Rib and vertebral deformities in rainbow trout (Oncorhynchus mykiss) explained by a dominant-mutation mechanism Hannes Gislason a, ⁎, Helena Karstensen a , Debes Christiansen b , Kirsti Hjelde c , Synnøve Helland c , Grete Bæverfjord c a Aquaculture Research Station of the Faroes, Við Áir, FO-430 Hvalvík, Faroe Islands b Faroese Food and Veterinary Agency, V.U. Hammershaimbsg. 11, FO-100 Tórshavn, Faroe Islands c Nofima Marin AS, 6600 Sunndalsøre, Norway abstract article info Article history: Received 28 December 2009 Received in revised form 14 September 2010 Accepted 15 September 2010 Keywords: Rainbow trout Oncorhynchus mykiss Deformities Intermuscular bones Dominant mutations We suggest and investigate a hypothesis of a dominant-mutation mechanism as a possible cause for rib and vertebral deformities found in farmed rainbow trout. We report on an X-ray characterization and a genetic study of a sample of 45 individuals from a rainbow trout broodstock population in the Faroe Islands, which was found to be affected by externally visible rib deformities. 41 fish were from 36 progeny families of 23 sires and 33 dams and 4 fish were of unknown family origin. These fish were from a family-based selection program, year-class 2005, produced from 144 females and 72 males, and the total number of families was 252. In the X-ray characterization both rib deformities and vertebral deformities were found. Only 8 fish were found to be unaffected: 15 fish had both rib and vertebral deformities, 20 fish had only rib deformities and 2 fish had only vertebral deformities. The abnormal ribs were observed as anomalously short and thin bones located inside the abdominal wall. Some had their ends pointing towards the skin, and in the most serious cases, they were perforating the skin. The numbers of rib and vertebral deformities in each individual were assigned rib and vertebral deformity scores. We estimate the proportions in year-class 2005 of deformed fish, of fish with rib deformities and of fish with vertebral deformities, to be 0.82 ± 0.11, 0.78 ± 0.12 and 0.38 ± 0.14, respectively. The 45 individuals were genotyped at nine microsatellite marker loci to investigate potential inbreeding problems. The actual and effective numbers of alleles, n a = 5.8 and n e = 4.2, the heterozygosity, H o = 0.718, and the inbreeding coefficient, F is = 0.034, are similar to the corresponding quantities for Danish strains. The heterozygosity and the inbreeding coefficient indicate a low inbreeding level. A phylogenetic tree and pairwise-relatedness estimates were also constructed from the genotype information and compared with the information from the family-based selection program for rainbow trout. We investigate the dominant-mutation hypothesis and show that genotype counts inferred from the deformity scores are consistent with Hardy–Weinberg equilibrium. From this model, we predict phenotypic segregation ratios of the deformity traits, which are in close agreement with the experimental observations. Finally, we discuss both environmental and genetic causes and conclude in favor of the dominant-mutation mechanism as the most likely cause of the observed deformities. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Fish in their natural wild environment or produced by aquaculture can be affected by various deformities and sometimes a very high incidence of deformities is found, e.g., an incidence of 81% of spinal deformities was found in a recent study of sea bass (Bardon et al., 2009). Regarding skeletal deformities, the salmonids are known to be affected by gill-cover defects, jaw deformities and most importantly by spinal deformities (Branson and Turnbull, 2008). In general, the causes of these deformities are complex and not fully understood. Known factors affecting the incidence of skeletal deformities in salmonids are: diet composition, toxins, timing of vaccine treatments, specific infections, vitamin and mineral deficien- cies, egg incubation temperature to first feeding, temperature from first feeding as well as genetic factors (Gjerde et al., 2005; Lall and Lewis-McCrea, 2007; Branson and Turnbull, 2008). Deformities in farmed fish can cause both economical losses due to a lower disease resistance, higher mortalities and lower processing yields, as well as ethical concerns regarding fish welfare. Therefore, it is desirable to keep the deformities at the lowest possible levels. Hence, an increased understanding of deformities in farmed fish and how they can be reduced is much desired. Aquaculture 309 (2010) 86–95 ⁎ Corresponding author. Present address: University of the Faroe Islands, J.C. Svabos Gøta 14, FO-110 Tórshavn, Faroe Islands. Tel.: + 298 352576; fax: + 298 352501. E-mail address: hannesg@setur.fo (H. Gislason). 0044-8486/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaculture.2010.09.016 Contents lists available at ScienceDirect Aquaculture journal homepage: www.elsevier.com/locate/aqua-online