The original line trisomic for a large fragment of chromosome 6 in a Betzes background was obtained from R. T. Ramage (see Ramage et al., 1975). A trisomic plant was crossed as a female with a six-rowed genetic stock containing the alleles o (orange lemma) and uc2 (uniculm) both on chromosome 6 and showing relatively close linkage (see chromosome 6 coordinator reports in previous issues of BGN). F plants with 2n=15 chromosomes were grown. In the F generation, a plant with an orange lemma (o) was isolated with 2n=15 chromosomes and multiple tillers (Uc2--). When selfed, all progeny were orange lemma, about 60% were uniculm and 2n=14 while the remaining 40% were multi-tillered and 2n=15. In this line, multitillered plants have been 2n=15 for 3 more generations and all segregated for about 50% uniculm plants with 2n=14 chromosomes. No primary trisomics have been recovered while one multi-tillered, 2n=16 plant was detected with 2 fragment chromsomes. The segregation ratios are very close to those reported by Ramage et al. (1975) for this fragment trisomic.
The line which has been isolated allows for detection of trisomic (fragment) plants in the seedling stage as all diploids are uniculm while the plants which produce tillers are trisomic for the fragment of chromsome 6. The fragment carries the Uc2 allele which gives multiple tillers in the trisomic while the diploids are homozygous for the uc2 allele on the two normal chromosomes and do not tiller. The fragment, which is missing a segment of the short arm of chromosome 6 including the satellite, appears to also be missing the segment which contains the o locus as both diploids and trisomics exhibit the recessive condition (orange lemma). Given that the o locus is thought to be quite close to the centrome of chromosome 6 (see chromosome 6 coordinator's reports in previous issues of BGN) and that the uc2 locus shows tight linkage with the o locus (see chromosome 6 coordinator's reports in previous issues of BGN). These results suggest that either the cytological distance between the centromere and the o locus in the short arm is much greater than the genetic distance or that there has been a crossover between the fragment short arm and the normal short arm carrying the o locus. Such a crossover, however, should have resulted in transfer of the satellite also; although a double crossover is a possibility. The 2n=15 plants all have only 2 normal 6 and 2 normal 7 chromosomes with satellites, so it appears that the fragment is not complementing the o locus. Further crosses are underway to attempt to shed more light on the cytology and genetics of the short arm of chromosome 6.
Ramage, R. T., M. Paluska, and R. F. Eslick. 1975. Cytological and breeding behavior of a large fragment of chromosome 6. BGN 5:52-53.