Here I show the results for three experiments designed to determine whether parasites are adapted to infecting hosts from sympatric (local) populations, relative to hosts from remote populations. All experiments used the New Zealand snail Potamopyrgus antipodarum as the host, and an undescribed trematode (genus Microphallus) as the parasite. All three experiments show strong local adaptation by the parasite.
Local adaptation across the southern alps of New Zealand. The figure on the left shows the frequency of infection in a reciprocal cross-infection experiment. Snails from two lake populations (L. Mapourika and L. Alexandrina) were experimentally exposed to parasites from the same two lakes. The results show that: 1) snails from L. Mapourika are more susceptible to parasites from L. Mapourika, and 2) that snails from L. Alexandrina are more susceptible to parasites from L. Alexandrina. In other words, the parasites are adapted to infecting snails from their local host population.
These two lakes are on opposite sides of the southern alps, across which migration by either the host or the parasite seems unlikely.
Note: comparison of infectivity between the two parasite populations is possible, but not helpful in this case, as the doses may have differed. Thus the apparently higher infectivity of the Mapourika population may have been the result of a higher dose of parasites in that treatment. Hence, I confined the analysis to comparing host populations within parasite treatments. (From Lively 1989).
The vertical bars give one SE of the mean.
Local adaptation on the West Coast of the South Island. The figure on the right shows the frequency of infection in a three-way reciprocal cross-infection experiment. Snails from three lake populations (L. Mapourika, L. Wahapo, and L. Paringa) were experimentally exposed to parasites from the same three lakes. The results show local adaptation for all three parasites populations.
These three lakes are all located west of the Southern Alps in New Zealand. Two of the lakes are within 7km of each other (Mapourika and Wahapo), while the other lake (Paringa) is about 100km to the southwest. The movement of parasites between these lakes (especially Wahapo and Mapourika) seems likely, and indeed genetic data indicate that the parasite is not genetically differentiated for presumably neutral alleles (allozymes). Nonetheless, the pattern shows strong local adaptation by all three parasite populations. (From Lively 1989).
More local adaption. The graph on the left shows local adaptation by parasites from two additional lakes. The mixed parasite treatment suggests that hybrids between the two parasite sources are less infective than local parasites. (From Lively and Dybdahl 2000). (See also Lively et al. 2004 for a meta-analysis.)
Lively, C.M. 1989. Adaptation by a parasitic trematode to local populations of its snail host. Evolution 43:1663-1671
Lively, C.M. and M.F. Dybdahl. 2000. Parasite
adaptation to locally common host genotypes. Nature 405:679-681.
Lively, C.M., M.F. Dybdahl, J. Jokela, E. Osnas, L.F. Delph. 2004. Host sex and local adaptation by parasites in a snail-trematode interaction. American Naturalist 164:S6-S18.
This work supported by grants from the US National Science Foundation
C. M. Lively, Dept. of Biology, Indiana
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