Phenotypic selection on Dalechampia blossoms:: Honest signaling affects pollination success

Pollinators may choose which flowers to visit by direct assessment of rewards or by indirect assessment of honest advertisements or other traits correlated with the quantity or quality of reward. We wished to know whether selection generated by pollinators acted directly or indirectly on floral rewards, on advertisement traits, and/or traits affecting pollinator efficiency (their fit with flowers) in Dalechampia vines (Euphorbiaceae) and whether the advertisement (bract size) was correlated honestly with reward amount (measured by resin-gland area). In Gabon we studied bee visitation and pollen arrival rates to blossoms of D. ipomoefolia Benth. and found that, despite the apparent visibility of the resin reward (and its volume), the strongest bee-mediated natural selection acted directly on bract size rather than gland area. Blossoms with larger bracts were visited more often by the only pollinators, female Heriades nr. spiniscutis (Apoidea: Megachilidae), and these blossoms received more pollen on their stigmas. Blossoms with larger resin glands were also visited more often and received more pollen overall, but this effect disappeared when bract size (which was phenotypically correlated with gland size) was controlled for statistically. These observational data were confirmed by experimental reduction of bract size, which significantly decreased pollen arrival rates. Thus, the bees appear to rely on the honest'' correlation between advertisement and reward in choosing the best blossoms to visit, and this behavior generates direct selection for larger bracts and indirect selection for larger resin glands. Bees visiting blossoms with larger separation between the gland and stigmas contacted the stigmas less frequently, and such blossoms received less pollen on their stigmas. Because gland area, bract size, and gland-stigma separation are positively correlated phenotypically, response to selection for larger bracts may be limited in this population by conflicting selection against large gland-stigma separation.