Extended taxon sampling reveals conflicting mitochondrial phylogenomics in the holarctic Reticulitermes termites (Blattodea: Termitoidae).

Reticulitermes is a genus of subterranean termites that experienced a rapid radiation during the late Miocene, resulting in a widespread distribution across the Holarctic region. Despite decades of phylogenetic research that has delineated some well-supported lineages, relationships among them remain unresolved. Previous studies based on a handful of mitochondrial or nuclear markers have produced conflicting topologies and failed to robustly resolve deep nodes within the genus, particularly among the East European-West Asian (EE + WA) and Western Europe lineages. More recently, despite improved resolution achieved through mitochondrial phylogenomics, a major limitation in Reticulitermes studies has been the complete absence of mitochondrial genomes for the EE+WA lineage. To address this, we sequenced 6 new mitochondrial genomes from 5 Reticulitermes taxa, including the first representatives of the EE + WA clade, and analyzed them alongside previously published data. Phylogenies were inferred across a range of analytical conditions. All analyses supported the monophyly of Reticulitermes, though the placement of EE + WA varied depending on the data and model. Analysis of the nucleotide dataset recovered EE + WA as sister to all other clades. In contrast, the amino acid dataset found it nested within the genus, placing East Asia as the earliest diverging lineage. Our results underscore the sensitivity of phylogenetic inference to dataset and model choice in the context of Reticulitermes rapid radiation and provide new insights for resolving the evolutionary history of this genus. The inclusion of EE + WA fills a key geographic gap and suggests a complex history of diversification, consistent with either an ancient refugial split in southeastern Europe-Western Asia or an extraordinarily rapid westward expansion.