QTL affecting morphometric traits and stress response in the gilthead seabream (Sparus aurata)
Introduction
The gilthead seabream (Sparus aurata) (hereafter referred to as “seabream”), is a marine teleost fish of the Sparidae family (order Perciformes). Highly prized for its flavour, and farmed in Southern Europe, seabream has become one of the most important species for modern aquaculture. Recent technical developments in captive breeding have allowed for increases in Mediterranean seabream output with an estimated 137,200 tonnes produced in 2008 valued at 718,866 million USD (FAO 2007–2010). Females are batch spawners, each laying 20,000–80,000 eggs/kg body weight daily for the 3 months October–December, and capable of producing up to 1 million eggs per season (Zohar and Gordin, 1979). Current market outlooks for saleable fish predict oversupply and falling prices, factors that will inevitably lead to industry drives for greater efficiency and improved quality; objectives that can be partly addressed by selective breeding.
Much of the additive genetic variation in seabream has yet to be exploited (Brown, 2004). Because seabream is a protandrous hermaphrodite, management of mating and selective breeding for desirable morphometric traits is severely handicapped. In captivity, sex reversal is conditioned by hormonal and social factors (Happe and Zohar, 1988, Moretti et al., 1999, Reyes-Tomassini, 2009) and as sex ratio determination is complex and not yet fully understood the control of sex change in practice may be challenging (Brown, 2004). Additionally, individual performance is difficult to measure, and offspring–parentage assignment can only be performed retrospectively. The development of controlled mating designs to enable genetic selection is consequently complex, although not impossible. The genetic control of economic traits for use in selective breeding programmes can, however, be explored through the use of genomic tools, and seabream may be considered a model organism for the family Sparidae. Cautadella et al. (1980) first reported 24 pairs of chromosomes for seabream. Genetic linkage maps (Franch et al., 2006, Loukovitis et al., 2011, Massault et al., 2011) largely concur with radiation hybrid maps (Sarropoulou et al., 2007, Senger et al., 2006), with 25–27 LG latterly acknowledged.
In recent years the welfare of farmed fish has become an issue of importance to the industry, not only in terms of public perception, product marketing and acceptance, but also the effective production of both quantity and quality (Ashley, 2007). Deterioration in quality, including flavour, results from pre-slaughter stress in seabream (Bagni et al., 2007). However, the mechanism of stress response in seabream is well described (Arends et al., 1999, Arends et al., 2000, Rotllant et al., 2000, Rotllant et al., 2001) with acute stress resulting in increased plasma cortisol levels. Plasma cortisol level is therefore widely used in studies reporting the reaction of fish to influences that may excite a primary stress response (Barton, 2002, Barton and Iwama, 1991, Pickering, 1992). Prior evidence for genetic control of stress response in seabream has not been reported. Thus, our study aimed to examine the genetic architecture of seabream growth traits and stress response to confinement, and to map major loci affecting these traits.
Section snippets
Experiment
Seabream eggs were obtained from a single-day broodstock spawning of 177 adult fish hatched in October 2005. Larvae were reared for 50 days at the Nireus indoor facilities at Nafpaktos, Greece, in 10 m3 fibreglass tanks and fed on a normal diet of algae, rotifers and brine shrimp (Artemia nauplii). After this period, weighing on average 0.12 g, a random sample of 2200 fish were transferred to a tank of 9 m3 and weaned from their live diet to industrial dry food; feeding regimes were subsequently
Summary statistics
Following removal of four extreme outliers (> 900 nm/ml), plasma cortisol levels in individual fish ranged from 1.22 to 603.36 ng/ml, with a mean of 75.84 (± 85.93). There was no statistically significant drift of cortisol levels over the period of sampling. Heritabilities of all the morphometric and both size traits – body weight (BW) and standard length (SL) – were moderate to high, ranging from 0.24 (tail depth — EndDOAN) to 0.58 (head depth — HD). Heritability for CortLOG was not significantly
Discussion
This study has provided some insight into the genetic control of morphometric traits, body weight and stress response in seabream and initiated the genetic dissection of these traits. Similar results for genetic and phenotypic correlations between body weight and length were found by Navarro et al. (2009). Whilst heritabilities are slightly higher in our study, they are of a similar magnitude and therefore indicate scope for selection in breeding programmes.
We identified 16 LG by analysing the
Conclusion
Our study is the first to detect several QTL affecting morphometric traits of potential economic importance to seabream aquaculture and has provided the foundation to confirm and refine these QTL and look in more detail at the genetic regulation of morphology and stress response in this species.
Acknowledgements
This study has been supported by the European Commission through the Aquafirst project (FP6-STREP-2004-513692). Microsatellites in singletons and contigs were generated through the ‘Marine Genomics Europe’ Network of Excellence (COGE-CT-2004-505403). DJK, CSH and RH acknowledge financial support from the BBSRC through an Institute Strategic Programme Grant. E Couto, B Louro, S Darivianakis, J Fuentes, P Guerreiro, M Kampakli and DM Power helped with on site phenotyping whilst K Oikonomaki
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