Supplementary data

Three-spined stickleback (Gasterosteus aculeatus) was the most abundant prey of the Arctic charr in Lake Peltojärvi: Of the 50 charr examined for stomach content 30 had eaten sticklebacks (smallest charr was 210 mm), three had foraged other fish (whitefish, Coregonus lavaretus and unidentified fish), and 17 individuals had empty stomachs. The mean number of ingested sticklebacks was significantly higher in the spring (ANCOVA, F1,49 = 28.339, p < 0.001) probably indicating the reduced feeding activity of the charr prior to spawning period (Table 1). Malmquist et al. (1992) found that limnetic Arctic charr morphs (like Lake Peltojärvi charr) had an ontogenic niche shift from planktivory to piscivory at the size of 220 mm. Thus, in the size class of the present Arctic charr (from 210 to 595 mm in standard length) Diphyllobothrium parasites were probably transmitted to charr via ingested sticklebacks (Halvorsen & Wissler 1973). The mean standard length of charr with different MHC genotypes did not differ each other (ANOVA, F1,49 = 1.263, p = 0.292, electronic supplementary material, Table S2). Hence, it seems likely that there was no difference in the exposure of different MHC genotypes to Diphyllobothrium but rather that the observed differences in parasite load are due to differences in parasite resistance.

Malmquist, H. J., Snorrason, S. S., Skulason, S., Jonsson, B., Sandlund, O. T. & Jonasson, P. M. 1992 Diet differentiation in polymorphic Arctic charr in Thingvallavatn, Iceland. J. Anim. Ecol. 61, 21-35.

Halvorsen, O. & Wissler, K. 1973 Studies of the helminth fauna of Norway. XXVIII. An experimental study of the ability of Diphyllobothrium latum (L.), D. dendriticum (Nitzsch), and D. ditremum (Creplin) (Cestoda, Pseudophyllidae) to infect paratenic hosts. Norw. J. Zool. 21, 201-210.

Conejeros, P., Phan, A., Power, M., Alekseyev, S.,O’Connell, M., Dempson, B., Dixon, D. 2008 MH class IIα polymorphism in local and global adaptation of Arctic charr (Salvelinus alpinus L.). Immunogenetics 60, 325-337.

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