Small chromosomal regions position themselves autonomously according to their chromatin class

The spatial arrangement of chromatin is linked to the regulation of nuclear processes. One striking aspect of nuclear organization is the spatial segregation of heterochromatic and euchromatic domains. The mechanisms of this chromatin segregation are still poorly understood. In this work, we investigated the link between the primary genomic sequence and chromatin domains. We analyzed the spatial intranuclear arrangement of a human artificial chromosome (HAC) in a xenospecific mouse background in comparison to an orthologous region of native mouse chromosome. The two orthologous regions include segments that can be assigned to three major chromatin classes according to their gene abundance and repeat repertoire: (1) gene-rich and SINE-rich euchromatin; (2) gene-poor and LINE/LTR-rich heterochromatin; and (3) genedepleted and satellite DNA-containing constitutive heterochromatin. We show, using fluorescence in situ hybridization (FISH) and 4C-seq technologies, that chromatin segments ranging from 0.6 to 3 Mb cluster with segments of the same chromatin class. As a consequence, the chromatin segments acquire corresponding positions in the nucleus irrespective of their chromosomal context, thereby strongly suggesting that this is their autonomous property. Interactions with the nuclear lamina, although largely retained in the HAC, reveal less autonomy. Taken together, our results suggest that building of a functional nucleus is largely a self-organizing process based on mutual recognition of chromosome segments belonging to the major chromatin classes

Organisation of the epidermal syncytial mosaic in Diceratocephala boschmai (Temnocephalida: Platyhelminthes)

The epidermis of Diceratocephala boschmai Baer, 1952 (Temnocephalida: Platyhelminthes) was studied using silver-nitrate staining and electron microscopy. The epidermis consists of six syncytia separated by lateral membranes: the frontal, trunk, stalk, adhesive disc syncytia, and a pair of post-tentacular syncytia. Neighbouring syncytia differ in many characters including (1) the presence or absence of locomotory cilia, (2) the degree of the differentiation of the apical cytoplasm layer, (3) the presence or absence of bundles of cytoskeletal filaments, imaginations of basal membrane and other specialised cytoplasmatic structures, (4) the abundance of hemidesmosomes at the basal membrane, and (5) the abundance and nature of gland ducts penetrating the syncytium. These structural differences reflect functional differences between the syncytia. Thus, multisyncytial organisation of the epidermis may be explained by functional differences between the syncytia. Only between the frontal and trunk syncytia has no apparent ultrastructural difference been found