Continental phylogeography, cryptic diversity, and taxonomic revision of the genus Calamobius (Coleoptera, Cerambycidae)

Abstract

The genus Calamobius contains a single species, Calamobius filum, a widespread longhorn beetle whose larvae bore into Poaceae stems and can cause significant economic losses in cultivated cereals. During the last two decades, the species has undergone a rapid northward expansion across Europe, raising concerns regarding its increasing agricultural importance. Despite its broad distribution, the evolutionary history and phylogeographic structure of Calamobius have remained poorly understood. This study investigated the continental-scale genetic diversity, phylogeography, taxonomy, and historical biogeography of Calamobius using mitochondrial COI DNA barcodes. Seventy-seven barcode sequences from Europe, Asia Minor, and the Middle East, including newly generated sequences from Spain, Türkiye, and Ukraine, were analyzed using maximumlikelihood phylogenetic reconstruction and haplotype network analysis. Ecological niche modeling was applied to reconstruct potential Last Glacial Maximum refugia, postglacial range dynamics, and future distribution under climate warming scenarios. Morphological comparisons of adult external characters and male genitalia were conducted to evaluate taxonomic boundaries. Phylogenetic analyses revealed two deeply divergent evolutionary lineages separated by approximately 6–7% mitochondrial sequence divergence. These lineages comprise five haplogroups that exhibit strong geographic segregation corresponding to major European mountain systems and Mediterranean basins, indicating prolonged historical isolation. The molecular divergence is accompanied by morphology and distinct behavioral traits, supporting recognition of two cryptic species within Calamobius. Consequently, the western lineage is restored as Calamobius tenuis Blanco-Fernández, 1859, stat. et nom. res., whereas the eastern lineage retains the name Calamobius filum (Rossi, 1790). Within these species, five geographically coherent subspecies are recognized: Calamobius tenuis tenuis, Calamobius tenuis decoripes comb. et stat. nov., Calamobius filum filum, Calamobius filum gracilis comb. et stat. nov., and Calamobius filum alasiyae Zamoroka ssp. nov. Species distribution modeling indicates that both species survived the Last Glacial Maximum in several southern refugia. However, mitochondrial phylogeography demonstrates that the two widespread northern haplogroups are absent from Mediterranean refugia, suggesting their origin in extra-Mediterranean refugial areas, most probably southern France and the Danube Basin. These findings challenge the traditional hypothesis that postglacial recolonization of Europe originated exclusively from Mediterranean refugia. Instead, they highlight an important role of northern refugia in shaping the present-day distribution of European longhorn beetles. Historical and contemporary distribution data demonstrate exceptionally rapid climate-driven range expansion. Over approximately the last two decades, Calamobius filum has expanded its northern distribution by about 500 km in Eastern Europe, while Calamobius tenuis stat. et nom. res. has advanced approximately 300 km in Western Europe. Ecological niche models further predict continued expansion toward the Baltic region and southern Fennoscandia under moderate warming scenarios. Additionally, DNA barcoding revealed secondary contact between both species in the Rhine Valley, suggesting a developing contact zone requiring further investigation using nuclear genomic markers.