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Nurse cell polytene chromosomes of Drosophila melanogaster otu mutants: morphological changes accompanying
N I Mal'ceva1, E S Belyaeva, R C King
1Laboratory of Molecular Cytogenetics, Institute of Cytology and Genetics, Novosibirsk, Russian Federation.
Abstract:
Combinations of certain mutant alleles of the ovarian tumor gene permit the production of viable eggs. Two alleles that behave in this way are otu7 and otu11. Females homozygous for either allele are sterile, and their ovarian nurse cells (NC) contain giant polytene chromosomes of various morphologies. Fertile flies (otu+/otu+, otu+/otu7, otu+/otu11) have endopolyploid nurse cells with typical dispersed chromosomes. Fertile hybrids (otu7/otu11) 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 activation and expression of genes functioning during oogenesis. The otu gene encodes a long and a short isoform. The normal long isoform appears to be responsible for the dispersion of chromosomes during the endomitolic DNA replications occurring in ovarian NCs. The genetic inactivation of euchromatic genes placed next to pericentric heterochromatin by a chromosomal rearrangement is accompanied by the compaction of corresponding chromosome regions. A comparative study of the manifestation of position-effect variegation for the polytene chromosomes of SG cells and NCs was made using the Dp(1;1)pn 2b and Dp(1;f)1337 rearrangements. The percentage frequencies of block formation in the SG and NC nuclei for Dp(1;1)pn 2b rearrangement were 92.6% vs. 15.8%, respectively; for Dp(1;f) 1337, these values were 56.8% vs. 9.7%. Therefore heterochromatin belonging to germ line chromosomes is in a configuration that is far less likely to inactivate inserted segments of euchromatin than is heterochromatin from somatic chromosomes.
Insights
Mutant ovarian tumor (otu) gene alleles in flies create unique polytene chromosomes in nurse cells, enabling gene expression studies during oogenesis. These otu hybrids reveal differences in heterochromatin
Area of Science:
- Developmental Biology
- Genetics
- Cell Biology
Background:
- Mutations in the ovarian tumor (otu) gene affect female fertility and nurse cell chromosome morphology in flies.
- Specific otu alleles (otu7, otu11) combined can produce fertile hybrids with highly polytenized nurse cell chromosomes.
- Ovarian nurse cells (NCs) normally undergo endomitotic DNA replication, leading to polytene chromosomes.
Purpose of the Study:
- To investigate the chromosomal basis of gene expression during oogenesis using otu mutant hybrids.
- To compare the behavior of heterochromatin and its effect on gene expression in somatic (salivary gland) and germline (nurse cell) chromosomes.
Main Methods:
- Generation of fertile otu7/otu11 hybrid flies.
- Analysis of polytene chromosome morphology in ovarian nurse cells and salivary gland cells.
- Comparative study of position-effect variegation (PEV) using specific chromosomal rearrangements (Dp(1;1)pn 2b, Dp(1;f)1337).
Main Results:
- Otu hybrids exhibit extensive polytenization in nurse cells, suitable for detailed chromosomal studies.
- Position-effect variegation is significantly less frequent in nurse cell chromosomes compared to salivary gland chromosomes.
- Heterochromatin in germline chromosomes is less effective at inactivating adjacent euchromatic genes than somatic heterochromatin.
Conclusions:
- Otu hybrids provide a powerful model for studying gene regulation during oogenesis at the chromosomal level.
- Germline heterochromatin exhibits distinct properties influencing gene expression compared to somatic heterochromatin.
- The long isoform of the otu gene is implicated in maintaining chromosome dispersion during endoreplication in nurse cells.