Quantitative trait loci for body growth and sex determination in the hermaphrodite teleost fish Sparus aurata L. D. Loukovitis* ,†,1 , E. Sarropoulou ‡ , C. Batargias § , A. P. Apostolidis † , G. Kotoulas ‡ , C.S. Tsigenopoulos ‡ and D. Chatziplis* *Animal Breeding and Genetics, Department of Animal Production, School of Agricultural Technology, Alexander Technological Educational Institute of Thessaloniki, Sindos, 57400, Greece; † Laboratory of Ichthyology and Fisheries, Department of Animal Production, Faculty of Agriculture, Aristotle University of Thessaloniki, Thessaloniki, 54124, Greece; ‡ Institute of Marine Biology and Genetics, Hellenic Centre for Marine Research, Heraklion, 71003, Crete, Greece; § Laboratory of Applied Genetics and Fish Breeding, Department of Aquaculture and Fisheries Management, School of Agricultural Technology, Technological Educational Institute of Messolonghi, Messolonghi, 30200, Greece Summary Gilthead sea bream (Sparus aurata L.) is an important marine fish in Mediterranean aqua- culture. Sex determination by age and/or body weight is a critical life-history trait, the genetic basis for which is largely unknown in this sequential hermaphrodite species. Herein, we performed a partial genome scan to map quantitative trait loci (QTL) affecting body weight and sex using 74 informative microsatellite markers from 10 paternal half- sib families to construct nine linkage groups (LG). In total, four growth-related QTL (two chromosome-wide and two genome-wide) and six QTL related to sex determination (three pairs in three different LGs) were detected (two chromosome-wide and one genome-wide). The proportion of phenotypic variation explained by the body-weight QTL ranged from 9.3% to 17.2%, showing their potential for use in marker-assisted selection. The results obtained offer solid ground to investigate the structure and function of the genomic regions involved in the mechanisms of sex reversal. Keywords aquaculture, body weight, quantitative trait loci, sex reversal, Sparus aurata Introduction Quantitative genetic variation characterizes many traits of economic importance in livestock. Variation in such com- plex traits is often influenced by a number of different quantitative trait loci (QTL), as well as environmental fac- tors. QTL identification in commercially important species would enhance the application of marker-assisted breeding for the genetic improvement of production traits. Simula- tion studies have shown that the utilization of marker information might be helpful to time- and cost-efficient breeding programs by increasing the accuracy of selection and decreasing the generation interval compared to selec- tion based only upon phenotype (Smith & Simpson 1986; Lande & Thomson 1990). QTL-mapping studies have suc- cessfully led to the identification of many genomic regions associated with QTL in most domestic animal species (Vel- mala et al. 1999; Diez-Tascon et al. 2001) and more recently in aquaculture species such as Atlantic salmon (Salmo salar), rainbow trout (Oncorhynchus mykiss), and tilapia Oreochromis spp. (for review, see Korol et al. 2007). A molecular toolbox recently has become available for the mass-spawning gilthead sea bream (Sparus aurata), one of the most important species for Mediterranean aqua- culture. This includes first-generation linkage group (LG) maps based on 204 microsatellite markers (Franch et al. 2006) and 324 molecular markers (Tsigenopoulos C. S., Chatziplis D., Lagnel J., Louro B., Vogiatzi E., Franch R., Bargelloni L., Patarnello T., Sarropoulou E., Power D. M., Canario A., Magoulas A. and Kotoulas G, in prep.); the Address for correspondence D. Chatziplis,Animal Breeding and Genetics, Department of Animal Production, School of Agricultural Technology, Alexander Technological Educational Institute of Thessaloniki, Sindos 57400, Greece. E-mail: chatz@ap.teithe.gr 1 Present address: Laboratory of Applied Genetics and Fish Breeding, Department of Aquaculture and Fisheries Management, School of Agricultural Technology, Technological Educational Institute of Messolonghi, Messolonghi 30200, Greece. Accepted for publication 17 January 2012 doi: 10.1111/j.1365-2052.2012.02346.x 753 © 2012 The Authors, Animal Genetics © 2012 Stichting International Foundation for Animal Genetics, 43, 753–759