Phenotypic plasticity in the holoparasitic mistletoe Tristerix aphyllus (Loranthaceae): Consequences of trait variation for successful establishment

The ability of a genotype to respond to changes in the environment through modifications in the phenotype is adaptive when the plastic genotypes attain a higher fitness than non-plastic genotypes. In this study we examine whether parasite traits involved in host infection exhibit adaptive phenotypic plasticity to the heterogeneous host microenvironment. We focused on a host-parasite relationship characterized by the holoparasitic mistletoe Tristerix aphyllus and the cactus host Echinopsis chilensis. Unlike most mistletoes, whose seeds are deposited on the host branches, seeds of T. aphyllus are often deposited on the spines of the cactus. The extremely long radicles of T. aphyllus have been suggested to represent a parasite adaptation to overcome the barriers to infection imposed by the spines of cacti. However, plastic rather than canalized phenotypes may represent a better strategy in changing environments. We evaluated whether T. aphyllus exhibits adaptive plasticity in radicle length through a sire half-sib genetic design under field conditions in two contrasting microenvironments (seeds deposited on spines 4 and 28 mm from the host surface). We used phenotypic and genotypic selection analyses to evaluate the relationship between radicle length and seed establishment. Our results revealed significant phenotypic plasticity for radicle length and family level variation among maternal but not paternal families. Short radicles and large seeds were favored in the short-distance environment, and long radicles were favored in the long-distance environment, suggesting that no single optimal phenotype exists for T. aphyllus. The observation that the heritability of radicle length and seed mass did not differ from zero was consistent with this finding. Overall, our results indicate that plastic rather than long radicles seem to be a better parasite strategy to overcome the microenvironmental heterogeneity imposed by host defensive traits.