Transmission of mitochondrial mutations and the action of purifying selection in the fly

By generating various Drosophila lines with two mitochondrial genotypes, we show that purifying selection drove complete replacement of the deleterious mutations by a wild type genome, but selection can stabilize propagation of two defective genomes that are complementary. Interestingly, such a selection does not occur at the organismal level, but rather occurs during a confined stage of oogenesis, likely due to a preferential proliferation advantage of the co-existing genome, which is more functional.

HFSP Long-Term Fellow Hansong Ma and colleagues
authored on Mon, 24 March 2014

The impact of mutations depends on their transmission as modulated by selection. Mendel and Darwin described the principles of segregation and natural selection, but these principles do not apply to the mitochondrial genome as they do to nuclear genes.  Each cell contains multiple mitochondrial genomes that appear to randomly replicate and distribute to daughter cells, and it is not known how selection operates to purge the mitochondrial genome of detrimental mutations.  

In this paper, we show that selection does not act on single genomes but on the collective activity of multiple genomes. We transferred cytoplasm between Drosophila melanogaster eggs carrying mitochondrial mutations to create various heteroplasmic lines with two mitochondrial genotypes and followed their ratios via qPCR over generations (see Figure).  We then altered the temperature to manipulate the selective pressure on a temperature-sensitive genome.  For all lines studied, negative selection drove a decline in the abundance of the temperature-sensitive genome over successive generations. Selection progressed to complete elimination of the temperature-sensitive genome if the second genome was wild type, but not if the second genome carried a mutation in a different gene (see Figure).  Transmission of two differently mutated genomes at a consistent ratio suggested that selection operates to optimize the function of a collective of genomes rather than acting autonomously on each genome.Thus, while this selection limits the abundance of the more defective genomes, it can also actively support persistence of such mutant genomes at low levels if they have another asset that contributes positively to the activity of an ensemble of genomes.

(click here to enlarge image)

Figure: The mtDNA map of the temperature-sensitive (ts) genome.  Flies with only the ts genome (homoplasmic) are healthy at permissive temperature (e.g. 22°C, but die in 4 days after being transferred to 29°C). Various heteroplasmic flies (flies have more than one mtDNA genotype) were made by cytoplasm transplantation and the abundance of the ts genome was followed at 29°C for multiple generations.  When paired with wild-type genome, the ts genome declined over generations and eventual it was completely eliminated. When paired with a less defective genome, the abundance of the ts genome decreased over generations as well, but it was maintained in a low percentage after many generations. We conclude that neither genome is eliminated when they make complementary contributions to function. 

We detected no selection at the level of the organism under conditions where we see selection for functional mitochondrial genomes. Therefore, selection occurs within the organism. By shifting temperature at different stages of development, we showed that the decline of a deleterious mutation was due to negative selection during mid oogenesis. Since selection is based on the function of electron transport, selection is acting on mitochondrial function.  Three categories of mechanism could give mitochondria a transmission advantage: selective localization to germplasm, resistance to an elimination mechanism, or preferential proliferation.  Our data disfavour the first two models due to the time of selection and insensitivity to parkin mutation, suggesting that preferential proliferation underlies selection.

Oogenesis is a period of tremendous growth in the population of mitochondria.  Like most eggs, a Drosophila egg is enormous.  It is 100,000 times the size of a normal cell and it has 10 million mitochondrial genomes.  It attains this bulk during oogenesis.  Roughly 16 doublings of mtDNA occur between the stem cell that gives birth to the egg chamber and the final oocyte.  We suspect that healthy mitochondria proliferate preferentially.  Supporting the possibility that a defect in electron transport might compromise replication of the mtDNA, an accompanying paper shows that the early egg chambers of homoplasmic temperature-sensitive mutants have greatly reduced mtDNA replication (Hill et al, 2014). We thus suggest that the temperature-sensitive genome has a marginally slower doubling rate in heteroplasmic strains.  This slight increase in doubling time will not detectably alter genome abundance in the first round of replication.  However, in exponential growth, even a slight difference in doubling is amplified to a large effect. Accordingly, if healthy mitochondria have a slightly faster proliferation, their advantage will be progressively magnified in subsequent doublings to increase their final contribution to the genomes populating the oocyte.

Reference

Transmission of mitochondrial mutations and action of purifying selection in Drosophila melanogaster.  Hansong Ma, Hong Xu & Patrick H O'Farrell. Nature Genetics (2014) doi:10.1038/ng.2919.

Other References

Selective propagation of functional mitochondrial DNA during oogenesis restricts the transmission of a deleterious mitochondrial variant. Hill, J.H., Chen, Z. & Xu, H. Nat. Genet.doi:10.1038/ng.2920 (9 March 2014).

Nature link

Pubmed link