Ancient chromosomal inversion helps stick insects thrive on Redwood trees

According to evolutionary geneticist Zachariah Gompert from Utah State University, genetic variation is the crucial driving force behind evolution. However, over time, this reservoir of genetic variation diminishes due to natural selection and random genetic drift.

Scientists have been pondering whether and how genetic variation can persist in the long run. Gompert and his colleagues from the University of Montpellier in France, the John Innes Center in the United Kingdom, the National Autonomous University of México in Querétaro, the University of Nevada, Reno, and the University of Notre Dame have addressed this question in a study published in the online edition of the Proceedings of the National Academy of Sciences on June 12, 2023.

The researchers focused their investigation on stick insects of the genus Timema, which have a diverse diet encompassing various plant species.

“We studied how genetic variation is maintained within a species and how it influences adaptation,” explains Gompert, an associate professor in USU’s Department of Biology and the USU Ecology Center.

In their study, the team examined Timema stick insects, specifically Timema knulli, which has the unique ability to feed and thrive on Redwood trees. Other Timema species struggle to survive on these trees or cannot do so at all.

The researchers discovered that T. knulli possesses this adaptation due to a chromosomal inversion—a structural change in its genome. Unlike a gene mutation that alters the DNA sequence, a chromosomal inversion occurs when a segment of a chromosome breaks at two points, flips 180 degrees, and reattaches at the original breakpoints.

“In the case of this inversion, a large segment of 30 million DNA bases on the chromosome gets inverted,” explains Gompert.

Interestingly, the team determined that this chromosomal inversion in T. knulli is ancient, estimated to have occurred approximately 7.5 million years ago. Even today, T. knulli populations carry both versions of the alleles—the one enabling feeding and thriving on Redwoods as a host plant and the original version that enhances survival on the ancestral host plant, a flowering plant. The heterozygous form of the inversion may be particularly advantageous.

Gompert states that environmental variations and gene exchange among migrating populations of stick insects contribute to the persistence of both the new and ancestral chromosomal variants or polymorphism. This persistence can provide the organisms with an advantage in an ever-changing world by facilitating ongoing evolution and adaptation.

“Rather than being a disadvantage, the intricate evolutionary processes involving this inversion offer protection against the loss of genetic variation and potentially support long-term survival,” suggests Gompert.

Source: Utah State University

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