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Nos evolution in light of recent gene findings in vertebrates. The proposed evolution of nos genes in gnathostomes (a) supposes an ancestral loss of a predicted fourth nos gene, based on the linkage of human Nos and Hox clusters (b). Loss of nos3 occurred in stem Clupeocephala and loss of nos2 in stem Neoteleostei (a). Species-specific nos2 duplications occurred in some Otomorpha, including Cyprinidae and Characidae families. (Online version in colour.)
