Pollinator-mediated phenotypic selection does not always modulate flower size and number in the large-flowered Mediterranean shrubCistus ladanifer(Cistaceae)

Abstract

Flower size and number usually evolve under pollinator-mediated selection. However, hot, dry environments can also modulate display, counteracting pollinator attraction. Increased pollen deposition on larger flower displays may not involve higher female fitness. Consequently, stressful conditions may constrain flower size, favouring smaller-sized flowers. The large-flowered, self-incompatible Mediterranean shrub Cistus ladanifer was used to test that: (1) this species suffers pollen limitation; (2) pollinators are spatially–temporally variable and differentially visit plants with more/larger flowers; (3) increased visits enhance reproduction under pollen limitation; (4) stressful conditions reduce female fitness of larger displays; and (5) phenotypic selection on floral display is not just pollinator-mediated. We evaluated pollen limitation, related floral display to pollinator visits and fruit and seed production and estimated phenotypic selection. Flower size was 7.2–10.5 cm and varied spatially–temporally. Visitation rates (total visits/50 min) ranged from 0.26 to 0.43 and increased with flower size. Fruit set averaged 80% and seed number averaged 855, but only fruit set varied between populations and years. Selection towards larger flowers was detected under conditions of pollen limitation. Otherwise, we detected stabilizing selection on flower size and negative selection on flower number. Our results suggest that selection on floral display is not only pollinator-dependent through female fitness in C. ladanifer.