Fitness consequences of floral variation in Senecio jacobaea (Asteraceae):evidence from a segregating hybrid population and a resource manipulation experiment

The present study examines some of the ecological factors that might exert selection on floral morphology in Senecio jacobaea, a self-sterile composit e which exhibits geographic variation in the frequency of rayed and discoid individuals. Regression analyses of phenotypic data from a large, segregat ing hybrid population, established in a semi-natural (garden) environment a nd studied over a 2-year period, revealed a negative relationship between t he size of the rays and the average germination rate of the maternal seed c rop, a pattern that can be attributed to the reduced germination speed of a chenes from ray florets. There was no effect of ray size on the amount of c ross-pollination achieved, the proportion of heads infested by larvae of se ed flies (Pegohylemyia) and the amount of resources retained for the next f lowering season. The lack of resource costs was also apparent in a manipula tion experiment with greenhouse-grown plants of the rayed phenotype: artifi cial removal of all rays at the early bud or flowering stage had no detecta ble effect on subsequent flower and fruit development, regardless of whethe r the plants experienced high or low water stress. Given these and other ob servations, I hypothesize that plant-animal interactions and resource costs sometimes play a minor role in exerting selection on flower morphology and that spatially varying selection on germination behaviour accounts for som e of the morph frequency variation in S. jacobaea. (C) 2001 The Linnean Soc iety of London.