| It will be seen from Table 1 that the absence
of the nucleolar organizer present in chromosome 1B, 6B or 5D makes little
difference to the turnover of RNA in the root tip cells. In contrast to
this, the absence of the organizer present in chromosome 1A reduces the
synthesis of RNA very greatly. A comparision of the grain count in the root tip cells of the four species, representing different ploidy levels, summarised in Table 2 also suggests that the nucleolar organizer of the D genome contributes little to RNA synthesis. However, these observations show that the two organizers contributed by the B genome are active in RNA synthesis, and their combined activity compares with that of the organizer present in the A genome. It is well known that bulk of the RNA in cells (nearly 80 per cent) is of the ribosomal type; it may, therefore, be inferred that the turnover analysed in the above lines and varieties relates largely to this fraction of the cellular RNA. This inference finds support from the work of several authors, who have shown that the nucleolar organizer is actively involved in the synthesis of ribosomal RNA (BROWN and GURDON 1964; RITOSSA and SPIEGELMAN 1965). The present analysis thus shows that in the evolution of hexaploid wheat, the nucleolar organizer present in the A genome has continued to play a dominant role in the synthesis of ribosomal RNA. It should be interesting to find whether the organizers present in the B and D genomes have an inherently weaker synthetic activity, or alternatively, there has been an interaction between the different nucleolar organising loci in the polyploid species. Literature BROWN, D.D. and J.B. GURDON 1964. Absence of ribosomal RNA synthesis in the anucleolate mutant of Xenopus laevis. Proc. Nat. Acad. Sci., Wash. 51 : 139-146. CROSBY, A.R. 1957. Nucleolar activity of lagging chromosomes in wheat. Ibid. 44 : 813-822. DARLINGTON, C.D. and L.F. LA COUR 1962. The handling of chromosomes. Allen and Unwin Ltd., London. JAIN, H.K. 1966. Correlated synthetic activities of chromosomes. Chromosome Today 1 : 145-149. RITOSSA, F.M. and S. SPIEGELMAN 1965. Localization of DNA complementary to ribosomal RNA in the nucleolus organizer region of Drosophila melanogaster. Proc. Nat. Acad. Sci., U.S. 53 : 737-745. SEARS, E.R. 1952(a). Misdivision of univalents in common wheat. Chromosome 4 : 536-550. SEARS, E.R. 1952(b). The behaviour of isochromosomes and telocentrics in wheat. Chromosome 4 : 551-562. (Received January 16, 1968) |
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