A multi-species comparison of genetic diversity between island and mainland ant populations

Ida Naughton and David Holway, Division of Biological Sciences, University of California, San Diego, La Jolla, CA, Neil D. Tsutsui, Environmental Science, Policy and Management, University of California, Berkeley, Berkeley, CA, Philip S. Ward, Entomology, University of California Davis, Davis, CA

Background/Question/Methods
Island biotas provide unparalleled opportunities to examine evolutionary processes. Founder effects and bottlenecks, for example, can decrease genetic diversity in island populations, and drift and selection further act to shift allele frequencies. One selection pressure common to many island systems involves selection for reduced capacity for dispersal, especially for species with aerially dispersing propagules. Although such generalities pervade the literature on island populations, assemblage-level tests of their importance are rare. Here, we use genomic datasets from nine ant species for which we obtained paired island-mainland samples, to test the following predictions. (i) Island populations support reduced levels of genetic diversity compared to conspecific mainland populations. (ii) Island populations exhibit greater population genetic structure, which would be consistent with the evolution of reduced dispersal capacity. For each species, we collected samples from ten evenly spaced sites along ~25 km, east - west transects on Santa Cruz Island, California and the adjacent mainland (Lompoc Valley). We used targeted enrichment of ultraconserved elements (UCEs) to obtain sequences of orthologous nuclear loci, and extracted SNP data from phased UCE reads.
Results/Conclusions
Estimates of heterozygosity and allelic richness did not significantly differ between mainland and island populations, but mainland populations did support higher levels of haplotype diversity. Moreover, mainland populations exhibited greater pairwise genetic distances between samples. STRUCTURE plots and pairwise Fst values showed that island and mainland populations exhibited interspecific variation. Fst values were above 0.10 for most taxa, but for two species (Monomorium ergatogyna, Prenolepis imparis) little differentiation was evident. Our results illustrate that island populations may not always conform to theoretical expectations. The age (7.5 my) and size (249 km2) of Santa Cruz Island could both be factors that would help to explain current levels of genetic diversity, whereas strong prevailing winds could play a role in the dispersal of reproductives.