Under-replication of intron+ rDNA cistrons in polyploid nurse cell nuclei of Calliphora erythrocephala

Abstract

We have examined the possibility that the intron-containing (intron+) rDNA cistrons of Dipteran flies are active in the germ-line derived polyploid nurse cell nuclei of the ovarian follicles. Using the organism, Calliphora erythrocephala, we describe here a procedure which yields very pure nurse cell nuclei and compare the intron-free (intron−) and intron+ rDNA cistron contents of nurse cell nuclei prepared by this procedure to those of 2–18 h embryo nuclei and 3 day pupal nuclei. DNA from three preparations of each nuclear type was examined and the intron− and intron+ cistron contents quantitated using a Southern transfer procedure. The number of intron− and intron+ rDNA cistrons per haploid genome in the presumed diploid 2–18 h embryo DNA was first established, and then the intron− and intron+ cistron contents of nurse cell nuclear DNA and 3 day pupal DNA were determined relative to these values. The intron− cistron content of nurse cell nuclear DNA was indistinguishable from that of embryonic DNA but the intron+ cistrons showed an 8-fold under-replication relative to the presumed diploid DNA. A slight under-representation of the intron− cistrons and 3-fold under-replication of the intron+ cistrons were demonstrated for 3 day pupal DNA. These findings strongly suggest that intron+ rDNA cistrons are non-functional in nurse cell nuclei and substantiate the generality of this implication for the whole organism during early pupal life.