Localization of the os gene on the short arm of chromosome 2

--T. M. Song and X. W. Lu

The chromosome location of a recessive opaque endosperm small germ gene (os) was tested by using the B-A translocation technique. The heterozygous mutants (+/os) were used as female parents and a set of 16 B-A translocations (kindly provided by Dr. E. B. Patterson from Coop Center, USA) as males. All the F1 ears were carefully checked. Among the 16 crosses made, 15 did not have any os seeds. Kernels with the recessive trait were discovered in every pollinated ear of only one cross, that is: +/os x TB-3L-2S(6270). Among a total of 1003 kernels, 288 showed the recessive traits. However, their endosperm trait and germ trait did not occur correspondingly. Some kernels have opaque endosperm but normal size germ, while others have small germ but normal endosperm (with a ratio about 6:4), indicating the unique characteristics of nondisjunction and preferential fertilization of BA chromosomes as well as the pleiotropic effect of the os gene. Noncorresponding germ and endosperm phenotype clearly reflected their different genotype. Since no os seed was found on the ears of +/os x TB-3La crosses, the os gene must be located on the short arm of chromosome 2. This conclusion was further confirmed by wx-marked translocation testing. A series of 11 reciprocal chromosome 9 translocations carrying the recessive waxy allele with one breakpoint close to the waxy locus was used to cross with the os mutant. In their F2 population, a significant chi-square value for independence was found only for wx T2-9b at the 1% level. The breakpoint of T2-9b is 2S.18 and 9L.22.

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