Selection through male function favors smaller floral display size in the common morning glory Ipomoea purpurea (Convolvulaceae)

In self-compatible, hermaphroditic plants, display size the number of flowers open on a plant at one time - is believed to be influenced by trade-offs between increasing geitonogamous selfing and decreasing per-flower pollen export as display size increases. Experimental results presented here indicate that selection through male function favors smaller display sizes in Ipomoea purpurea. In small arrays, plant display size was manipulated experimentally, and female selfing rate, male outcross success, and total male fitness were estimated using genetic markers and likelihood and regression analyses. As would be expected if larger displays experience greater geitonogamy, selfing rate increased with display size. However, the per-flower amount of pollen exported to other plants decreased with display size. The magnitude of this effect is more than sufficient to offset the increase in selfing rate, resulting in reduced per-flower total male fitness with increasing display size. The low values of inbreeding depression previously reported for this species would enhance this effect.