Heritability estimates for male proportion in hybrids between Nile tilapia females (Oreochromis niloticus) and blue tilapia males (Oreochromis aureus) Carlos A. Lozano a,b, ⁎, Bjarne Gjerde b,c , Jørgen Ødegård b,c , Morten Rye a , Tran Dinh Luan d a Akvaforsk Genetics Center AS, N-6600 Sunndalsøra, Norway b Department of Animal and Aquacultural Sciences, Norwegian University of Life Sciences (UMB), P.O. Box 5003, 1432 Ås, Norway c Nofima, P.O. Box 5010, 1432 Ås, Norway d Research Institute for Aquaculture No.1 (RIA1), Dinh Bang, Tu Son, Bac Ninh, Viet Nam abstract article info Article history: Received 29 May 2013 Received in revised form 24 February 2014 Accepted 26 March 2014 Available online 4 April 2014 Keywords: Hybrids O. niloticus O. aureus Heritability Male proportion Sex ratio Estimates of the genetic variation in male proportion (MP) were obtained from a total of 83 hybrid families, pro- duced by crossing 82 Nile tilapia females (Oreochromis niloticus) with 35 blue tilapia males (Oreochromis aureus), which were stocked in 132 hapas in a common pond. The parents of each species originated from three different countries (Israel, Taiwan, China), but only seven of the nine possible sire-dam origin combinations (crosses) were produced. After on average 53 days in the hapas (April–July) a random sample of approximately 94 fish per hapa was slaughtered and sexed by examining gonad tissues under the microscope. Overall MP was 77%, and thus below the expected value of 100% assuming a major sex determination system (XX/XY in Nile tilapia and WZ/ ZZ in blue tilapia). The effect of cross on MP was highly significant, but not very reliable as some of the crosses were represented with a limited number of families. The variation in MP among the families was substantial (from 0% to 100%) and within-cross heritability estimates for MP on the observed scale were 0.38 ± 0.07 and 0.42 ± 0.09 when as- suming either equal or different additive genetic sire-dam variances for the parental species, respectively; and, as expected, higher on the underlying liability scale (0.79 ± 0.11 and 0.82 ± 0.13). The effect common to full-sibs other than additive genetics was significant, but explained a low proportion of the total variance (0.04 ± 0.01 and 0.08 ± 0.02 on the observed and liability scales, respectively). The magnitude of the estimated heritabilities sug- gests that MP in hybrids is partly under additive (polygenic) genetic control. However, these estimates may be biased since parental strains may not be pure and since the unknown genetic sex of all parents (determined by the major genetic XX/XY and WZ/ZZ sex determination system) may not necessarily match the phenotypic sex. Possibilities for selection for increased MP in hybrids to commercially required levels (N 95% males) are discussed. © 2014 Elsevier B.V. All rights reserved. 1. Introduction “Tilapia” is a common name given to cichlid warm-water fishes be- longing to the genera Oreochromis, Sarotherodon and Tilapia. Tilapias are the second most important aquaculture fish species, reaching a glob- al production of approximately four million metric tons in 2011 (FAO, 2013). Sex dimorphism favoring males and unwanted early reproduc- tion are the two factors that have made all-male tilapia farming the in- dustry standard. All-male culture controls unwanted reproduction and increases the amount of fish of marketable size obtained per crop. The most common method currently used by the industry to obtain all- male fry is oral administration of hormones (Phelps and Popma, 2000); however, this method is banned in some countries (e.g. EU, China, India and Japan) and Best Aquaculture Management Practices (BAP) encourages the use of other methods (GAA, 2008). Mating of two different species which yield a hybrid with high male proportion (MP) is an environmentally friendly method used for the production of all-male tilapia fry in some countries. There are however three main constraints for the use of species hybrids: 1) Limited availability of pure tilapia species since the introgression of genes from other spe- cies contributes to variation in MP (Macaranas et al., 1986; Marengoni et al., 1998); 2) maintaining pure genetic species is challenging since hybrids can contaminate the pure species (Beardmore et al., 2001; Lovshin, 1982); and 3) large variation in MP among crosses of the same inter-specific hybrid combination (Garcia Pinto, 1982; Mair et al., 1991b; Marengoni et al., 1998; Pruginin et al., 1975) and also Aquaculture 430 (2014) 66–73 ⁎ Corresponding author at: Akvaforsk Genetics Center AS, N-6600 Sunndalsøra, Norway. Tel.: +47 95816815; fax: +47 64949502. E-mail address: carlos.lozano@afgc.no (C.A. Lozano). http://dx.doi.org/10.1016/j.aquaculture.2014.03.036 0044-8486/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Aquaculture journal homepage: www.elsevier.com/locate/aqua-online