Mapping genetic factors affecting heterosis in maize

Mapping genetics fators affecting heterosis in maize Elisabetta Frascaroli*, Maria Angela Canè*, Pierangelo Landi*, Giorgio Pea†, Michele Morgante‡, Mario Enrico Pè† * Dipartimento di Scienze e Tecnologie Agroambientali, Università di Bologna, Bologna, Italy † Dipartimento di Scienze Biomolecolari e Biotecnologie, Università di Milano, Milano, Italy ‡ Dipartimento di Scienze Agrarie ed Ambientali, Università di Udine, and Istituto di Genomica Applicata Parco Scientifico e Tecnologico di Udine "Luigi Danieli", Udine, Italy Abstract The term heterosis describes the superiority of heterozygous genotypes for one or more characteristics in comparison with the corresponding parental homozygotes. The higher productivity of the heterozygotes is exploited through the development of hybrid varieties in several crop species, and historically it represented one of the most outstanding advancement in plant improvement. Despite the long history of successes, especially in maize (Zea mays L.), the genetic bases of heterosis are not well understood yet. The application of molecular markers for the dissection of the genetic basis of many quantitative traits prompted the development of different approaches for the evaluation of heterosis. Our research was conducted on the genetic material developed from the single cross between maize inbred lines H99 and B73 in order to: (i) study the level of heterosis for traits of agronomic importance; (ii) identify the genomic regions (QTL quantitative trait loci) most involved in heterosis; (iii) investigate the relationships between the level of molecular marker heterozygosity and the phenotypic performance; (iv) estimate the genetic effects involved (i.e., allelic and non-allelic interactions). Materials were the basic generations, the derived 142 recombinant inbred lines (RILs) and the three testcross populations obtained by crossing the 142 RILs to each parent and their F1. RIL population was genotyped with a total of 158 marker loci arranged in a genetic linkage map. All genotypes were field-tested in three locations. The field layout was a randomized complete block design for basic generations, and a modified split-plot design for the four populations. Least square means over locations were used for subsequent analyses. Classical genetic analises were conducted following the North Caroline Model III (NCIII) and Triple Test Cross (TTC) designs. Moreover, Composite Interval Mapping was performed to search for QTL by combining plant’s phenotypic and marker’s genotypic data. QTL analysis was conducted on testcross populations’ means and on derived datasets concerning additive and dominance effects only. Thresholds for declaring putative QTL were defined by permutations. A mixed linear model was then used to map digenic epistatic QTL. Among the examined traits, seedling weight (SW), number of kernels per plant (NK) and grain yield (GY) showed heterosis greater than 100% and dominance degree higher than one. Several QTL were identified prevailingly in the additive-dominance range, for traits with low heterosis, and prevailingly in the dominance-overdominance range for plant height (PH), SW, NK and GY. Only a few QTL with digenic epistasis were identified. Some chromosome regions presented overlaps of overdominant QTL for SW, PH, NK and GY, suggesting pleiotropic effects on the overall plant vigor.