Only Attract Ants? The Versatility of Petiolar Extrafloral Nectaries in <i>Passiflora</i>

Abstract

Passiflora species presents a coevolutive relationship with Heliconiini butterflies, their primary herbivores. The Heliconiini caterpillars are able to detoxify toxic compounds produced by Passiflora, thus morphological defense strategies stand out over chemical innovations. In this framework, we highlight the presence of mimetic structures and extrafloral nectaries (EFN) as morphological strategies. Heliconian butterflies oviposit only on leaves that do not possess previous eggs, so the presence of egg mimics could prevent the oviposition. EFN are glands that offer nectar to territorial and aggressive ants, establishing mutualistic relationships. Here, we present a structural and chemical analysis of petiolar EFN and nectar from Passiflora alata and P. edulis in order to have insights about the implications of these features in deterring heliconian caterpillars. P. alata have one to four stipitate-crateriform EFN while P. edulis possess a pair of convex glands. Butterflies lay their eggs isolatedly or in up to three on leaves of both species. Our morphological results suggest that EFN from P. alata may act as egg mimics. Ontogenetic data suggest that the variation in the number of glands observed in this species is a serial homology, wherein the selection pressure for this variation is possibly the oviposition pattern. P. alata retain alkaloids, flavonoids and terpenoids inside nectariferous cells; sugars and flavonoids are found in the nectar of both species, while alkaloids are also detected in P. edulis. There is a selective retention/release of secondary metabolites from the EFN tissues to nectar. Knowing that these compounds can be dissuasive to some herbivores and inoffensive to others, we plotted this relationship in a consumer growth versus secondary metabolite concentration diagram. Our results suggest a more active role in the modulation of the gland defense from plants besides the establishment of a mutualistic relationship with ants, an important response in a coevolutive scenario.