Hybrid salmonids: ploidy effect on skeletal meristic characteristics and sea lice infection susceptibility By M. Fleming 1 , T. Hansen 2 , O. F. Skulstad 2 , K. A. Glover 3 , C. Morton 3 , L. A. Vøllestad 1 and P. G. Fjelldal 2 1 Center for Ecological and Evolutionary Synthesis, Department of Biology, University of Oslo, Oslo, Norway; 2 Institute of Marine Research (IMR), Matre Research Station, Matredal, Norway; 3 Institute of Marine Research (IMR), Nordnes, Bergen, Norway Summary This study examined how triploidization and species hybrid- ization affects skeletal related meristic characteristics in sal- monids with the goal of understanding the maternal and paternal contribution. The study also investigated the hybrid sea louse infection rate in hybrid salmonids. In order to do so, a number of vertebrae, scales along the lateral line, and dorsal fin rays were measured in diploid and triploid Atlantic salmon, Arctic char, and Atlantic salmon (female) X Arctic char (male) hybrids. The success of triploidization was 100%, and some spontaneous triploid individuals (~15%) were found among the hybrids. In general, hybrids displayed intermediate counts for all three characteristics with salmon displaying the highest number of dorsal fin rays, and char displaying the highest number of scales and vertebrae. The effect of triploidization was always strongest among the hybrids. However, the direction of the effect differed between structures in the hybrid; triploidization increased the number of vertebrae and reduced the number of fin rays towards the level observed in diploid char, and reduced the number of scales towards the level observed in diploid salmon. We con- ducted a second experiment whereby diploid Atlantic salmon and Atlantic salmon (female) X Arctic char (male) hybrids were exposed to an experimental challenge with the parasitic salmon louse (Lepeophtheirus salmonis). There was no differ- ence in the sea-lice infection level between Atlantic salmon and the hybrids. This study shows that the present model with diploid and triploid Atlantic salmon (female) X Arctic char (male) hybrids may be a useful tool for the study of which traits are maternally and paternally inherited in salmo- nids. The understanding of morphological development with regard to meristic traits could be advantageous for both hybrid and triploid fish domestication. Introduction The salmon aquaculture industry in Norway is one of the largest producers of Atlantic salmon (Salmo salar) in the world. However, in a 9-year span from 2001 to 2009, a reported 3.93 million Atlantic salmon escaped from Norwe- gian fish farms (Jensen et al., 2010). Not only do these escap- ees represent a massive loss in production, they pose a serious threat to native populations. For example, farmed escapees may enter rivers and interbreed with wild conspecif- ics (Skaala et al., 2006; Glover et al., 2013). Thus, escapees represent one of the largest environmental challenges for the salmon aquaculture industry. The production of sterile fish offers the opportunity to stop direct genetic interactions and is currently being considered by the industry as a way of mitigating genetic interactions. At the moment, there are only two acceptable ways of pro- ducing sterile fish: triploidization (Tiwary et al., 2004) and hybridization (Bartley et al., 2001). The method of triploidi- zation has become a refined and reliable tool to ensure steril- ity in large batches of fish. The method involves the manipulation of the cell shortly after fertilization to retain the second polar body, a procedure which renders the embryo triploid (Benfey and Sutterlin, 1984). The original method involved heat shocks, but over time, the procedure has evolved; the present method consists of applying hydro- static pressure at specific time intervals following fertilization (Maxime, 2008; Fjelldal and Hansen, 2010). The hydrostatic pressure interferes with the spindle fibers, which allow for the egg to retain the second polar body during meiotic divi- sion (Tiwary et al., 2004). The result is an egg containing two sets of maternal chromosomes and one paternal. The second method of producing sterile fish is by hybridiza- tion with closely-related species. Hybridization within salmonids often yields viable organisms that are unable to reproduce due to problems with gonad development and chro- mosome pairing (Bartley et al., 2001). Interspecific hybrids offer the potential to combine the desirable characteristics of two species (Bartley et al., 2001). Thus, such hybrids can cre- ate potentially new possibilities for aquaculture. For example, a cross between white bass (Morone chrysops) and the striped bass (M. saxatili) produced offspring that, under commercially cultured conditions, grow faster and react better to tanks and cages than the parental species (Hallerman, 1994). Further- more, within salmonids, increased disease resistance has been shown with crosses between rainbow trout (Oncorhynchus mykiss) and char (Salvelinus sp.; Dorson et al., 1991). In addition to the problem of farmed escapees and genetic interactions with wild conspecifics, the parasitic salmon louse (Lepeophtheirus salmonis) represents another environmental problem faced by the salmon farming industry. This external U.S. Copyright Clearance Centre Code Statement: 0175-8659/2014/3004–746$15.00/0 J. Appl. Ichthyol. 30 (2014), 746–752 © 2014 Blackwell Verlag GmbH ISSN 0175–8659 Received: November 1, 2013 Accepted: May 5, 2014 doi: 10.1111/jai.12530 Applied Ichthyology Journal of