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BackgroundThe myocyte enhancer factor 2 (MEF2) gene family is broadly expressed during the development and maintenance of muscle cells. Although a great deal has been elucidated concerning MEF2 transcription factors' regulation of specific gene expression in diverse programs and adaptive responses, little is known about the origin and evolution of the four members of the MEF2 gene family in vertebrates.Methodology/Principal FindingsBy phylogenetic analyses, we investigated the origin, conservation, and evolution of the four MEF2 genes. First, among the four MEF2 paralogous branches, MEF2B is clearly distant from the other three branches in vertebrates, mainly because it lacks the HJURP_C (Holliday junction recognition protein C-terminal) region. Second, three duplication events might have occurred to produce the four MEF2 paralogous genes and the latest duplication event occurred near the origin of vertebrates producing MEF2A and MEF2C. Third, the ratio (Ka/Ks) of non-synonymous to synonymous nucleotide substitution rates showed that MEF2B evolves faster than the other three MEF2 proteins despite purifying selection on all of the four MEF2 branches. Moreover, a pair model of M0 versus M3 showed that variable selection exists among MEF2 proteins, and branch-site analysis presented that sites 53 and 64 along the MEF2B branch are under positive selection. Finally, and interestingly, substitution rates showed that type II MADS genes (i.e., MEF2-like genes) evolve as slowly as type I MADS genes (i.e., SRF-like genes) in animals, which is inconsistent with the fact that type II MADS genes evolve much slower than type I MADS genes in plants.ConclusionOur findings shed light on the relationship of MEF2A, B, C, and D with functional conservation and evolution in vertebrates. This study provides a rationale for future experimental design to investigate distinct but overlapping regulatory roles of the four MEF2 genes in various tissues.

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Total dissolved gas (TDG) supersaturation downstream has a negative environmental effect on fishes. It is caused by discharge from high dams and increases the incidence of gas bubble disease and fish mortality. Downstream of a high dam, there is an area with low TDG saturation due to the gradual mass exchange of gases between the separation zone and the mainstream and the long retention time in the confluence, which contributes to the dissipation of saturated TDG at the confluence of the mainstream and its tributaries. This area can provide a temporary shelter for fish to avoid the effects of TDG supersaturation during dam discharge. A depth-averaged, two-dimensional model of TDG dissipation at a river confluence was established. The concentration field was verified by a flume experiment. A numerical simulation of the TDG at the confluence of the Zumuzu River and its tributary, the Mozigou River, was conducted. The simulation showed that the convergence of the tributary, which had a low TDG saturation level, could reduce the TDG saturation level of the mainstream. However, the low-saturation area was not large enough for fish to avoid the negative effects of TDG saturation due to a sharp river slope and a large flow ratio between the mainstream and its tributary. To expand the suitable shelter area with low TDG supersaturation levels in order to provide a suitable shelter for fish, some engineering measures were explored, including the excavation of the riverbed and the construction of resistance obstacles. After the engineering measures were introduced, we observed a 30-fold increase in the size of the area with low TDG saturation, which was as high as 10005 m 2 at 110% of the TDG saturation. Combined with the comprehensive analyses of the flow velocity and the water depth, the 3 confluence region was thought to be suitable to protect the fish from the effects of TDG supersaturation. This study provides an important reference for protecting fish during high dam discharge.