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Interphotoreceptor retinoid-binding protein (IRBP) plays a key role in vertebrate vision by transporting retinoids between photoreceptor and retinal pigment epithelium cells in the eye. Kalluraya et al.(1) report that vertebrate IRBP arose via horizontal gene transfer (HGT) of a bacterial S41 peptidase into the vertebrate ancestor, followed by domain duplications and neofunctionalization. These events likely occurred> 500 ma, concomitant with the emergence of the vertebrate eye (1), exemplifying functionally coopted HGT in animals.

Kalluraya et al.(1) propose independent HGTs into amphioxus and vertebrates, despite their close relationship within Chordata. However, they also noted difficulty in identifying the closest relatives to vertebrate IRBP and acknowledge that shared HGT is “difficult to completely rule out”(1). We performed new phylogenetic analyses of Kalluraya et al.’s