The packaging unit: a basic structural feature for the condensation of late cricket spermatid nuclei.

The alignment, folding and packaging of cricket chromatin was examined during late spermiogenesis by an electron-microscope study of nuclei dispersed by air--liquid surface tension forces after detergent treatment. Late developing spermatid genomes arrange themselves in multiple packaging units in a stepwise process which includes: (1) a loss of the beaded repeating structure of chromatin as nucleoprotein fibres become smooth and gradually assume a uniform diameter; (2) a side-by--side alignment of structurally modified chromatin fibres; and (3) a regular folding into packaging units. Alignment and folding of chromatin fibres are presumably mediated by intermolecular bonds easily disrupted by spreading forces. In very late spermatids, interfibre binding forces are difficult to overrride by spreading alone, indicating a stronger cross-linking of increasingly coalescent packaging units. 'Unit to unit' coalescence stabilizes the nuclear structure, first limiting and afterwards denying penetration of phosphotungstic acid, as displayed in thin sections of extremely cricket spermatid nuclei. Binding of phosphotungstate by nuclear basic proteins can be facilitated by limited protein solubilization after disulphide reduction of unfixed cricket tests with sodium dodecyl sulphate and dithiothreitol. Results of this study permit the proposal of model experiments useful for clarifying the organization of highly condensed spermatid genomes and for evaluating the structure of genome segments in systems wherein changes of chromatin-associated protein occur.