Major histocompatibility polymorphism associated with resistance towards amoebic gill disease in Atlantic salmon (Salmo salar L.) James W. Wynne a,b, * , Mathew T. Cook a , Barbara F. Nowak b , Nicholas G. Elliott a a CSIRO Marine and Atmospheric Research, CSIRO Food Futures Flagship, Hobart 7001, Tasmania, Australia b School of Aquaculture, Aquafin CRC, University of Tasmania, Launceston 7250, Tasmania, Australia Received 12 June 2006; revised 30 August 2006; accepted 30 August 2006 Available online 15 September 2006 Abstract The association between major histocompatibility (MH) polymorphism and the severity of infection by amoebic gill disease (AGD) was investigated across 30 full sibling families of Atlantic salmon. Individuals were challenged with AGD for 19 days and then their severity of infection scored by histopathological examination of the gills. Fish were then genotyped for the MH class I(Sasa-UBA) and MH class II alpha (Sasa-DAA) genes using polymorphic repeats embedded within the 3 0 untranslated regions of the Sasa-UBA and Sasa-DAA genes. High variation in the severity of infection was observed across the sample material, ranging from 0% to 85% gill filaments infected. In total, seven Sasa-DAA-3UTR and ten Sasa-UBA-3UTR marker alleles were identified across the 30 families. A significant association between the marker allele Sasa-DAA-3UTR 239 and a reduction in AGD severity was detected. There was also a significant association found between AGD severity and the presence of two Sasa-DAA-3UTR genotypes. While the associations between MH allele/genotypes and AGD severity reported herein may be statistically significant, the small sample sizes observed for some alleles and genotypes means these associations should be considered as suggestive and future research is required to verify their biological significance. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Salmo salar; Major histocompatibility complex; Amoebic gill disease; Disease resistance 1. Introduction Amoebic gill disease (AGD) is the most significant disease affecting Atlantic salmon in Tasmania, Australia [1]. AGD is believed to be caused by the protozoans Neoparamoeba spp. which, acting as the primary pathogen, infect the gills of marine cultured salmonids [2,3]. Clinical signs of AGD include lethargy, respiratory distress, rapid opercular movement, and ultimately, if not treated, death [4]. Histologically, AGD is initially characterised by attachment of amoeba to healthy gill epithelial tissue followed by the progression of lamellar fusion, epithelial desquamation and oedema, epithelial hyperplasia and interlamellar vesicle formation [5,6]. An infiltration of leucocytes into the central venous sinus is also observed adjacent to infected regions and often increases with disease progression [5]. In some * Corresponding author. CSIRO Marine and Atmospheric Research, CSIRO Food Futures Flagship, Hobart 7001, Tasmania, Australia. Tel.: þ61 3 6232 5222; fax: þ61 3 6232 5000. E-mail address: james.wynne@csiro.au (J.W. Wynne). 1050-4648/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.fsi.2006.08.019 www.elsevier.com/locate/fsi Fish & Shellfish Immunology 22 (2007) 707e717