SUSCEPTIBILITY OF BROOK CHARR, SALVELINUS-FONTINALIS TO THE PATHOGENIC HEMOFLAGELLATE, CRYPTOBIA-SALMOSITICA, AND THE INHERITANCE OF INNATERESISTANCE BY PROGENIES OF RESISTANT FISH

Eighty-seven of 175 laboratory raised F1 generation brook charr (Salve linus 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 un der in vitro conditions while those of susceptible fish did not. Sperm from 4 susceptible and 4 resistant male charr, from 8 F1 families, we re used to fertilize eggs from 2 hatchery raised susceptible charr. Su sceptibility of progenies (F2 families) was determined 4 weeks after i noculation of the parasite. All progenies (seven F2 families) of 4 sus ceptible males were susceptible to experimental infection and their pl asma did not lyse the parasite. It was presumed that these susceptible progenies were homozygous recessive (rr), and that the resistant alle le 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 suscep tible charr in 6 other F2 families from the other 3 resistant males (p resumed Rr) and two susceptible females was about 1:1. The results sug gest 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.