Pollination by fungus gnats (Diptera: Mycetophilidae) and self-recognition sites in Tolmiea menziesii (Saxifragaceae)

Abstract

Observations of insects visiting strongly protandrous flowers of Tolmiea menziesii (Saxifragaceae) show that Gnoriste megarrhina (Mycetophilidae), a large fungus gnat with a body ca. 7 mm long and a proboscis ca. 6.5 mm long, is the primary pollinator. Pollen is transferred to ventral portions of the gnat's thorax while it probes for an unusually dilute nectar (8.3% to 12% sucrose equivalents) produced at the base of the floral tube. Grains are transferred to the two receptive stigmas on each pistil when it forages for nectar on a female phase flower. Pollen load analyses indicate that Tolmiea menziesii may be the only nectar source visited by the gnat while it is in bloom. While hoverflies (Syrphus spp.) and bumblebees (Bombus spp.) also visit T. menziesii flowers, they do not forage for nectar and usually behave as pollen robbers visiting only male phase flowers after earlier visits by G. megarrhina. When the modes of floral presentation of T. menziesii are compared to other species pollinated primarily by micro-dipterans, and mycetophilids in particular, there appears to be a floral convergence in some spring-flowering, perennial herbs of moist, evergreen woodlands and forests. Analyses of open (gnat-pollinated) vs. hand-pollinated pistils show that manual cross-pollinations are slightly more successful than open pollinations. Fluorescence microscopy confirms two sites of self-recognition and rejection within the pistil: following controlled self-pollination experiments almost half of the pollen tubes produced either stop growing once they reach the top of the ovary and/or grow horizontally. The remaining tubes produced by self-pollination penetrate ovules but seed is never set. This double mode of self-recognition and rejection is similar to the one described for Heuchera micrantha var. diversifolia (Saxifragaceae) but may also confirm an earlier report of post-zygotic rejection made for Tolmiea.