Unravelling host-mediated effects on hemiparasitic Mexican mistletoePsittacanthus calyculatus(DC.) G. Don traits linked to mutualisms with pollinators and seed dispersers

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While the influence of mistletoe–host interactions on each other’s evolution is well-recognized, the role of interactions between mistletoes and its mutualistic pollinators and seed dispersers mediated by host species is relatively unexplored. Here, we examine the effects of host species (Mexican hawthorn Crataegus mexicana DC, black cherry Prunus serotina Ehrh., leather-leaf Mexican oak Quercus crassipes Bonpl.) on flower morphology, nectar production, pollinator visitation rate and female reproductive fitness in hemiparasitic Mexican mistletoe Psittacanthus calyculatus (DC.) G. Don (Loranthaceae) populations at three different locations. We first measured the lengths of corolla, style, stamen, exerted stamen and anther, the length and width of the ovary and nectar availability for mistletoe flowers in natural populations. Then, we evaluated flower visitation and measured (length and width) and weighed the fruits and its seeds of mistletoes growing on each of the three host species. Finally, we evaluated the effects of host species, location and flower traits on fruit or seed size variation (both as proxies of reproductive fitness). We found mistletoes growing on natural hosts P. serotina and Q. crassipes produced larger flowers, fruits and seeds than mistletoes on cultivated C. mexicana. However, these differences varied across space. The amount of available nectar and hummingbird visitation rates in flowers of mistletoes on C. mexicana was higher at the three sites than in flowers of mistletoes on P. serotina or Q. crassipes. The effects of host species, study site and floral trait covariates affected significantly all fitness measures, indicating that mistletoes’ reproductive fitness is affected differently depending on the host species and their site of occurrence. These host-associated differences in reproduction might have implications for interactions with mutualistic vectors.