Evolutionary fuel: Researchers study maintenance of an ancient chromosomal inversion

Newswise — LOGAN, UTAH, USA --Genetic diversity is the prime force behind evolution, claims Zachariah Gompert, an evolutionary geneticist from Utah State University. However, throughout time, this diversity pool diminishes due to the effects of natural selection and random genetic drift.

The ongoing existence and mechanisms of long-term genetic variation pose a significant inquiry for scientists. In the June 12, 2023, online release of the Proceedings of the National Academy of Sciences, Gompert and his colleagues from the University of Montpellier in France, the John Innes Centre in the United Kingdom, the National Autonomous University of México in Querétaro, the University of Nevada, Reno, and the University of Notre Dame present their investigation into this matter. The study received support from Gompert's 2019 National Science Foundation CAREER Award, as well as funding from the European Research Council.

Gompert, an associate professor in USU's Department of Biology and the USU Ecology Center, explains, "Our study focused on the mechanisms behind the preservation of genetic variation within a species and the subsequent effects of such variation on adaptation."

To conduct the study, the team specifically examined stick insects of the genus Timema, known for their diverse diet that encompasses a wide range of plant species.

Gompert elaborates, stating, "In western North America, there exist over a dozen species of Timema. These species are generalists and can consume various plant types. However, Timema knulli is unique in that it exclusively feeds and flourishes on Redwood trees, which are unable to sustain other Timema species to the same extent or not at all."

According to Gompert, the distinctive ability of T. knulli is attributed to a chromosomal inversion, which refers to a structural alteration in its genome. Unlike a gene mutation that involves changes in the DNA sequence, a chromosomal inversion occurs when a segment of the chromosome experiences two breaks followed by a 180-degree rotation before being reinserted at the initial breakpoints.

Gompert further explains, "In the case of an inversion, significant segments of the chromosome, in this instance approximately 30 million DNA bases, undergo a reversal, effectively flipping backwards."

And this inversion in T. knulli, the team determined, is ancient.

Gompert reveals, "Based on our analysis, we estimate that the chromosomal inversion likely occurred around 7.5 million years ago. What's fascinating is that populations of T. knulli still retain both versions of the alleles. One version enables them to feed and thrive on Redwoods as a host plant, while the original version enhances survival on the ancestral host plant, a flowering plant. It appears that the heterozygous form, possessing both versions, may confer additional advantages."

Gompert explains that the persistence of both the new and ancestral chromosomal variants or polymorphisms in stick insects is influenced by environmental heterogeneity and gene exchange among migrating populations. This phenomenon plays a crucial role in facilitating ongoing evolution and adaptation, providing these organisms with an advantage in a dynamic and changing world. By maintaining genetic diversity through these mechanisms, stick insects can continue to evolve and adapt to their surroundings.

Gompert suggests that the intricate evolutionary processes associated with this chromosomal inversion, far from being a disadvantage, actually contribute to the resilience of the species by safeguarding against the loss of genetic variation. This complexity may ultimately promote the long-term survival of the species, ensuring its ability to adapt and persist in changing environments.