Embryonic temperature and the genes regulating myogenesis in teleosts

Abstract

In this study, full coding sequences of Atlantic salmon (Salmo salar L.) muscle genes were cloned, including myogenic regulatory factors (MRFs) (myod1c, myog, mrf4, myf5), inhibitors of Myostatin (fst, decorin), markers of myogenic progenitor cell (MPC) proliferation (sox8) and fusion (calpastatin), a marker of slow muscle fibre differentiation (smlc1) and a novel eukaryotic gene involved in regulating growth (cee). Several of these genes were then characterised using a range of experimental and computational analyses with the aim to better understand their role in myogenesis and their evolution in teleosts.

A series of experiments supported previous findings that teleosts have extra copies of many genes relative to tetrapods as a result of a whole genome duplication (WGD) event that occurred some 320-350 Mya. For example, it was shown that genes for myod and fst have duplicated in a common teleost ancestor, but were then specifically lost or retained in different lineages. Furthermore, several characterised Atlantic salmon genes were conserved as paralogues, likely from a later WGD event specific to the salmonid lineage. Phylogenetic reconstruction and comparative genomic approaches were used to characterise the evolution of teleost paralogues within a framework of vertebrate evolution. As a consequence of one experiment, a revised nomenclature for myod genes was proposed that is relevant to all diploid and polyploid vertebrates.

The expression patterns of multiple myogenic genes were also established in Atlantic salmon embryos using specific complementary RNA probes and in situ hybridization. For example, co-ordinated embryonic expression patterns were revealed for six salmon MRFs (myod1a, myod1b, myod1c, myog, mrf4, myf5), as well as markers of distinct MPC populations (pax7, smlc1), providing insight into the regulatory networks governing myogenesis in a tetraploid teleost. Furthermore, it was shown that Atlantic salmon fst1 was expressed concurrently to pax7 in a recently characterised MPC population originating from the anterior domain of the epithelial somite, which is functionally analogous to the amniote dermomyotome. In another experiment, the individual expression domains of three Atlantic salmon myod1 paralogues were shown to together recapitulate the expression of the single myod1 gene in zebrafish, consistent with the partitioning of ancestral cis-acting regulatory elements among salmonid myod1 duplicates. Additionally, the in situ expression of cee a novel and highly conserved eukaryotic gene was revealed for the first time in a vertebrate and was consistent with an important role in development including myogenesis.

Additionally, Atlantic salmon were reared at 2, 5, 8 or 10 ºC solely to a defined embryonic stage, which was just subsequent to the complete pigmentation of the eye. After this time, animals were provided an equal growth opportunity. Remarkably, changing temperature during this short developmental window programmed the growth trajectory throughout larval and adult stages. While 10 and 8 ºC fish were larger than those reared at 2 and 5 ºC at the point of smoltification, strong compensatory growth was subsequently observed. Consequently, after 18 months of on growing, size differences among 5, 8 and 10 ºC fish were not significant, although each group was heavier than 2 ºC fish. Furthermore, significant embryonic-temperature induced differences were observed in the final muscle fibre phenotype, including the number, size distribution and myonuclear density of muscle fibres. A clear optimum for the final muscle fibre number was observed in 5 ºC fish, which was up to 17% greater than other treatments. In a sub-sample of embryos, temperature induced heterochonies were recorded in the expression of some MRFs (myf5, mrf4) but not others (myod1a, myog). These results allowed the proposition of a potential mechanism explaining how temperature can program the muscle phenotype of adult teleosts through modification of the somitic external cell layer, a source of MPCs throughout teleost ontogeny.