Three-dimensional organization of Drosophila melanogaster interphase nuclei. I. Tissue-specific aspects of polytene nuclear architecture.

Abstract

Abstract. Interphase chromosome organization in four different Drosophila melanogaster tissues, covering three to four levels of polyteny, has been analyzed. The results are based primarily on three-dimensional reconstructions from unfixed tissues using a computerbased data collection and modeling system. A characteristic organization of chromosomes in each cell type is observed, independent of polyteny, with some packing motifs common to several or all tissues and others tissue-specific. All chromosomes display a righthanded coiling chirality, despite large differences in size and degree of coiling. Conversely, in each cell type, the heterochromatic centromeric regions have a unique structure, tendency to associate, and intranuclear location. The organization of condensed nucleolar chromatin is also tissue-specific. The tightly coiled prothoracic gland chromosomes are arrayed in a similar fashion to the much larger salivary gland chromosomes described previously, having polarized orientations, nonintertwined spatial domains, and close packing of the arms of each autosome, whereas hindgut and especially the unusually straight midgut chromosomes display striking departures from these regularities. Surprisingly, gut chromosomes often appear to be broken in the centric heterochromatin. Severe deformations of midgut nuclei observed during gut contractions in living larvae may account for their unusual properties. Finally, morphometric measurements of chromosome and nuclear dimensions provide insights into chromosome growth and substructure and also suggest an unexpected parallel with diploid chromatin organization. IN a polytene interphase nucleus, chromosomes undergo repeated rounds of DNA replication without nuclear division. The homologous chromatids remain in close lateral alignment, rendering chromosomes with distinctive banding patterns visible in the light microscope. These nuclei provide a useful model system for directly analyzing interphase chromosome organization. Previously, we determined the spatial organization of chromosomes in a set of nuclei from Drosophila melanogaster salivary glands, which have the highest level of polyteny in the larva (22, 34). Characteristic packing motifs were found, including a polarized (Rabl) orientation of chromosomes across the nucleus, separation of chromosomes into noninterwound spatial domains, nearest-neighbor packing of the metacentric autosome arms, high-frequency contacts between a specific set of loci and the nuclear surface, invariant apposition of the aggregated centromeric regions (the chromocenter) to the nuclear surface, and a striking right-handed coiling chirality that does not require homologue pairing. Despite these nonrandom features, precise chromosome configurations appear not to be specified. Thus, while the folding of chromosomes is not identical in every salivary gland cell, several other facets of their organization are quite well defined.Dr. Hochstrasser's present address is

Developmental control of the DNA replication and transcription programs

“…We calculated the average ploidy values of fat body, midgut, and salivary gland cells to be 225C 6 9, 171C 6 19, and 1669C 6 104, respectively (mean 6 SEM). This is consistent with previous analysis on these tissues, including the presence of two types of polyploid cells in the larval midgut (Rodman 1967;Butterworth and Rasch 1986;Hochstrasser and Sedat 1987). The diploid cells from the imaginal islands in the midgut were clearly distinguishable by nuclear size and not quantified.…”

Analysis of genes and chromosomes by nonisotopic in situ hybridization

“…First, it is still not known to what extent chromatin distributions observed after various fixation and in situ hybridization procedures reflect the in vivo Situation. Second, in spite of great improvements in 3D microscopy of chromosomes [137][138][139], there are still limitations that impair the precision with which the shape, size, and distribution of intranuclear chromosome targets can be measured. Third, it is far from trivial to create 3D models of chromosome distribution in order to decide for or against a random distribution.…”

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

“…Tight pairing of homologues is a feature of dividing cell types in the diptera, including Drosophila melanogaster (41); this association can be maintained into differentiated cell types (68,74). Using FISH, Kleckner (90) and Burgess et al (29) report that homologue association and pairing occurs in diploid cells of S. cerevisiae (yeast), where homologues appear to have multiple interstitial pairing contacts in dividing cells at G 1 and G 2 , although this is disrupted in the S phase of the cell cycle.…”

“…In many organisms the chromatids remain tightly associated, forming polytene chromosomes, and these have been found in a diverse range of tissues and taxa (see the table in reference 122). In Drosophila, many tissues of the embryo contain cells with polytene chromosomes (74), and in vascular plants, they regularly occur in synergid and tapetal cells (122). (ii) Another mechanism is endomitosis, where replicated chromosomes condense as if entering mitosis but then do not segregate; instead, they remain together within a single nucleus.…”