tanzania and Nigeria revealed 224 genotypes with no visible foliar
symptoms for CMD and 176 genotypes did not show any visible foliar
symptoms for either CMD or CGM. Similar results were also obtained from
evaluations in India. MAS is routinely used to breed for resistance to CMD
in neotropical gene pools and to share the results with sub-Saharan Africa
and India.
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