Maize is the chief support of most of Africa’s rural economies (Oluwafemi et al., 2008;
Stevens, 2008), including Mozambique, in which it is the staple food and is grown in all of its agro-ecological zones (Denic et al., 2001). Despite this, the average yield of maize
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in Africa is the lowest in the world and, as a result, fails to meet the high demands in most Sub-Saharan countries (Denic et al., 2001; Magenya et al., 2008). Numerous viral pathogens affect maize productivity, with maize streak virus (MSV) disease being considered as the most significant biological threat to food security in Africa (Bosque- Perez, 2000; Shepherd et al., 2007; Martin and Shepherd, 2009). This is a major concern and can only be addressed by solving production constraints to improve crop yields.
Virus resistance is usually associated with one or two major resistance loci, which facilitate marker-assisted selection (MAS), but resistance genes have been found to cluster in the maize genome (Redinbaugh et al., 2004). Genomic regions associated with resistance to the MSV disease have been identified in several studies using different populations in diverse environments and these studies have revealed that resistance is quantitatively inherited with a varying number of genes involved (Pernet et al., 1999; Welz et al., 1998; Mawere et al., 2006). Mawere et al. (2006) reported that MSV resistance is expressed by a major gene and two or three modifying genes. Pernet et al. (1999) also proposed that MSV resistance was controlled by two genetic systems, one from a major gene on the short arm of chromosome 1 and the other conditioned by minor genes on chromosomes 2, 3 and 10, that confer quantitative resistance. Minor quantitative trait loci (QTL) effects have been detected at bins 3.06, 5.03 and 8.07 (Asea et al., 2008). The major QTL, designated msv1, was identified on the short arm of chromosome 1 (1S – bin1.04) (Welz et al., 1998; Kyetere et al., 1999; Mawere et al., 2006). The stability of QTL across populations has been shown to be variable; however, this is not the case for maize streak virus disease (MSVD) QTL (Pernet et al., 1999).
A study conducted by Danson et al. (2006) used three DNA markers: bnlg1811, umc1917 and umc1144, which are contained between bin 1.04 and 1.05 of maize chromosome 1 to screen 115 recombinant inbred lines (RILs) for resistance to MSV disease. These markers were able to differentiate resistant from susceptible lines. A study by Asea et al. (2008) further examined a consensus MSV QTL in bin 1.04 as a potential target for selection in improving host resistance. Maize streak field evaluations and subsequent selections were conducted in Zimbabwe in a population of 410 F2:3 lines
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derived from hybridisation between inbred line CML202 with known resistance to MSV and the susceptible line VP31. It was concluded that the major locus conferring resistance to MSV on chromosome 1 was significant (P<0.05) for resistance across seasons and explained 23% of phenotypic variations in the F2:3 generation. Markers used for this current study were developed by Danson et al. (2006); Asea et al. (2008), the maize database (http://www.maizegdb.org) and from a study by Lagat et al. (2008) in which the QTL for resistance to MSVD in one resistant source MAL13 crossed to one elite line, MAL9, were mapped using SSRs. Conventional maize breeders may benefit from the use of molecular markers in order to improve selection intensity and maximise genetic gain (Collard et al., 2005).
The adoption of hybrids in maize production has resulted in increased yields across the world (Warburton et al., 2002). Maize breeding relies on the available genetic diversity (Karanja et al., 2009). Improved hybrids of maize are developed by making use of information on the genetic relationships and diversity among elite materials (Dias et al., 2003; Diniz et al., 2005). Evaluating genetic diversity among the elite lines aids in the estimation of genetic variation and thus the degree of heterosis to be expected among segregating progeny for pure-line cultivar development (Biswas et al., 2008; Salem et al., 2008; Karanja et al., 2009).
Heterosis, also known as hybrid vigour, is a phenomenon in which the offspring show superiority over their parents either in yield, vigour, increased size, rate of growth or other reproductive factors (Duvick, 1999; Virmani et al., 2003). The term coined by Shull (1952) can be used for the expression of adaptive traits like increased resistance to disease and drought tolerance, with the hybrid of choice exceeding the best parent in superiority. However, superiority is lost with every successive generation of self- fertilisation, thus maximum heterosis is expressed in the F1 generation (Meyer et al., 2004). The manifestation of heterosis depends on genetic divergence of the two parental varieties (Hallauer and Miranda, 1988). Morphological, pedigree, physiological, biochemical and molecular data can be used to identify elite inbred lines to be crossed for a superior hybrid (Smith and Smith, 1989). However, molecular markers can detect variation at the DNA sequence level (Diniz et al., 2005) and genetic distances (GD) are
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used to group similar germplasm as the first step in identifying potentially useful heterotic patterns (Melchinger, 1999).
Simple sequence repeat (SSR) markers, also known as microsatellites (He et al., 2003;
Molnar et al., 2003), have been extensively used to characterise germplasm collections in major cereal crops including wheat (Salem et al., 2008; Ijaz and Khan, 2009) and maize (Taramino and Tingey, 1996; Smith et al., 1997; Li et al., 2002; Danson et al., 2006; Aguiar et al., 2008; Cholastova et al., 2011). Different repeat numbers in SSRs can be treated as separate “alleles” and the site can be treated as highly polymorphic with multiple alleles for the detection of variation in populations (Akkaya et al., 1992).
Microsatellites are highly abundant, simple to analyse, co-dominant, economical and are easily assayed using PCR with primers specific to conserved regions flanking the repeat array (Yu et al., 2000). Compared with other marker types, SSRs are advantageous due to their abundance in plant genomes and large number of alleles per locus making them highly polymorphic even among closely related cultivars due to naturally occurring mutations, and thus they can distinguish between closely related species (Brown et al., 1996; Weising et al., 2005), providing greater power of discrimination. Hence, they are useful for assigning heterotic groups for maize lines (Enoki et al., 2002; Li et al., 2002;
Xia-Su et al., 2004).
The objectives of the study were, therefore, to determine the genetic diversity among the 25 maize inbred lines using 19 SSR markers which are known to be associated with MSV disease resistance in maize. The information will be used in the selection of the most appropriate parents out of the potential MSV resistance donors for the introgression of the MSV resistant gene, msv1 into the different Mozambican lines that are adapted to the lowland environment but are susceptible to MSV. The information would be crucial in devising future hybrid breeding programmes that will emphasise MSV resistance, in Mozambique.
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2.2 Materials and methods