One of the most interesting chromosomal aberrations in plants is the centric ring--a chromosome whose arms are joined to one another so as to produce a continuous ring. Thorough analyses of ring behavior have been performed by McClintock (Genetics 23:315-376 and Cold Spr. Harb. Symp. 9:72-80) and Schwartz (Am. Nat. 87:19-28). They found that a ring chromosome may be lost or multiplied during mitosis and meiosis. Or, as a result of recombination, modified rings may arise which are either deficient or duplicated for the chromatin present on the original ring. When the ring chromosome carries a dominant allele of a mutant present on the homologous normal chromosome this behavior may be followed genetically. Loss or modification of the ring results in the expression of the recessive allele, producing a mosaic plant, i.e., one that shows both the dominant and recessive phenotypes.
Because recessive embryo lethal mutants die at an early stage of development they are difficult to study. The goal of this project was to produce a set of genetically marked ring chromosomes carrying the dominant alleles of these embryo lethal mutations. Such chromosomes could then be used to produce mosaic plants with both normal and lethal tissue. This would make it possible to determine whether or not an embryo lethal mutation is active in a mature plant and if so, provide a way of studying its effects. (Ring chromosomes could also be used to study whether recessive disease mutants show cell autonomy or whenever one needs a situation of producing a mosaic plant made up of sectors of dominant and recessive tissue for a particular gene.)
Plants recessive for eight plant color genes (a1, a2, c2, bz1, bz2, b1, pl1, r-r) were fertilized by x-rayed pollen from plants homozygous for the dominant alleles of these genes. Nine thousand forty-eight seeds were planted and 21 mosaic plants were found. The mosaic plants were selfed and crossed onto a set of six tester stocks: a1, a2, c2, bz1, bz2, and r-g. All the tester stocks were also b1, pl1. One could determine which of the mosaic-looking plants were caused by physiological conditions or because they contained a ring by examining the outcrossed ears. This is because ring chromosomes are not usually transmitted (or are greatly reduced) through the male. Therefore, if an ear showed the expected 1:1 ratio for a particular gene, one knew a ring carrying that gene was not involved. But if an ear showed only a recessive phenotype (or a greatly reduced number of dominant phenotypes) it was an indication that the mosaic plant carried a ring with that particular dominant gene on it. The data to date indicate we have produced five centric chromosome fragments: one involving chromosome 2 and the B1 locus; one involving chromosome 4 and the C2 locus; two involving chromosome 5 and the A2 locus; and one involving chromosome 6 and the Pl1 locus. These five plants and their progeny are now being studied cytologically to determine if they contain a ring chromosome. If they do, it should now be possible to undertake an analysis of the embryo lethal mutations or any other situation where a mosaic plant is needed involving these four chromosomes.
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