Expression of six mitochondrial genes during Drosophila oogenesis: analysis by in situ hybridization

We have done a comparative analysis of RNA from six mitochondrial genes (rDNA, ND2, COI, COIII, ND4–ND5, Cyt b) during Drosophila oogenesis, using in situ hybridization. This study showed the same variation for each of these transcripts, which is similar to that obtained with the total mitochondrial RNA (Tourmente et al. (1990) Biol. Cell 60, 119–127). A constant RNA density until stage 9, followed by a rapid decline, was observed in follicle and nurse cells. These results confirm those previously obtained (Tourmente et al., (1990) Biol. Cell 60, 119–127), in favor of the existence of a correlation between the mtRNA level and the cell volume and / or the nuclear DNA content, and suggest a global extra-mitochondrial, transcriptional control mechanism. We also show that the relative proportions of the different RNA are similar, whatever the stage and cell type examined, even though the total mtRNA quantity is different. They are comparable to those previously obtained by Northern analysis of Drosophila embryos (Berthier et al. (1986) Nucleic Acids Res. 14, 1400–1412, suggesting a posttranscriptional control independent of the cell type. Surprisingly, we have detected an extra-mitochondrial hybridization for COIII, both in light and electron microscopy. Northern analysis of poly(A) ⁺RNA from ovaries or cultured cells revealed an 1.7 kb extra-mitochondrial RNA, which is probably of nuclear origin.