Non-disjunction of B chromosome centromeres in the maize male gametophyte has been used to develop powerful methods for chromosome gene location and gene expression studies. The presence of B chromatin in the nucleus may also cause non-disjunctions of the A chromosomes during endosperm development. The event can be detected as sectors on the endosperm when heterozygous combinations for chromosome markers expressed in this tissue are used. A method for chromosome gene location based on this phenomenon can be developed; the advantage is twofold: 1) it can be used to locate genes lying on chromosome segments not covered by the actual TB-A stocks, and 2) comparisons for biochemical traits can be made between endosperm sectors of the same kernel differing only in the chromosome involved in the non-disjunction.

The method has been tested by using two marker genes both located on the short arm of chromosome 7: o2 and Zp20-1, where Zp20-1 is proximal to the centromere at 13 map units from o2. Kernels from the cross between the inbreds 33-16 (o2, zp20-2) and OH-43 (TB-9; O2, Zp20-1) were scored for opaque sectors; the frequency of kernels showing half opaque endosperm phenotype was about 0.001. Four endosperms in which the two phenotypical parts could easily be separated were analyzed using the immunological test to detect the presence of the cytoplasmic protein b-32 (this protein is absent when the locus opaque-2 carries the recessive allele) and the IEF method to reveal the zein polypeptide coded by Zp20-1 While the biochemical opaque-2 phenotypes fitted the expectation (in the opaque sectors the protein b-32 was absent and in the normal sectors it was clearly revealed) the IEF patterns of the sector phenotypes did not differ, i.e. opaque and normal endosperm tissues both showed the zein band 20-1.

These results indicate that the whole chromosome 7 was not involved in the genetical event leading to the formation of the endosperm opaque sectors analyzed, and could be explained on the basis of a chromosomal loss or a mitotic crossing-over.

G. Binelli, C. Soave and E. Ottaviano
 
 

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