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consistent with pedigree information. These lines were developed from a common genetic background for MSV resistance. The CIMMYT maize lines, CML505 and CML509, did not cluster in MC1 with the rest of the CML lines, which is also in agreement with pedigree information. These observations are supported by the study by Yuan et al. (2002) in which all the CIMMYT lines clustered together according to pedigree. According to Warburton et al.
(2002), CIMMYT inbred lines are usually drawn from a mixture of populations that contain a broad genetic base. The CML505 and CML509 lines were derived from the same population with adaptation to the tropical lowland and resistance to downy mildew disease. In agreement with pedigree information lines E46 and E47 were placed in the same cluster 2, because both lines were derived from ZM521. However, it was observed that the E-group of lines which were derived from the same population “ZM621” (except E7) were allocated to different clusters with three lines in MC3, two in MC2 and three in MC1, which is not congruent with pedigree information. This can be explained by the fact that the base population ZM621 is broad-based. It was developed as a synthetic hybrid population between heterotic group A and group B lines involving more than eight lines, and has gone through several cycles of recombination at CIMMYT in Harare.
The clustering of the Mozambican inbred LP lines also partly reflects the pedigree information and origin of the lines. The lines LP37D and LP37F are sister lines derived from the same base population Pop44 from CIMMYT, but they are placed in different sub-clusters since LP37D has dent grain texture while the LP37F has flint grain. Lines LP23 and LP19 were developed under lowland tropical conditions in Mozambique, and Nigeria, respectively, and were derived from different populations; hence they were grouped in different sub- clusters within MC1. The actual pedigree data for the line LP21 could not be established, so the observation that it was classified in the same sub-cluster as LP19 suggests that they have a similar genetic background or have similar allele frequencies. These lines may be placed in the same heterotic group in the programme in Mozambique. The importance of the origin of the lines is also reflected by the classification of PA1 in sub-cluster 2.2 together with line E46, E47 and E66, among others, which were derived from the mid-altitude populations ZM521 and ZM621 in Zimbabwe. The genetic distance matrices derived from the SSRs were highly correlated (r = 0.85), indicating that the SSRs have distinguishable power to detect polymorphism and are appropriate for genetic diversity analysis among maize inbred lines.
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2.4.2 Implications for breeding new hybrids
Studies report that for the production of hybrids with better yield performance, it is best to use lines with larger genetic distances as parents as these increase genetic variation (Yuan et al., 2002; Shahnejat-Bushehri et al., 2005; Biswas et al., 2008). Small genetic distances reveal that lines are closely related thus should not be crossed with each other if hybrid vigour is to be maximised. Among the 25 inbred lines, E75 and LP21 as well as E77 and LP21 were the most distantly related parental lines with the lowest similarity values of 0.15, indicating that the two lines are divergent and contain different allele frequencies which can be exploited to make hybrids. The lines E24 and LP19 with a low similarity value of 0.26 would also be recommended for use in making hybrids.
The lowland adapted and MSV resistant donor lines (CML505 and CML509) had low similarity values, ranging between 27 and 54%, with the set of lines from Mozambique (P and E series), indicating the existence of large genetic distances and, therefore, are divergent.
Hence, it would be expected that productive hybrids could be obtained by making crosses between the CMLs and the Mozambican lines because they have complimentary gene frequency that might result in high heterosis. The fact that LP21 and LP23 were placed in a different cluster with the MSV donor lines CML505 and CML509 suggests that productive hybrids can be produced by combining the DM resistant lines with the MSV resistant lines, respectively. The lines CML505 and CML509 were previously characterised, and displayed MSV resistance with ratings of 1.5 and 2, respectively (CIMMYT, 2009); whereas Denic (2005) characterised the LP inbred lines, including LP23 and LP21, and reported that they are highly resistant to DM. Most importantly, the results indicate that LP19 and LP21 are in the same sub-cluster while LP23, which originates in Nigeria, is in a different sub-cluster, which suggests that a three-way cross hybrid among these lines (LP19/LP21/LP23) should be productive. This analysis has been confirmed by the maize breeders in Mozambique from practical experience with these lines (1Fato, 2011; pers. comm.).
1Dr Pedro Fato, Chokwe Research Station, Institute of Agriculture of Mozambique (IIAM).
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2.4.3 Implications for developing new inbred lines
Additionally, classification of MSV resistance donor lines in two different clusters MC1 (P13 through to P18) and MC2 (CML505 and CML509) indicates that they can be regarded as different heterotic groups. This has implications for managing the breeding programme in Mozambique. In order to fix inbred lines over shorter periods, it is crucial to develop breeding populations by crossing lines with similar allele frequencies. Lines CML505 and CML509 are the potential donors for MSV resistance to the Mozambican lines in the cluster 2 (E66, E7, E46, E47 and E24). On the other hand, the MSV potential donor lines P13, P14, P15, P16, P17 and P18 could be used to improve MSV resistance in the other Mozambican lines in the cluster 1 (E80, E27, E71, LP19, LP37D, LP37F and LP23) because they are likely to have a similar gene frequency. Unfortunately there are no MSV resistance donor lines which were fitted in cluster 3; hence another set of potential donor lines for use to improve resistance of these lines (E72, E75 and E77) to MSV should be found.