Environmental, social, and genetic factors predisposing Xenopus laevis tadpoles to infection
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This work examines the ecological, social and genetic factors that predispose amphibians to infection. In the last 30 years many amphibian populations have declined due to infectious disease, although few researchers have studied the factors involved in mediating amphibian infection. My research is designed to explore some of these factors. I first examined the effects of crowding, kin composition (the relatedness of individuals in a group), and habitat complexity on the growth and survival of Xenopus laevis tadpoles exposed to the bacterial pathogen Aeromonas hydrophila. In tadpoles, stress, and in particular corticosterone, a hormone associated with stress, is known to inhibit growth. Crowding, kin composition, and habitat complexity have all been linked to tadpole growth. As corticosterone exposure is also linked to reduced immune function, I examined how these ecological factors influence tadpoles' disease resistance. Tadpoles exposed to the bacterium were significantly smaller and more likely to die than control tadpoles. Tadpoles reared only with siblings (pure sibship groups) were larger, less variable in size, and had lower mortality rates than tadpoles reared in mixed sibship groups. The size difference between pure and mixed sibship groups was greatest when they were exposed to the pathogen. Habitat complexity reduced size variation within tanks but did not affect mean tadpole size. Mixed kinship composition and high tadpole density can increase competition, reduce growth, and increase disease susceptibility. These results indicate that growth was inhibited by pathogen exposure but kin association may ameliorate this effect. The Major Histocompatibility Complex (MHC) is an integral part of the vertebrate adaptive immune system. To determine the importance of the MHC in conferring disease resistance in amphibians, I challenged X. laevis tadpoles, bearing different combinations of four MHC haplotypes (f, g, j, and r), with A. hydrophila in two experiments. Exposure to A. hydrophila affected the growth and survival of these tadpoles and that the MHC moderated these effects. Tadpoles with two MHC haplotypes (r and g) were susceptible to this pathogen and tadpoles with the other two haplotypes (f and j) were resistant. Heterozygous tadpoles with both susceptible and resistant haplotypes were always intermediate to either homozygotes in size and survival. These results demonstrate that MHC genotype plays a major role in determining the impact of bacterial pathogens on the growth and survival of X. laevis tadpoles. To test whether the effect of exposure to pathogens differs according to the similarity of the hosts I challenged tadpoles with natural levels of the microorganisms associated with different MHC genotypes by exposing the tadpoles to water preconditioned by adults of different MHC genotypes. If the pathogens are adapted to the MHC genotype of their hosts, tadpoles exposed to water from adults with which they shared MHC haplotypes would be more susceptible than those exposed to water from MHC-dissimilar adults. Alternatively, if the hosts are adapted to their pathogens tadpoles may be more resistant to pathogens from MHC-similar frogs than those from MHC-dissimilar frogs. I found that tadpoles exposed to water from MHC-dissimilar animals developed faster, but without increased growth, and were more likely to die than those exposed to water from MHC-similar animals. Furthermore, there was an optimal difference between the tadpoles’ and the donors’ MHC where tadpoles were sufficiently different to the donor to defend against its locally adapted pathogens, and sufficiently similar to not be exposed to especially virulent foreign pathogens. Finally, I present an inventory of bacteria found in the gut and skin (nonsystemic sites) and heart, muscle, and abdominal cavity (systemic sites) of captive frogs. I found several species of bacteria previously identified as amphibian pathogens and many bacteria in systemic sites that have not been considered pathogenic to amphibians. None of the frogs tested positive for the amphibian chytrid fungus, Batrachochytrium dendrobatidis. I discuss the potential importance of these species of bacteria as amphibian pathogens and as protective probiotics, using New Zealand frogs as a case study. In its sum, this work describes some of the factors that can affect amphibians’ ability to resist disease. I show that the genetic constitution of an individual, specifically in terms of the MHC, affects the impact of a disease, and so too does its social and ecological conditions, including the level of crowding, the kinship of its groupmates and the specific microbial challenges of its immediate environment. I also show that many of the factors linked to tadpole growth and development that are well described in other amphibians also affect Xenopus tadpoles.