Heritability estimates for growth-related traits in the Pacific oyster (Crassostrea gigas) using a molecular pedigree Ning Kong, Qi Li, Hong Yu & Ling-Feng Kong Key Laboratory of Mariculture Ministry of Education, Ocean University of China, Qingdao, China Correspondence: Q Li, Key Laboratory of Mariculture Ministry of Education, Ocean University of China, Qingdao 266003, China. E-mail: qili66@ouc.edu.cn Abstract Pacific oyster is one of the leading species in world aquaculture, but heritability estimation applying mixed-family approach has not been actively pur- sued. In this study, heritability for growth-related traits in the Pacific oyster was first estimated by creating a single cohort of 45 families in a full- factorial mating design consisting of nine sires and five dams. A total of 270 offspring were analysed and parentage assignment inferred by six micro- satellite markers achieved 100% success. All par- ents contributed to the spawn and a total of 42 full-sib families were represented. Using an animal model, heritability estimates at 12 months of age were 0.49  0.25 for shell height, 0.36  0.19 for shell length, 0.45  0.23 for shell width and 0.35  0.17 for wet weight. Genetic correlation between shell height and wet weight was quite high (0.79  0.25), suggesting that direct selec- tion of shell height, which is an easily measurable trait, also improves wet weight. The results obtained in this study indicate that growth-related traits could be improved by exploiting additive genetic effects through selective breeding. Keywords: Pacific oyster, growth-related traits, microsatellites, heritability, genetic correlations, parentage assignment Introduction The Pacific oyster Crassostrea gigas is now the most widely farmed oyster in the world, whose world- wide production reached 663 thousand metric tonnes in 2010 (FAO 2012). In China, aquaculture of C. gigas has become widespread over the past 30 years, and C. gigas has rapidly become the dom- inant farmed oyster (Li, Yu & Yu 2006). For centu- ries, selective breeding has been an important contributor to enormous advances in genetic improvement and productivity of farmed animals (Dekkers 2002). After one and two generations of mass selection, an average response to selection of 4% and 18% in live weight has been obtained in Saccostrea commercialis respectively (Nell, Sheridan & Smith 1996; Nell, Smith & Sheridan 1999). Langdon, Evans, Jacobson and Blouin (2003) found that yield of C. gigas had been improved 9.5% after one generation of selection, compared with that of nonselected control. In China, the grow-out sys- tems of C. gigas usually use lantern nets hung on long lines which allow easy separation of stocks or families and facilitate selective breeding pro- grammes. Thus, the Pacific oyster industry is acces- sible to benefit from a genetic improvement programme. Given the growing consumer demand for C. gigas, increasing growth rate through selec- tive breeding has become a top priority for growers. Heritability, which measures the relative magni- tude of additive genetic variation, predicts the suc- cess of improving traits through selective breeding (Falconer & Mackay 1996). Reliable estimates of heritability can provide valuable information for making reasonable breeding plans and estimating breeding values of candidate traits (Wang, Li, Xiang, Wang, Liu, Pang, Duan and Xu 2006). A number of studies have been performed to estimate herita- bility of growth rate in C. gigas. Lannan (1972) used full-sib analysis to estimate broad-sense heritabilities for total weight (H 2 = 0.31) and meat weight (H 2 = 0.37). Hedgecock, Cooper and © 2013 John Wiley & Sons Ltd 499 Aquaculture Research, 2015, 46, 499–508 doi: 10.1111/are.12205