Identification of a single major genetic locus controlling the resistance to lymphocystis disease in Japanese flounder (Paralichthys olivaceus)

Japanese flounder (Paralichthys olivaceus) is an economically important fish as a food, and has been widely cultured in Asian countries such as Japan, Korea and China. We chose to study lymphocystis disease (LD) because it has become widely spread in these countries and seriously damaged fish farms. LD is an Iridovirus-induced infection. Japanese flounder affected with LD develop characteristically hypertrophied cells, called lymphocystis cells, on skin, fins and/or mouth. Fish with lymphocystis cells lose commercial value because they are ugly. Lymphocystis cells on the mouth prevent proper feeding and in the worst case can lead to starvation. The disease is thus costly to farmers. There are presently no treatment for LD nor commercially available vaccines. In this study, we tried to identify the LD-resistance locus (LD-R) by linkage analysis because it may facilitate the effort for the establishment of LD-resistance strains.We used 50 microsatellite markers to search for a locus associated with resistance to LD in Japanese flounder. Linkage tests were conducted in a backcross progeny (n = 136) produced by crossing a susceptible male with a (susceptible × resistant) hybrid female.One major locus for LD-R was detected on linkage group (LG) 15 of the Japanese flounder genetic linkage map. On LG15, a microsatellite marker (Poli.9-8TUF) explained 50% of the total phenotypic variation in the 136 individuals screened. Therefore, it can be considered as a candidate locus for marker-assisted breeding in order to enhance selective efficiency for LD resistance.