Pollinator behaviour, mate choice and the realised mating systems of Grevillea mucronulata and Grevillea sphacelata

Successful long-term conservation and management of populations of plants requires successful management of the suite of factors that determine their 'realised' mating systems. Within the genus Grevillea, mating systems are potentially complex. They may vary among species and among populations within a species, reflecting variation in pollinator behaviour and diversity ('potential' mating systems) and in breeding system (the 'preferred' mating system). We used a combination of pollinator observations, pollination experiments and electrophoretic analysis of seed from open pollinations, to examine variation in potential and preferred mating systems in two populations of each of two 'spider-flowered' Grevillea species: G. mucronulata (visited by honeyeaters and honeybees, although only birds effected pollen removal and transfer) and G. sphacelata (visited only by honeybees, which made frequent contact with pollen and stigmas). Almost all observed bird and insect foraging bouts on either species involved movements among inflorescences within plants or among inflorescences on closely neighbouring plants. On the basis of these data, the mating system was predicted to involve a high level of selfing or inbreeding. However, the pollination experiment revealed that both species were highly self-incompatible and showed a clear preference for outcross pollen in mate-choice experiments. For both species, fruit set through autogamy was lower than 0.7% and fruit set from self-pollinations was always significantly lower than for outcross pollinations (0-11% cf. 25-33% for G. mucronulata and 0% cf. 4.2-8.8% for G. sphacelata). Allozyme studies revealed that genotypes in open-pollinated seeds on 20 G. mucronulata and 20 G. sphacelata plants were surprisingly uniform, best explained by outcrossed matings between close neighbours. We found little between-population variation in any aspect of the mating system for either species. These studies reveal that the preferred mating system of the plant and the pattern of pollinator behaviour interact to determine the mating system in a population, emphasising the need for a multifaceted investigation of mating systems, especially in predicting the fates of populations that have pollination systems altered by disturbance, small size, isolation and introduced species such as the honeybee.