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Alexandre Bauer PhD thesis

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Gammarids and acanthocephalans, a multi-(cryptic) hosts and multi-parasites system?

Started in october 2017

Supervisor: Thierry Rigaud, laboratoire Biogéosciences, Dijon

 

Abstract

Multi-host parasites can exploit various host species that differ in abundance and susceptibility to infection, which will contribute unequally to their transmission and fitness. “Key hosts” are those contributing significantly more to the completion of the life cycle of such parasites. Three non-exclusive criteria may be used to identify a key host: its high abundance, its high exposure/susceptibility to infection, and the large number of infectious stages produced per infected individual. Many parasites with complex life cycles have evolved the ability to manipulate several traits of their intermediate hosts’ phenotype, including behavior, thereby increasing the likelihood of transmission to their definitive hosts. Host susceptibility to manipulation must therefore be included in the definition of a “key host” species, because of its involvement in parasite transmission. Several species of acanthocephalan manipulative parasites (among which Pomphorhynchus laevis and P. tereticollis) use amphipod crustaceans of the genus Gammarus as intermediate hosts. It has recently been shown that many G. pulex and G. fossarum cryptic lineages are living in sympatry in Eastern France rivers, bringing a new vision of the spectrum of intermediate hosts that acanthocephalans can use, and their relative contribution to their life cycles.

The first aim of the thesis was to test whether the presence of host species/lineages in the wild is stable over time (determining therefore the possibility of simultaneous use of hosts by parasites, or sequential use if not the case). It was shown that the several host species living in sympatry in the studied rivers (G. roeselii and several lineages of the G. pulex/fossarum complex) are constantly available for acanthocephalans over the entire duration of our monitoring, but in relatively variable proportions. Different patterns of infections in acanthocephalans were highlighted during this temporal monitoring. Therefore, variation in susceptibilities to infection by the different parasite species between gammarids species are suggested, allowing us to propose that the different hosts contribute unequally to the parasites life cycles.

The second objective of this work was to assess the relative susceptibilities of the two G. roeselii and G. fossarum morphospecies to infection by P. laevis, using experimental infections. If G. roeselii suffers higher prevalence in the wild than G. fossarum, but is nevertheless less susceptible to infection in the laboratory. This host, already known to be less efficiently manipulated, therefore constitutes a kind of dead-end host for this parasite (the parasites do not transmit to their definitive host and accumulate in intermediate hosts). The “sink” effect of these infections seem nevertheless moderate given the low infectivity of the parasites in G. roeselii.

In the third part of the thesis, the respective roles of three factors – susceptibility to infection, parasite virulence, manipulation – in the interactions between the two parasites P. laevis and P. tereticollis and their community of sympatric hosts of the cryptic species complex G. pulex/fossarum were studied experimentally. Differences were revealed, in terms of both infectivity and virulence, between host cryptic lineages. The mortality of infected individuals increased significantly after several weeks of development of both parasite species, which did not allow accurate comparisons of behavioural changes between host lineages.

This thesis shows that the system studied here is indeed a multi-host and multi-parasite system. Acanthocephalans, traditionally considered as generalists, actually show marked preferences, probably local, in the exploitation of certain intermediate hosts lineages which are not necessarily the most abundant ones.

 

Keywords

cryptic diversity; multi-hosts parasites ; multi-parasitized hosts; specificity

 

Jury

Jean-Nicolas Beisel, université de Strasbourg – reviewer
Jérôme Boissier, IHPE UMR 5244, université de Perpignan – examiner
Thierry Lefèvre, UMR MIVEGEC Montpellier – reviewer
Marie-Jeanne Perrot-Minnot, Biogéosciences, université de Bourgogne – examiner
Thierry Rigaud, Biogéosciences, université de Bourgogne – supervisor

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Thumbnail image Gammarids and acanthocephalans, a multi-(cryptic) hosts and multi-parasites system?

