Proceedings of the XLVII Italian Society of Agricultural Genetics - SIGA Annual Congress

Verona, Italy - 24/27 September, 2003

ISBN 88-900622-4-X

Poster Abstract - 1.30

Addressing Heterosis in Maize by Molecular and Quantitative Genetics Approaches

G. PEA*, E. Frascaroli**, F. Cattonaro***, L. Gianfranceschi*, P. Landi**, M. Morgante***, M. Villa*, M. A. Canè**, M. E. PÉ*

*) Dip. Scienze Biomolecolari e Biotecnologie, via Celoria 26, 20133 Milano

**) Dip. Scienze e Tecnologie Agroambientali, via Fanin 40, 40127 Bologna

***) Dip. Produzione Vegetale e Tecnologia Agraria, via delle Scienze 208, 33100 Udine

heterosis, maize, QTLs, gene expression

The exploitation of heterosis in plant breeding is considered one of the most revolutionary advancement in plant improvement. However, despite a long story of successes, there is still a striking gap between the extensive agricultural practice of hybrid vigour utilization and our understanding of the cause of such phenomenon. An integrated approach comprising both molecular and quantitative analyses is being applied to gain some understanding of the heterotic phenomenon in maize: combination and integration of large scale transcriptional profiling data and modern quantitative genetics is being performed, in order to assess the importance of gene expression in determining heterosis and to shed light on the genetic and molecular mechanisms underlying it.

Appropriate plant material was produced, to provide a common set of genotypes for each experimental approach and to allow data integration. The material meets two contrasting needs: sufficient complexity in order to allow an accurate genetic interpretation of the phenotypic values, and simple genetic design to ease molecular biology analysis. In particular a population of 142 recombinant inbred lines (RILs), obtained from the original cross B73 x H99 was used for the production of three pseudo-back cross (PBC) populations by crossing each of the 142 RIL with both the original parental lines (PBC-P1 and -P2) and their F1 hybrid (PBC-F1). In addition, a tester population (TC) was also produced, by crossing each of the 142 RIL with tester line Mo17. Owing to the mating design adopted, the mapping population of reference is the B73 x H99 RILs. This population has been already characterized for more than 200 molecular markers, mainly Simple Sequence Repeats and AFLPs.

All the populations described (i.e. RILs per se, PBC-P1, PBC-2, PBC-F1 and TC, for a total of 716 genotypes) were grown in three locations (2 reps per location). Data for components of vegetative growth, plant cycle and yield were collected and analyzed. To identify the underlying QTLs, Composite Interval Mapping was employed (LOD > 2) by using PLABQTL (Utz, H.F. & Melchinger, A.E. 1996). Analyses of their effects and relationships across all populations allowed us to place emphasis on heterotic effect evaluation and thus to chose from 6 to 9 relevant QTLs. Results suggest that there are loci actually involved in heterosis and that dominance plays a role in it. For those QTLs, Near Isogenic Lines (NILs) are going to be produced for further genetic dissection and molecular analysis. Results concerning plant vigour and yield are discussed.

A comparison of the expression profiles of sets of genes in the hybrid and in the corresponding inbred parents is being used to identify the components of the genetic circuits that control heterosis. DNA microarrays on glass slides (University of Arizona), spotted with thousands of maize ESTs, were hybridized with fluorescently labelled cDNA obtained from 15 dap kernels and young leaves (endosperm and Unigene1 microarrays, respectively). Preliminary results of Significance Analysis of Microarray (SAM) are presented. Allele-specific expression levels in inbreds and hybrids will also be determined. The approach is based on the detection of Single Nucleotide Polymorphisms (SNPs) located within the transcript by single base extension methods followed by capillary electrophoresis separation using RNA as a template.