Phylogeographic history of the ring distribution in Malcus flavidipes (Heteroptera: Malcidae) shaped by directional dispersal and island adaptation in the South China Sea region

The ring distribution pattern offers a unique perspective for analysing the mechanisms of species or population differentiation. The semi-closed South China Sea (SCS) region, with its islands, peninsulas, and continental margins, is ideal for studying the formation of ring distribution patterns and the evolutionary dynamics of island populations. This study focused on Malcus flavidipes, a species distributed in the SCS region. Based on double-digest restriction-associated DNA data and geographic distribution data, population genetic approaches and species distribution modeling were employed to investigate its distribution pattern formation and the evolutionary characteristics of island populations. The results revealed five geographic clades around the SCS which are located on the Yunnan–Indochina Peninsula, southern China, Hainan Island, Kalimantan and the Philippines. Genetic diversity evidence supported that the ring distribution pattern originated in the Yunnan-Indochina Peninsula and diffused through the land bridges exposed during the Pleistocene glaciation along two routes: the northern route extended from southern China to Hainan Island, whereas the southern route stretched from Kalimantan to the Philippines. Island populations show significantly lower genetic diversity and higher within-population inbreeding coefficients than mainland ones. The genetic structural differences among different island populations are closely related to their geographic distance from mainland, responses to historical climatic fluctuations, and selective pressures from island environments. This study provides important empirical evidence for the ring differentiation of species in the SCS region, and offers a new perspective for understanding the evolutionary mechanisms of island populations under the combined effects of geographical isolation, genetic drift, and environmental selection.