Somite formation and expression ofMyoD,myogeninandmyosinin Atlantic halibut (Hippoglossus hippoglossusL.)embryos incubated at different temperatures: transient asymmetric expression ofMyoD

Abstract

Genes encoding the myogenic regulating factors MyoD and myogenin and the structural muscle proteins myosin light chain 2 (MyLC2) and myosin heavy chain (MyHC) were isolated from juvenile Atlantic halibut (Hippoglossus hippoglossus L.). The impact of temperature on their temporal and spatial expression during somitogenesis were examined by incubating halibut embryos at 4, 6 and 8°C, and regularly sampling for whole-mount in situ hybridisation and reverse transcription (RT)-PCR.There were no significant effects of temperature on the onset of somitogenesis or number of somites at hatching. The rate of somite formation increased with increasing temperature, and the expression of MyoD, myogenin and MyHC followed the cranial-to-caudal somite formation. Hence, no significant effect of temperature on the spatial and temporal expression of the genes studied was found in relation to somite stage. MyoD, which has subsequently been shown to encode the MyoD2 isoform, displayed a novel bilaterally asymmetric expression pattern only in white muscle precursor cells during early halibut somitogenesis. The expression of myogenin resembled that previously described for other fish species, and preceded the MyHC expression by approximately five somites. Two MyLC2 cDNA sequences were for the first time described for a flatfish, probably representing embryonic (MyLC2a) and larval/juvenile (MyLC2b) isoforms.Factors regulating muscle determination, differentiation and development have so far mostly been studied in vertebrates with external bilateral symmetry. The findings of the present study suggest that more such investigations of flatfish species could provide valuable information on how muscle-regulating mechanisms work in species with different anatomical, physiological and ecological traits.

Temperature influences the coordinated expression of myogenic regulatory factors during embryonic myogenesis in Atlantic salmon (Salmo salarL.)

“…Thus, whereas smyoD1a and smyoG expression showed no consistent differences with temperature with respect to developmental-stage, the expression of sMRF4 and smyf5 and the slow muscle differentiation marker s-smlc1 were retarded at 2°C compared to 8°C. Our finding that the relative timing of smyoD1a and smyoG expression was independent of temperature parallels observation in Atlantic cod (Hall et al, 2003), Atlantic herring , common carp (Cole et al, 2004) and Atlantic halibut (Galloway et al, 2006), but differs from the result reported in rainbow trout (Xie et al, 2001). As a consequence of the heterochronies in sMRF4 and smyf5 expression, the ratio of the individual myoD family members at each developmental stage was a function of environmental temperature.…”

“…Our working hypothesis is that the role of the teleost myoD1 gene was subfunctionalised in Atlantic salmon according to the model of Force et al (Force et al, 1999), with each paralogue regulated by a sub-set of the cis-acting elements found in the promoter region(s) of the single myoD1 gene (Macqueen and Johnston, 2006). A second myoD gene, myoD2, has been retained in five percomorphic teleosts (Tan and Du, 2002;Galloway et al, 2006;Macqueen and Johnston, 2006;Fernandes et al, 2007). To date myoD2 paralogues have not been identified in salmonids or zebrafish.…”

“…To test this hypothesis, the expression of myoD family members has been investigated by in situ hybridisation in fish embryos reared at different temperatures. The majority of studies have found no difference in the relative timing or intensity of myoD or myoG expression with respect to somite stage in embryos reared at a range of temperatures [Atlantic cod Gadus morhua (Hall et al, 2003); Atlantic herring Clupea harengus ; common carp Cyprinus carpio (Cole et al, 2004) and Atlantic halibut Hippoglossus hippoglossus (Galloway et al, 2006)]. However, in rainbow trout Oncorhynchus mykiss it was reported that myoD and myoG expression was more intense at the mRNA and protein levels and also more advanced with respect to somite stage in embryos incubated at 12°C versus 4°C (Xie et al, 2001).…”

“…Subsequently two myoD paralogues of lower percentage identity were identified in five Percomorpic teleost fish (Tan and Du, 2002;Galloway et al, 2006;Macqueen and Johnston, 2006;Fernandes et al, 2007). A third myoD paralogue was recently characterised in Atlantic salmon, rainbow trout and brown trout (Salmo trutta) and a phylogenetic analysis showed that each salmonid myoD paralogue was orthologous to a universal teleost myoD gene (named myoD1) and distinct from the second myoD paralogue (named myoD2) found in some fish (Macqueen and Johnston, 2006).…”

Molecular regulation of muscle development and growth in Senegalese sole larvae exposed to temperature fluctuations

“…During early stages of Atlantic halibut somitogenesis, Galloway et al (2006) showed that myod2 has a transient left-right asymmetric expression, whereas myod1 always presents a symmetric expression in presomitic and somatic adaxial cells (Andersen et al, 2009) thus hypothesizing that myod2 could be somehow related to the development of external asymmetry in this flatfish species.…”