Dramatic nucleolar dispersion in the salivary gland of Schwenkfeldina sp. (Diptera: Sciaridae)

Micronucleoli are among the structures composing the peculiar scenario of the nucleolus in salivary gland nuclei of dipterans representative of Sciaridae. Micronucleolar bodies contain ribosomal DNA and RNA, are transcriptionally active and may appear free in the nucleoplasm or associated with specific chromosome regions in salivary gland nuclei. This report deals with an extreme case of nucleolar fragmentation/dispersion detected in the salivary gland of Schwenkfeldina sp. Such a phenomenon in this species was found to be restricted to cell types undergoing polyteny and seems to be differentially controlled according to the cell type. Furthermore, transcriptional activity was detected in virtually all the micronucleolar bodies generated in the salivary gland. The relative proportion of the rDNA in polytene and diploid tissues showed that rDNA under-replication did not occur in polytene nuclei suggesting that the nucleolar and concomitant rDNA dispersion in Schwenkfeldina sp. may reflect a previously hypothesised process in order to counterbalance the rDNA loss due to the under-replication. The chromosomal distribution of epigenetic markers for the heterochromatin agreed with early cytological observations in this species suggesting that heterochromatin is spread throughout the chromosome length of Schwenkfeldina sp. A comparison made with results from another sciarid species argues for a role played by the heterochromatin in the establishment of the rDNA topology in polytene nuclei of Sciaridae.