Reproductive biology of a mutualist-vectored parasitic plant differs with host species
Parasites capable of parasitizing many species may benefit from an increased abundance or density of potential hosts, but specialization allows for adaptation to specific host defenses Host race formation occurs when populations of parasites specialize on host species and may lead to sympatric or parapatric speciation. For parasitic plants, divergence among populations on different host species may be a cause or consequence of differences in flowering time. That divergence will allow for a further reduction in gene flow and, potentially, local adaptation to host species. When reproduction depends upon the services of mutualist partners, the timing of mutualist activity will also influence the relationship between fitness and phenology of the parasite. In order to determine whether the conditions for host race formation exist, I asked how the reproductive output and timing of desert mistletoe (Phoradendron californicum), an arboreal, dioecious, xylem hemiparasite differed over two host species and two sites. Each week over the flowering period, I obtained quantitative measures of fruit and flower production, flower to fruit conversion success, and fruit removal. These measures were related to the activity periods of floral visitors and the specialized mistletoe seed disperser, the phainopepla (Phainopepla nitens).
Desert mistletoe differed in flowering time when growing on mesquite (Prosopis velutina) versus acacia (Acacia greggii) hosts. Overlap between the flowering periods on different hosts was minimal. Mistletoe on mesquite were significantly larger, produced more flowers per length of branch, and had a higher flower to fruit conversion ratio than did mistletoe on acacia. In contrast, a greater proportion of ripe fruit were removed from mistletoe on acacia; in addition, establishment may be greater on acacia, as infection intensity was greater on average than for mesquite. The timing of fruiting and the dominant pollinator species did not differ between mistletoe populations on different hosts, however. Differences in flowering phenology may provide a mechanism of pre-zygotic isolation, allowing for host race formation and local adaptation in desert mistletoe.