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MARTINEZ-PERALTA, C. AND M. C. MANDUJANO. (Laboratorio de Genetica y Evolucion, Departamento Ecologia de la Biodiversidad, Instituto de Ecologia, Universidad Nacional Autonoma de Mexico). Reproductive ecology of the endangered living rock cactus, Ariocarpus fissuratus (Cactaceae). J. Torrey Bot. Soc. 138: 145-155. 2011.-The genus Ariocarpus comprises seven endangered species endemic to the Chihuahuan Desert. We studied the reproductive ecology of Ariocarpus fissuratus at Cuatrocienegas region, Mexico, in order to explore if reproductive traits influence population viability. The size of perianth, pistils and stamens, floral behavior and controlled pollination treatments were assessed to determine Cruden’s outcrossing index (OCI), the outcrossing rate and the mating system. The breeding system was estimated with pollen/ovule ratios (P/O) and outcrossing rate was estimated by comparing progeny from self- and outcross-pollination treatments. Frequency and behavior of floral visitors was determined, and nectar production recorded. Ariocarpus fissuratus blooms synchronously during three weeks in autumn. Flowers display herkogamy and homogamy, and OCI and P/O ratio suggest xenogamy. Generalized linear mixed models were fitted to evaluate the effect of pollination treatment on fruit set and number of viable seeds yielded per fruit. Fruit set was significantly higher in the outcrossing treatment than in the selfing treatments. The mating system indicates that A. fissuratus is mainly an outcrosser, showing a partial self-incompatibility with low fruit set for hand self-pollination. Floral visitors are native solitary and introduced bees, and beetles cause high levels of florivory damage. Reproductive ecology suggests that the need for pollinators, presence of exotic pollinators, partial self-incompatibility and florivory negatively affect sexual reproduction and thus the persistence of this endangered species.