Abstract
Brood pollination is one of the more unusual ways in which plants reproduce. The plant has to trade off damage to its flowers and reduction in future offspring in exchange for the production of seeds. Brood pollination occurs along a continuum from parasitism to mutualism. In two closely related monocot species of Thysanotus in southwest Western Australia one autogamous species is parasitised by visiting beetles that eat the ovules and affect viable seed production. In the other species, which is normally buzz pollinated, visiting beetles can be facultative brood pollinators. In many cases, adults of brood pollinator mutualisms feed on nectar or more rarely pollen. The Thysanotus system is unusual, as the anthers of T. manglesianus are poricidal so pollen is not freely accessible to the beetle which is incapable of sonication and no nectar is produced. However the beetles damage the flowers and consume some pollen, which is necessary as the pollen will not be available unless the anther walls are breached. A beetle may lay an egg on a plant's ovary, where the hatched larva burrows through the ovary wall and eats the developing ovules. However, not all flowers receive an egg despite the flower being visited by beetles. Thus, when there is no bee pollinator, beetle visitation allows seed to be produced, albeit less than would be produced if the beetles were absent and a normal pollinator present. As this type of pollination interaction is unknown in other species of Thysanotus, it is not possible to decide if the brood pollination mutualism has developed from a parasitic relationship or the reverse. The autogamous species does not need a pollinator and always had a higher fruit set and fewer beetles than the population of the other species at a site where the more usual bee pollinator was absent. It may be that brood pollination mutualism is a development from a parasitic relationship.