Evolution of danaines' relationships to plants
Neglecting associations to flowers for obtaining nectar, danaine butterflies exhibit a dual relationship to plants: on the one hand, their larvae require specific hostplants (primary host associations). On the other hand, adults (often only the males) require pyrrolizidine alkaloids (PAs) from plants usually unrelated to the larval hostplants (secondary host associations).
This dual insect-plant relationship and the peculiar significance of hostplants' secondary metabolites make Danainae an ideal system to study the role of sequestration of secondary plant chemicals in an evolutionary perspective.
- It is a typical feature of Danainae that their larvae feed on Apocynaceae – Asclepiadoideae (some species utilize certain Moraceae (in addition)) ('primary hosts').
Classical studies on Danaus plexippus, the American Monarch, have demonstrated that the adult butterflies are protected by cardiac glycosides (CGs) from larval hostplants which is the basis for mimicry. However, it turned out that not all of the Monarch's larval hostplants contain CGs and that there is great variation in the CG content and composition in the respective Asclepiadoideae making protection very variable. Thus, a "palatability spectrum" exists and also Batesian mimicry within a species ("auto mimicry").
Two further Danaus species (D. chrysippus and D. gilippus) have been shown to store CGs from larval hostplants, but other species which feed on CG-containing plants have been found not to sequester CGs or have not been investigated yet.
The generalizations about sequestration of CGs in Danainae often found in the literature are simply wrong – many Asclepiadoideae used as larval hostplants do not contain CGs.
- Another feature typical for Danainae is that adult males require (as 'secondary hosts') plants of certain Boraginaceae, Fabaceae, Asteraceae and Apocynoideae to obtain protective compounds, pyrrolizidine alkaloids (PAs), pharmacophagously, i.e. independent of nutritional requirements but to increase fitness: PAs are stored for protection against predators and are transferred to females; at the same time, PAs are required as precursors for synthesizing relevant male courtship pheromone components.
The utilization of PAs appears to be the basic protective mechanism of danaines and is a significant factor in the complex mimicry relationships.
Some species gather PAs not as adults but as larvae from Parsonsia (Apocynaceae – Apocynoideae).
Analysis of apocynad larval hostplants of milkweed butterflies (Lepidoptera: Danainae) in search for pattern
This joint project with Prof. Dr. Sigrid Liede-Schumann of the University of Bayreuth analyses the hostplant records of Danainae in the light of new knowledge of the phylogeny of Apocynaceae s.l..
Utilization of larval hostplant chemicals in Danainae (Lepidoptera)
This joint project with Prof. Dr. Ulrich Mebs of the University of Frankfurt addresses the open question of sequestration of cardenolides in Euploea as part of a wider study on danaine genera previously not studied from a chemoecological point of view.
Petschenka G, Fandrich S, Sander N, Wagschal V, Boppré M, Dobler S (2013) Stepwise evolution in the Na+/K+-ATPase of milkweed butterflies (Lepidoptera: Danaini). Evolution: in press