Asymmetric Migration Shapes Genetic Structure of the Invasive Avian Vampire Fly ( Philornis downsi ) Across the Galápagos Islands

Biological invasions on islands provide a natural framework to study how dispersal and connectivity influence evolutionary and ecological processes. The avian nest parasitic fly, Philornis downsi-first recorded in Darwin's finch nests in 1997-causes high mortality in endemic land birds, yet its inter-island and sex-specific patterns of dispersal and genetic structure remain poorly understood. We use low-coverage whole genome sequencing to investigate genome-wide patterns of genetic diversity, directional migration and effective population size in P. downsi across five major Galápagos Islands and its native range in mainland Ecuador. We find evidence for a genetic bottleneck in the Galápagos, isolation by distance, and evidence that the island closest to the Ecuadorian mainland, San Cristóbal, is genetically divergent from the other four islands sampled, despite retaining the highest genetic diversity. No evidence was found for sex-biased dispersal; however, sex-biased genetic structure was detected using only markers from inferred autosomal scaffolds. We found asymmetric gene flow with higher migration rates from San Cristóbal westward to the other islands, matching the direction of both southeast trade winds and major cargo shipping routes. Our results suggest both natural and human-mediated colonisation of P. downsi from the mainland through San Cristóbal to the other islands, followed by high inter-island dispersal among closely situated sink islands. Our findings are critical for prioritising islands for control strategies that will reduce P. downsi impacts on vulnerable endemic birds and underscore the value of understanding directional migration patterns for managing invasive species in metapopulations.