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Background
To minimize the need for costly and
time-consuming phenotypic selection in multiple environments,
considerable research has been undertaken to investigate suitable
molecular markers linked to resistant loci to MSV (Kyetere et al.,
1999; Pernet et al., 1999a and b; Weltz et al., 1998), GLS
(Bubeck et al., 1993; Clements et al., 2000; Lehmensiek
et al., 2001 and Saghai-Maroof et al., 1996) and NLB (Dingerdissen
et al., 1996; Freymark et al., 1993; Schechert et al.,
1999; Weltz et al., 1998). Only the loci for MSV have been found
consistent in multiple field tests in Zimbabwe (Kyetere et al.,
1999; Pernet et al., 1999a and b), Reunion (Pernet et al.,
1999a and b; Weltz et al., 1998) and Uganda (Kyetere et al.,
1999). However, the genetic map for MSV resistance referred to above was
based on restriction fragment length polymorphisms (RFLPs) that have
restrictive throughput potentials.
Despite its
widespread use as human food and animal feed, the maize consumed in
Uganda is low in essential amino acids. Recently, the national maize
program acquired a number of nutritional enhanced (with elevated levels
of lysine and tryptophan) elite maize lines from the CIMMYT known as
quality protein maize (QPM). While agronomically acceptable, these QPM
maize lines are susceptible to one or the other of the major foliar
diseases in Uganda. For example, the recently released QPM variety,
Longe 5 (Obatanpa), is susceptible to NLB. Converting well adapted maize
varieties such Longe 1 (MSV and NLB) could quickly solve this problem.
However, QPM line conversions through conventional breeding methods
requires costly and time-consuming phenotypic evaluations (Dreher
et al, 2000).
Single sequence repeats (SSRs) are a
new generation of markers based on the polymerase chain reactions that
has significantly increased the map densities of a number of animals and
plants. SSR are the principal assay used in human and animal genetics,
largely because they are abundant and have a high information content.
The Maize Mapping Project has so far developed a large number of
scorable SSR markers from expressed sequence tags (Prof. Rich Pratt,
personal communication). Attempts to determine associations of these
SSRs with important agronomic traits are underway. CIMMYT-Mexico has
already identified SSR markers linked MSV resistance and to the QPM
trait (Dr. J.M Ribaut, personal communication). However,
additional research will be required to test the effectiveness of these
SSR marker in populations other than those from which they were
developed.
Breeding in Uganda
The National Agricultural Research
Organization-Maize Program like many breeding programs in Uganda relies
on traditional breeding methods. While this has produced important
genetic gains in maize in Uganda e.g. release of agronomically
acceptable and disease resistant lines such as KWCA, Longe1 and Longe2,
the breeding progress has been slow. The MAS program we are proposing
can potentially enhance the efficiency and reduce the time required to
develop suitable germplasm products for Uganda.
MAS is most likely to confer an advantage over
conventional breeding techniques when phenotypic screening is
particularly expensive or difficult, as is the case with MSV and the QPM
opaque2 gene.
Effective selection for MSV resistance and the QPM opaque2
allele requires
costly and time consuming phenotypic evaluations
(Dreher et al., G. 2000;
Kyetere et al,
1999). In addition MSV frequently occurs together with NLB and GLS making
accurate assessment of breeding lines difficult. Also where the desired
disease did not develop for whatever reason, the researcher would have
to wait for another season to acquire needed data.
Overall objective
To incorporate MSV resistance and the
QPM opaque2 allele into elite maize lines using SSR markers.
However, much debate is still continuing concerning the utility and cost
of molecular marker assisted selection (MAS). MAS does not provide a
“silver bullet” but it can provide time savings and cost-effective
strategies for many types of projects
(Dreher et al, 2000). The project we are proposing is
such a project, but questions must still be answered before we
understand how best to optimize and implement a MAS strategy.
Specific Objectives
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To determine simple sequence repeat
(SSR) polymorphisms for Maize
streak virus (MSV) resistance and the Quality Protein Maize (QPM)
traits.
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To introduce the opaque-2
allele and improve MSV resistance for a least two Ugandan maize
genotypes.
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To compare MAS to
conventional phenotypic selection for MSV resistance and the
opaque-2.
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To train two graduate students in
polymerase chain reaction (PCR) based SSR molecular marker
techniques.
Collaborators
Justus
Imanywoha
Namulonge
Agricultural Research Institute, P.O. Box 7084, Kampala,
Uganda
E-mail:
naari@afsat.com Mobile Phone: 256-077-430072
Kevin Pixley
Maize Program, CIMMYT, P.O. Box MP 163 Mount Pleasant,
Harare ZIMBABWE. E-mail:
CIMMYT-ZIMBABWE@CGNET.COM. Phone: 263 (4) 301 807
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