Geographical variations of odour and pollinators, and test for local adaptation by reciprocal transplant of two European Arum species

1. Interactions between entomophilous plants and their pollinators are one of the major factors shaping the evolution of floral features. As species are distributed in more or less connected populations, they have evolved in a geographical mosaic of co-evolution were the outcome of the plant-pollinator interaction is likely to vary as a result of local adaptations. 2. Arum italicum and Arum maculatum are two species of Araceae which deceive their fly pollinators by mimicking the odour of their oviposition sites. Whereas A.italicum is known to be pollinated by flies belonging to different families (i.e. opportunist), A.maculatum relies on only two pollinating species of the family Psychodidae throughout its European repartition area (i.e. specialist). 3. The interannual and geographical variations of pollinators and pollinator-attractive odours were described in several populations of the two species over two consecutive years. Furthermore, local adaptation to pollinators was tested by transplanting inflorescence-bearing plants between two different sites and by recording the number and composition of the insect fauna trapped inside the inflorescences during anthesis as a measure of a fitness component. 4. Pollinators and pollinator-attractive odours of the two Arum species varied in time and space, but there was no clear odour structure between populations. When transplanted, inflorescences of both species trapped the same composition and number of insects as native inflorescences at a given site; this indicates that pollinator composition is highly dependent on the local availability of insects. 5. No pattern of local adaptation was found for these two species, but local pollination conditions were shown to strongly affect the degree of geographical variations of these interactions. The lack of a clear odour geographical structure might be due to high gene flow or to similar selective pressures exerted by pollinators, and the high interindividual odour variation may be linked to the deceptive strategy adopted by the two plant species.