Breeding investigations on utility of maize streak virus resistant germplasm for hybrid development in the tropics.

Abstract

Maize (Zea mays L.) supports millions of livelihoods in sub-Saharan Africa (SSA) in terms of

food and feed. Production of the crop is however limited by several factors, among these, maize

streak virus (MSV) disease. Although extensively studied, MSV remains a serious problem in

SSA due to several challenges in breeding MSV resistant maize varieties. These include

integration of MSV resistant germplasm from different backgrounds, reliance on a few resistant

sources, and genotype x environment interactions. This study was designed to assess the

breeding potential of several MSV resistant lines in hybrid combinations. Understanding

architecture of genetic divergence and background of these genotypes would greatly aid in

breeding high yielding and stable MSV resistant hybrids. Experiments were conducted during

2010 to 2012 seasons in Kenya. Diallel crosses and SSR markers were used to characterize

MSV resistant maize inbred lines from three programs of CIMMYT, KARI and IITA.

In general, this study revealed that MSV is still an important problem in Kenya with high

incidence and severity levels in the farmers’ fields. The levels of MSV resistance in locally

grown hybrids needs to be improved. Farmers challenged breeders to develop new hybrids that

combine early maturing, high yield potential and MSV resistance.

The study was successful in identifying the best eight inbred lines for use in breeding new maize

hybrids with MSV resistance. The nature of gene effects was established for the first time, in

particular the role of epistasis and G x E in conditioning MSV resistance in hybrids. Results

indicate serious implications for previous models that ignored epistasis in studying MSV

resistance in maize. The inbreds Z419, S558, CML509 and Osu23i, displayed high levels of

epistasis for MSV resistance. Unless strong sources of MSV resistance, such as MUL114 and

CML509, are used, breeding resistant hybrids will require parents that carry dominant

resistance genes. The additive-dominance model was adequate to explain northern leaf blight

(NLB) resistance in hybrids, indicating fewer complications in breeding NLB resistant hybrids.

The study also reveals that SSR genetic distance data can be used to predict hybrid

performance, especially when the correct set of markers is used. Many previous studies have

not found any significant relationship between genetic distance and heterosis, due to large

G x E and use of a wrong set of markers. The diallel analysis and SSR data established the

important heterotic groups, which will be exploited for efficient development of MSV resistant

maize hybrids. These strategies will be recommended to programs that emphasize MSV

resistance in maize hybrids.