Unexpected levels of gene flow in a wallflower occupying an island-like habitat (Erysimum teretifolium, Brassicaceae)

Background/Question/Methods

Island biogeography predicts that populations occupying island-like habitats near genetic reservoirs will contain higher levels of diversity than more isolated populations. The distribution of genetic diversity among islands should reflect isolation by distance. The Zayante Sandhills of Santa Cruz County, California, are island-like xeric habitats separated by mesic redwood and mixed evergreen forests. These naturally patchy habitats are home to many endemic plant and animal species, including the Ben Lomond Wallflower (Erysimum teretifolium), and are threatened by sand mining and development. Approximately 13 populations of E. teretifolium remain, several of which contain fewer than 100 individuals. This study investigates the distribution of genetic diversity within and among eight extant populations of E. teretifolium (n=11-32 individuals per population) using two approaches to estimate genetic diversity (four heterospecific microsatellite loci and Next-Generation sequencing of the mitochondrial and chloroplast genomes and nuclear ribosomal regions). It examines whether E. teretifolium’s island-like habitat influences its distribution of genetic diversity, which can help prioritize populations for conservation and guide imminent reintroduction efforts. 

Results/Conclusions

Despite the island-like nature of the habitat and detectable population structure, populations of E. teretifolium do not exhibit isolation by distance. Populations show cryptic subdivision, with F_st for 24 of 28 population comparisons being significantly greater than zero and 82% of the genetic variation existing within populations. These data ultimately question previous suggestions that patchy habitats can be modeled as islands. Smaller populations show lowered allelic richness, indicating that maintaining large population sizes is important for preserving genetic variation.