A Temporal Dimension to the Influence of Pollen Rewards on Bee Behaviour and Fecundity in Aloe tenuior

Abstract

The net effect of pollen production on fecundity in plants can range from negative – when self-pollen interferes with fecundity due to incompatibility mechanisms, to positive – when pollen availability is associated with increased pollinator visitation and fecundity due to its utilization as a reward. We investigated the responses of bees to pollen and nectar rewards, and the effects of these rewards on pollen deposition and fecundity in the hermaphroditic succulent shrub Aloe tenuior. Self-pollinated plants failed to set fruit, but their ovules were regularly penetrated by self-pollen tubes, which uniformly failed to develop into seeds as expected from ovarian self-incompatibility (or strong early inbreeding depression). Bees consistently foraged for pollen during the morning and early afternoon, but switched to nectar in the late afternoon. As a consequence of this differential foraging, we were able to test the relative contribution to fecundity of pollen- versus nectar-collecting flower visitors. We exposed emasculated and intact flowers in either the morning or late afternoon to foraging bees and showed that emasculation reduced pollen deposition by insects in the morning, but had little effect in the afternoon. Despite the potential for self-pollination to result in ovule discounting due to late-acting self-sterility, fecundity was severely reduced in artificially emasculated plants. Although there were temporal fluctuations in reward preference, most bee visits were for pollen rewards. Therefore the benefit of providing pollen that is accessible to bee foragers outweighs any potential costs to fitness in terms of gender interference in this species.

High levels of fecundity in small and isolated populations of a self‐compatible Aloe pollinated by opportunistic birds and bees

“…; Duffy et al . ). Due to their frequent visitation and effectiveness, bees likely play an important role in the pollination of this species, although their foraging will likely result in geitonogamous self‐pollination (Karron et al .…”

“…; Duffy et al . ), while in other cases their contribution to pollination is negligible (Johnson et al . ; Hargreaves et al .…”

“…Bees that are able to access aloe nectar often contribute to fecundity, while bees that feed mainly on pollen often visit only male‐phase flowers (Duffy et al . ) and make smaller contributions to seed production (Hargreaves et al . ; Duffy & Johnson ).…”

Staminal hairs enhance fecundity in the pollen-rewarding self-incompatible lilyBulbine abyssinica

“…A logical inference is thus that staminal hairs convey a signal of potential pollen rewards to these pollen‐collecting bees. Previous studies in other genera within the Xanthorrhoeaceae show that bee visitation rates decrease when pollen rewards are artificially removed (Vaughton et al ., ; Duffy & Johnson, ; Duffy et al ., , ). Bees have been shown to benefit from making rapid decisions at the expense of choice accuracy (Chittka et al ., ; Burns, ).…”

“…In hermaphroditic plants that are self‐compatible or have late‐acting self‐incompatibility, self‐pollen can interfere with female function through a process known as ovule discounting (Aizen & Harder, ; Vaughton & Ramsey, ; Duffy, Patrick & Johnson, ). This complicates potential selection on pollen production in such plants, because any increase in production can enhance pollinator attraction and siring of offspring on other plants, but also lengthen pollinator visits on plants and increase rates of seed abortion through ovule discounting (Duffy, Johnson & Peter, ). Such trade‐offs in male and female function may lead to sexual conflict in hermaphroditic plants and can be exacerbated when plants rely on pollen rewards to attract bee pollinators (Duffy et al ., ) or produce an excess of pollen relative to the number of available mates (Duffy & Johnson, ).…”