Exploiting heterosis in maize through molecular marker technology

Heterosis is the superiority in performance of hybrid individuals compared with their parents. Although its genetic basis has not been fully understood, breeders have exploited heterosis resulting to significant increase in the yields of crops particularly maize. Heterotic group and heterotic pattern are tools that are used to exploit heterosis in hybrid maize breeding. Heterotic group is a group of related or unrelated genotypes from the same or different populations, which display similar combining ability or heterosis when crossed with genotypes from other genetically distinct germplasm groups. Heterotic pattern on the other hand denotes a specific pair of heterotic groups, which express high heterosis and consequently high hybrid performance in their cross. Heterotic groups and patterns are tools for sorting, managing and sampling germplasm and if such tools were not used, breeding programs might resort to crossing and testing parents in a more haphazard manner. Heterotic groups could be established by diallel analysis, testcrossing, reciprocal recurrent selection, and through molecular markers. Molecular markers provide an extremely powerful tool for grouping germplasm based on genetic similarity. Molecular marker systems such as Restriction Fragment Length Polymorphisms (RFLPs), Amplified Fragment Length Polymorphisms (AFLPs), Random Amplified Polymorphic DNAs (RAPDs) and Simple Sequence Repeats (SSRs) allow the detection of differences among germplasm at the DNA level. The authors used SSRs to assess the genetic diversity among thirty-three elite yellow maize inbred lines of the Corn Breeding Program of IPB [Institute of Plant Breeding]. The inbreds were clustered based on the matrix of genetic dissimilarities. The objective of the study was to verify the usefulness of the clusters, generated through molecular marker assay, as possible heterotic groups by evaluating intra and inter-group crosses. Cluster analysis placed the inbreds into five distinct groups, which is consistent with pedigree data. Field experiment showed that in general, yield, heterosis and specific combining ability (SCA) are higher between inter-group than within group crosses. This is consistent with theoretical expectations that SCA and heterosis are lower within a heterotic group compared to between group crosses. The result of the study made the authors pool inbreds of Group 1 (G1), Group 2 (G2) and Group 3 (G3) into one heterotic group, designated as Group A and Group 5 (G5)-inbred as Group B. This is now the new heterotic pattern of the corn breeding program of IPB. Based on the general combining ability (GCA) estimates the lines used, the authors selected Pi23 and P2S2 as the testers from Group A and B, respectively. The results allowed them to sort their inbred lines by testcrossing them to Pi23 and P2S2. The authors identified hybrids that performed well in combination with Pi23 and P2S2 and they will evaluate these hybrids further to determine their merit as cultivars. Broad-based synthetics were also developed based on the heterotic groups that they generated. Their study have shown that SSRs can exploit heterosis by partitioning breeding materials into well-defined or new heterotic groups. However, field experiments should be conducted to confirm the groupings generated from molecular marker data