SELECTIVE FORCES ON THE MAINTENANCE OF OUTCROSSING IN AN ALMOST EXCLUSIVELY CLEISTOGAMOUS VIOLET SPECIES.

PREMISE OF THE STUDY Cleistogamous species constitute interesting study systems to resolve the longstanding question of how outcrossing is maintained given that seed production is assured through selfing. In this work, we investigate the selective forces that allow the persistence of producing self-pollinated cleistogamous (CL) and chasmogamous (CH) flowers in Viola jaubertiana Mares & Vigin. METHODS We monitored three populations at different elevation for two years, and studied the flowering phenology and the relative contribution of each flower morph to parental fitness. We tested whether allocation to CH and CL flowers differed across populations and if it covaried with herbivory and water stress conditions. We also performed hand-pollination and bagging experiments in CH flowers to estimate inbreeding depression and heterosis. KEY RESULTS CH flowers open in winter, under unfavorable conditions for pollination, show high pollen limitation and no-delayed selfing, and thus produce a low amount of seeds. Conversely, CL flowers appear in early spring, are physiologically cheaper to produce (i.e. dry weight 3.4 times lower than that of CH flowers), and yield c. 100 times more seeds than CH flowers. CH flowers were favored under water stress and low herbivory. Crosses between populations showed up to 25% greater fitness than those within populations. CONCLUSIONS Despite the great pollen limitation in CH flowers, we suggest that the interaction among different environmental determinants and heterosis are probably sufficient forces to maintain chasmogamy in this long-lived species, reducing deleterious fixed mutations in the selfed lines. This article is protected by copyright. All rights reserved.