Island-mainland variation in bees: Applying geometric morphometrics to wing venation in one species

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Charles Thrift and Katja C. Seltmann, Cheadle Center for Biodiversity and Ecological Restoration, University of California Santa Barbara, Santa Barbara, CA

Background/Question/Methods
A single species geographically separated into island and mainland populations often exhibits differences in phenotype between those populations. Size differences, such as dwarfism and gigantism, are well-studied particularly in mammals, but island-mainland population phenotype differences are not well-studied in insects.
Bees (Hymenoptera: Anthophila) have unique wing venation and imaged bee wings can be used to accurately identify bees to species. Wing venation patterns alone may be sufficient to classify variation between populations of the same species using geometric morphometrics. In this study, we applied these methods to determine if populations of island and mainland bees are significantly different. Populations are from Santa Cruz Island and Santa Barbara, California, a coastal mainland town. Santa Cruz Island is a 35-kilometer long island about 32 kilometers off the coast of Santa Barbara in the Pacific Ocean. Thus, there is a physical barrier between mainland and island populations. One species of sweat bee, Halictus tripartitus (Hymenoptera: Halictidae), was chosen. This species is commonly collected on both the island and the mainland, and specimens were obtained from both natural history collections and new collections.
To analyze wing venation patterns, both forewing and hindwing were removed, slide mounted, imaged, and annotated with digital landmarks using TPS morphometric software for approximately 500 specimens and 9 landmarks. In R, landmark coordinates were aligned with a generalized procrustes analysis (GPA) and simplified with a principal component analysis (PCA). The output of the PCA was plotted on biplots, and the output of the GPA was used in an analysis of similarities (ANOSIM) test.
Results/Conclusions
Current ANOSIM results show that there is a statistically significant difference between island and mainland Halictus tripartitus populations (p-value < 0.05). Ongoing work includes adding additional specimens and running new tests, including non-metric multidimensional scaling (NMDS), and “envfit” regression (in R library vegan) to identify the strength and direction each of the 9 landmarks has on the group. This project shows that wing morphometrics can distinguish between populations of bees and may be a viable method of automated bee identification. Additionally, these found differences in bee wing morphology can lead to further work in island-mainland population variation, including evolutionary questions of gene flow between island and mainland populations.