Journal of Heredity Advance Access originally published online on January 11, 2006
Journal of Heredity 2006 97(1):89-93; doi:10.1093/jhered/esj010
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Brief Communication |
A Blue Variant in the Rainbow Trout, Oncorhynchus mykiss Walbaum
From INRA, Station d'Hydrobiologie, St-Pée-sur-Nivelle, France (Blanc and Poisson); and INRA, Laboratoire de Génétique des Poissons, 78350 Jouy-en-Josas, France (Quillet)
Address correspondence to Dr. Edwige Quillet at the address above, or e-mail: Edwige.Quillet{at}jouy.inra.fr.
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Abstract |
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A blue variant of the rainbow trout, which appeared in a French fish farm, displayed an iridescent body color that was cobalt blue on the back, lighter on the undersides, and silvery on the belly and which held up to adult stage. This color was supposed to result from a Tyndall effect involving a structural arrangement of melanin pigments because it disappeared when it was associated with a depigmenting gene. This blue variant appeared to be governed by an autosomal recessive gene. Blue fry survival and body weight were about 25% less than those of wild-type sibs, but no major problem was observed in further breeding performances, including reproduction. These features do not correspond with those of the blue variants previously described in the rainbow trout.
Genetic variants for body color are not frequent in salmonids. In the rainbow trout (Oncorhynchus mykiss Walbaum), which has been much studied, recorded variants belong to two main groups: (1) the depigmented (yellow) phenotypes, such as albino (Bridges and von Limbach 1972) and golden (Wright 1972), and (2) the blue phenotypes, such as iridescent metallic blue (Kincaid 1975) and cobalt blue (Yamazaki 1974). Yellow phenotypes are caused by defects in the synthesis of melanin, the pathway of which is complex and not yet completely understood (Boonanuntanasarn et al. 2004; Nakamura et al. 2001). In blue variants, it is generally accepted that the iridescent color of the body is not caused by a chemical pigment but by a Tyndall scattering of incident light, resulting from the structural arrangement of integumental pigments (Yamaguchi and Miki 1981).
Color variants were often found to have detrimental pleiotropic effects. For instance, yellow variants are inferior in juvenile survival and growth (Blanc 2002; Dobosz et al. 2000), and the cobalt blue is associated with obesity (Yada et al. 2002), disorders in liver and kidneys (Oguri 1976, 1992, 1993), and complete sterility in both sexes because of the lack of a large part of the pituitary gland (Kaneko et al. 1993; Oguri 1974). Only the iridescent metallic blue variant (Kincaid 1975) showed better growth than the wild-type one and could therefore be recommended for farming. Yet, because of their distinctive phenotypes, these variants are useful animal material in various experimental approaches, such as experimental tagging (e.g., Blanc and Poisson 2003), monitoring of efficiency in chromosome manipulations (Chourrout 1980; Thorgaard et al. 1995), or genetic mapping (Nakamura et al. 2001).
The present report describes a new blue variant of the rainbow trout which appeared in a French fish farm. The objectives of this study were (1) to determine its mode of inheritance, (2) to measure eventual pleiotropic effects on survival and growth, and (3) to contribute to the understanding of the causation (chemical versus structural) of the blue color.
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Materials and Methods |
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Gametes were obtained from blue-variant (B: two dams and one sire) and wild-type (W: two dams and a few sires) stocks held in the Moulin-des-Prés fish farm (Bolazec, France-29216), where this genetic variant had appeared. Gametes were pooled within each type (B-eggs, W-eggs, and W-sperm). Three fertilizations were carried out, that is, B-eggs x B-sperm (BB), B-eggs x W-sperm (BW), and W-eggs x B-sperm (WB). Fertilized eggs were incubated in the hatchery of the Brittany Oceanological Center (COB) in Brest (F-29200) and then eyed eggs were transferred to the INRA Hydrobiology Station in St-Pée-sur-Nivelle for hatching and rearing in the experimental facilities and attached fish farms.
At adult stage, seven BW and six WB females were individually backcrossed with BB males. Incubation of eggs, hatching, and rearing were carried out in INRA facilities provided with water from the Nivelle river. At 2.5 months posthatching, the progeny from each of the 13 crosses was sorted according to body color, counted, and bulk weighed for estimation of the mean individual body weight. Then, six of these progenies (with good condition and similar body weights) were used for a 4-week test on blue and wild-type fry (randomly sampled in equal numbers, averaging 150, in common rearing). In each sib group, initial and final numbers and bulk weights of each phenotype provided estimates of survival and specific growth. Statistical methods (Snedecor and Cochran 1967) were as follows: phenotypic counts were tested for goodness-of-fit to the theoretical "1:1" ratio (autosomal inheritance) using the chi-square test; blue and wild-type performances in survival and growth were compared by the Student's t test for paired data.
