Update on the genetic mapping of the opaque2-modifier genes
--Moro, GL; Carneiro, N and Larkins, B

opaque2-modifiers are genes with the ability to convert the soft chalky endosperm, as found in maize opaque2 mutants, to a hard, vitreous phenotype. Modified opaque2 genotypes or Quality Protein Maize (QPM) have increased levels of the essential amino acid lysine and a normal appearing kernel. QPMs have been developed independently at CIMMYT (Mexico) and University of Natal (South Africa). Our lab is working on the biochemical, genetic and molecular characterization of endosperm modification (for details see Lopes et al., MNL 69:125, 1995). Genetic mapping using CIMMYT's QPM identified two loci associated with modification, the first near the telomere of chromosome 7L and the second at the 27-kD g-zein locus, near the centromere on 7L (Lopes et al., Mol. Gen.Genet. 247: 603-613, 1995). We are now extending the mapping effort to QPM lines from South Africa. Two crosses are being analyzed: G10 QPM x W64Ao2 and G6 QPM x W64Ao2. Our strategy is to perform bulked segregant analysis in the F2 generation in order to identify RFLPs associated with the modified phenotype. So far we found only one modifier closed linked to the g-zein locus.  We could not find any polymorphism near the extreme of 7L.  Also, our results suggest that the duplicated g-zein locus (AB) is not necessary for modification, as previously thought.  Among the 27 F2 individuals of the modified bulk in the G10 (AB locus) x W64Ao2 (ReA locus) cross we found one plant heterozygous for the g-zein locus (ReAAB).  Its seeds had all clearly vitreous endosperm with no phenotypic segregation for modification.  The zein profile of these seeds was typical of modified opaque2 endosperm, with high levels of g-zein and low levels of a-zein.  Some F3 plants originated from these seeds had the ReA g-zein locus and their seeds were also fully modified.  We are now performing the biochemical analysis of these seeds to verify their zein profile.  Also we continue to cover other areas of the genome looking for other loci involved in the process of modification.


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