Stoneflies are among the most ancient lineages of winged insects, serving as crucial bio-indicators for the health of freshwater ecosystems worldwide. For decades, however, the evolutionary history of this important insect order, known as Plecoptera, has been a persistent puzzle for entomologists, with key aspects of their family tree remaining unresolved due to a sparse fossil record and complex physical traits.
A new comprehensive analysis of mitochondrial genomes has finally brought clarity to these long-standing questions. By sequencing and studying genetic data from 97 species, representing every known stonefly family, a collaborative research team has reconstructed the group’s evolutionary backbone, settled debates over its major sub-orders, and established a detailed timeline of its origins and diversification. The findings, published in the journal iScience, provide a robust framework for understanding the evolution of these ancient insects.
An Enduring Entomological Puzzle
Scientists have long struggled to build a reliable phylogenetic tree for the nearly 4,000 species of stonefly. The challenge stemmed from several factors, including an incomplete fossil record that offered few clues, the intricate and sometimes misleading evolution of their physical forms, and limited genetic sampling in previous studies. A central, unresolved issue was the basal relationship between the two primary suborders of Plecoptera, Euholognatha and Systellognatha. Limitations in both the scope of gene sampling and the evolutionary models used in prior research prevented a clear resolution.
A Comprehensive Genomic Approach
To overcome these past hurdles, researchers embarked on the most thorough genomic study of stoneflies to date. The effort was co-led by Prof. CAI Chenyang of the Nanjing Institute of Geology and Palaeontology at the Chinese Academy of Sciences and Prof. DU Yuzhou of Yangzhou University.
Data and Sequencing
The research team newly sequenced the complete mitochondrial genomes of 29 stonefly species. They combined this new information with 68 publicly available datasets to create a massive genetic library. This effort achieved a critical milestone: for the first time, all 17 extant families of stonefly were represented in a single genomic analysis, providing an unprecedented level of detail.
Analytical Framework
The study focused on the mitochondrial genome, a small, circular piece of DNA found outside the nucleus in an organism’s cells. In insects, it typically contains 37 genes, including 13 protein-coding genes, 22 transfer RNA genes, and two ribosomal RNA genes. Because it provides a wealth of nucleotide data and other genome-level characters, the mitogenome is a powerful tool for phylogenetic analysis. The team applied an integrative framework that combined this rich dataset with multiple, sophisticated evolutionary models to reconstruct the family tree with high confidence.
Reconstructing the Stonefly Family Tree
The results of the analysis provided a clear and robust reconstruction of the stonefly’s evolutionary backbone, resolving previously murky relationships. The study’s findings are largely consistent with classifications based on morphology, showing that the order Plecoptera divides into two main clades: the suborder Antarctoperlaria and the suborder Arctoperlaria.
A New Timeline for Stonefly Evolution
The genomic data allowed the researchers to establish a refined and detailed timeline for the origin and spread of stoneflies. The analysis indicates that the crown-group stoneflies—the group containing all living species and their most recent common ancestor—first originated during the Pennsylvanian period, approximately 323 to 299 million years ago. Following this origin, the major diversification events that gave rise to the modern families occurred between the Cisuralian period (299 to 272 million years ago) and the Early Triassic (252 to 247 million years ago).
The Power of Mitogenomes
This study highlights the immense potential of using mitochondrial genomic data to untangle complex and ancient evolutionary histories, especially when analyzed with optimized computational models. By providing a large number of genetic sites and stable genomic characters, mitogenomes can overcome the limitations of relying on just a few genes or purely on physical characteristics. This successful resolution of the stonefly family tree serves as a powerful demonstration of how mitogenomic approaches can be applied to clarify higher-level phylogenetic relationships in other challenging insect groups.
Collaborative Research Effort
The successful reconstruction of the stonefly phylogeny was the result of a dedicated collaboration between multiple institutions. The project was led by researchers at the Chinese Academy of Sciences and Yangzhou University. The work received support from the National Natural Science Foundation of China and the National Key R&D Program of China.