Undescribed species diversity in Brewer’s Jewelflower illuminates potential mechanisms of diversification associated with serpentine endemism

Premise of study: Documenting species-level diversity is a fundamental goal of biology, yet undescribed species remain hidden even in well-studied groups.  Inaccurate delimitation of species boundaries can limit our understanding of ecological and evolutionary processes and patterns of biodiversity, and may further impede conservation and management efforts.

Methods: We utilized an integrative approach, combining techniques from speciation biology, molecular phylogenetics, and geometric morphometrics to assess diversity in the Californian serpentine endemic Streptanthus breweri (Brewer’s Jewelflower).  We assessed reproductive isolation resulting from flowering time differences, mating system differences, and interfertility among four distinct geographic clusters of S. breweri that span the geographic range of the species.  We generated a gene tree based on the ribosomal DNA internal transcribed spacer region (ITS), a diagnostic species-level marker for this clade of jewelflowers, and additionally quantified leaf morphology in plants grown under common conditions.

Key Results: Four geographic clusters of S. breweri in northern California represent not a single species, but instead a species complex of at least three putative species.  Independent data associated with Biological, Phylogenetic, and Morphological Species Concepts support these conclusions. 

Conclusions: This work illustrates that latent biodiversity may be concealed even in well-studied groups and underscores the contribution of edaphic endemism generally, and serpentine endemism specifically, to California’s rich plant biodiversity.  The existence of unrecognized species diversity within the S. breweri species complex highlights multiple factors, including 1) the spatial context of geologic discontinuities, 2) a selfing mating system, and 3) differential selection pressures across discontinuous specialized habitats as potential drivers of evolutionary divergence on serpentine.

Methods
Datasets were 1) generated in greenhouse common gardens at the University of California, Davis (Davis, California, USA); and 2) using Sanger sequencing of the ribosomal DNA internal transcribed spacer region (ITS).