Taxonomy of monogenean parasites and their coevolution with Australian atheriniform fishes

Corlis, David Brian (2004) Taxonomy of monogenean parasites and their coevolution with Australian atheriniform fishes. PhD thesis, James Cook University.

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Abstract

Does a phylogenetic association exist between Australian freshwater fishes and their monogenean parasites and if so what is the nature of the association? Are the associations an example of coevolution, phylogenetic tracking or some other phenomenon? In this thesis I explore these questions.

Knowledge of monogeneans from Australian freshwater fishes is very sparse. To date 26 species of monogenean have been described from 16 species of native freshwater fishes (roughly 5% of the approximately 300 species of freshwater fish known). In the current study I examine a further 19 species of fish from Australian freshwaters.

Studies on phyletic associations can be confounded by taxonomic problems. This is an under-acknowledged problem. Taxonomic understanding of the host group, if vertebrates, is generally better than that of the parasite group. This is certainly true of the atheriniform fish studied here. Here, detailed taxonomic work on previously undescribed parasites is presented. This is based on both morphological and morphometric analysis of dimensions. I erect four new genera of Monogenea in the Dactylogyridae: Longidigitis gen. nov., Recurvatus gen. nov., Iliocirrus gen. nov.and Helicirrus gen. nov.and describe 19 new species. The genera are defined by morphology of the copulatory apparatus while species are defined by haptoral sclerite morphology.

Studies on phyletic associations (coevolution, phylogenetic tracking and cospeciation) typically use comparisons of the phylogeny of the hosts with an independently derived phylogeny of the parasites. Where a single parasite species inhabits a single host species (i.e. is mono-host-specific), comparisons of phylogenies will provide information on the extent of phyletic association, extinction of parasites etc. It is often the case that a single species of parasite inhabits more than one species of host. This can complicate interpretation of the results, but provides an opportunity to discuss phenomena such as host addition, host-switching, rates of cospeciation etc. In the absence of a molecular or morphological-based phylogeny of the parasites, I used data from morphometric analyses to infer a phylogeny.

In this study I recognised that the assumption of mono-host specificity (parasite occurs only on a single host species) is inappropriate as a model for examining coevolution. A much lower level of mono-host-specificity, than previously documented or assumed, was detected which creates many problems for comparing host and parasite phylogenies. Consequently, I developed an approach using the distribution of parasite species among hosts.

A strict one-host one-parasite cospeciation scenario is clearly not apparent for all host species; however a high congruence with host clades is seen in inter and intra specific morphometric variation of parasite and for their associated distribution patterns among hosts. This congruence may be explained by delayed speciation of parasites, since the alternative, extensive host addition, should produce a high level of incongruence with the host phylogeny especially for morphometric variation-derived phylogenies. Whether the patterns of congruence shown represent coevolution or phylogenetic tracking is uncertain. There is clearly a response induced by the host on the parasite in the form of morphometric variation of sclerite variables. Coevolution may be occurring but the high prevalence of parasite infection appears to reject the idea of a parasite-induced host immune response. Thus phylogenetic tracking seems to be the process controlling parasite speciation.

Clustering patterns produced by both methods mentioned above imply a rather high level of congruence with the host clades. These two methods, however, require a well-planned sampling regimen to enable collection of sufficient parasites from sufficient host species and sample sites for pattens to become apparent. It was noted that small sample sets from multiple sites would indicate levels of host specificity better than a large sample set from a single site since, in this study, parasite community structures within sample sites show a high level of prevalence.

Apparent cospeciation between Australian atheriniform fishes and their monogenean parasites is more common in older lineages than in more recently evolved lineages. Two major parasite speciation events were identified, one co-occurred with the speciation event that produced Melanotaenia trifasciata and the second with the speciation event that produced the “nigrans” clade. Delayed cospeciation is also evident and is reflected in the distributions of parasite species and the inferred parasite phylogenies that used intraspecific variation. If the parasites have not evolved at the same rate, but appear to be one taxonomic level behind the host, then host clades can be identified by the presence of a particular parasite species. Host addition or switching does not appear to be significant in the monogeneans studied here although possible examples are discussed. Morphometric variation of parasite haptor can be used in phylogenetic studies. Parasite distributions among host species can reflect host phylogeny. Parasite inter-specific associations are strong especially among low host-specific species. Host body length does not appear to affect these interactions. It is clear that when examining phyletic associations of Australian teleosts and their parasites, the host family should be studied as a basic unit.

A phyletic association is present between hosts and parasites and evidence suggests phylogenetic tracking is the process. Cospeciation is common in older host lineages while delayed cospeciation is common in more recently derived lineages.

Item ID: 14914
Item Type: Thesis (PhD)
Keywords: freshwater fishes, Atheriniformes, parasites, Monogenea, taxonomy, phylogeny, morphology, phyletic associations, host-parasite relationships, coevolution, cospeciation
Date Deposited: 10 Nov 2010 23:13
FoR Codes: 06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology > 060307 Host-Parasite Interactions @ 60%
06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology > 060301 Animal Systematics and Taxonomy @ 40%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960807 Fresh, Ground and Surface Water Flora, Fauna and Biodiversity @ 50%
97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 50%
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