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Revista chilena de historia natural

Print version ISSN 0716-078X


BOTTO-MAHAN, CAREZZA; MEDEL, RODRIGO; GINOCCHIO, ROSANNA  and  MONTENEGRO, GLORIA. Factors affecting the circular distribution of the leafless mistletoe Tristerix aphyllus (Loranthaceae) on the cactus Echinopsis chilensis. Rev. chil. hist. nat. [online]. 2000, vol.73, n.3, pp.525-531. ISSN 0716-078X.

We describe the pattern of emergence of the holoparasitic mistletoe Tristerix aphyllus from its cactus host Echinopsis chilensis in a semiarid Chilean ecosystem. The observed circular distribution of the parasite inflorescence differed significantly from a uniform distribution based on a random process. We quantified the circular distribution of the seeds defecated on the cactus surface by Mimus thenca, the only bird responsible of seed dispersal. Our data did not support the idea of a directional seed deposition by the bird. To test the hypothesis that the observed circular distribution can be attributable to a differential seed survival due to microsite temperature variation, we infected cacti with seeds of T. aphyllus every 30ºand quantified the temperature associated to each angle. Our results revealed that even though seeds located in angles with higher sun exposure had the lowest haustorial disk formation, this variation in mortality is not sufficient to explain the angular polarity observed in this species. Inspection of inflorescences of T. aphyllus that emerged 17 months after the experimental infection, revealed mean angular values indistinguishable from the natural circular distribution. Assessment of the anatomical structure at two opposing angles of the cactus revealed striking differences in epidermal constitution, such as a four-fold thicker epidermis in north than in south facing samples due to formation of highly lignified bark. We suggest that bark formation is likely the most important factor determining the biased circular distribution of T. aphyllus

Keywords : parasitic plant; inflorescence; seed deposition; bark formation; circular statistics.

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