Wild-caught rainbowfish Melanotaenia spp. originating from three isolated populations were infected with a quantified dosage of parasites Ichthyophthirius multifiliis in a controlled environment. The Melanotaenia eachamensis from Dirran Creek were much more susceptible to ichthyophthiriasis than were M. splendida from the Lake Tinaroo or Bluewater Creek populations. When the highly susceptible Dirran Creek rainbowfish were crossed with rainbowfish from a fourth population, Lake Eacham M. eachamensis, they produced hybrids with significantly higher resistance than pure-bred Dirran Creek, but not higher than pure-bred Lake Eacham fish. Hence, intraspecific hybridization increased resistance to I. multifiliis infection in M. eachamensis. Hosts from all three populations were much less susceptible to infection on their second exposure to the parasite. However, the Bluewater Creek population was better able to acquire immunity to I. multifiliis than either the Dirran Creek or Lake Tinaroo populations. It is tentatively suggested that there may be a link between the heterozygosity of populations of rainbowfish and their initial ability to resist infection by Ichthyophthirius multifiliis. 2000 The Fisheries Society of the British Isles

Background: Condition-dependence is a ubiquitous feature of animal life histories and has important implications for both natural and sexual selection. Mate choice, for instance, is typically based on condition-dependent signals. Theory predicts that one reason why condition-dependent signals may be special is that they allow females to scan for genes that confer high parasite resistance. Such explanations require a genetic link between immunocompetence and body condition, but existing evidence is limited to phenotypic associations. It remains unknown, therefore, whether females selecting males with good body condition simply obtain a healthy mate, or if they acquire genes for their offspring that confer high immunocompetence.

Variation in avian immune response can be influenced by social environment. This is of particular interest in the context of immunomediated sexual behaviour because social environment may subsequently affect a bird’s relative investment in immunocompetence versus sexual signalling. I tested whether the effect of social environment on immune response and sexual signalling depends on socio-sexual status using male zebra finches (Taeniopygia guttata). To do this, I manipulated social environment (‘same sex’ versus ‘dual sex’) and socio-sexual status (‘high’ versus ‘low’) of the males. I then determined what effect these manipulations had on an index of immunocompetence, namely cell-mediated immune response, and two indices of sexual signalling (bill colour and song rate). I found that social environment influenced cell-mediated immune response and sexual signalling in low-status males. These males had lower immune responses and increased sexual signalling in the dual-sex environment compared with the same-sex environment. In contrast, high-status males had similar immune responses and sexual signalling regardless of social environment. These results suggest that social environment can influence immune response and sexual signalling; however, the nature of this effect was context-dependent, with low-status males more affected than high-status males.

Ichthyophthirius multifiliis, or white spot, is a well known and widely distributed parasite of freshwater fish. However, it is not know whether it can infect other aquatic vertebrates such as amphibians. This study uses a series of laboratory-based experiments to demonstrate that I. multifiliis can infect the tadpole stage of an amphibian, the striped marshfrog (Limnodynastes peronii) of Eastern Australia. The tadpoles did not appear to develop ichthyophthiriasis at low parasite levels (200 parasites per tadpole), but at high parasite levels (2,000 parasites per tadpole) 100% of the tadpoles developed ichthyophthiriasis. This is the first time that it has been demonstrated that I. multifiliis can infect a nonpiscine vertebrate host.