Nuclear differentiation in the filamentous caulonema of the moss Funaria hygrometrica

Abstract

SUMMARYNuclei from difTerent eel! types in plants and animals show many features of differentiation; they differ in shape, volume, structure, ultrastructure and in the distribution of nuclear components. Using the filamentous caulonema of the moss Funaria hygrometrica Hedw. this study records the changes in cytoplasmic organization alongside the reorganization of the interphase nucleus. Events taking place in the meristematic cells at or near the tip of the ad\ ancmg caulonemal filaments {e.g, acquisition of polarity, tip growth, nuclear and cell division, side branch initiation) are associated uith haploid nuclei (IC DXA amount Oi pg] that are spherical or slight!\-oval, with no blocks of condensed chromatin, and a large central nucleolus with a large granular component. Maturation of the caulonemal cells invoh es wall thickening and pigmentation concomitant with suspension of elongate plastids in linear arrays along endoplasmic strands. Many cells become highly polarized with the majority of the organelles at their apical ends. These c\'toplasmic changes are associated with endoreduplication of the genome to about 8C. Endoreduplication occurs b>' amplification of the IC genome to give nuclei with !C-8C DN.^ amounts. There is no evidence of differential amplification of the genome. The amplification in the copy number of ribosomal RNA genes is associated with the heterochromatinisation of the genes within the nucleolus. At the same time the nucleolus reduces in volume owing to a diminution of the granular component and all components of the nucleolus become spatially .separate. There is an increased nuclear yo\ume a.ssociated nith endoreduplication and tbe nucleus elongates causing an increase in the surface area of the nuclear envelope. These major nuclear reorganizations are associated with a stable distribution of tbe ' D' polypeptide involved in pre-mRNA splicing. Scrutiny of published data suggests that similar differentiation events might be encountered commonly in other organisms. The changing nuclear morphology probably reflects the changing activity of the nucleus and the cell. It might be that nuclear reorganization changes the balance of genes or gene products and the spatial distribution of the component parts to enable the new nuclear functions. These results suggest that nuclear differentiation is a fundamental feature of cell differentiation.

Higher Levels of Organization in the Interphase Nucleus of Cycling and Differentiated Cells

“…8C). This suggests differentiation between the haploid genomes such that only one genome at a time replicates (92). Such specialization of component genomes may be mediated through DNA methylation or other genomic imprinting mechanism.…”

“…Moss caulonemata lend themselves to developmental studies of the nucleus because cell and nuclear differentiation can be observed over time in vivo (92). Caulonemata are filaments of cells growing by cell division of an apical cell.…”

The role of DNA methylation on nuclear and cell differentiation in the filamentous caulonema of the moss Funaria hygrometrica

“…This paper aims at further understanding of relationships between nuclear and cytoplasmic differentiation in plant development by exploiting our recent discovery of regular and profound nuclear and cytoplasmic changes occurring during the development of moss caulonemata (Kingham et al, 1995).…”

“…During caulonemal differentiation, the nuclei endoreduplicate and elongate and their ultrastructure changes substantially. Concomitantly, the cytoplasm reorganizes, with major microtubule-associated changes in the distribution of plastids, mitochondria and endomembrane domains (Kingham et al, 1995).…”

“…(Funariales, Musci), wild-collected from the Ashdown Forest, Sussex, UK, were sown on 15 ml of solid Parker medium (Klekowski, 1969) overlain with Cellophane2 discs. Under laboratory conditions (18 mC in constant light from fluorescent tubes, 50 W m −# ), spores germinated within 10 d and produced protonemal colonies c. 15 mm in diameter comprising both chloronemata and fully differentiated caulonemata (described in Kingham et al, 1995) after 3-4 wk. Gametophores developed from the caulonemata after 6-8 wk.…”