Pollen Limitation And The Evolution Of Selfing In Clarkia Unguiculata

Within the genus Clarkia, individual species have evolved floral traits that enable self-fertilization (selfing), as opposed to pollinator-dependent mating between genetically distinct individuals (outcrossing). Insufficient pollen deposition, which may reduce individual seed production, is hypothesized to have contributed to the evolution of selfing. If seed set is limited by pollen deposition (i.e., pollen limited), then natural selection may favor the evolution of traits associated with selfing. Our experiment confirmed that pollen limitation occurs in the annual self-compatible wildflower Clarkia unguiculata, a predominantly outcrossing species that often occurs in sympatry with its selfing sister species Clarkia exilis.  In Spring 2010, we selected 600 plants at three different sites in the southern Sierra Nevada. We subjected two flowers per plant to one of the following experimental treatments: (1) naturally pollinated or (2) hand-pollinated with supplemental pollen from neighboring plants. We compared seed production between fruits derived from flowers that were pollinated naturally and those that were pollen supplemented. We detected pollen limitation in natural populations of Clarkia unguiculata early in the flowering season, and to a lesser degree in the late season. Advantages of receiving additional pollen may be nullified in the late season when plants allocate limited resources to a fixed number of seeds. As anthropogenic factors may influence the interactions and evolution of plants and pollinators in the Sierra Nevada in the future, it is important to note that a plant’s ability to evolve selfing mechanisms may be influenced by resource availability.