We have previously reported (MNL 67:25-26) the recovery of a chromosome fragment (mini-1) that rescues lethal oil yellow genotypes. This mini-1 is thought to be a partial isochromosome that covers the proximal part of 10S at least as far as the oil yellow locus (Oy). This conclusion is based on three observations: (i) oil yellow somatic variegation correlated with lagging and loss of the mini-1 at mitotic and meiotic anaphase; (ii) gene dosage experiments indicating that the mini-1 chromosome has two doses of the wild type allele Oy+; (iii) the recovery of plants with derivative mini chromosomes such as those with two small or 1/2 minis (telocentrics) and those with a large mini. Two independent recoveries of the large mini, called Lg-1 and Lg-2, have been made and both appear to be heteromorphic with one arm about the same size as the original mini-1 while the other approximates the size of the short arm of chromosome 10. Our interpretation is that we have recovered a crossover between the mini-1 and the regular 10S. This gives rise to a chromosome in which the large arm comprises most or all of 10S and the small arm is the 10S proximal region up to Oy+, which was present in the original mini-1. Unlike mini-1, the Lg-1 chromosome frequently forms trivalent associations with chromosome 10 at meiosis confirming the presence of 10S. However, the full extent of this 10S will be determined genetically and by the ability of this chromosome to rescue a homozygous lethal terminal deletion (def(bnl3.04-Rp5-Rp1-M).
The Lg-1 chromosome has been recovered in both the olive (oy/oy) and the lethal yellow (Oy/Oy) backgrounds. These stocks can be maintained by selecting green seedlings segregating among selfed progeny. The Lg-1 chromosome shows about 25% pollen transmission and 12% via the egg. These stocks will provide a useful source of trisomic 10S material for genetic study.
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