Grants and current projects

2020: DFG grant BR 2930/6-1

The prothoracic repellent glands of stick and leaf insects: Reconstructing the morphological and chemical evolution of an elaborate arthropod defensive system

 Animals are permanently under strong selection to avoid predation, therefore predator-prey interactions are major driving forces of evolution and have given rise to a plethora of effective defensive strategies. Colouration is arguably the most obvious among primary defenses and might enhance crypsis to reduce detection or warn predators of the prey’s unpalatability or toxicity. Both strategies can be found in stick and leaf insects (Phasmatodea), a mesodiverse lineage of large terrestrial herbivorous arthropods well known for their remarkable capability to camouflage themselves as plant parts. Most species are nocturnal, remain motionless during the day and exhibit masquerade crypsis by mimicking twigs, leaves and bark. In contrast, a minority of species such as the Peruvian fire stick (Oreophoetes) displays striking forms of aposematism (warning colouration) insinuating the use of their efficient prothoracic repellent glands. Chemical spraying from these glands is just one of numerous secondary defense mechanisms in phasmatodeans (i.e., those initiated when a predator attacks), comprising startle display, defensive stridulation, thanatosis or escape by flight, limb autotomy or counterattack via heavily armend legs. Although most species largely rely on cryptic appearance to avoid predator detection, their secondary defense system is quite elaborate, and repugnatorial glands in the prothorax are supposed to be widely present among extant species, in fact constituting a derived ground pattern feature (autapomorphy) of all Phasmatodea.

The chemical substances involved in this powerful defensive mechanism are known only for a dozen species out of the 3.000+ described taxa, so far with no information available for major and pivotal lineages. The goal of the present project is (i) to generate a profound information base for the presence and anatomical diversity of the prothoracic repellent glands across a broad and representative taxon sampling of 100 stick and leaf insect species, (ii) to identify the chemical substances involved, and (iii) to reconstruct the step-wise anatomical and chemical evolution of this elaborate defense system and to detect key innovations in a phylogenetic framework. The obtained information will be correlated with further data on body form and size, armature, colouration and behavioural observations of the examined specimens in order to reveal transformations of crucial traits and to detect key innovations that might have had an effect on diversification rates and shaped the evolution of stick and leaf insects. Since the few hitherto reported repellent substances show an unexpectedly huge diversity between taxa, exhibiting highly specific characteristics such as odor or causing skin irritations (as perceived by humans), and include some previously unreported natural products, we also expect our project to demonstrate the value of stick and leaf insects as sources of new bioactive compounds.

Madagascar is one of our planet's leading biodiversity hotspots exhibiting an extraordinarily distinct and diverse, but also severely endangered biota. Many elements of its fauna are poorly documented and remain largely unexplored. For example, the evolutionary history of the Malagasy stick insects (order Phasmatodea) is entirely obscure. Stick insects are large terrestrial arthropods with limited dispersal abilities, forming at least four distinct taxonomic groups on Madagascar, i.e., endemic families and subfamilies with completely unresolved phylogenetic affinities in respect to continental lineages. An extensive multigene phylogenetic analysis and molecular clock approach is supposed to infer whether (i) the Malagasy phas-matodean fauna can be explained by vicariance biogeography (forming relics of the Gond-wana fragmentation), (ii) if it is the product of a single post-Gondwanan colonization, or (iii) the result of multiple recent oceanic dispersal events. This project is also designed to investi-gate the adaptive clade diversifications and microendemism of phasmatodeans on Mada-gascar. By using Bayesian reconstruction methods in combination with quantitative mor-phometric and discrete anatomical characters the project is aimed to significantly advance our understanding of the phylogeography and evolution of distinct ecomorphs of stick insects on islands, e.g., ground-dwellers and canopy-dwellers, flying and flightless forms.