Quantitative analysis, by ultrastructural in situ hybridization, of mitochondrial genomes and their expression in mid-gut and ovarian cells of a mutant strain of Drosophila subobscura

In the mitochondrial deletion mutant strain studied here, two types of DNA coexist (heteroplasmy): intact mtDNA (15.9 kb) and mutant mtDNA (10.9 kb), which represents about 80% of the mitochondrial genomes in somatic tissues. The heteroplasmy level is lower in ovary (63%). Mutation is transmitted un changed through generations. Quantitative analysis of in situ DNA hybridiza tion demonstrated that for the 12SrDNA probe, of a gene outside the deletio n, the mitochondrial DNA cellular content in the studied cells of the mutan t strain is 1.5 times higher than in the wild-type strain. For the probe en coding Cyto b, a mitochondrial gene affected by the mutation, the ratios (m utant versus wild-type content) differ according to cell type: close to 0.4 in MGE cells and 0.7 in ovary cells. These values indicate heteroplasmic l evels of about 72% in MGE cells and 50% in stage 10 oocytes, which is lower than that previously reported for stage 14 oocytes (60%) and embryos (69%) . Analysis of in situ RNA hybridization showed that for the 12SrDNA probe, the transcript concentrations do not differ significantly between MGE cells and cells of germinal origin from the two strains. For the Cyto b probe, t he mutant RNA/wild-type RNA ratios are lower in somatic cells than in stage 10 nurse cells and oocytes, but in each case less than expected. These stu dies indicate that the progressive heteroplasmy increase may be related to intense phases of mitochondria biogenesis and that different compensatory p henomena may exist. (C) 2000 Editions scientifiques et medicales Elsevier S AS.