Experimenting on the blue color causation was carried out through breeding fish carrying simultaneously the determinants of the blue and the yellow (depigmented) colors. In that purpose, eight BB sires were crossed with four dams (INRA strain) that were light-colored (so-called palomino phenotype, P) because they were heterozygous for the depigmenting golden gene. This gene, which had been introduced in INRA stocks for experimental purposes (Blanc 2002; Blanc and Poisson 2003; Chourrout 1980), was probably the same as that originally described in a West Virginia strain (Clark 1970; Craig and Crossman 1977; Wright 1972), but this assumption was never validated by a formal genetic test. In the progeny from BB sires and P dams, the normally pigmented fry was discarded, so as to keep only the fish (palomino colored) expected to carry both the blue and palomino genetic factors. These fish were raised to adult stage and then three females and eight males were separately backcrossed with BB sires and dams. At 3 months posthatching, the progeny from each cross was sorted according to body color and counted. These data were tested for homogeneity using the chi-square test (Snedecor and Cochran 1967), and the corresponding recombination percentages between blue and palomino traits were evaluated.
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Results |
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Inheritance and Performances of the Blue Phenotype
Blue color appeared on every fish in the BB progeny, while every fish in both heterozygous lots (BW and WB) developed the normal rainbow trout phenotype (wild type), indicating a recessive inheritance. The blue color on the body of the BB variants appeared progressively from 6 to 7 weeks posthatching, becoming quite conspicuous in fingerlings. The back and sides of the fish were blue iridescent, cobalt blue on the back and lighter on the undersides, and the belly was silvery (Figure 1). This bright color was maintained during growth, irrespective of the fish transfer to outdoor tanks or ponds. Only at sexual maturation did this color darken and become less conspicuous. However, it was still distinguishable from the wild-type coloration.
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The numbers of wild-type and blue phenotypes observed in the backcross progeny are presented in Table 1. In every sib group, blue fish were less numerous than wild-type ones, this inferiority being significant (P < .05) in eight cases. The relative number of blue to wild type averaged 76.8%, with a large familial variation (55.1%–94.4%). Blue fry were also smaller than wild-type ones in every sib group, the relative body weight of blue to wild-type averaging 75.4 %, with a familial variation (62.1%–92.1%) significantly correlated with that of the blue fry relative number (Figure 2, r = .73 with 11 df, P < .01). Also, the blue fry showed a characteristic spinning flight reaction when disturbed (because spinning is a symptom of infectious pancreatic necrosis, a virological check was run as a precaution and gave a negative result); this behavior was not observed in their heterozygous sibs.
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The results of survival and growth tests are presented in Table 2. In every replicate, the initial body weight of blue fry was less than that of wild-type fry (random sampling). Survival during the tests was good, without significant difference between blue and wild-type means (95.7% and 95.3%, respectively). Mean specific growth was better in wild-type (5.12%) than in blue fry (4.84%), but this difference was short of statistical significance (t = 2.016, P = .100). In further fish farming (which was not monitored with the same care as the above tests), both blue trout and heterozygotes showed ordinary growth and, at adult stage, production of apparently normal gametes in both sexes, although with large individual differences.
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Investigation on the Blue Color Causation
In the progeny of the blue-golden carriers backcrossed with the blue parents, two different phenotypes (palomino and blue) could be expected under the (unlikely) hypothesis that these traits are governed by two alleles at the same locus. Under the hypothesis that they are governed by two different genes, four phenotypes could be expected in the backcross (Table 3). Three of them (wild type, blue, and palomino) had been previously observed, but the fourth one, combining the determinants of both blue and palomino colors, was unknown so far. Examination of the fry revealed that this fourth phenotype was highly depigmented (albino-like, although with pigmented eyes) and therefore distinguishable from the palomino type, which presented visible melanophores on the head and dorsal area. The observed numbers of fry in the four phenotypes are presented in Table 4. Relatively to the expected "1:1:1:1" ratio (independent autosomal inheritance), there was a deficiency of depigmented fry (P and BP: –26.1%), and a deficiency of blue homozygotes (B and BP: –19.2%). The recombination percentage between blue and palomino traits averaged 49.4%, with the standard error being 0.9%, and therefore did not differ significantly from 50%. Tests for homogeneity were significant (P < .05) in four cases, suggesting limited interactions between blue and palomino pleiotropic effects.
