Beetles adapted to caves millions of years before entering them

2023-06-30 09:30:49

A study by the Institute of Evolutionary Biology (IBE), a joint center of the Higher Council for Scientific Research and Pompeu Fabra University, compared different lineages of terrestrial and aquatic beetlesof which several species had managed to colonize the underground world independently.

The team found that both groups shared a increase in genetic variability that facilitated their later adaptation to the caves. According to the results, changes in the gene repertoire that would later facilitate their adaptation occurred millions of years before to enter this medium.

Contrary to the most widespread theory, the study has shown that it was this increase in genetic diversity that allowed adaptation to cavesand not only the genetic loss associated with the traits that determine some adaptations, such as loss of pigmentation or vision.

Increased genetic diversity allowed beetles to adapt to caves

The study has also determined that this evolutionary process was both parallelwith similar changes in the same genes from different species, such as convergentwith similar changes in different genes performing the same function.

The order of beetles divided into two main lineages more than 300 million years ago: one consisting mainly of ground beetles and another that would also include most of the water beetles. As a result, this great order of insects continued to diversify for millions of years until several of its lineages conquered the subterranean environment.

This study compared comparative genomics tools extant species of six different lineages of subterranean and surface beetles, which come from widely separated ancestral lineages. However, he did not find a large genetic difference between the subterranean and surface species, instead in all cases the changes were observed much earlier.

“Genomic changes in the gene repertoire are observed before the colonization of the caves. In these ancestors there were more genetic changes than between the close species of the caves and their terrestrial sister species”, affirms Rosa FernandezIBE researcher, who has led the study.

Genomic changes in the gene repertoire are observed before the colonization of the caves

Rosa Fernández, study leader

The results point to a case of evolutionary exaptationwhich occurs when a trait, characteristic, or structure of an organism or taxonomic group assumes a function that did not previously exist or that differs from the original function derived from its evolution. An example of this could be the use of feathers for mating or flight in birds, which originally developed feathers to keep warm.

Deconstructing Darwin’s ‘evolutionary wrecks’

Loss of vision and pigmentation in subterranean animals, among others, is related to the loss of genes related to these traits due to an environment as restrictive as caves, as defended by the motto “Either you use it, or you lose it”. For this reason, Darwin believed that the animals that ventured into this environment were ‘evolutionary shipwrecks’ that would gradually degenerate and they could not emerge again. The results of the study, however, contradict this. The research found that there was more gene gain than loss in this adaptation process of the beetles.

The research found that there was more gene gain than loss in the beetles’ adaptation to caves.

“The duplication and gain of genes had a fundamental role for the colonization and later adaptation to the deep subterranean medium. This mechanism could be the key to better understanding the adaptations of these animals, beyond the loss of genes,” he adds. Pau Balart-Garcíaresearcher at the IBE and first author of the study.

In this way, the study refutes the theory of simplification of the genetic repertoire as a mechanism of adaptation to this subterranean environment.

The keys to the evolution of underground animals

Until now, it was believed that the convergent evolution was the main force shaping the genomes of subterranean animal species, and would result from similar changes in different genes or gene families that would carry a very similar function, such as the loss of vision of subterranean beetles. Parallel evolution, on the other hand, is observed among the same families that underwent similar changes, such as expansions or contractions.

This study shows that both modes of evolution are responsible for facilitating cave adaptation in totally different subterranean beetle species. This discovery facilitates the investigation of the evolution of other animals underground in future studies, such as molluscs, scorpions or salamanders, and may help to better understand the genomic basis of adaptation to caves.

Reference:

Balart-García, P. “Genomic exaptation and convergent evolution paved the way to independent subterranean colonization across beetle lineages” Nature Communications (2023)

Rights: Creative Commons.