Loss of buzz pollination results in chronic pollen limitation in an enantiostylous plant

Abstract

Plants with specialised pollination systems may be susceptible to reduced fertility if they are incapable of autonomous self-pollination and are visited infrequently by pollinators capable of promoting effective cross-pollination. Here, we investigate this question by examining the reproductive ecology of the Cape endemic Cyanella alba subsp. flavescens (Tecophilaeaceae), which has mirror-image flowers (enantiostyly) and dimorphic anthers (heteranthery). These traits are usually associated with buzz pollination mediated by bees that extract pollen from the poricidal anthers of flowers through vibration. We quantified insect visitation to flowers in three populations over two years in the vicinity of the Biedouw Valley, Western Cape, South Africa to determine whether plants were visited by buzz-pollinating bees. We also recorded visits by pollinators to other co-flowering species at each site and constructed a network on insect visitation. We conducted controlled hand pollinations and bagging experiments to determine whether plants were self-compatible and capable of autonomous self-pollination, and compared natural levels of fruit and seed set with those obtained from hand cross-pollination to evaluate evidence for pollen limitation of seed fertility. Despite 74 h of field observations, only two actively buzz-pollinating bees were recorded visiting flowers, although they were observed visiting other species in the community. The most frequent visitors to C. alba were opportunistic pollen-consuming beetles although we obtained no direct evidence that they were effective in promoting cross-pollination. Plants in each population exhibited chronic pollen limitation of seed set. Controlled pollinations demonstrated that plants were moderately self-sterile and incapable of autonomous self-pollination. Although fruit set was high following self-pollination, few seeds were evident within developing fruit. Observations of pollen-tube growth following controlled hand self- and cross-pollination revealed abundant pollen tubes at the base of styles in both treatments. Thus, self-sterility in C. alba probably results from late-acting ovarian self-incompatibility, and possibly also the abortion of developing seed due to inbreeding depression. We discuss the possible causes of scarce buzz pollination in C. alba and consider its ecological and evolutionary consequences.