Marker-Assisted Breeding for Cassava Mosaic Disease Resistance - Translational Genomics for Crop Breeding: Biotic Stress - page 312

severity index than the latter. The disease
response profile of the moderately resistant geno-
types suggests evidence of good and efficient
recovery mechanism in the disease response pat-
tern of the CMD2 gene.
The results also indicate that the resistance
status of these genotypes was influenced by
their different genetic backgrounds, considering
that the 64 genotypes evaluated are of differ-
ent cross combinations representing 29 families.
However, disease symptom expression observed
among the CRs was not distinctly different from
those of the ARs. Resistance mechanisms for
the CMD2 source appear to be related to con-
stricted long-distance movement or gene silenc-
ing of the virus, as plants may be infected but ulti-
mately recover (Akano et al. 2002). CMD2 has
been shown to provide resistance for all CMGs,
including those from India, indicating that the
mechanism of resistance is probably generic for
all CMG and pointing to a single virus aviru-
lence protein as a resistance (R) protein interact-
ing elicitor of resistance (Fauquet pers. com.).
The CMD-resistant TME3, with the resis-
tant locus CMD2 , was challenged with differ-
ent species of CMGs along with the CMD sus-
ceptible cultivar 60444 (Fauquet pers. com.).
TME3 showed a very high level of resistance to
ACMV, with very mild symptoms in the inocu-
lated leaves, and it did not show systemic infec-
tion. In contrast EACMV-like viruses showed
varied levels of infection in TME3, which later
recovered completely. There is a very strong
correlation between the visual symptom recov-
ery and the virus content (Fauquet pers. com.).
Although EACMV-UG did not produce any vis-
ible symptoms in TME3 at 35dpi, PCR analysis
showed the presence of virus in the systemic
leaves. Thus TME3 does not confer immunity
to CMGs, but does result in drastically reduced
virus accumulation and symptoms in compari-
son to susceptible cultivars for all tested gem-
iniviruses. Viral DNA accumulation was posi-
tively correlated to symptom severity for ACMV
and EACMV-like viruses, as in cultivar 60444
(Chellappan et al. 2004).

These studies clearly demonstrate that the par-
ticipation of a virologist is necessary in a CMD
and CBSD resistance program. Understanding
the response of each type of resistance to each
virus is essential in order to breed better cul-
tivars and to evaluate the response of the lines
under selection to make the best choice possible.
Too many times the exact nature of the viruses
in the fields influencing resistance selection is
unknown and grouped under the term CMD, and
this is not sufficient. Such grouping leads to mis-
interpretation of data, to wrong hypotheses, or to
wrong explanations. It is clear that CMD2 pro-
vides an extremely high level of resistance to
ACMV - close to immunity - but only mod-
erate resistance to EACMV-like viruses. Until
the nature of the triggering viral protein(s) for
resistance is/are known, response of plants to
these two different types of viruses has to be
considered separately. A synergistic response of
the plant to a dual infection can easily be inter-
preted as a resistance breakdown, while it could
be an immediate response to the mixture of the
two types of viruses. A flush of symptoms to
EACMV-like viruses can easily be interpreted
as a susceptible response, while the plant may
recover after some time. These field data could
mask and somewhat undermine the MAS data.
Evidently the situation could be more complex if
new CMG recombinants emerge, and here too the
participation of a virologist would be essential. A
record of the vector populations in the breeding
fields could also be a very good indicator and is
highly recommended, as it seems obvious, albeit
for unknown reasons, that pandemics in East and
Central Africa greatly depend on the population
build up.

BAC Sequencing Reveals New
Markers for CMD2 Gene

To elucidate the function of CMD2, further
attempts were made to clone the gene via
positional cloning, using a large full-sib cross-
developed from noninbred parents and used
for fine mapping (as described above) and a