[Activity of W-sex heterochromatin and accumulation of the nuage in nurse cells of the lepidopteran Ephestia].

In the Lepidopteran Ephestia, in the female sex, the W chromosome, with which no function is known to be associated, is, in whole or part, maintained in a heterochromatic state in all cellular categories, except for the oocyte. In the present work, however, W-sex heterochromatin (W-SH) is demonstrated by ultrastructural autoradiography to be transcriptionally active in nurse cells during previtellogenesis. This activity is accompanied by accumulation of "nuage" in the perinuclear cytoplasm, both phenomena arresting at the beginning of vitellogenesis. We have performed kinetic analysis of the labeling associated with W-SH and with nuage through a pulse-chase experiment, together with high resolution examination of the structures visible in the vicinity of active W-SH. Our results suggest that W-SH transcripts are packaged and transported to the cytoplasm within polyparticles typically resembling the hnRNP particles--the structures packaging (pre)messenger RNA which are isolated from the nuclei of numerous Eukaryotes--and that they are concentrated within the nuage upon leaving the nucleus. Such findings explain the close relationship observed in Ephestia, both in space and time, between the nuage and active W-SH. In different organisms, including Drosophila, the nuage has been proposed to be a site of assembly of germ plasm. Considering this hypothesis as well as our results with Ephestia, we speculate that the activity of W-SH detected in nurse cells reflects the expression of one or several genes located within the heterochromatin of the W chromosome, and that the function of these genes is related with the elaboration of germ plasm. The implications raised by these unexpected proposals are mentioned.