ACTIVITY OF W-SEX HETEROCHROMATIN AND ACC UMULATION OF THE NUAGE IN THE NURSE CELLS OF THE LEPIDOPTERAN EPHESTIA

In the Lepidopteran Ephestia, in the female sex, the W chromosome, wit h which no function is Known to be associated is, in whole or part, ma intained in a heteuochromatic 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 transcr iptionally active in nurse cells during previtellogenesis. This activi ty is accompanied by accumulation of ''nuage'' in the perinuclear cyto plasm, 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 re solution examination of the structures visible in the vicinity of acti ve W-SH. Our results suggest that W-SH transcripts are packaged and tr ansported to the cytoplasm within polyparticles typically resembling t he hnRNP particles - the structures packaging (pre)messenger RNA which are isolated fi om the nuclei of numerous Eukaryotes - and that they are concentrated within the nuage upon leaving the nucleus. Such findi ngs 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 asse mbly of germ plasm. Considering this hypothesis as well as our results with Ephestia, we speculate that the activity of W-SH detected in nur se cells reflects the expression of one or several genes located withi n the heterochromatin of the W chromosome, and that the function of th ese genes is related with the elaboration of germ plasm. The implicati ons raised by there unexpected proposals are mentioned.