Molecular assessment of resistance to amoebic gill disease

• Main Author: Wynne, JW
• Format: Thesis
• Language: English
• Published: 2008
• Subjects: Atlantic salmon; Fishes; Atlantic salmon fisheries; Gills; Salmon farming; Salmo salar
• Online Access: https://eprints.utas.edu.au/22220/; https://eprints.utas.edu.au/22220/7/Wynne_whole_thesis_ex_pub_mat.pdf; https://eprints.utas.edu.au/22220/1/whole_WynneJamesWatkin2008_thesis.pdf

Amoebic gill disease (AGD) is the most significant health issue affecting the culture of Atlantic salmon (Salmo salar L.) in Tasmania, Australia. Recent research has suggested that heritable variation in AGD resistance exists within the Tasmanian Atlantic salmon population. Subsequently, enhancing this resistance through selective breeding has become a major research focus in Tasmania. The mechanisms controlling this commercially important trait remain poorly understood. To this end, an investigation of the molecular mechanisms controlling AGD resistance was conducted. Due to their high polymorphism and important immune function genes of the major histocompatibility complex (MHC) - known as the MH genes in Atlantic salmon - represent some of the best candidate loci with a possible influence upon AGD resistance. With this in mind, the amount of MH variation and its association with resistance to AGD was investigated. In contrast to what has been previously reported at non-coding microsatellite loci, a high level of MH class II diversity has been maintained in the Australian Atlantic salmon population compared to the ancestral population. The use of an AGD challenge test with subsequent MH genotyping demonstrated that the presence of one MH class II alpha allele known as Sasa-DAA-3UTR 239 was significantly associated with reduced disease severity. Individuals containing a copy of this allele had 4.0% less gill filaments infected by AGD compared to individuals without this allele. Next utilising a cDNA microarray with real-time PCR verification the transcriptional changes associated with AGD and AGD resistance were investigated. Comparing the gene expression profiles within the gill, liver and anterior kidney between naive and AGD affected (at 19 days post inoculation) Atlantic salmon suggests the host response to AGD upon acute first infection is largely suppressive and localised to the site of infection, the gill. Next, the gill transcriptome response between Atlantic salmon deemed putatively resistant and putatively susceptible to AGD following chronic natural infection was investigated. Results suggested that compared to the susceptible individuals, Atlantic salmon resistant to AGD demonstrate an up-regulation of adaptive immune genes and negative regulators of the cycle cell. Further characterisation of the full length mRNA sequence and expression distribution of one unknown transcript which was significantly up-regulated in both previous microarray experiments was investigated. This research has provided the first molecular assessment of resistance to AGD in Atlantic salmon. The implications of this research in terms of the understanding of the molecular mechanisms of AGD resistance and the ultimate development of genetic markers linked to resistance will be considered.