NURSE CELL POLYTENE CHROMOSOMES OF DROSOPHILA-MELANOGASTER OTU MUTANTS - MORPHOLOGICAL-CHANGES ACCOMPANYING INTERALLELIC COMPLEMENTATION AND POSITION EFFECT VARIEGATION

Combinations of certain mutant alleles of the ovarian tumor gene permi t the production of viable eggs. Two alleles that behave in this way a re otu(7) and otu(11). Females homozygous for either allele are steril e, and their ovarian nurse cells (NC) contain giant polytene chromosom es of various morphologies. Fertile flies (otu(+)/otu(+), otu(+)/otu(7 ), otu(+)/otu(11)) have endopolyploid nurse cells with typical dispers ed chromosomes. Fertile hybrids (otu(7)/otu(11)) produce large numbers of polytene chromosomes comparable to, and often larger than, classic salivary gland (SG) chromosomes. Therefore, these otu hybrids provide a unique system for studying, at the chromosomal level, the activatio n and expression of genes functioning during oogenesis. The otu gene e ncodes a long and a short isoform. The normal long isoform appears to be responsible for the dispersion of chromosomes during the endomitoti c DNA replications occurring in ovarian NCs. The genetic inactivation chromatin by a chromosomal rearrangement is accompanied by the compact ion of corresponding chromosome regions. A comparative study of the ma nifestation of position-effect variegation for the polytene chromosome s of SG cells and NCs was made using the Dp(1; 1)pn2b and Dp(1;f) 1337 rearrangements. The percentage frequencies of block formation in the SG and NC nuclei for Dp(1; 1)pn2b rearrangement were 92.6% vs. 15.8% r espectively; for Dp(1;f) 1337, these values were 56.8% vs. 9.7%. There fore heterochromatin belonging to germ line chromosomes is in a config uration that is far less likely to inactivate inserted segments of euc hromatin than is heterochromatin from somatic chromosomes. (C) 1997 Wi ley-Liss, Inc.