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A species tree of mammals marked with multiple losses of functional Tas1r2. The Tas1r2 genes studied by amplification and sequencing of Tas1r2 are underlined, whereas those studied by genome sequence analysis are not. Birds are used as the outgroup of mammals. Note that Tas1r2 originated in the common ancestor of bony vertebrates (Grus and Zhang 2009), and therefore, the absence of Tas1r2 in individual mammalian and avian species are due to gene losses.
