ANTHER-SMUT INFECTION IN SILENE DIOICA - VARIATION IN FLORAL MORPHOLOGY AND PATTERNS OF SPORE DEPOSITION

The anther-smut Microbotryum violaceum, causes a systemic perennial in fection of its host Silene dioica. Infection results in sterility and the production of teliospores in flowers. These spores are transmitted to healthy plants by flower visiting insects. The behavioural respons es of flower visiting insects to a variation in floral characters are therefore likely to affect rates of pollen export/import, the rate of spore deposition and probability of disease. Since infected plants are sterilised, they are effectively removed from the gene pool. It is th erefore often assumed that in this host-pathogen system there is a con siderable potential for a pathogen-pollinator mediated selection on fl oral characters. In a transplantation experiment, which included plant s from four different populations, we tested for correlations between variation in female floral morphology and patterns of spore and pollen deposition, and a resulting risk of disease. The source populations i n this experiment were located on four islands in Skeppsvik archipelag o in northern Sweden, and represented a gradient of disease incidence from completely healthy (Island 1), low incidence (Island 2) to high i ncidences (Islands 3 and 4) of disease. Fifty plants from each populat ion were transplanted to the centre of the population on Island 4. Pla nts from the non-diseased population had larger flowers and longer sty les than plants from the highly diseased populations. Numbers of polle n grains and spores deposited on flowers were strongly and positively correlated. We found that plants originating from the healthy populati on received approximately four times more pollen and nine times more s pores per flower than the individuals from the resident diseased popul ation. The resulting incidences of disease among plants from the four populations differed significantly, and was 20% among plants originati ng from the healthy population and 0% among plants from the resident h ighly diseased population. Plants from the other two diseased populati ons showed intermediate values (13% and 11% respectively). In a survey of ten populations we found among healthy plants a significant negati ve correlation between mean style length (positively correlated with c orolla size and ovule number) and incidence of disease. The potential for pathogen-pollinator mediated selection on floral characters in thi s host-pathogen system is discussed.