| Nucleolar activity of lagging chromosomes in wheat
Arlene CROSBY University of Missouri, Columbia, Missouri, U.S.A. The monosomic series in wheat makes possible the examination of large numbers of micronuclei for any chromosome or chromosome arm desired. At meiosis each monosomic gives a large percentage of telophase laggards, some of which then form micronuclei. A study of the nucleolar activity of these lagging chromosomes was undertaken (1) to determine whether or not all the chromosomes were capable of forming a nucleolus in their micronuclei and to what extent, and (2) to identify if possible the nucleolus organizing chromosome of the D genome. Iso-XR (the isochromosome for the right arm of chromosome ten) showed a small nucleolus in 36.9% of its micronuclei, telo-XR in 24.7%, normal X in 17.8%, and I in 17.1%. The D-genome monosomics showed none to 0.8%, with the exception of mono-XVIII, which showed 5.5%. These results indicate that in wheat only those micronuclei which contain nucleolar chromosomes can form nucleoli. Diakinesis studies confirm the nucleolar organizing abilities of chromosome I, X, and XVIII. Both telo-XR and normal I have been found attached to the nucleolus. Although the third nucleolar organizer in this material is evidently relatively inactive, a cell was found which shows monosome XVIII attached to the nucleolus, and this monosome was once found at first metaphase with a very pronounced secondary constriction. Also, 2.2% of the microspores of tetrasomic XVIII showed 4 nucleoli, as contrasted with a negligible percentage in that class for di-XVIII. Indications are that it is the short arm of XVIII which is responsible for nucleolus formation. The other chromosomes of the A and B genomes are being investigated to determine whether one or more of them, as well as chromosome XVIII, may sometimes be responsible for the development of the third nucleolus. Also, nucleolus size is being studied in the sporocytes of plants with both increased and decreased dosages of known nucleolar chromosomes. No evidence has been found that the disappearance of the so-called " matrix " material of wheat chromosomes is dependent upon the formation of a nucleolus, or that the reappearance of the matrix is dependent upon the contribution of nucleolar material to the chromosomes. |