MICROSATELLITE DIVERSITY OF M3S MAIZE POPULATION UNDER RECURRENT SELECTION

Many studies reported that genetic diversity in maize has been decreasing at an alarming rate resulting in genetic uniformity of commercial hybrids. Thus, characterisation and introgression of exotic germplasm to increase the genetic diversity of current maize germplasm has been continuous in recent years. In the 1990 a synthetic population of maize designated Maksimir 3 Synthetic (M3S) was formed at the Faculty of Agriculture University of Zagreb. The origin of the parental lines (progenitors) of the M3S population traces back to several open pollinated varieties and local populations from different parts of the Republic of Croatia and other parts of the former Yugoslavia and most of them showed equally good combining ability for grain yield with both the BSSS and the Lancaster testers (Pejić and Kozumplik, 1990). In order to improve grain yield and other agronomic traits in the M3S population a recurrent selection program was initiated in the population. Two cycles of selection have been successful in improving grain yield in the population (Šarčević et al., 2004). The objectives of this study were: i) to determine genetic distances among the parental inbred lines of the M3S population (M3S progenitors), the M3S population before and after two cycles of recurrent selection, and elite inbred lines representing the BSSS and Lancaster heterotic group; and ii) to examine the effect of two cycles of recurrent selection on changes of allele frequency and population structure. Nine M3S progenitors, three BSSS lines, and three Lancaster lines were genotyped at 24 SSR loci, out of which nine randomly chosen loci were used for genotyping 96 individuals from both C0 (the M3S population before selection) and from C2 (M3S population after two cycles of selection). Mean genetic distance among nine M3S progenitors was 0.61 indicating a broad genetic base of the M3S population. High mean genetic distance was found between M3S progenitors and BSSS lines (0.69) and M3S progenitors and Lancaster lines (0.71). This indicates that the M3S population represents a germplasm source unrelated to both the BSSS and Lancaster germplasm. Mean genetic distance between the M3S population and BSSS as well as Lancaster lines decreased slightly after two cycles of recurrent selection suggesting the need to introduce testers from both groups in future selection in the M3S population in order to maintain heterotic complementarity of the M3S population to these groups. A test of selective neutrality identified several non-neutral loci in the population whose allele frequency changes from the C0 to the C2 cannot be explained by genetic drift. Mean gene diversity decreased from 0.57 in the C0 to 0.45 in the C2, resulting in an interpopulation inbreeding coefficient of 0.15. The number of pairs of loci in significant linkage disequilibrium decreased after two cycles of selection. Statistically significant departures from Hardy-Weinberg equilibrium toward an excess of homozygous genotypes were observed at several loci likely as a result of assortative mating.