Evolutionary significance of phytochemical diversity:Cardenolides as a model system

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Plants produce a variety of toxic compounds which function as a defense against herbivorous insects. One classically studied group of toxins are the cardenolides (aka cardiac glycosides) which bind to and specifically inhibit the ubiquitous animal cation pump Na+K+-ATPase which is involved in essential physiological processes.Since cardenolides were subject of many chemoecological studies during the last five decades, they provide an excellent model to understand evolutionary adaptations of herbivorous insects to host plant toxins. A prime example for the adaptation of an insect to cardenolides is the monarch butterfly (Danaus plexippus). The caterpillars of this species are not only able to tolerate cardenolides contained in their host plant Asclepias (Apocynaceae) but also store these compounds for their own defense (sequestration).Plants often possess a spectrum of structurally different toxins belonging to the same chemical class. Some species of Asclepias, for example, produce up to 30 different cardenolides. However, it is unknown whether such a variety is of ecological or evolutionary significance.I therefore want to test whether phytochemical diversity has a negative impact on a specialist herbivore. For this I will use the monarch and its host plants (Asclepias spp.) as a model system. Besides constitutive production plants also produce higher levels of cardenolides upon insect attack (induction). Hence, I will also test whether caterpillars after plant induction possess higher cardenolide concentrations in their body and whether plants, upon induction, produce specific cardenolides which are especially toxic for D. plexippus. Moreover, I will investigate whether selective sequestration of particular cardenolides in the monarch is an optimization to avoid autotoxicity.To answer these questions I will perform an integrative approach combining ecological, biochemical and chemical-analytical techniques.