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

Abstract

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 unchanged through generations. Quantitative analysis of in situ DNA hybridization demonstrated that for the 12SrDNA probe, of a gene outside the deletion, the mitochondrial DNA cellular content in the studied cells of the mutant strain is 1.5 times higher than in the wild-type strain. For the probe encoding Cyto b, a mitochondrial gene affected by the mutation, the ratios (mutant 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 levels 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, the 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 studies indicate that the progressive heteroplasmy increase may be related to intense phases of mitochondria biogenesis and that different compensatory phenomena may exist.

The nuclear genome is involved in heteroplasmy control in a mitochondrial mutant strain of Drosophila subobscura

“…We have shown [25,24], that heteroplasmy level is lower in the germinal line. It is 50% in the stage 10 oocytes of the mutant strain, 55% in the stage 14 oocytes, 55–60% in the embryos, and then increases during the larval stages before levelling off around 80% at the third larval stage, after which it remains constant until the death of the flies.…”

“…However, the mutation does not give rise to an evident phenotype, except for a decrease in activity of complex I (40–50%), which is particularly affected by the mutation, and of complex III (20% decrease), but the ATP synthesis capacity and the cellular energy balance are preserved [20,21]. The heteroplasmy is probably intramitochondrial [22,23], and is less marked in the oocytes (55–60%) [24,25].…”