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 togther 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

1. To determine simple sequence repeat (SSR) polymorphisms for Maize streak virus (MSV) resistance and the Quality Protein Maize (QPM) traits.

2. To introduce the opaque-2 allele and improve MSV resistance for a least two Ugandan maize genotypes.

3. To compare MAS to conventional phenotypic selection for MSV resistance and the opaque-2.

4. 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