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Assessing the genetic diversity of Portuguese maize germplasm using microsatellite markers

Portugal, with its privileged historical and geographical position as an enter point of new species in Europe, soon has contacted with and cultivated maize. A very extended collection of maize germplasm has been preserved in the Portuguese Bank of Plant Germplasm (BPGV) in Braga, Portugal. The traditional Portuguese maize landraces here maintained, may represent valuable source of interesting genes to introduce into modern cultivars. In the present study, microsatellite markers were used for germplasm analysis and estimation of the genetic relationship between 104 accessions of Zea mays L. from the Portuguese gene bank collection. 102 maize accessions were chosen to represent the diversity of dent and flint maize germplasm found in Portugal in the past. The American elite inbreds, B73 and Mo17, were included in the analysis for comparison. Seven SSRs loci, located on different chromosomes and with different repeat types and repeat base compositions, were selected from MaizeDB (www.agron.missouri.edu) for the analysis. The number of alleles per microsatellite marker varied from 2 to 25, with an average of 8.29, and a total of 58 different alleles were detected. The polymorphism information content (PIC), a measure of the allelic diversity at a locus, was estimated as described on Botstein et al. (1980) for each of the polymorphic SSR loci detected in the present study. The detected PIC values varied from 0.375 to 0.9011, with an average of 0.7010. The proportion of shared alleles distance (Dps) (Bowcock et al., 1994), defined as 1 - the proportion of shared alleles between the multiple locus genotype of two individuals, was calculated. Among the 102 Portuguese maize accessions, the average distance was 0.7622, with a minimum of 0 (indistinguishable pairs) for 3 pairs of lines and a maximum of 1 (with no common alleles) for 696 pairs of lines showing the existence of large variability between accessions. The average distance of Mo17 to the 102 other accessions was 0.8065, and of B73 was 0.7497. In order to visualize the relationship among the maize accessions, cluster analysis was performed with NTSYS-pc based on the unweighted pair-group method with arithmetic average (UPGMA). The cophenetic correlation coefficient was calculated, and Mantel's test was performed to check the goodness of fit of a cluster analysis for the matrix on which it was based. The same procedure was employed to calculate cophenetic correlation coefficients among matrices based on different distance measures. Cluster analysis showed a good fit to the matrix on which it was based giving significant cophenetic correlation coefficient (r = 0.5684 ; p < 0.001 ; 1000 permutations). The resulting dendrogram point up that almost all of the accessions could be distinguished and clustered into two large groups. Within these two broad germplasm groups, accessions were grouped in a manner that is to some extent consistent with their known pedigree. The indistinguishable pairs of accessions were accessions with a common origin. The pattern of genetic variation among these maize accessions was investigated by the analysis of molecular variance (AMOVA) approach (Excoffier et al., 1992) using WINAMOVA 1.55 program. Total genetic variation was partitioned according to a number of traits important both in breeding and for line identification (as endosperm type, kernel and cob colour, plant height and initial plant vigour). Different groups of accessions were established in agreement to these different grouping patterns. The significance of &#61542; -statistics was obtained non-parametrically by 1000 permutations. Although most of the genetic diversity was attributable to differences among accessions within the different groups established (around 96% in all the cases), significant &#61542; -values among groups (p<0.01) suggesting the existence of phenotypic differentiation, corroborating our grouping hypothesis. There is an urgent necessity for both maintaining genetic diversity for crop improvement and improving the quality of our genetic resource management. Molecular markers can be fundamental to support these issues. On this study, SSR markers showed to be effective to characterize and identify maize accessions, and demonstrate association among them, confirming the good suitability of SSR markers for the analysis of germplasm collections.

maize; microsatellites; genetic diversity; germplazm

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