Diverse floral scents in Malagasy Bulbophyllum evolve in a bounded fashion

Floral scent is a fundamental feature in angiosperm pollination, reproductive isolation, and speciation. However, little is known about the neutral or selective processes shaping long-term patterns of floral scent evolution within particular lineages. Here we investigated the floral scent composition and variation of 41 species of Malagasy Bulbophyllum orchids (crown age: c. 12.7 Ma). In addition, we inferred the mode of floral scent evolution across 32 species of this lineage, using phylogenetic comparative methods (PCMs) that represent a novel approach to this field of research. Both qualitative and semi-quantitative variability of floral scent was high among species. Of the 297 volatile organic compounds (VOCs) detected, 225 (c. 76%) were species-specific, including a novel floral scent compound (N,N-dimethylleucine O-methyl ester), and the relative scent composition varied significantly among subclades. Whole scent bouquets but also most single VOCs best fitted a constrained (Ornstein-Uhlenbeck) model of trait evolution. The high diversity and specificity of floral scent compounds compounds observed, with some known to be bioactive during orchid pollination, suggests that scent has an important role in the pollinator attraction of Malagasy Bulbophyllum. Our results further support the hypothesis that floral scent has not evolved neutrally in this lineage but in a bounded fashion, possibly due to strong selective constraints imposed by the group's main pollinators (small Dipteran flies).