Diversity and evolution of mirror-image flowers in Gesneriaceae

Gesneriaceae is a pantropical family with diverse floral traits and forms a diversification and endemism center in Southwest China. Mirror-image flowers, a highly specialized pollination system, were discussed in Gesneriaceae with an emphasis on its diversity, systematic distribution, and evolution. Mirror-image flowers was found in seven genera in the subfamily Didymocarpoideae, among which three genera have reciprocal mirror-image flowers (Rhabdothamnopsis, Henckelia, Didymocarpus) and other three genera possess nonreciprocal mirror-image flowers (Paraboea, Ornithoboea, Saintpaulia). Subg. Streptocarpella Engler of Streptocarpus contains both reciprocal and nonreciprocal mirror-image flowers. Gesneriaceae was dominated by nonreciprocal mirror-image flowers with united anthers and hidden sexual organs in floral tubes, distinctive from the typical mirror-image flowers in other families. The main pollinators to mirror-image flowers in these species are small-bodied bees such as Amegilla spp. These unusual floral traits suggest the mirror-image flowers in Gesneriaceae probably had experienced distinctive evolutionary history. Phylogenetic relationships indicate that mirror-image flowers are restricted to the subfamily Didymocarpoideae, which is endemic to the Old World. According to molecular phylogenetic studies, at least five independent origins of mirror-image flowers were found. The highly-specialized pollination mechanism associated with mirror-image flowers might facilitate pollination accuracy and consequently accelerate speciation rate of Didymocarpoideae. It is suggested that molecular phylogenetic methods can be used to explore the evolutionary relationship of reciprocal and nonreciprocal mirror-image flowers in Gesneriaceae to determine the possible role of evolutionary transitions of these two types of mirror-image flowers in the family’s speciation histories and long-distance dispersal.