Résumé
In the study of multi-host parasites, it is often found that host species
contribute asymmetrically to parasite transmission. Yet in natural
populations, identifying which hosts contribute to parasite transmission
and maintenance is a recurring challenge. Here, we approach this issue by
taking advantage of natural variation in the composition of a host
community. We studied the brine shrimps Artemia franciscana and Artemia
parthenogenetica and their microsporidian parasites Anostracospora rigaudi
and Enterocytospora artemiae. Previous laboratory experiments had shown
that each host can transmit both parasites, but could not predict their
actual contributions to the parasites’ maintenance in the field. To
resolve this, we gathered long-term prevalence data from a metacommunity
of these species. Metacommunity patches could contain either or both of
the Artemia host species, so that the presence of the hosts could be
linked directly to the persistence of the parasites. First, we show that
the microsporidian A. rigaudi is a spillover parasite: it was unable to
persist in the absence of its maintenance host A. parthenogenetica. This
result was particularly striking, as A. rigaudi displayed both high
prevalence (in the field) and high infectivity (when tested in the
laboratory) in both hosts. Moreover, the seasonal presence of A.
parthenogenetica imposed seasonality on the rate of spillover, causing
cyclical pseudo-endemics in the spillover host A. franciscana. Second,
while our prevalence data was sufficient to identify E. artemiae as either
a spillover or a facultative multi-host parasite, we could not distinguish
between the two possibilities. This study supports the importance of
studying the community context of multi-host parasites, and demonstrates
that in appropriate multi-host systems, sampling across a range of
conditions and host communities can lead to clear conclusions about the
drivers of parasite persistence.