Global DNA Barcoding of Sigara (Hemiptera: Corixidae) Reveals Cryptic Species Formation and Climatic-Niche Divergence

The genus Sigara Fabricius, 1775 (Hemiptera: Corixidae) is an important freshwater bioindicator, but its conservative morphology complicates traditional species identification. This study combines DNA barcoding, traditional taxonomy, and climatic factors within an integrative framework to provide new insights into species delimitation. We analyzed cytochrome c oxidase subunit I (COI) sequences from 501 Sigara specimens collected from 15 countries, including 95 newly generated COI barcodes. Our integrated approach included phylogenetic analyses, genetic distance assessments, and ecological niche modeling. The results revealed remarkable diversity, with sequences clustering into 46 Barcode Index Numbers (BINs). COI genetic distances showed a pronounced bimodal pattern, with most intraspecific divergences below 1% and interspecific divergences mainly ranging from 8-16%, while the few intermediate divergences (4-8%) suggested potential cryptic diversity or recent species divergence. Most morphologically defined species corresponded to distinct molecular lineages, supporting traditional taxonomy, whereas some morphospecies exhibited genetic discordance potentially associated with species divergence. Integrative analyses revealed that climatic factors, particularly isothermality and seasonal temperature and precipitation, are major drivers of Sigara MOTU distributions, highlighting the important role of climate in shaping their geographic patterns. These findings indicate that although COI barcodes are not sufficient to define the species boundaries of Sigara alone, they provide a key entry point for the accurate division of species, which in turn promotes the development of molecular tools for freshwater ecological monitoring and conservation.