Identifying molecular determinants of drug susceptibility in salmon lice (Lepeophtheirus salmonis)

鲑鱼虱（Lepeophtheirus salmonis）是一种感染野生和养殖鲑鱼的甲壳类外寄生虫，对大西洋海洋鲑鱼养殖造成重大问题。养殖场的鲑鱼虱感染需要控制，以保持良好的鱼类健康和福利，并尽量减少养殖场来源的寄生虫对野生鱼类种群的潜在影响。目前，在养鱼场控制鲑鱼虱在很大程度上依赖于使用药物，辅之以养殖场管理措施。目前正在制定一些非化学控制战略，其中包括通过清洁鱼对虱子进行生物控制，并试图通过接种疫苗和选择性育种方案使鲑鱼对虱子具有抵抗力。然而，这些和其他替代控制策略目前尚未充分发展，无法在工业规模上全面实施。虽然抗寄生虫药物能有效控制鲑鱼虱，但可用的除虫剂种类相对较少。从其他害虫和寄生虫中可以得知，在查尔斯·达尔文发现的自然选择机制的推动下，重复使用相同或类似的化学品会增加寄生虫产生耐药性的风险。某些被称为“抗性等位基因”的遗传特征在生物体中通常是罕见的，因为它们在正常条件下对其携带者没有任何好处。然而，拥有抗性等位基因的寄生虫能够比没有耐药性的寄生虫更好地在药物治疗中存活下来，因此，如果过度使用同一种药物，抗性等位基因在接受治疗的寄生虫种群中出现的频率会增加，因为易感个体会因治疗而死亡。这可能最终导致整个人群产生耐药性。人们对鲑鱼虱产生耐药性的机制知之甚少。在拟议的项目中，实验室中产生的鲑鱼虱的耐药菌株将用于调查潜在的耐药机制。将采用两种策略来找出这些虱子是如何产生耐药性的。首先，将在耐药和药物敏感的鲑鱼虱之间进行育种杂交，并使用最新的DNA测序方法确定与耐药相关的基因序列。其次，将确定耐药和药物敏感鲑鱼虱之间的基因表达差异，这将为特定分子途径参与特定药物耐药性的发展提供新的见解。这些知识将对鉴定能够打破耐药机制的药物非常有用。目前，鲑鱼养殖场现有治疗方案的选择是基于“生物测定”的结果，即对鲑鱼虱进行小规模实验室处理，以确定对治疗的敏感性。然而，鲑鱼虱的生物测定是容易出错的，并且经常不能正确预测后续农场处理的成功。本项目将开发快速测试，以检测对治疗的遗传易感性，预计在确定特定农场情况的最佳治疗选择方面，这种测试将比生物分析更具体和准确。使用现有的最佳治疗方法具有明显的经济和环境效益，并有助于预防耐药性的发展。新的、未开发的药物类别是对未来粮食安全具有高价值的有限自然资源。通过帮助对抗耐药性的发展，该项目将有助于延长现有和新型除虫剂的使用寿命，从而提高鲑鱼集约化养殖的可持续性。

The salmon louse, Lepeophtheirus salmonis, is a crustacean ectoparasite infecting wild and farmed salmonid fishes, which causes significant problems in marine Atlantic salmon aquaculture. Salmon louse infections on farms require control to maintain good fish health and welfare, and to minimise potential impacts of farm-origin parasites on wild fish populations. Currently, salmon louse control on fish farms depends heavily upon the use of drugs, supplemented by farm management measures. A number of non-chemical control strategies are currently under development, and these include the biological control of lice through cleaner fish and attempts to render salmon resistant to lice through vaccination and selective breeding programmes. However, these and other alternative control strategies are presently not sufficiently developed to allow full implementation at an industrial scale.While anti-parasitic drugs offer efficient salmon louse control, relatively few types of delousing agents are available. It is known from other pests and parasites that the repeated use of the same or similar chemicals increases the risk that parasites may develop drug resistances, driven by the mechanisms of natural selection discovered by Charles Darwin. Certain genetic features, called "resistance alleles", are usually rare in organisms as they provide no benefits to their carriers under normal conditions. Parasites possessing resistance alleles, however, are able to survive drug treatments better than those without, hence if the same drug is over-used, resistance alleles can increase their frequency in treated parasite populations as susceptible individuals are killed by treatment. This may ultimately result in drug resistance of the entire population.Very little is known about the mechanisms responsible for drug resistance in salmon lice. In the proposed project, drug-resistant strains of salmon lice generated in the laboratory will be used to investigate potential resistance mechanisms. Two strategies will be followed to find out how these lice have managed to become drug resistant. Firstly, breeding crosses will be made between resistant and drug-susceptible salmon lice, and genetic sequences that are associated with resistance will be identified using the latest DNA sequencing methodologies. Secondly, differences in gene expression between resistant and drug-susceptible salmon lice will be determined, which will provide new insights into the involvement of particular molecular pathways in development of resistance to particular drugs. Such knowledge will be extremely useful in the identification of drugs which can break the resistance mechanism.Choices among available treatment options on salmon farms are currently based on the results of "bioassays", which are small-scale laboratory treatments of salmon lice used to establish susceptibility to treatment. However, salmon louse bioassays are error-prone and regularly fail to correctly predict the success of subsequent farm treatments. The present project will develop fast tests to detect genetic susceptibility to treatment that are expected to be more specific and accurate than bioassays in identifying the best treatment choice for a given farm situation. Using the best available treatment has obvious economic and environmental benefits, and can help to prevent resistance development. New, unexploited drug classes represent a limited natural resource of high value for future food security. By helping to combat the development of drug resistance, this project will help to extend the life of current and novel delousing agents, and thus improve the sustainability of intensive salmon farming.