A promiscuous wallflower: The rare sandhill endemic plant, Erysimum teretifolium (Brassicaceae), is self-incompatible and relies on a diversity of pollinators
Islands and island-like habitats are biodiversity hotspots. The insular nature of these environments limits gene flow and facilitates local adaptation. The Zayante sandhills of Santa Cruz County, California, contain many plants and animals restricted to these exceptionally xeric soils surrounded by mesic forests. Erysimum teretifolium (the Ben Lomond wallflower) is an endangered sandhill endemic plant exhibiting recent reproductive failure. To determine its mating system, we performed 161 crosses comparing reproductive success of self-pollinations to within- and among-population crosses. We then examined correlations between seed set and geographic distance as well as seed set and genetic divergence using four microsatellite loci. To assess the diversity and frequency of pollinators, we collected ~151 flower hours of pollinator observations with digital video recording.
Reproductive failure in E. teretifolium is likely caused by its predominantly self-incompatible mating system (self-pollinations produced ~6.5x fewer seeds than outcross pollinations). The species benefits from among-population crosses, which produced 13% more seeds compared to within-population crosses. There was no correlation between seed set and geographic distance of the parents (R² = 0.00584), nor was there was any correlation between seed set and genetic divergence of the parents (R² = 0.0151). Pollinator observations revealed a diversity of pollinators from Hymenoptera, Lepidoptera, and Diptera with a visitation rate of 0.827 visits/flower/hour. Self-incompatibility in E. teretifolium likely explains the reproductive failure, yet the crossing results suggest these populations do not exhibit island-like population dynamics. The benefits of among-population crosses and the reliance on pollinators will guide future reintroduction efforts.