Functional assessment of the ionotropic receptors IR25a and IR93a in predator perception in the freshwater crustaceanDaphniausing RNA interference

Abstract

Acquiring environmental information is vital for organisms as it informs about the location of resources, mating partners, and predators. The freshwater crustacean Daphnia detects predator specific chemical cues released by its predators and subsequently develops defensive morphological features that reduce the predation risk. The detection of such chemical information is generally processed via distinct chemoreceptors that are located on chemoreceptor cells. Lately an ancestral type of ionotropic receptors (IRs) has been identified in crustaceans. IRs and the putative co-receptors IR25a and IR93a are postulated to be involved in chemoreception However, functional roles have not been assessed. Here, using three Daphnia species as model, we report that the two co-receptors are expressed within the chemosensory antennules and gene expression is increased with predator perception. Importantly, RNA interference mediated knock-down of the two IRs impedes species-specific defense expression in the three Daphnia species. Our results suggest that (albeit not testing the enigmatic receptor protein directly), the reduction of two associated proteins has impaired the functional aggregation of the postulated chemoreceptor complex. This in turn has hampered the perception of environmentally relevant chemical cues resulting in a substantial reduction of defensive morphological features.

Tropical rock lobster (Panulirus ornatus) uses chemoreception via the antennular lateral flagellum to identify conspecific ecdysis

“…Transcriptomic analysis of single ORN cells from P. ornatus aesthetascs, as previously done by Kozma 54 for P. argus, would differentiate between IRs expressed in ORNs and CRNs in this region of the antennules. Functional assessment of IRs in chemosensory behaviour has been achieved through RNA interference (RNAi) knock-out of IR25a and IR93a in several Daphnia species 60 . In the case of P. ornatus , reverse genetic studies with RNAi have proven to be ineffective, due in part to animal-wide low expression of the enzymes and receptors required for functional and systemic gene silencing 61 .…”