Mitochondrial DNA (mtDNA) is an important molecular marker for insect ecology. For instance, mtDNA markers have been used to analyze the genetic diversity of melon aphid (Aphis gossypii) populations on different host plants, informing understanding of their dispersal patterns. However, recent research suggests the utility of mtDNA may be more complicated than once thought. In a new review in the Annals of the Entomological Society of America, a team of researchers conclude that it remains a valuable basic tool but stress that scientists must consider the properties of mtDNA when using the marker and drawing conclusions from the data. (Photo by Jim Baker, North Carolina State University, Bugwood.org)

Melissa Mayer

By Melissa Mayer

Insects, like all eukaryotes, have specialized organelles called mitochondria tucked into nearly every cell. Most famously, these act like tiny generators, transforming the energy from food into energy that powers the cell.

Mitochondria have their own set of DNA separate from the DNA held in the cell’s nucleus. That mitochondrial DNA (mtDNA) is an important molecular marker for insect ecology, but new research suggests it may be more complicated than once thought.

In a review published May 15 in the Annals of the Entomological Society of America, researchers with the Qingdao Agricultural University, Jilin Academy of Agricultural Sciences, and Shandong Academy of Agricultural Sciences in China looked at the utility of mtDNA as a molecular marker, given those new complications. They conclude that the marker remains a valuable basic tool but stress that scientists must consider the properties of mtDNA when using the marker and drawing conclusions from the data.

mtDNA and Ecology Studies

One of the attractive features of mtDNA is that it’s easy to work with in the lab. That’s because there are multiple copies in each cell, and mitochondrial genes are conserved across animal species. It also has some super useful properties that have now come into question: simple inheritance and near-neutrality.

Unlike nuclear DNA, mtDNA doesn’t come from both parents and recombine; it simply passes from mother to offspring. It also mutates at a much higher rate than nuclear DNA. Those mutations usually don’t make a species more or less fit (and are thus termed “nearly neutral”) and can help trace genetic lineages through time.

That’s made mtDNA a powerful tool for insect ecology studies, especially phylogenetics, interactions between insects and plants or parasitoids, biological invasion, and species identification.

mtDNA Status: It’s Complicated

But it turns out it’s not quite that simple. New research shows that mtDNA does recombine sometimes. And it isn’t as nearly neutral as once believed.

“Mitochondrial DNA is no longer considered absolutely neutral,” says Xing-Yuan Men, Ph.D., professor at the Institute of Plant Protection at Shandong Academy of Agricultural Sciences and senior author of the paper. “Some varieties in mtDNA are usually related to fitness.”

The authors say that fitness can be affected by differences in mtDNA sequences and transcription levels, especially when it comes to selective pressures like climate stress and food availability. It’s also possible for insect lines that separated to come back together and hybridize, sometimes onboarding new genes that increase fitness.

This new information doesn’t change the fact that mtDNA is a convenient and relatively inexpensive molecular marker—but it does mean that researchers must be careful when using mtDNA and drawing conclusions from their data. It also means that a strong background in biochemistry and cell biology are valuable for emerging entomologists.

Looking Toward the Future

The authors make specific recommendations for researchers, noting that phylogenetics, species evolution, and population dynamics are subjects in which understanding the nuanced properties of mtDNA is essential. They suggest using other approaches along with mtDNA to study gene flow or isolation by distance.

They also say that DNA barcoding with mtDNA—using short stretches of DNA to identify species or detect cryptic species—may be best-suited for confirming known species and used along with other data like nuclear DNA, morphology, and ecology. Mitochondrial DNA remains a good source of markers for metabarcoding, which sequences bulk samples or environmental samples to identify multiple species in a community.

A better understanding of mtDNA could also help the next generation of molecular markers emerge. “The sequencing technology is developed fast,” Men says. “Comparative genomics often uses the whole genome as markers and differs from the current marker system, which uses only limited fragments of mtDNA. The new generation of markers should be composited of whole or part-mtDNA genomics and thus contain more information.”

That’s good news for molecular ecology, especially as the pressures of climate change amp up the need to understand how insects responded to environmental shifts in the past in order to better predict their future.

Melissa Mayer is a freelance science writer based in Portland, Oregon. Email: melissa.j.mayer@gmail.com.