Integrative taxonomy of herbaceous plants with narrow fragmented distributions: A case study on Primula merrilliana species complex

An accurate understanding of species diversity is essential to studies across a wide range of biological subdisciplines. However, species delimitation remains challenging in evolutionary radiations, particularly in those herbaceous plants associated with microendemic, naturally fragmented distribution systems, where genotypic and phenotypic traits likely evolved discordantly. The Primula merrilliana complex, which is endemic to eastern China and has high horticultural value, used to be treated as one species but several clues suggested it might be composed of multiple species. Here we used multiple lines of evidence, including molecular, morphological, reproductive isolation, and geographic data, to assess independently evolving lineages within this complex. Our results indicated that the species diversity in the complex was underestimated previously, and four species (independently evolving lineages) can be recognized, including two new species described here. The extensive variation of the breeding system, especially the floral morph transition from distyled (outcrossing) to homostyled (selfing) multiple times, possibly promoted the rapid speciation within such a small geographic scale. This study case indicated that the phenomenon of genetically highly divergent but morphologically indistinguishable is perhaps shown in herbs with fragmented distributions; the alternative extreme evolutionary phenomenon, in which complete reproductive barriers have been accumulated but with little genetic differentiation, also exists. Thus we highlight the importance of incorporating other characters, such as postzygotic reproductive isolation and geographic data, with commonly used molecular and morphological traits to infer species boundaries through an integrative taxonomic approach in such systems.