Image provided by Dr. Patel.
Within cells, specialized molecular compartments called organelles are hubs of activity. Mitochondria, the organelles responsible for the conversion of nutrients into energy, originated from one cell engulfing another and forming a symbiosis that has evolved over millions of years. The mitochondria have lost their autonomy but retained their own DNA, called mtDNA, which mainly contains genes required for energy production. While the majority of our DNA is inherited in equal measure from mother and father, mtDNA comes only from the mother. Therefore, mutations that negatively affect males but not females can accumulate in the mtDNA since its inheritance is, for the most part, unaffected by male-harming mutations. Evolutionary biologists have named this theory "Mother's Curse". Theoretically, it sets up an evolutionary "arms race" such that the nuclear DNA must acquire mutations to combat the negative effects of male-harming mutations in the mtDNA. But is there evidence for this? Researchers led by former postdoctoral fellow Maulik Patel in Dr. Harmit Malik's Laboratory (Basic Sciences Division) identified and described a mutation in a mitochondrial gene that specifically causes such a male fertility defect and lends support to this hypothesis. Their investigation was published in eLife.
To enrich for male-harming mutations, the researchers devised an experimental evolutionary strategy in which female flies are prevented from mating with their siblings and repeatedly crossed (mated with) males from an external stock. This is meant to prevent mutations that mask the male-harming mutations that arise in the female mtDNA from appearing in the nuclear DNA of the population. After a year of crosses, the authors tested for the appearance of a specifically male-harming mutation by looking for a defect in male fertility, measured by counting offspring. In 1 out of 12 experimental populations, the authors measured a defect in male fertility and found no evidence for a defect in fertility among the females of that population. The authors deep sequenced total DNA from 100 male flies from this experimental line as well as a control line. They found a single mutation, changing glycine 177 to a serine in the cytochrome c oxidase gene (COII), at a high frequency in the mtDNA of the experimental line. This gene is involved in the oxidative phosphorylation process that generates energy for the cell.
Cells contain multiple mitochondria each with its own version of the mtDNA and individuals can harbor a mtDNA mutation in all or some of their mitochondria so the scientists isolated male flies with the COIIG177S mutation in all of their mitochondria, at least as determined by the lack of non-mutant reads by deep-sequencing. These males were around 40% less fertile than their wild-type (non-mutant) counterparts and the difference in fertility between COIIG177S and wild-type increased to a 90% difference at a higher temperature. In contrast, they did not observe a defect in the health or fertility of the females by any measure they tested.
The fertility defects in the males homoplastic for COIIG177S were due to smaller seminal vesicles and impaired development of sperm, which ended up forming tangles rather than individual sperm. Additionally, individual COIIG177S mutant sperm were less motile than control sperm.
Finally, the authors measured the function of the mutated gene product and found that the activity of cytochrome c oxidase enzyme with the G177S mutation was lower than control COXII in flies kept at a higher temperature. The COIIG177S mutation did not compromise COXII activity specifically in males, as females with the COIIG177S mtDNA also had lower COXII activity. While the effect of the mutation on the activity of the enzyme is not male specific, it may follow that a mutation that decreases energy production efficiency affects males more acutely due to the high energy requirement for sperm movement.
When the scientists crossed COIIG177S flies to various other populations of Drosophila melanogaster isolated from different areas, they found that many of the male progeny had either fully or partially restored fertility despite still harboring the COIIG177S mutation in their mtDNA. This demonstrates that "suppressor" mutations in the nuclear DNA that mask the effects of mtDNA mutations exist in natural populations. "One of the more remarkable findings from our study is that the effect of the mitochondrial mutation on male fertility depends on the nuclear genome of the males, as mutant males with some nuclear genomes have normal fertility," said Dr. Patel. Identifying these "suppressor" mutations in the nuclear DNA will reveal the molecular pathways involved in this male fertility defect.
Patel MR, Miriyala GK, Littleton AJ, Yang H, Trinh K, Young JM, Kennedy SR, Yamashita YM, Pallanck LJ, Malik HS. 2016. "A mitochondrial DNA hypomorph of cytochrome oxidase specifically impairs male fertility in Drosophila melanogaster." eLife. 5:e16923.
See also the Hutch News article: https://www.fredhutch.org/en/news/center-news/2016/08/mothers-curse-mutation-identified-animals.html
This research was funded by the National Institutes of Health, the Helen Hay Whitney Foundation, the Mathers Foundation, and Howard Hughes Medical Institute.