Abstract
Distyly is a floral polymorphism, characterized by a reciprocal positioning between stigmas and anthers in different flowers, where two floral morphs, long-styled (pin) and short-styled (thrum) occur within the population. Distyly is suggested as one of the routes leading to the evolution of separate sexes in plants. In this evolutionary pathway, pollinators may disrupt the complementarity of pollen transfer between morphs. Consequently, the floral morphs gradually specialize as either male or female. A key process required for gender specialization in distylous plants is a deviation of the realized functional gender (i.e. the proportion of genes transmitted to the next generation via pollen donation and seed production) from the potential functional gender (i.e. the expected contribution of male and female function to reproductive success from the number of ovules or pollen grains produced by each morph). I selected the distylous herb Arcytophyllum lavarum (Rubiaceae) to determine if asymmetry in pollen flow promotes differences in seed production, pollen donation and a discrepancy between the potential and the realized functional genders in pin and thrum floral morphs. Pollen flow in A. lavarum is asymmetric and the pin morph is more efficient at performing cross-pollination than the thrum morph. Conversely, the thrum morph produced two times more seeds than the pin morph. Male and female contributions to the potential functional gender were equivalent in both morphs. However, the pin morph transmitted more genes through pollen donation and the thrum morph more through seed production than expected from their potential functional genders. These results support the hypothesis that if pollinators consistently promote asymmetric pollen flow between morphs over generations, it is possible that gender specialization may evolve to the extreme of dioecism from an original distylous condition.
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Acknowledgements
I especially thank Federico Valverde, Hilario Abarca and Eliecer Abarca (Estación Biológica Cerro de La Muerte) J. Mora and F. Kepfer for logistic support and F. Mora for assisting during part of the data collection. Image analyses were performed in the Auditory Neurobiology Laboratory, University of Miami and the Entomology Laboratory, University of Costa Rica. S. Tomchik and Dr. Z. (John) Lu provided invaluable advice during image analyses. Charlotte M. Taylor determined the Arcytophyllum species. I thank A. C. Villegas as the advisor of this project at the Organization for Tropical Studies. Comments by S. Koptur, E. K. Kuprewicz and three anonymous reviewers improved this manuscript. Part of the equipment used in this research was provided by IDEA-WILD This research was supported by the Organization for Tropical Studies – Glaxo fellowship for Latin–American researchers (Fund No.502) to C.G. Laboratory and data analyses were supported by the Aldridge Assistantship and the J. McLamore Fellowship, - University of Miami, both to C.G. This project was honored with the Alwyn Gentry Award 2004, Association for Tropical Biology and Conservation.
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García-Robledo, C. Asymmetry in pollen flow promotes gender specialization in morphs of the distylous neotropical herb Arcytophyllum lavarum (Rubiaceae). Evol Ecol 22, 743–755 (2008). https://doi.org/10.1007/s10682-007-9198-0
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DOI: https://doi.org/10.1007/s10682-007-9198-0