Provenance, guts, and fate: Field and experimental evidence in a host-mistletoe-bird system

Theory suggests that host—parasite interaction should lead to local adaptation of parasites to their host. In mistletoe—host tree interactions the foraging patterns of fruit-eating birds can play an important role in host local adaptation, depending on the tree species on which the mistletoe fruits were consumed (host provenance) and the tree species on which the mistletoe seeds were deposited (host fate). Yet this interaction has not been previously documented. To determine the effects of host provenance, bird gut processing, and host fate on mistletoe seed germination and seedling establishment of Psittacanthus calyculatus, we conducted a seed inoculation experiment in Tlaxcala, Mexico. Mistletoe fruits were collected from Crataegus pubescens (Cp), Prunus serótina (Ps), and Salix bonplandiana (Sb) and measured. We then manually removed the exocarp and pulp of half the fruits, and the other half were given to grey silky-flycatchers (Ptilogonys cinereus) for gut processing. Obtained seeds of both groups of fruits were measured and deposited on branches of other trees of the same 3 host species in a full factorial design. Mistletoe fruit size varied significantly among host tree species, but differences did not coincide with those in gut retention time. Bird gut processing and the host provenance X host fate interaction were the best predictors for the occurrence of germination and establishment of seedlings. Seed germination and seedling establishment were more likely when seeds were gut processed by birds, but gut retention time had no significant effects. The significant host provenance X host fate interaction was largely due to higher success of mistletoe seeds from Sb to Sb and lower success from Sb to Ps. Our results suggest that to understand the efficiency of infection of P calyculatus and possibly other mistletoe species, the linked effects of the digestive processes undergone by mistletoe seeds and the species of host trees where the seeds were consumed and ultimately deposited should be considered.