Microsatellite markers based assessment of genetic diversity and construction of heterotic groups in maize

A study was undertaken to elucidate the nature and extent of differentiation and divergence among eighteen inbred lines of maize and to separate them into heterotic groups based on the analysis of targeted microsatellite sites. Experimental materials of the present study comprised fifteen inbred lines, three testers and forty five single cross experimental hybrids, which were evaluated in randomized block design with three replications under rabi and kharif seasons. Statistical analyses to derive inferences regarding divergence pattern of parental lines and heterosis manifestation in experimental hybrids were carried out using the data recorded on days to tasseling, days to silking, days to brown husk, plant height, ear height, ear length, number of kernels per ear and grain yield per plant. Numerical taxonomic approach of classification using these eight quantitative characters of agronomic importance based average taxonomic distance was done to discriminate the inbred lines into different clusters. Inbred lines were also raised in pots for extraction of genomic DNA from the young seedlings. Amplification of the genomic DNA was carried out using a panel of twenty eight microsatellite sites specific primer pairs covering all the chromosomes existing in the genome. Microsatellites based allelic diversity data were utilized for classification and formation of heterotic groups of the inbred lines. Analysis of variance revealed the existence of significant differences among the inbred lines and hybrids for all the characters recorded under both the seasons. Mean performance of parents and hybrids significantly varied for the eight quantitative characters under study. Among the inbred lines, CML467 recorded significantly higher grain yield per plant followed by tester CML165, which was statistically at par to inbred lines LM13, HKI586, CML465, HKI162 and HKI323B over the environments. Sixteen crosses recorded significantly higher mean performance and significantly positive heterosis for grain yield per plant. Among these sixteen crosses, the highest yielding cross HKI162 × CML161 was statistically at par to five crosses, namely, CML468 × CML161, CML471 ×CML161, HKI323B × CML161, CML469 × CML165 and CML471 × CML165 for grain yield per plant. Taxonomic distance based cluster analysis revealed ample genetic diversity among inbred lines, which occupied different clusters during rabi and kharif season and across the seasons. The results indicated a close correspondence with respect to clustering pattern of inbred lines during rabi and kharif seasons. A perusal of dendrogram generated on the basis of eight quantitative traits indicated that the inbred lines CML467, CML470 and DH2012 were relatively more diverse from the remaining inbred lines under evaluation. Altogether 296 allelic variants were detected amongst amplified products generated with 28 primer pairs. A total of 49 loci were assigned to 28 primer pairs with an average of 6.04 alleles per locus The number of alleles ranged from 6 in the cases of umc1136, phi072 and umc1179 to 16 in case of phi053 with a range between 56 to 352 bp. Considering the number of alleles generated by different primer pairs in conjunction with the level of polymorphism, the primers umc1297, phi053, umc1266, phi093, bnlg118, phi034, phi115, phi065 and phi084 appeared to be more informative primers. Among the inbred lines under molecular characterization, CML163 and CML467 appeared as the most diverse genotypes. A remarkably higher level of genetic polymorphism was revealed by the use of 28 microsatellite markers. Both quantitative and molecular data based cluster analyses revealed that inbred lines CML 468 and CML469 are closely related to each other. Remarkably greater extent of similarity was also noticed between inbred lines HKI323B and HKI586. All the three testers, namely, CML161, CML165 and CML163, were included in same cluster in both the cases and seemed to be closely related. However cluster analysis using quantitative characters and microsatellite markers based data did not exhibit a complete correspondence. Parental genetic distance based on quantitative data or genetic similarity based on molecular data did not exhibit significant association with grain yield heterosis in hybrids. Results clearly indicated that a high grain yield is not always obtained from hybrids that have high parental genetic distance value, but generally from those with moderate to high genetic distance. A comparative assessment of heterotic groups formed by using microsatellite markers based genetic similarity, hybrid index value and hybrid mean value, indicated that CML471 and CML115 or HKI323B and HKI586 or DH2012 and LM13 belonged to same heterotic group in all the cases. The distribution pattern of the entries into different heterotic groups formed on the basis of hybrid index value and hybrid mean value corresponded with the microsatellite markers based heterotic groups to the extent of 75% and 67% in the first heterotic group, 40% and 29% in the second heterotic group and 33% and 40% in the third heterotic group, respectively. Overall coincidences of inbred lines in heterotic groups formed on the basis of microsatellite markers with the hybrid index value and hybrid mean value based heterotic groups were considerably higher. Results, therefore, indicated that microsatellite markers may be effectively and efficiently utilized to assign the inbred lines to heterotic groups for grain yield and to establish heterotic groups of maize inbred lines.