Abstract
The specialisation of organisms for their primary resources is a fundamental concept in evolutionary ecology. For parasites, resource specialisation is essentially equivalent to host specificity. Generalist parasites exploit a wide range of host species, while specialists concentrate on a few. Elucidation of host specificity constraints is needed for a greater understanding of both evolutionary and ecological aspects of parasite transmission.
In this thesis, I investigate how phenotypic and genetic variables constrain host specificity in the trematode Maritrema novaezealandensis. Using a field survey and several experimental studies, I explore environmental, immunological, morphological, behavioural, physiological, and genetic aspects of host specificity.
A field survey was used to quantify patterns of host use by parasites throughout a community of intermediate hosts, in a locality known for its high prevalence of M. novaezealandensis, and to reveal the flow of parasites throughout the major crustacean species. The patterns observed depended on several biological and ecological factors, including the physiology, morphology, immunology, and behaviour of the various host species. Certain decapods were compatible hosts for M. novaezealandensis, while other crustaceans demonstrated lower host suitability as shown by high levels of melanised cysts and parasite stages not completing their development.
In a mesocosm study multiple clones of M. novaezealandensis, ranging from low to high heterozygosity, were exposed simultaneously to the amphipods Paracalliope novizealandiae and Heterophoxus stephenseni to examine intraspecific genetic variation in host preferences. All clones preferred H. stephenseni to varying degrees, and no clear association was found with heterozygosity. Cercariae were exposed to the amphipods in the presence and absence of sand (refugium for H. stephenseni). Without sand, infection levels were significantly higher in H. stephenseni than in P. novizealandiae. With sand, H. stephenseni was able to hide, offsetting the parasite’s intrinsic preferences for this host, a finding illustrating the importance of habitat features as determinants of host usage.
The immune response elicited by M. novaezealandensis in the crab Macrophthalmus hirtipes was investigated to see if significant differences exist in the immune responses generated by ten distinct clones on the basis that these clones could potentially differ in host specificity. Interclonal differences in immune response elicited were indeed detected, however, additional predictor variables (heterozygosity and the presence in the host of pre- existing parasites) did not influence this response.
To investigate the degree of phenotypic variation within and among 42 clonal parasite lines of M. novaezealandensis, morphological traits, photoreactivity, and survivorship were measured and their variability was also quantified. Furthermore, the hypothesis that homozygotes are more variable than heterozygotes was tested. Substantial differences among clones in morphology, photoreactivity and survivorship were found, yet no significant differences among clones in levels of intra-clonal phenotypic variability were seen. In addition, no correlation was found between heterozygosity and phenotypic variation among clones, such that the prediction of highly variable homozygotes was unsupported.
A subset of these clones was used in four infection experiments involving two crab and two amphipod species. Infection success and post-infection measures of fitness were quantified and related to phenotypic and genetic traits measured previously. Infection success varied among clones, dependent on which host was considered. Some clones acted as specialists, performing well in only some hosts, whereas others acted as generalists, performing equally well in all hosts. None of the genetic or phenotypic traits considered had consistent effects on infection success of clones across all host species. Overall, the clone- level host specificity of M. novaezealandensis for its second intermediate hosts was shown to depend on a wide range of factors, all combining to maintain a population-level generalist strategy in this trematode.