Differences in initial and acquired resistance to Ichthyohthirius multifiliis between populations of rainbowfish

Wild-caught rainbowfish Melanontaenia spp. originating from three isolated populations were infected with a quantified dosage of parasites Ichthyophth irius multifiliis in a controlled environment. The Melanotaenia eachamensis from Dirran Creek were much more susceptible to ichthyophthiriasis than we re M. splendida from the Lake Tinaroo or Bluewater Creek populations. When the highly susceptible Dirran Creek rainbowfish were crossed with rainbowfi sh from a fourth population, Lake Eacham M. eachamensis, they produced hybr ids with significantly higher resistance than pure-bred Dirran Creek, but n ot higher than pure-bred Lake Eacham fish. Hence, intraspecific hybridizati on increased resistance to I. multifiliis infection in M. eachamensis. Host s from all three populations were much less susceptible to infection on the ir second exposure to the parasite. However, the Bluewater Creek population was better able to acquire immunity to I. multifiliis than either the Dirr an Creek or Lake Tinaroo populations. It is tentatively suggested that ther e may he a link between the heterozygosity of populations of rainbowfish an d their initial ability to resist infection by Ichthyophthirius multifiliis . (C) The Fisheries Society of the British Isles.