Unstable chromosomes may be used to uncover recessive embryonic lethal mutations in sectors of the plant. This allows analysis of post-embryonic effects of these mutations in the sporophyte. Such chromosomes have been produced from TB translocations, as has been previously reported by Wayne Carlson (Theor Appl Genet 43:147, 1973) and Achille Ghidoni (Theor Appl Genet 43:151, 1973). Their putative ring chromosomes appeared in the progeny of crosses where the male carried a supernumerary B-A chromosome (A A x A A B-A).
We used a recessive allele of viviparous9 (vp9) as a marker for instability of TB-7Sc. Sectors hemizygous for vp9 are white in a green background. We derived two stocks that heritably uncover vp9 in variably sized sectors. Each unstable B-7 chromosome appeared in the progeny of a TB-7Sc hyperploid crossed by a wild type tester (7 7B B7 B7 x 7 7).
One of these putative ring chromosome stocks has been subjected to preliminary molecular and cytological analysis. Tissue was collected from two plants showing large leaf to leaf sectors and DNA was prepared from both within the sector and from surrounding green tissue. Southern analysis using rs1 sequence as a probe showed loss of one copy of this gene, which lies on chromosome 7S, in the white sectors. Examination of meiocytes revealed pachytene configurations consistent with ring chromosomes. However the expected anaphase I double bridges that would indicate the presence of ring chromosomes have not yet been observed.
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