Mitochondrial DNA maintenance in Drosophila melanogaster

Abstract

All 37 mitochondrial DNA (mtDNA)-encoded genes involved with oxidative phosphorylation and intramitochondrial protein synthesis, and several nuclear-encoded genes involved with mtDNA replication, transcription, repair and recombination are conserved between the fruit fly Drosophila melanogaster and mammals. This, in addition to its easy genetic tractability, has made Drosophila a useful model for our understanding of animal mtDNA maintenance and human mtDNA diseases. However, there are key differences between the Drosophila and mammalian systems that feature the diversity of mtDNA maintenance processes inside animal cells. Here, we review what is known about mtDNA maintenance in Drosophila, highlighting areas for which more research is warranted and providing a perspective preliminary in silico and in vivo analyses of the tissue specificity of mtDNA maintenance processes in this model organism. Our results suggest new roles (or the lack thereof) for well-known maintenance proteins, such as the helicase Twinkle and the accessory subunit of DNA polymerase γ, and for other Drosophila gene products that may even aid in shedding light on mtDNA maintenance in other animals. We hope to provide the reader some interesting paths that can be taken to help our community show how Drosophila may impact future mtDNA maintenance research.

Keywords: DNA synthesis and repair; Drosophila melanogaster; mitochondria; nucleic acids.

Figure 1. Schematic representation of the Drosophila melanogaster and human mitochondrial genomes

The non-coding regions (A+T-rich region in flies, and D-loop in humans) are shown in gray. Genes encoded in the heavy and light strands (or their equivalents in the Drosophila mtDNA) are shown in black and blue, respectively. Transfer RNA genes are indicated by single-letter symbols, and the small and large ribosomal RNA genes by 12S and 16S, respectively. The genome size indicated is based on Lewis et al. [42] and Anderson et al. [130].

Figure 2. Hierarchical cluster analysis of the relative expression levels of Drosophila mtDNA maintenance genes across different fly tissues

Transcript levels were obtained from FlyAtlas 2 [95], processed as described in the Methods, and shown in the heatmap as log₂(fold change). The analysis was performed using EXPANDER [126]. L, AM, and AF indicate larval, adult male and female, respectively. The tissues sampled were: head, brain, eye, trachea, salivary gland (SG), crop, heart, midgut, hindgut, malpighian tubules (MT), garland cells (GC), rectal pad (RP), thoracicoabdominal ganglion (TAG), fat body (FB), carcass, ovary, virgin spermatheca (VS), mated spermatheca (MS), testis and accessory glands (AG). Groups 1 and 2 indicate respectively the ‘transcription/transcript processing’ and ‘replication’ clusters of genes formed with the analysis. See text for more details.

Figure 3. PolG2 expression across Drosophila tissues correlates with mitochondrial RNA metabolism, and not mtDNA replisome genes

Lack of significant correlations between PolG2 transcript levels and those of the indicated mtDNA replisome genes is shown in (A). Significant correlations among the transcript levels of the other mtDNA replisome genes, and between PolG2 transcript levels and those of the indicated mitochondrial RNA metabolism genes are shown, respectively, in (B,C). Transcript levels were obtained from FlyAtlas 2 [95] and were processed as described in the Methods. The R², Pearson’s r, and the significance F values were obtained from linear regression analyses performed using Microsoft Excel. Tissues from which expression levels were calculated are listed in Figure 2.

Figure 4. Twinkle dysfunction has distinct effects on the Drosophila nervous system and musculature

Lifespan (A), climbing (B) and quantitative PCR (C) assays were performed as described in the Methods, using flies or fly tissue extracts as indicated. mtDNA copy number of Twinkle overexpressors in (C) was respectively normalized by the levels in each control UAS-Twinkle line, which were arbitrarily set to 1.0 (indicated with dashed line).