The potato cyst nematode Globodera pallida possesses an expanded serotonergic nervous system

Abstract The neurotransmitter serotonin (5-hydroxytryptamine; 5-HT) controls behaviours that are essential for the parasitic lifestyle of plant parasitic nematodes (PPNs). Here, we used serotonin immunochemistry and in situ hybridization for the serotonin synthesis genes, Gpa-tph-1 and Gpa-bas-1, to map its signalling pathways in Globodera pallida J2 infective stages. We identified fourteen serotonin-immunoreactive cell bodies around the pharyngeal median bulb and revealed that eleven of them co-express Gpa-tph-1 and Gpa-bas-1, confirming these are serotonin-synthesising neurons. Serotonin immunoreactivity was also localised in structures consistent with labelling of amphids, and in sixteen non-neuronal cells located in the middle and posterior region of the nematode. Depletion of serotonin with reserpine reduced the serotonin immunoreactivity in all fourteen cell bodies confirming their neuronal identity, but not in non-neuronal tissue. We identified additional, but infrequent expression of Gpa-tph-1 in two cells and expression of Gpa-bas-1 in one pair of symmetrically bilateral cells anterior to the median bulb, that lacked serotonin immunoreactivity. The results reveal distinct pools of serotonin distribution in G. pallida and indicate serotonin signalling is more complex than in other parasitic nematodes and C. elegans, highlighting evolutionary divergence in neurotransmitter identity.