The hypothesis that coelacanth is the closest living relative of tetrapods 3 was rejected based on three genome-scale approaches

21 2 1 Since its discovery of the " living fossil " in 1938, the coelacanth (Latimeria chalumnae) has generally 2 been considered to be the closest living relative of the land vertebrates, and this is still the prevailing 3 opinion in most general biology textbooks. However, the origin of tetrapods has been the subject of 4 intense debate for decades. The three principal hypothesis (lungfish-tetrapod, coelacanth-tetrapod, 5 or lungfish-coelacanth sister group) have been proposed. We used the maximum gene-support tree 6 approach to analyze 43 nuclear genes encoding amino acid residues, and compared the results of 7 concatenation and majority-rule tree approaches. The results inferred with three common 8 phylogenetic methods and three genome-scale approaches consistently rejected the hypothesis that 9 the coelacanth is the closest living relative of tetrapods. 10 11 The origin of land vertebrates (tetrapods) has been subject to debate for many decades, yet the 12 relationship of the extant basal taxa remains contentious. The origin of the tetrapod is of 13 importance and always commands considerable popular interests in public and academic fields. 14 However, since the discovery of the " living fossil " in 1938, Latimeria chalumnae (1, 2), the last 15 known surviving species of a lineage of lobe-finned fish, was generally considered to be the 16 closest living relative of the land vertebrates, the missing link between aquatic and terrestrial 17 vertebrates. This hypothesis is still the prevailing opinion in most general biology textbooks (3). 18 Three hypotheses have been proposed for the phylogenetic relationship: e.g., lungfish-tetrapod 19 (hypothesis 1, Fig 1a), coelacanth-tetrapod (hypothesis 2, Fig 1b), or lungfish-coelacanth sister 20 grouping (hypothesis 3, Fig 1c). The lungfish-coelacanth-tetrapod trichotomy (Fig 1d) is not 21 generally considered as a hypothesis. 22 23 Fig. 1. 24 3 1 Regarding which of the three extant basal taxa was the closest living relative to tetrapods, the 2 coelacanth hypothesis (Fig 1b) was supported by some comparative morphologists and 3 paleontologists (4-7), although the lungfish was historically thought to have that claim, which 4 was also supported by more recent researchers (8-12). The hypothesis that coelacanths and 5 lungfishes form a monophyletic group that is equally closely related to the tetrapods (Tree III) was 6 also proposed (13-15). 7 Molecular data have gradually been collected to infer phylogenetic relationship for the last two 8 decades. With mitochondrial DNA, lungfishes were linked as the closest relatives of tetrapods …