Biology Reference
In-Depth Information
(e) Pyramiding CMD resistance
three cultivars are believed to be of African ori-
gin. To reduce the risk of introducing susceptible
germplasm into Asia, it has been suggested that
CMD-resistance germplasm from Africa should
be introduced with the aid of MAS (Bi et al.
2010).
Pyramiding genes is vital for increasing disease
resistance durability in cassava cultivars and pro-
vides the most potent strategy for reducing the
threat of CMGs. A key target in breeding is to
combine the different sources of CMD resis-
tance with the aid of markers. In the cassava
breeding CoP, F
1
populations developed using
parents of two sources of CMD genes (
CMD2
and
CMD3
) have been developed. Six genotypes
of the
CMD2
source (CR36-5, CR42-4, AR12-
45, CR52A-41, AR15-5, and AR36-2) were used
in crosses with TMS97/2205. Another clone,
96/1089A, identified as a high-resistance cassava
cultivar and described as a likely different source
of CMD resistance by IITA, is being used to pyra-
mid CMD-resistance genes. TMS96/1089A has
been used to develop a mapping population to tag
CMD QTLs in this parent in the cassava research
initiative of the CGIAR GCP. It is expected that
once additional markers for new sources of CMD
resistance are identified, MAS can be effectively
used to select for multiple sources of CMD resis-
tance in breeding populations.
CMD2
Resistance Profile
The best LA genotypes (64 in total), selected
after initial analysis of CMD resistance in Nige-
ria, were reevaluated and integrated into the
breeding scheme of the National Root Crops
Research Institute, Nigeria. The genotypes were
evaluated in Nigeria in order to validate their
CMD resistance profile and their stability at three
to four years. The LA genotypes were evalu-
ated using a symptom severity index (SSI) of
1- 5 (where 1 is symptomless and 5 is highly
severe). The results indicate that resistance lev-
els and response patterns vary among the geno-
types. While some genotypes were relatively
stable in CMD resistance, maintaining low
symptom expression (SSI 1 or 2) throughout the
growing season, some genotypes were found to
show a significant increase (to SSI 3) in symp-
tom expression (moderate) at disease peak period
and then recovered from the disease rapidly with
less symptom expression levels later. The highly
resistant genotypes often showed good resistance
at the peak pressure of the disease (often midway
into the season) and were therefore relatively sta-
ble. Of the 64 genotypes validated, 48 genotypes
were found to show good resistance whereas the
other 18 genotypes showed moderate resistance
to the disease (Okogbenin et al. 2012).
The 64 genotypes evaluated showed good
resistance to the disease, although with differ-
ential response. Three types of disease response
were observed in the
CMD2
genotypes: (1) the
highly resistant genotype, (2) the resistant geno-
type, and (3) moderately resistant genotype. Both
the highly resistant and the resistant genotypes
are generally low in disease symptoms with the
former almost symptomless for almost all of
the growing season and therefore with a lower
(f) Germplasm Screening for Parent
Selection in CMD Resistance Breeding
in Asia
CMD is a potential threat to cassava cultivation in
Southeast Asian (ASEAN) countries. Plantlets of
18 cassava cultivars collected from China, Thai-
land, and other ASEAN countries were infected
with CMD by means of agro-inoculation medi-
ated infection, and these plantlets developed var-
ious levels of CMD symptoms, indicating a lack
of resistance to CMD. There was a positive asso-
ciation between symptom severity scores and
accumulation levels of viral DNA in the dif-
ferent cultivars tested. Molecular marker anal-
ysis of these cultivars, using markers RME1,
SSRY28, and/or NS158, tested positive for the
marker allele associated with the
CMD2
gene in
only three of the cultivars (11Q, T7, and N13),
which showed moderate resistance to CMD. The
Search WWH ::
Custom Search