Kudoid parasites are common in marine fish and their affects range from relatively benign to pathogenic. They are notorious in the seafood industry for spoiling fish meat, either by muscle liquefaction or by producing unsightly macroscopic cysts. Kudoids also infect several other tissues such as brain, heart, gills, connective tissue, intestinal smooth muscle, and epithelia. Since the revised classification of the Kudoidae in 2004, kudoid myxospores have a range of shapes, but with two main characteristics: they have four or more spore valves with the equivalent number of polar capsules. With relatively few morphological traits to base kudoid species descriptions, DNA has proven to be increasingly useful to specifically diagnose infections. However, to properly utilise genetic information, there is a need to understand how genetics relates to the biological characteristics of species. This will ensure the genetic markers used are appropriate for species characterisation. This project aimed to develop an understanding of how biological characteristics including morphology, tissue tropism, host specificity, and geographic distribution correlate with genetic relatedness of kudoid species, and how this information could be employed to facilitate diagnosis and characterisation of species. A taxonomic investigation of kudoid isolates, mostly from the east coast of Queensland, Australia, was conducted using information on host identity, locality, infection status, tissue of infection and histological response to infection. Together with spore morphology and DNA relationships, comparisons were made with existing data from literature. Forty-three small subunit and 68 large subunit ribosomal DNA sequences were generated from kudoid isolates for this project. The two gene regions showed similar relationships amongst the kudoids, however the genetic variability between closely related species was less conserved in the large subunit, and showed superior species resolution that correlated with subtle morphological differences of spores. Broadly speaking, genetic relatedness of kudoids is closest for species infecting the same type of tissue with similar spore morphology. Within these tissue/morphology groups, some relationships were emerging from genetic correlations with pathology, however there are weaknesses in utilising this characteristic since pathology may change with host species or host age. Similar relationships to pathology were seen from geographic locality; however, the limited number of areas that genetic data originates from may have introduced some bias. Lastly, there were very few genetic relationships that correlated with host relatedness. The taxonomic affinities of the new kudoid isolates lead to the characterisation of six novel kudoid species and four additional host-parasite combinations. The investigation into these new species highlighted the broad host ranges and geographic distribution of some kudoid species. Genetic mixing of kudoid species populations from the east coast of Queensland, Australia with Japan was noted. For example, Kudoa amamiensis was discovered from carangid and pomacentrid fish from Queensland, increasing the host range from eight (seven from Japan, one from north Queensland) to 11 host species. Also, geographic distribution was extended to Heron Island on the south Great Barrier Reef (Queensland). The distribution did not extend to rocky temperate reefs further south. Another example is Kudoa yasunagai which was previously recorded from four fish species in Japan and one from the Philippines. Now the distribution has been extended to North Stradbroke Island in Southern Queensland from three additional host species. The connection was made between these host isolates with the assistance of DNA and a novel way of analysing morphological data in species that have variable morphotypes i.e. different numbers of polar capsules in the spores. It was confirmed that the dominant morphotype can vary between different hosts and even host samples of the one species. Genetic data has also proven useful in the determination of host range. An analysis of kudoid host specificity showed just over two thirds of the described kudoid species have been recorded from a single host; the remainder have been recorded from two to 38 host species. By using DNA data, 17 new hosts were recorded for K. thalassomi (total 18 hosts). The new hosts came from six different host families, but these fish shared the same habitat on the Great Barrier Reef. Other high host range kudoid species may be disproportionately broad as similar morphological species may have been misidentified from some hosts. For example, the host range of K. thyrsites (38) is likely to represent a species complex that may be split pending further genetic analyses. From this study, it has been reinforced that a holistic approach to species definition is important. Genetic analysis has become a particularly valuable tool; however, in conjunction with biological data such as spore morphology, pathology, host range, and geographic distribution, our understanding of kudoid parasites becomes more robust and provides important knowledge for diagnostics and aquaculture management. In the future, significant improvements in kudoid understanding will coincide with the solving of kudoid life cycles and transmission patterns.
| Identifer | oai:union.ndltd.org:ADTP/288366 |
| Creators | Mieke Burger |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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