The process of biological invasion represents an excellent opportunity for exploring organismal evolution over contemporary timescale. The yellow crazy ant, Anoplolepis gracilipes is a globally distributed invasive species that is believed to originate from the tropics but has extended its territory into humid subtropical climate regions via human-mediated jump dispersal. To investigate whether the adaptive evolution occurs during the colonization, we assessed the population structure of A. gracilipes across 12 geographical regions (including Southeast Asia, East Asia, Australia, and several Indo-Pacific islands) based on nuclear SNPs and mitochondrial DNA variation. Genomic SNPs of an endosymbiont Wolbachia were also generated as an additional marker. All nuclear, mitochondrial, and Wolbachia phylogenies are congruent with each other in supporting the presence of two distinct genetic clades. We found no evidence of gene flow between the two clades, even at a fine geographic scale where they are found in sympatry. Phylogenomic analyses indicate a strong geographic structure of the two genetic clades, with one restricted in the tropics and the other in the subtropics. Using outlier approaches, we further pinpointed a genome region that is likely linked to a gene involved in freezing tolerance. Preliminary results of chill coma recovery assay showed that individuals from different genetic clades possess different levels of thermal tolerance. Altogether, our study provides new evidence for a potential adaptive evolution in an invasive ant.
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