Parasite-induced inversion of geotaxis in a freshwater amphipod: a role for anaerobic metabolism?
Résumé
1. Many parasites with complex life cycles alter the phenotype of their intermediate hosts in
ways that seem to favour transmission to a final host. Although there is a large literature on
host manipulation, how parasites alter the phenotype of their hosts remains poorly known.
2. The bird acanthocephalan Polymorphus minutus is known to alter geotaxis in its amphipod
host, Gammarus roeseli. Here, we examine the potential roles of low oxygen availability and
the excretion, by the parasite, of two products from its own anaerobic metabolism (lactate and
succinate) in altered geotaxis.
3. Under hypoxia, uninfected G. roeseli showed negative geotaxis and lower metabolic rate,
two traits also altered by infection with P. minutus, albeit with different intensities. The
injection of a mixture of lactate and succinate in uninfected amphipods mimicked the
parasite-induced reversion of geotaxis, without affecting the metabolic rate.
4. In addition, both P. minutus-infected gammarids and uninfected ones conditioned to
hypoxia for 2 days showed elevated levels of lactate in the brain, but not in the haemolymph.
5. Overall, our results indicate that the pathways involved in anaerobic metabolism and
hypoxia signalling might be responsible for the changes in geotaxis and metabolic rate induced
by P. minutus infection.
6. Our study emphasizes the need to consider the tight and complex connections between physiological
processes and behavioural adjustments, in particular at the brain level, in the understanding
of parasitic manipulation, and more broadly of behavioural changes in infected hosts.