. The Gnamptogenys taivanensis group (Hymenoptera: Formicidae: Ectatomminae) is a species-group that contains eight described species known from southern China, Taiwan, and northern Vietnam. The taxonomy of the group remains poorly understood due to limited morphological evidence (mostly relying on the morphology of the worker caste), the complete lack of molecular evidence, as well as poor sampling effort: most of the species except G. taivanensis have so far been known only from one or two localities in southern China each, and are allopatric to each other. However, our recent collection of many colonies of G. taivanensis group from three localities (Tay Con Linh, Phia Oac-Phia Den, and Hoang Lien Son) in the northern mountainous region of Vietnam showed remarkable diversity in the external morphology of workers, even within a single locality. The present study thus aims to explore species delimitation of the G. taivanensis group in the region using a combination of phylogenetic and morphometric analyses. Phylogenetic analyses for the genome-wide sequence datasets generated by MIG-seq and morphometric analysis using Nest-Centroid (NC) clustering consistently recovered seven major lineages of the G. taivanensis group (six Vietnamese and one Taiwanese lineages), with four singleton colonies for which these analyses yielded inconsistent and complex pictures. Moreover, the sympatric occurrence of the two or three lineages in each of the three localities of northern Vietnam is indirect evidence of the presence of reproductive isolation among these lineages. These lines of evidence strongly support heterospecificity of the lineages. However, phylogenetic analysis for mitochondrial COI gene showed an almost incompatible pattern with these lineages, possibly due to incomplete lineage sorting and/or introgressive hybridisation. A future comprehensive study on a larger geographic scale is needed to clarify the placement of the singleton colonies and conduct a formal taxonomic revision of the group.
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