Supervisor: Thierry Rigaud
Multi-host parasites may use host species differing in abundance and susceptibility to infection. These various hosts will contribute unequally to the transmission and the fitness of the parasites. The "key hosts" are those contributing significantly more to the completion of the parasite life cycle. Three non-exclusive criteria can be used to identify a key host contributing disproportionately to parasite transmission: high abundance, high exposure / susceptibility to infection, and/or large number of infective stages produced per infected individual.
Complex life-cycle parasites are, by definition, multi-host parasites because they require at least two successive host species to achieve their development. However, may also use several different host species at any stage of their cycle. Many parasites with a complex life cycle have developed the ability to manipulate several aspects of the phenotype of their intermediate hosts, including behaviour, thus increasing the probability of transmission to their definitive hosts. For these parasites, host susceptibility to manipulation should be included in the definition of a "key host" species because of its involvement in parasite transmission.
Several species of acanthocephalan manipulative parasites use Gammarus species as intermediate hosts. It has recently been shown that many cryptic species of Gammarus pulex or fossarum live in sympatry in the rivers of eastern France, bringing a new vision of the spectrum of intermediate hosts that acanthocephalans can use, and their relative contribution.
The first objective of the thesis is to carry out in two rivers a temporal monitoring of the relative frequencies of the different cryptic species of Gammarus. The aim is to know whether these frequencies are stable over time (allowing simultaneous use of all host resources by parasites), or whether they vary over time (allowing sequential use of hosts by parasites). The second objective of the thesis is to estimate the susceptibility to infection of the different cryptic species, and then their sensitivity to behavioural manipulation, by carrying out experimental infections in the laboratory.
multi-host parasites, specificity, cryptic diversity