ANTHER SMUT DISEASE IN DIANTHUS SILVESTER (CARYOPHYLLACEAE) - NATURAL-SELECTION ON FLORAL TRAITS

Mating opportunities, pollination intensity, and pollen dispersal abil ity may vary with variation in floral traits such as color, size, and shape. Where these traits are selected by pollinators for enhanced ela boration, they should evolve toward the equilibrium between selection for further elaboration and selection against this through reduced fec undity or vitality. Here we show that pollinator-borne fungal diseases of plants may be a factor influencing the position of this equilibriu m. Populations of the rock pink, Dianthus silvester often contain indi viduals infected with the anther smut fungus Microbotryum violaceum (= Ustilago violacea). In a naturally infected population in the Alps of eastern Switzerland we investigated how intrapopulation variation in flower size and nectar rewards influenced spore deposition and how flo ral traits varied with disease status. We found that spore deposition increased with increasing petal size, suggesting that large-flowered p lants were at a greater risk of disease. Spore deposition was also hig her for plants growing in patches with many or a high proportion of di seased neighbors. Multiple regression analyses showed that petal size or nectar reward influenced spore deposition when the effects of neigh borhood disease abundance were controlled statistically. In sequential analyses, after removing the effects of disease density or frequency and plant gender, petal length explained significant variation in spor e deposition. Diseased plants had reduced female reproductive organs, but calyx size was intermediate between that of healthy perfect and fe male flowers of this gynodioecious-gynomonoecious species, and disease d plants bore flowers with the largest petals. This may reflect a symp tom of this disease or the cause, if larger-flowered plants are more l ikely to become infected. We conclude that investment to pollinator at traction may bring an enhanced risk of contracting this sterilizing po llinator-borne disease, so natural selection by the fungus M. violaceu m acts to lower attractiveness to pollinators.