The Survival Advantage of an Ancient Chromosomal Inversion in Stick Insects

by Klaus Müller
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chromosomal inversion

Caption: Stick insects, Timema knulli, perched on a branch of a Redwood tree. The research team led by evolutionary geneticist Zachariah Gompert from Utah State University explored the impact of a chromosomal inversion in these insects. (Image credit: Moritz Muschick)

Maintaining genetic variation is crucial for the fueling of evolutionary processes, according to Zachariah Gompert, an evolutionary geneticist at Utah State University. However, over time, this reservoir of genetic diversity can become depleted due to natural selection and random genetic drift.

Scientists have been grappling with the question of whether and how genetic variation can persist in the long run. In a collaborative study, Gompert and his colleagues from the University of Montpellier (France), the John Innes Centre (UK), the National Autonomous University of México, Querétaro, the University of Nevada, Reno, and the University of Notre Dame addressed this inquiry. Their findings were published online on June 13, 2023, in the Proceedings of the National Academy of Sciences. The research received support from the National Science Foundation CAREER Award, which Gompert received in 2019, as well as funding from the European Research Council.

“Our investigation focused on understanding how genetic variation can be preserved within a species and how it influences adaptation,” explains Gompert, who is an associate professor in the Department of Biology and the Ecology Center at Utah State University.

The team chose to study stick insects of the genus Timema, known for their ability to feed on a wide range of plants.

“In western North America, there are over a dozen species of Timema that are generalists, capable of consuming various plant types,” Gompert says. “However, one particular species, Timema knulli, thrives exclusively on Redwood trees, unlike other Timema species that struggle or cannot survive on them.”

The unique ability of T. knulli to exploit Redwood trees as a host plant is attributed to a chromosomal inversion—a structural alteration in its genome. Unlike a gene mutation that involves changes in DNA sequence, a chromosomal inversion occurs when a segment of a chromosome is broken at two points, turned 180 degrees, and then reinserted at the original breakpoints.

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

The team’s research revealed that this chromosomal inversion in T. knulli is ancient, estimated to have occurred around 7.5 million years ago.

“We believe that T. knulli populations still possess both versions of the alleles—the one enabling them to feed and thrive on Redwoods and the original version that enhances survival on the ancestral host plant, a flowering plant. The heterozygous form of this inversion may be particularly advantageous,” Gompert adds.

Environmental diversity and gene exchange between migrating populations of stick insects contribute to the persistence of both the new and ancestral chromosomal variants or polymorphism. This persistence could provide an advantage to these organisms in a changing world by facilitating ongoing evolution and adaptation.

“Rather than being a hindrance, the intricate evolutionary processes associated with this inversion offer resilience against the loss of genetic variation and may promote long-term survival,” suggests Gompert.

Reference: “Complex evolutionary processes maintain an ancient chromosomal inversion” by Patrik Nosil, Victor Soria-Carrasco, Romain Villoutreix, Marisol De-la-Mora, Clarissa F. de Carvalho, Thomas Parchman, Jeffrey L. Feder, and Zachariah Gompert, 13 June 2023, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2300673120

Funding: National Science Foundation

Frequently Asked Questions (FAQs) about chromosomal inversion

What is the significance of the chromosomal inversion in stick insects?

The chromosomal inversion in stick insects, specifically in Timema knulli, is significant as it provides a unique advantage for the species. It allows T. knulli to feed and thrive on Redwood trees, a host plant that other Timema species struggle with or cannot survive on. This inversion is an ancient genetic adaptation that has persisted for approximately 7.5 million years.

How does the chromosomal inversion contribute to genetic variation?

The chromosomal inversion in Timema knulli contributes to genetic variation by maintaining both versions of the alleles associated with feeding on Redwoods and the ancestral host plant. This polymorphism enhances the species’ ability to adapt to different environments and promotes ongoing evolution.

How does the chromosomal inversion foster long-term survival?

The complexity of evolutionary processes involving the chromosomal inversion provides resilience against the loss of genetic variation. This resilience allows stick insects with the inversion to adapt to changing environments and increases their chances of long-term survival by enabling ongoing evolution and adaptation.

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