Susceptibility of brook charr, Salvelinus fontinalis to the pathogenic haemoflagellate, Cryptobia salmositica, and the inheritance of innate resistance by progenies of resistant fish.

Eighty-seven of 175 laboratory raised F1 generation brook charr (Salvelinus fontinalis) were resistant to experimental Cryptobia salmositica infection. Susceptibility to the pathogen was not related to age nor to route of infection. Plasma of resistant charr lysed the parasite under in vitro conditions while those of susceptible fish did not. Sperm from 4 susceptible and 4 resistant male charr, from 8 F1 families, were used to fertilize eggs from 2 hatchery raised susceptible charr. Susceptibility of progenies (F2 families) was determined 4 weeks after inoculation of the parasite. All progenies (seven F2 families) of 4 susceptible males were susceptible to experimental infection and their plasma did not lyse the parasite. It was presumed that these susceptible progenies were homozygous recessive (rr), and that the resistant allele was dominant. All progenies (two F2 families) of 1 resistant male (presumed RR) and 2 susceptible females were all resistant to infection and their plasma lysed the parasite. The ratio of resistant to susceptible charr in 6 other F2 families from the other 3 resistant males (presumed Rr) and two susceptible females was about 1:1. The results suggest a single Mendelian locus which determines innate resistance to C. salmositica infection in brook charr and that it is possible to breed Cryptobia-resistant fish.