The adaptive function of touch-sensitive stigmas
Touch-sensitive movement in stigmas of angiosperm flowers is a phenomenon observable in a timescale of seconds or minutes. Despite its early documentation in a small fraction of angiosperm species, touch-sensitive stigmas (TSS) have been little studied. In this review, we examine its occurrence in the angiosperm phylogeny and investigate associations between plant reproductive traits and TSS. Prominent hypotheses regarding the evolution of TSS invoke the adaptive functions of outcrossing, pollination efficiency, and pollen germination. Using literature searches for species reported with TSS, we found that the distribution of TSS is phylogenetically clustered within the order Lamiales, while Bignoniaceae emerges as the family that abounds in TSS species. We found the strongest association with bisexuality in flowers. The other traits associated with TSS include self-incompatible mating system, dichogamy, large campanulate corollas, specialized pollination, and multiple ovules. Thus, we predict that TSS can be an effective reproductive strategy in conjunction with these specific traits.