TITLE:
Only Attract Ants? The Versatility of Petiolar Extrafloral Nectaries in Passiflora
AUTHORS:
Poliana Cardoso-Gustavson, Nathalia L. Andreazza, Alexandra C. H. F. Sawaya, Marilia de Moraes Castro
KEYWORDS:
Passiflora alata Curtis; Passiflora edulis Sims; Egg Mimics; Anatomy; Secondary Metabolites
JOURNAL NAME:
American Journal of Plant Sciences,
Vol.4 No.2A,
February
28,
2013
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.