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Discussion and Conclusions |
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TopAbstractMaterials and MethodsResultsDiscussion and ConclusionsReferences |
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The present report describes a rainbow trout variant characterized by (1) a blue iridescent body color appearing at 6–7 weeks posthatching, holding up to adult stage in outdoor rearing and (2) a detrimental pleiotropic effect on fry survival and body weight but no major problem in further breeding performances, including the production of apparently normal gametes. So far, two blue variants were reported in the rainbow trout. One was, in the United States, the iridescent metallic blue (Kincaid 1975), which had the distinctive features (1) to occur only between 8 and 20 months of age and in subdued light environment (the blue color fading when the fish were transferred in outdoor tanks), and (2) to have a positive pleiotropic effect on growth rate (+23%). The other variant was, in Japan, the cobalt blue (Yamazaki 1974), which had the distinctive feature of being associated with severe reproductive abnormalities inducing complete sterility in both sexes because of the lack of a large part of the pituitary gland (Kaneko et al. 1993; Oguri 1974). The blue phenotype described in the present report does not correspond with either of the above features, although its body color resembles that of the cobalt blue. Therefore, it must be considered as a new genetic variant.
Our experimental results show that the genetic determinant of this variant is neither a dominant or codominant gene (in this case, blue-colored individuals would have been found among both BW and WB heterozygotes) nor a recessive sex-linked gene (in this case, blue males and wild-type females would have been obtained in BW progeny). Therefore, simple autosomal recessive inheritance is the most likely hypothesis. Although the proportion of blue phenotype in the backcross progeny was less than expected, the correlation observed between the relative number and body weight of blue fry (Figure 2) indicates that this insufficiency was a matter of pleiotropic deficiency, rather than the indication of some unexpected complexity in the genetic determinism of the variant. We found no linkage with the golden gene used in our experiments, but still, the position of the causative allele relative to the loci responsible for the other blue variants is not known.
Also, our results show that the blue color of this new variant is probably of structural rather than chemical origin. Indeed, the new phenotype generated by associating the blue and golden genotypes was depigmented and not greenish (as would have resulted from adding a blue pigment to the palomino color). Thus, we may hypothesize that the blue color resulted from a structural arrangement of melanin inducing a Tyndall effect, so that it was impaired by the lack of melanin when it was associated with a depigmenting gene.
As in most cases previously reported, the new color variant we describe here is associated with negative pleiotropic effects. A problem in measuring such effects results from the limited size of the initial sample of parents inducing some degree of inbreeding in the progeny. This was obviously the case in the present study, which was initiated with only three blue and a few wild-type parents. Therefore, pleiotropic effects on blue fry number, body weight, survival and growth were measured with reference to wild-type sibs, that is, at equal inbreeding level. The relative inferiority of blue fry number and weight was found to vary widely among the sib groups considered (about 10%–40%, Figure 2), roughly averaging 25%. In the subsequent survival and growth test, blue and wild-type sibs did not differ significantly, and, in further farming, both types showed ordinary performances in growth and reproduction. This may indicate that the blue inferiority was mainly a matter of embryonic and larval development, the resulting hindrance being maintained, but not made worse, beyond the juvenile stage. However, in fish farming, no valid comparison with other trout stocks could be done because of probable inbreeding differences. Therefore, a doubt remains about possible pleiotropic effects affecting the blue variant at adult stage, particularly in its reproductive performances.
This new variant was studied with the hope that it would be suitable to the production of large-size trout because of its silvery blue color, which was considered as an attractive trait for marketing. However, this advantage did not compensate for the associated reduction of survival and growth performances. Therefore this variant could not be kept on in the INRA fish farms, for practical and financial limitations, despite the fact that it could be used (as the cobalt variant was) as a tool in physiological studies. This raises the more general question of the conservation of rare genetic material lacking economic value but having a potential interest in the scope of future scientific investigations.
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Acknowledgments |
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The authors are indebted to Mr. Quénéhervé for providing them with blue-variant gametes, to the Brittany Oceanological Center in Brest for incubating the eggs, and to the LANAPAT Veterinary Services for virological check of the fry. They also thank J.B. Laxague and the technical teams of Donzacq and Lées-Athas INRA fish farms for their valuable contribution to the rearing of the experimental animals. The photo in Figure 1 is from G. Choubert and was processed by D. Bazin.
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Footnotes |
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Corresponding Editor: Martin Tracey
Received July 2, 2005
Accepted November 28, 2005
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References |
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