A cryptic genetic boundary in remnant populations of a long-lived, bird-pollinated shrubBanksia sphaerocarpavar.caesia(Proteaceae)

Abstract

Plant species often exhibit genetic structure at multiple spatial scales. Detection of this structure may depend on the sampling strategy used. We intensively sampled a common, naturally patchy Banksia species within a 200 km2 region, in order to assess patterns of genetic diversity and structure at multiple spatial scales. In total, 1321 adult shrubs from 37 geographical populations were genotyped using eight highly polymorphic microsatellite markers developed for the species. Genetic structure was detected at three spatial scales. First, we identified a stark and unexpected division of the landscape into two genetic subregions, one to the north-east and one to the south-west of the sampling grid. This differentiation was based on sudden, highly structured differences in common allele frequencies, the cause of which is unknown but that may relate to physical and reproductive barriers to gene flow, localised selection, and/or historical processes. Second, we observed genetic differentiation of populations within these subregions, reflecting previously described patterns of restricted pollen flow in this species. Finally, fine-scale genetic structure, although weak, was observed within some of the populations (mean SP = 0.01837). When feasible, intensive sampling may uncover cryptic patterns of genetic structure that would otherwise be overlooked when sampling at broader spatial scales. Further studies using a similar sampling strategy may reveal this type of discontinuity to be a feature of other south-western Australian taxa and has implications for our understanding of evolution in this landscape as well as conservation into the future. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 115, 241–255.