Bshary, Redouan

2012, Brandenburg, Anna, Kuhlemeier, Cris, Bshary, Redouan

2009, Brandenburg, Anna, Bshary, Redouan, Kuhlemeier, Cris

A key component shaping plant-pollinator interactions is nectar. Its volume can regulate the length and frequency of pollination events. Nectar provisioning can be costly for the plant. Once secreted by the nectaries, the sugar-rich solution is usually consumed by a floral visitor and lost for “recycling” within the plant. Nectar reduction should thus be advantageous for the plant: non-secreted carbohydrates can be reallocated within the plant to other structures promoting growth, reproduction or attraction. However, most angiosperms provide nectar. It is assumed that certain pollinator behaviors, so called partner control mechanisms, favor nectariferous over deceptive plants and ultimately prevent the spread of “cheaters”. The partner control mechanisms identified in the context of plant-pollinator mutualisms are avoidance of nectarless species, reduction of drinking time and number of flowers visited on nectarless plants. Not all behaviors are performed simultaneously, and external conditions such as plant density as well as intrinsic factors of the foraging insect can determine to which extent certain behaviors are exerted. In the present study, we analyze foraging behavior of nocturnal hawkmoths on cheating Petunia axillaris axillaris plants under several conditions. The aim of this thesis was to assess which partner control mechanisms are executed by pollinators facing nectarless/low nectar plants. We observed hawkmoth behavior in two field sites and conducted experiments with naïve and experienced hawkmoths Manduca sexta under controlled conditions. We investigated which of the foraging decision rules might potentially reduce the fitness of cheaters and thus limit their spread in a population. In field assays, we observed that the density of naturally occurring P. axillaris plants and the presence of alternative food sources can influence hawkmoth behavior on nectarless Petunias: only when food plants were abundant and dense, pollinators would reduce the number of flowers on nectarless Petunias, whereas in the lower density there seemed to be no selection against cheaters. In learning experiments under controlled conditions, we observed that none of the tested behaviors (reduction of drinking duration on nectarless plants, avoidance of nectarless plants, reduction of number of flowers visited on nectarless plants) were improved over the course of the experiment. However, in all learning trials there was a significant reduction of drinking duration on nectarless plants, indicating that this control mechanism of hawkmoths is always exerted innately. Learning might therefore not be of major importance in discrimination against cheaters in our system. We constructed a plant with extremely high phenotypic similarity to P. axillaris yet only a third of the regular nectar volume (F25). Genotyping of F25 revealed a high genotypic imilarity to its parental plant but failed to answer questions about the genetic background of low nectar volume. The low nectar line was used in behavioral experiments with Manduca sexta. A major goal was to find out how pollination behavior affects female reproductive success of F25. Analogous to previous experiments, we found that the drinking duration was significantly reduced on cheaters. In hand pollination assays, F25 produced significantly more seeds than P. axillaris, however this effect was neutralized when pollinated by Manduca sexta. The benefits of nectar reduction are thus counterbalanced by a change in pollinator foraging behavior. In the future, we would like to assess which other fitness parameters are concerned when a plant ceases its nectar production. Altogether, we were able to show which foraging rules are exerted by hawkmoths on cheating P. axillaris and how one partner control mechanism, namely drinking duration, affects seed set of a plant with reduced nectar offerings. We hope that this work has contributed to answering questions about the costs and benefits of cheating.

2012, Brandenburg, Anna, Kuhlemeier, Cris, Bshary, Redouan

2009, Brandenburg, Anna, Dell’Olivo, Alexandre, Bshary, Redouan, Kuhlemeier, Cris

We all appreciate the beauty of flowers, but we seldom consider their function in the life cycle of the plant. The function of beautiful flowers is to advertise the presence of nectar. Floral nectar is the key component in the mutualism between flowering plants and their pollinators. Plants offer nectar as a reward for the transport of pollen by animal vectors. Studying nectar is challenging because of its complex physiology, complex polygenetic structure, and strong environmental variability. Recent advances set the stage for exciting future research that combines genetics and physiology to study ecological and evolutionary questions.

2011, Brandenburg, Anna, Bshary, Redouan

Pollination success of deceptive orchids is affected by the density and distribution of nectar providing plant species and overall plant density. Here we extended the framework of how plant density can affect pollination to examine how it may promote the success of plant intraspecific cheaters. We compared hawkmoth behaviour in two native populations of Petunia axillaris, where we simultaneously offered rewarding and manually depleted P. axillaris. We asked whether pollinator foraging strategies change as a function of plant density and whether such changes may differentially affect nectarless plants. We observed the first choice and number of flowers visited by pollinators and found that in the dense population, pollinators visited more flowers on rewarding plants than on nectar-depleted plants. In the sparse population, such discrimination was absent. As we found no differences in nectar volume between plants of the two populations, the observed differences in plant density may be temporal. We reason that if differences were more permanent, an equivalent of the remote habitat hypothesis prevails: in a sparse population, cheating plants benefit from the absence of inter- and intraspecific competitors because pollinators tend to visit all potential resources. In a denser population, a pollinator’s optimal foraging strategy involves more selectivity. This would cause between-plant competition for pollinators in a pollinator-limited context, which applies to most hawkmoth-pollinated systems. We propose that nectar-provisioning of plants can be density-dependant, with cheaters able to persist in low density areas.