NUCLEAR DIFFERENTIATION IN THE FILAMENTOUS CAULONEMA OF THE MOSS FUNARIA-HYGROMETRICA

Nuclei from different cell types in plants and animals show many featu res of differentiation; they differ in shape, volume, structure, ultra structure and in the distribution of nuclear components. Using the fil amentous caulonema of the moss Funaria hygrometrica Hedw. this study r ecords the changes in cytoplasmic organization alongside the reorganiz ation of the interphase nucleus. Events taking place in the meristemat ic cells at or near the tip of the advancing caulonemal filaments (e.g . acquisition of polarity, tip growth, nuclear and cell division, side branch initiation) are associated with haploid nuclei (1C DNA amount 0.5 pg) that are spherical or slightly oval, with no blocks of condens ed chromatin, and a large central nucleolus with a large granular comp onent. Maturation of the caulonemal cells involves wall thickening and pigmentation concomitant with suspension of elongate plastids in line ar arrays along endoplasmic strands. Many cells become highly polarize d with the majority of the organelles at their apical ends. These cyto plasmic changes are associated with endoreduplication of the genome to about 8C. Endoreduplication occurs by amplification of the 1C genome to give nuclei with 1C-8C DNA amounts. There is no evidence of differe ntial amplification of the genome. The amplification in the copy numbe r of ribosomal RNA genes is associated with the heterochromatinisation of the genes within the nucleolus. At the same time the nucleolus red uces in volume owing to a diminution of the granular component and all components of the nucleolus become spatially separate. There is an in creased nuclear volume associated with endoreduplication and the nucle us elongates causing an increase in the surface area of the nuclear en velope. These major nuclear reorganizations are associated with a stab le distribution of the 'D' polypeptide involved in pre-mRNA splicing. Scrutiny of published data suggests that similar differentiation event s might be encountered commonly in other organisms. The changing nucle ar morphology probably reflects the changing activity of the nucleus a nd the cell. It might be that nuclear reorganization changes the balan ce of genes or gene products and the spatial distribution of the compo nent parts to enable the new nuclear functions. These results suggest that nuclear differentiation is a fundamental feature of cell differen tiation.