II. 26. A balanced unstable metacentric trisomic for chromosome 3 in barley. (1)
R. J. Singh and T. Tsuchiya, Department of Agronomy, Colorado State University, Fort Collins, Colorado 80523 U.S.A.
(1) Supported in part by USDA/SEA Competitive Research Grant 5901-0410-9 0334-0 to T. Tsuchiya.
One metacentric trisomic for chromosome 3, designated metatrisomic 3B, was obtained in the F2 population of 14 + 1 acro 3 LS x ac xc (balanced lethal). The F2 population segregated for the gene ac (albino) only and a trisomic ratio was observed. This indicated that gene ac is on the short arm of chromosome 3 because it showed a disomic ratio with Triplo 3L (Singh and Tsuchiya, 1981). The plant was identified by somatic chromosome study.
Morphologically the plant with 2n = 14 + 1 meta 3B chromosomes was similar to the normal diploid. The seeds obtained from this particular plant segregated for albino (ac) and green seedlings. Chromosome count was made first for the six green mutants, all of which carried 2n = 14 + 1 meta 3B chromosomes. Based on this result it was assumed that normal two chromosomes carried recessive gene ac and all the albino plants were diploid. To confirm this assumption, remaining 87 seeds were germinated. Three green seedlings with 2n = 14 + 1 meta 3B and 84 albino seedlings with 2n = 14 chromosomes were recorded. This indicated that progenies of this plant will always segregate diploid albino and metatrisomic green plants. The frequency will change only if crossing over occurs between metacentric and normal chromosomes.
Two out of nine 2n = 14 + 1 meta 3B plants showed white stripes of various degrees on leaves, culms and spikes. It was also assumed that the extra metacentric chromosome which carried a dominant allele for green trait was eliminated in the somatic tissues, and that albino regions had diploid chromosome number. Meiotic analysis confirmed the assumption on the elimination of the extra metacentric chromosome in the above two plants because 7II and 7II + 1I chromosome configurations were observed in the same anther. The rate of elimination differed from one anther to another and floret to floret indicating that extra metacentric chromosome was unstable.
The remaining seven plants, which were green throughout their growth, showed extra metacentric chromosome in all the sporocytes at metaphase I with various frequencies of 7II + 1I and 1III + 6II chromosome configurations. However, these frequencies varied from plant to plant. Trivalent configurations were mainly ring and rod, tandem-V, tandem-rod and very few were ring shaped. This indicated that metacentric chromosome may be a pseudo-iso chromosome.
Reference:
Singh, R. J. and T. Tsuchiya. 1981. Preliminary studies on identification of telotrisomics of barley by N-banding technique. Barley Genet. Newsletter 11: