ZUFALL, REBECCA and MARK D. RAUSHER.* Evolution, Ecology and Organismal Biology Group, Department of Biology, Box 90338, Duke University, Durham, NC 27708-0338. - Diffuse coevolution and anthocyanin production.
The evolution of floral displays is generally believed to be molded
primarily by selection imposed by pollinators. Recently, however, it
has been suggested that the evolution of floral color may also be
influenced by herbivores and pathogens because several enzymes of the
biosynthetic pathway that produces anthocyanin pigments are also
involved in the production of various flavonoids, which can confer
resistance to natural enemies. We present evidence for this type of
pleiotropy in naturally occuring floral color variants of the common
morning glory, Ipomoea purpurea. Specifically, we examined
susceptibility to insect herbivores and fungal pathogens of genotypes
with functional or non-functional copies of chalcone synthase, the
first enzyme of the anthocyanin biosynthetic pathway. Genotypes with a
non-functional CHS received approximately 25% more herbivore damage
and exhibited twice the intensity of infection by Rhizoctonia solani.
Plant fitness, as measured by seed production, was negatively
correlated with amount of herbivore damage. These results indicate
that the equilibrium frequences of the CHS alleles in natural
populations may be governed by diffuse selection imposed by herbivores
and pathogens, as well as by pollinators.
Key words: anthocyanins, coevolution, diffuse selection, resistance