Started in october 2017

Supervisor: Thierry Rigaud, laboratoire Biogéosciences, Dijon

 

Abstract

Multi-host parasites can exploit various host species that differ in abundance and susceptibility to infection, which will contribute unequally to their transmission and fitness. "Key hosts" are those contributing significantly more to the completion of the life cycle of such parasites. Three non-exclusive criteria may be used to identify a key host: its high abundance, its high exposure/susceptibility to infection, and the large number of infectious stages produced per infected individual. Many parasites with complex life cycles have evolved the ability to manipulate several traits of their intermediate hosts' phenotype, including behavior, thereby increasing the likelihood of transmission to their definitive hosts. Host susceptibility to manipulation must therefore be included in the definition of a "key host" species, because of its involvement in parasite transmission. Several species of acanthocephalan manipulative parasites (among which Pomphorhynchus laevis and P. tereticollis) use amphipod crustaceans of the genus Gammarus as intermediate hosts. It has recently been shown that many G. pulex and G. fossarum cryptic lineages are living in sympatry in Eastern France rivers, bringing a new vision of the spectrum of intermediate hosts that acanthocephalans can use, and their relative contribution to their life cycles.

The first aim of the thesis was to test whether the presence of host species/lineages in the wild is stable over time (determining therefore the possibility of simultaneous use of hosts by parasites, or sequential use if not the case). It was shown that the several host species living in sympatry in the studied rivers (G. roeselii and several lineages of the G. pulex/fossarum complex) are constantly available for acanthocephalans over the entire duration of our monitoring, but in relatively variable proportions. Different patterns of infections in acanthocephalans were highlighted during this temporal monitoring. Therefore, variation in susceptibilities to infection by the different parasite species between gammarids species are suggested, allowing us to propose that the different hosts contribute unequally to the parasites life cycles.

The second objective of this work was to assess the relative susceptibilities of the two G. roeselii and G. fossarum morphospecies to infection by P. laevis, using experimental infections. If G. roeselii suffers higher prevalence in the wild than G. fossarum, but is nevertheless less susceptible to infection in the laboratory. This host, already known to be less efficiently manipulated, therefore constitutes a kind of dead-end host for this parasite (the parasites do not transmit to their definitive host and accumulate in intermediate hosts). The “sink” effect of these infections seem nevertheless moderate given the low infectivity of the parasites in G. roeselii.

In the third part of the thesis, the respective roles of three factors – susceptibility to infection, parasite virulence, manipulation – in the interactions between the two parasites P. laevis and P. tereticollis and their community of sympatric hosts of the cryptic species complex G. pulex/fossarum were studied experimentally. Differences were revealed, in terms of both infectivity and virulence, between host cryptic lineages. The mortality of infected individuals increased significantly after several weeks of development of both parasite species, which did not allow accurate comparisons of behavioural changes between host lineages.

This thesis shows that the system studied here is indeed a multi-host and multi-parasite system. Acanthocephalans, traditionally considered as generalists, actually show marked preferences, probably local, in the exploitation of certain intermediate hosts lineages which are not necessarily the most abundant ones.

 

Keywords

cryptic diversity; multi-hosts parasites ; multi-parasitized hosts; specificity

 

Jury

Jean-Nicolas Beisel, université de Strasbourg - reviewer
Jérôme Boissier, IHPE UMR 5244, université de Perpignan - examiner
Thierry Lefèvre, UMR MIVEGEC Montpellier - reviewer
Marie-Jeanne Perrot-Minnot, Biogéosciences, université de Bourgogne - examiner
Thierry Rigaud, Biogéosciences, université de Bourgogne - supervisor

extrait:
lien_externe:
titre:
La manipulation parasitaire dans un contexte multi-hôtes
date_de_debut:
2017
titre_these:
La manipulation parasitaire dans un contexte multi-hôtes
date_de_debut_these:
2017
nom:
Bauer
date_de_debut_these_numerique:
201700

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