Changes in tissue cellularity are associated with growth enhancement in genetically modified arctic char (Salvelinus alpinus L.) carrying recombinant growth hormone gene

Transient up- and down-regulation of expression of myosin light chain 2 and myostatin mRNA mark the changes from stratified hyperplasia to muscle fiber hypertrophy in larvae of gilthead sea bream (Sparus aurata L.)

Hyperplasia and hypertrophy are the two mechanisms by which muscle develops and grows. We study these two mechanisms, during the early development of white muscle in Sparus aurata, by means of histology and the expression of structural and regulatory genes. A clear stage of stratified hyperplasia was identified early in the development of gilthead sea bream but ceased by 35 dph when hypertrophy took over. Mosaic recruitment of new white fibers began as soon as 60 dph. The genes mlc2a and mlc2b were expressed at various levels during the main phases of hyperplasia and hypertrophy. The genes myog and mlc2a were significantly up-regulated during the intensive stratified formation of new fibers and their expression was significantly correlated. Expression of mstn1 and igf1 increased at 35 dph, appeared to regulate the hyperplasia-to-hypertrophy transition, and may have stimulated the expression of mlc2a, mlc2b and col1a1 at the onset of mosaic hyperplasia. The up-regulation of mstn1 at transitional phases in muscle development indicates a dual regulatory role of myostatin in fish larval muscle growth.

Muscle growth in teleost fish is regulated by factors utilizing the activin II B receptor

The activin type IIB receptor (Acvr2b) is the cell surface receptor for multiple transforming growth factor β (TGF-β) superfamily ligands, several of which regulate muscle growth in mammals. To investigate the role of the Acvr2b signaling pathway in the growth and development of skeletal muscle in teleost fish, transgenic rainbow trout (RBT; Oncorhynchus mykiss, Walbaum) expressing a truncated form of the acvr2b-2a (acvr2bΔ) in muscle tissue were produced. High levels of acvr2bΔ expression were detected in the majority of P1 transgenic fish. Transgenic P1 trout developed enhanced, localized musculature in both the epaxial and hypaxial regions (dubbed "six pack"). The F1 transgenic offspring did not exhibit localized muscle growth, but rather developed a uniform body morphology with greater girth, condition factor, and increased muscle fiber hypertrophy. There was a high degree of variation in the weight of both P1 and F1 transgenic fish with several fish of each generation exhibiting enhanced growth compared to other transgenic and control siblings. The "six pack" phenotype observed in P1 transgenic RBT overexpressing a acvr2bΔ and the presence of F1 individuals with altered muscle morphology provides compelling evidence for the importance of TGF-β signaling molecules in regulating muscle growth in teleost fish.

Genetically modified growth affects allometry of eye and brain in salmonids

Effects of growth acceleration on eye development have been examined in genetically modified salmonids. Growth hormone (GH) transgenic coho salmon ( Oncorhynchus kisutch (Walbaum, 1792)) show dramatically elevated overall body growth and an absolute increase in eye size, but relative eye growth is shifted from negatively allometric to more isometric. Thus, transgenic fish possess significantly smaller eyes relative to nontransgenic fish of the same size. Ration-restricted limitation of growth in transgenic salmon to that of wild type restores relative eye growth rate, suggesting that effects on eyes are an indirect consequence of modification of growth rate rather than a direct effect of GH overexpression. Heart, spleen, and liver did not show changes in proportion among groups, whereas total brain size showed the same response as eye. Relative eye and brain size were also reduced in a fast-growing domesticated strain of rainbow trout ( Oncorhynchus mykiss (Walbaum, 1792)), suggesting modifications of allometry are a more general effect of growth acceleration. GH mRNA levels from the transgene were elevated in eyes, whereas IGF-I mRNA was not, suggesting this organ may be regulated in a different fashion than other organs. Neural tissues with critical structural requirements for optimal function may be subject to less modification of growth rate than are other organ systems.

Early thermal history significantly affects the seasonal hyperplastic process occurring in the myotomal white muscle of Dicentrarchus labrax juveniles

The effect of early (embryonic and larval) thermal history on subsequent (juvenile) white muscle hyperplasia was studied in a teleost fish, the European sea bass (Dicentrarchus labrax L.). D. labrax, incubated and reared at constant temperatures of 13 degrees C, 15 degrees C or 20 degrees C from the embryonic stage of half epiboly up to 18-19 mm in total length, were transferred to ambient seawater temperature and reared for the subsequent 14 months on commercial feed. The somatic growth of juveniles was linked to annual variations of ambient seawater temperature and inversely related to early rearing temperature, so that, after 14 months, the juveniles originally reared at low temperatures had compensated for the growth retardation experienced during early life. The white muscle growth process of juveniles was quantified after two periods of growth opportunity at ambient seawater temperature (100 and 400 days post-transfer) as well as, in order to follow total-length-dependent effects of early temperature and to discriminate total-length-independent effects of early temperature, on juveniles from the three batches sampled at six successive equivalent total lengths (31-33, 84-88, 141-145, 166-172, 196-206 and 211-220 mm). Our data demonstrate the existence of a seasonal recruitment of new white muscle fibres when seawater temperature increases and of a shrinkage of the largest white muscle fibres during the winter months. The seasonal recruitment of new white muscle fibres occurring in juveniles is linked to their early rearing temperature. Juveniles originating from low temperatures have a higher and longer capacity to recruit new white muscle fibres when seawater temperature increases, supporting their better somatic growth. This finding is discussed in relation to the early (embryonic and larval) myogenic processes of the three populations and is related to their sex ratio.

Interface of biotechnology and ecology for environmental risk assessments of transgenic fish

Genetically engineered fish with enhanced phenotypic traits have yet to be implemented into commercial applications. This is partly because of the difficulties in reliably predicting the ecological risk of transgenic fish should they escape into the wild. The ecological consequences of the phenotypic differences between transgenic and wild-type fish, as determined in the laboratory, can be uncertain because of genotype-by-environment effects (GXE). Additionally, we are limited in our ability to extrapolate simple phenotypes to the complex ecological interactions that occur in nature. Genetic background can also shape the phenotypic effects of transgenes, which, over time and among different wild populations, can make risk assessments a continuously evolving target. These uncertainties suggest that assessments of transgenic fish in contained facilities need to be conducted under as wide a range of conditions as possible, and that efficacious physical and biological containment strategies remain as crucial approaches to ensure the safe application of transgenic fish technology.