In the footsteps of Wallace: population structure in the breadfruit fruit fly, Bactrocera umbrosa (F.) (Diptera: Tephritidae), suggests disjunction across the Indo-Australian Archipelago

Understanding the interplay between plant host and insect herbivore diversification underpins many areas of pure and applied research. The tephritid fruit fly Bactrocera umbrosa is a primary pest of a small number of Artocarpus species throughout Southeast Asia and the West Pacific. Recent molecular evidence supports a pattern of eastward migration and species diversification in Artocarpus. Here, we aimed to test whether population structure in B. umbrosa was associated with historical biogeographical barriers such as Wallace's Line and discuss observed patterns in the context of Artocarpus diversification. We used an integrative approach to explore population structure within B. umbrosa based on morphological (wing shape and aedeagus length) and molecular (mitochondrial COI and COII) data. Overall, aedeagi and wing centroid sizes were generally larger and exhibited greater variation in the West Pacific than Southeast Asia. Molecular data agreed with this trend, and COI also showed a subtle but clear disjunction between regions associated with Weber/Lydekker's Lines. Taken together, the West Pacific was supported as the putative origin of B. umbrosa, whereas movement westward into Southeast Asia occurred more recently, likely via a single colonisation event followed by highly restricted gene flow. Population structure in B. umbrosa does not reflect an ancient history of tracking Artocarpus diversification eastward out of Southeast Asia.