Genome-wide SNPs identify a cryptic species and population genetic structure across Wallace’s Line in the Paleotropical butterfly Euploea mulciber (Nymphalidae: Danainae)

The archipelagos of Southeast Asia encompass three biodiversity hotspots, and its islands harbor much of Earth’s biodiversity. Insects comprise the majority of this diversity and a vertebrate bias in biogeographic studies limits a general understanding of insular biodiversity patterns. Two main factors contribute to the lack of population-level insect studies: dearth of natural history data (e.g., species distributions) and the availability of specimens for DNA sequencing. Butterflies are an excellent system for these studies as natural history data are readily available, and they are among the most collected and catalogued insects. The striped blue crow (Euploea mulciber) is a polymorphic and widely distributed butterfly found across mainland Southeast Asia and several islands within the Indo-Australian Archipelago (IAA). We examine the historical biogeography and evaluate the population structure of E. mulciber to explore how isolation on islands can shape the genetics of a widely distributed insular insect with high potential dispersal ability. Using low-coverage whole-genome SNP data generated from 85 specimens, we find that E. mulciber represents a cryptic species complex comprising three genetically distinct groups segregated by Wallace’s line and Huxley’s line. These groups correspond to the Sunda Shelf, the Philippines, and the Lesser Sundas. The populations within the Lesser Sundas are likely worthy of species-level recognition due to limited gene flow and morphological differences but an integrative approach is required for species delimitation. These results emphasize the importance of insect-based investigations in insular biodiversity studies as the true diversity in these regions is currently underestimated.