Fitness and Morphology Support Genetic Differentiation Across Different Geographic Scales in a Native Insect Utilising Native vs. Invasive Host Plants

Native species can evolve rapidly in response to utilising invasive species as novel resources. We investigated the genetic and trait differentiation of the Australian soapberry bug Leptocoris tagalicus across three biotypes: those feeding on invasive Cardiospermum grandiflorum in New South Wales (NSW) and Queensland (Qld), invasive C. halicacabum in the Northern Territory (NT), and on the native host Alectryon tomentosus (in Qld). Genetic analyses revealed moderate differentiation between NT insects and those from NSW and Qld (F_ST = 0.033). Conversely, insects from NSW and Qld had low genetic differentiation, irrespective of their host plant associations (F_ST = 0.008). Field data and data from a multi-generation experiment indicated ongoing adaptation in proboscis length in insects feeding on the two invasive host plant species, likely in response to the sizes of their fruits. Multi-generation hybridisation experiments demonstrated high narrow sense heritability in insect proboscis length and body size (H₂ = 0.48 and 0.4, respectively). Crosses involving F₁ hybrids of insect biotypes generally outperformed inter-biotype and control crosses. Taken together, these findings support ongoing genetic differentiation among L. tagalicus biotypes across different spatial scales, even in instances of high gene flow.