Lice live their entire lives with a set of genes that in humans would indicate a late-stage degenerative disorder such as Parkinson’s or Alzheimer’s disease.
How do lice tolerate this genome structure that in humans and many other animals would result in major neurodegenerative problems? “We’re a long way from connecting those dots,” said Stephen Cameron, professor of entomology at Purdue University.
Parkinson’s and Alzheimer’s are aging-related mitochondrial diseases, so called because of the malfunctioning mitochondria that produce cellular energy.
“If animals develop a neurological disorder too early in life, they’re not going to have offspring,” Cameron said. “Lice seem to have it from when they hatch. They’re clearly handling it and have handled it for 50 million years.”
The last decade has seen a flood of mitochondrial genomics data on lice and other insects. This data bolsters studies related to properly identifying and classifying insect species and developing insecticides that act on the mitochondria. Lice, meanwhile, offer a model for studying the impact of evolution on neurodegenerative disease.
The number of sequenced insect mitochondrial genomes has increased by 876% since 2014, while the total species diversity count has increased by 790%. Cameron’s article “Insect Mitochondrial Genomics: A Decade of Progress,” appeared in the Annual Review of Entomology. He published a similar article in 2014.
The mitochondria help organisms process oxygen and food, which are subject to natural selection. “And yet we have stunningly few examples of studies that actually take that into account,” Cameron said.
The sequencing technologies used today were just becoming available in 2014. Since then,
technologies have improved while cost has plummeted. Cameron sequenced his first genome in 2002. The task took six months of daily lab work and cost $4,000.
“For that money I could now do hundreds,” he said. “Genetic-scale sequencing these days is just ludicrously efficient.”
Today, Cameron’s graduate students do more genomic analysis in a week than he did in two years working as a postdoctoral researcher. Cameron broadly compared the geometric rate of increase to Moore’s Law, the idea that the number of transistors on a computer chip doubles about every two years.
His new review article identified genomic fragmentation and control region duplication as currently important research topics. Entomologists see genomic fragmentation, the breakdown of DNA into smaller pieces, as a model system to study neurological disease states.
“It’s rare. Anything that’s rare makes you wonder why,” Cameron noted. “Typically, when things are all one way, it’s because evolution has it in a box. It’s constrained. So, why is this constraint against fragmented genomes being released in the case of lice?”
Some evidence suggests that in extreme cases, fragmentation might benefit small, inbred populations by allowing an organism to purge harmful forms of its mitochondrial genome. This may apply to lice, which consist of small, inbred populations.
Cameron is conducting research both on fragmentation in lice and on the duplication of genetic control regions in a type of winged insect called thrips. Small and difficult to identify, thrips sometimes bloom into large infestations that plague greenhouses and field crops.
Molecular diagnostics can help identify the pest, but changes in its mitochondrial genome make this more difficult in thrips than in other insect types.
“We’re trying to work up some better methods that allow us to more reliably use DNA-based species delimitation, which can then be used for quarantine services to keep pests out of America or other areas,” he said.
Mitochondrial genome studies also can contribute to pest control methods that act by modifying insect metabolism. “Most pesticides are neurotoxins, so they don’t immediately interact with the mitochondria.”
Insect identification has both scientific and economic implications. Nearly all insect identifications that use molecular data are performed using a mitochondrial DNA method.
In the 1990s and early 2000s, South American fruit flies that managed to elude early detection and identification jumped into commercial fruits. That could happen again with new agricultural areas joining the global production system.
Agricultural biosecurity can predictively benefit from a better genomic understanding of insect pests, Cameron noted. Doing such work now parallels the coronavirus research that followed the severe acute respiratory syndrome (SARS) outbreak 20 years ago. Once the COVID-19 pandemic broke out in 2020, biomedical researchers were able to respond swiftly.
He also highlighted an emerging body of research on the effects of extreme environments, such as high altitudes and desert conditions, on insect genomes. The results could then be applied to various methods designed to control their populations.
“It’s good to know what their biology would allow them to evolve toward, to understand the escape hatches that evolution provides for them,” he said. “And, with climate change, those aspects contribute to our understanding of how beneficial insects might respond to changing environments.”
