CSIRO, Division of Plant Industry

A chromosome fragment that rescues lethal oil yellow genotypes
--Dick Brock and Tony Pryor

The oil yellow gene on the short arm of chromosome 10 of maize has both dominant (Oy) and recessive (oy) mutant alleles in addition to the normal green of the wildtype (+), and different genotypes give the following phenotypes:
            +/+ ;         +/oy         -- green
            Oy/+;      oy/oy     -- viable oil yellow
            Oy/Oy ;  Oy/oy       -- lethal yellow

A mutant oil yellow variegated seedling was recovered after 60Co R-irradiation of wildtype pollen used to cross a plant heterozygous for the dominant Oy gene (Oy/+). The variegated phenotype was recovered in progeny crosses and analysed by test crossing to various oil yellow mutant stocks.

Cytogenetic studies have demonstrated that the variegated phenotype is invariably associated with the presence of a small or 'Mini' chromosome and two classes of variegated plants can be identified:
Genotype Phenotype
Type I [Oy/+ ; Mini] oil yellow plus green stripes
[oy/oy ; Mini]
Type II [Oy/Oy; Mini] yellow plus green and oil yellow
[Oy/oy; Mini] stripes

The Mini divides regularly at PMC meiosis (cf. Maguire, MP, Genome 29:744-777, 1987) but with somewhat more delayed separation relative to normal chromosome segregation. The Mini is transmitted to 5% of progeny via the egg and to 10% via pollen. Plants containing two Mini's have not been recovered. Plants with tassel sectors lacking the Mini (determined from tapetal mitoses and PMC meiosis) are observed, but this has not been shown to coincide with the green/yellow plant sectors that can occur in the mature tassel. The reason for plant sectoring is unknown but is possibly due to late division and/or delayed separation of the Mini chromosome at mitosis resulting in cell lineages with and without the Mini chromosome. It is assumed that the Mini represents part of the short arm of chromosome 10 and carries the wildtype (+) oil yellow gene and thus rescues the otherwise lethal Oy/Oy and Oy/oy genotypes. Plants carrying the Mini and homozygous Oy/Oy produce Oy pollen and when crossed onto females homozygous for the recessive oy/oy all the F1 plants are yellow lethal (Oy/oy) except for the 10% which inherit the Mini via the pollen. This genetic system could be used to select for maternal inheritance events. 

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