Reversal versus specialization in floral morphological evolution in Petrocosmea (Gesneriaceae)

Abstract

A widely held hypothesis in evolution is that adaptive specialization constrains the potential direction of future evolutionary change and thus may be irreversible, also known as Dollo's law. However, this hypothesis has long been subject to debate in evolutionary biology. Floral specialization is intriguing as it is usually linked to reproductive isolation and could affect speciation. Here, following the discovery of four new taxa, we observed some interesting phenomena of reversal versus specialization in morphology in a clade with the most specialized flowers in the genus Petrocosmea. In the phylogenetic tree based on sequences of multiple DNA regions, the morphological reversals, especially the regain of a long corolla tube, are nested within the branches characteristic of normally specialized flowers with a short corolla tube and highly specialized zygomorphy. Our results indicate that the highly specialized floral organ of this clade is still actively evolving in multiple branches toward specialization while reversals to different ancestral states occur in some branches. Great disturbance of ecological environment is likely a crucial factor affecting trait reversibility, such as the rapid uplift of the Himalayan–Tibetan plateau. The four new taxa are treated herein taxonomically. The flowers of this clade represent an interesting model to explore the genetic basis underlying the evolutionary reversal versus specialization and the interplay between genetic factors and environmental variables.