Petrocodon Hance is a genus( ca. 30 species) in the angiosperm family Gesneriaceae, which are mainly distributed in the limestone region of South China. The genus is special for its remarkable floral diversity, and is also one of the most taxonomy-difficult groups in the family. Although the genus was redefined to accommodate species with distinctive floral morphologies from eight other genera based on molecular phylogenetic studies, the evolutionary trends of the floral characters have not been studied comprehensively yet; the major causes of the incongruence between traditional taxonomy and molecular phylogenetic studies remain largely unexplored; and the morphological characters that can support the redefined Petrocodon keep unknown, either. In the present study, we coded 35 morphological characters( including 26 floral characters) of 19 Petrocodon species and 9 outgroup taxa of Primulina, traced the evolutionary trends and addressed the taxonomic implications in Petrocodon based on phylogenetic tree. We found that most of the floral characters, especially those used for delimiting traditional genera, evolved frequently and highly homoplasiously within and across the genus, which were the key factors that misled the traditional classifications. Further, we proposed that among all the characters we examined, the shape of filaments and stigmas may be used to distinguish most taxa of Petrocodon from its closest relative Primulina. The present study indicates that caution must be taken when using these floral traits in future classification of Gesneriaceae. Moreover, extensive examinations of the morphological characters and their evolutionary trends need to be carried out and traced upon denser sampling and more robust phylogenetic tree. More importantly, efforts should also be taken to investigate the underlying molecular regulatory networks and driving forces underpinning the complicated floral evolution of Petrocodon. All these will finally improve the understandings of the evolutionary processes and mechanism of this group of typical karst flora and its allies.
Floral evolutionary trend of Petrocodon (Gesneriaceae) and its significance in classification