Small population of the largest water strider after the late Pleistocene and the implications for its conservation

Climate oscillation and its synergistic impacts on habitat fragmentation have been identified as threatening the survival of some extant species. However, the mechanisms by which semi-aquatic insects impacted by such events remain poorly understood. Herein, we studied the largest water strider in the world, Gigantometra gigas, to explore the effect of these two factors on its evolutionary history. The sequences of mitogenomic and nrDNA cluster were utilized to reconstruct phylogenetic relationship among G. gigas populations and its demographic history. Mitochondrial genes were separately reconstructed topologies of that populations and detected remarkable differences. We found that G. gigas populations conform to the isolation-by-distance model, and decline occurred at about 120 ka, which was probably influenced by the climate change during the late Pleistocene and eventually maintained a small effective population size (Ne) around 85,717. The populations in Guangdong Province of China are worthy of note in that they exhibit low genetic diversity, a small Ne around 18,899 individuals, and occupy an area with little suitable future habitat for G. gigas. This work recommends that conservation efforts are implemented to ensure the long-term survival of small G. gigas populations, and notes that further evaluation of their extinction risk under the impacts of human activities is required.