Differences in flower colors between spiny and non-spiny Asteraceae species: A possible case of aposematism?

Variation in flowering color among plant species is widely documented, but its ecological function is often unknown. Comparing phylogenetically related species that differ in ecology can help identify factors that select for flower colors. We compared the distribution of inflorescence colors of the 98 spiny versus the 189 non-spiny species of the Asteraceae in the flora of Israel, and found significant differences between the two groups. Yellow/white inflorescences dominate the non-spiny species, while pink/purple/blue flowers dominate the spiny plants. We hypothesize that the pink/purple/blue flowering of the spiny species may advertise their deterring spines to mammalian herbivores. This putative aposematic signal is particularly conspicuous in summer, when the surrounding landscape turns yellow and grazing pressure is highest. Additional potential adaptive functions for pink/purple/blue flowering in summer-blooming species include increased visibility to pollinators, and improved protection from radiation damage due to the flowers’ high anthocyanin content. The pollinator attraction and radiation defense hypotheses, however, do not account for the observation that the spiny species that flower in yellow/white and in pink/purple/blue have overlapping blooming periods, as do yellow/white and pink/purple/blue flowers of non-spiny species. Phylogenetic constraints may further influence Asteracean flowering colors, a hypothesis that is partially consistent with our data. Spine colors differ significantly from flower color in the spiny species, suggesting that spine and flower colors may have evolved in response to different selective agents. Proximately, the different colors of flowers and spines may reflect the cellular location of pigments in different cell types (anthocyanic red parenchyma versus yellow carotenized lignified hard cells).