An assemblage-level comparison of genetic diversity and population genetic structure between island and mainland ant populations

Island biotas provide unparalleled opportunities to examine evolutionary
processes. Founder effects and bottlenecks, for example, typically
decrease genetic diversity in island populations, while selection for
reduced dispersal can increase population structure. Given that support
for these generalities mostly comes from single-species analyses,
assemblage-level comparisons are needed to clarify how (i) colonization
affects the gene pools of interacting insular organisms, and (ii) patterns
of genetic differentiation vary within assemblages of organisms. Here, we
use genome-wide sequence data from ultraconserved elements (UCEs) to
compare genetic diversity and population structure of mainland and island
populations of nine ant species in coastal southern California with
respect to genetic diversity and population structure. As expected, island
populations (from Santa Cruz Island) had lower than expected
heterozygosity and Watterson’s theta compared to mainland populations
(from the Lompoc Valley). Island populations, however, exhibited smaller
genetic distances among samples, indicating less population subdivision
and a higher capacity for dispersal compared to mainland populations.
Within the focal assemblage, pairwise Fst values revealed pronounced
interspecific variation in mainland-island differentiation, which
increases with gyne body size. Our results reveal differences in genetic
diversity and population genetic structure across an assemblage of
interacting species, and illuminate general patterns of insularization in
ants. Compared to single-species studies, our analysis of nine species
pairs from the same island-mainland system offers a powerful approach to
studying fundamental evolutionary processes.