The molecular basis for the spectral tuning of longwave-sensitive (LWS) visual pigments in mammals have been described in a wide range of placental species, including the primates. However, little is known about the molecular mechanisms in marsupial LWS pigments. Here, we have studied and compared the LWS opsins in four Australian marsupials: two diprotodonts and two polyprotodonts. Phylogenetic analysis establishes that all LWS marsupial sequences form a distinct clade from the placental mammals that is subdivided into diprotodont and polyprotodont groups. Amino acid sequences reveal that substitutions at sites 277 and 285 are largely responsible for the spectral shifts in marsupial LWS pigments and species comparison indicates that the ancestral gene most likely encoded Tyr277 and Ala180. Amino acid substitutions are discussed in the context of spectral shifts in marsupial LWS and in relation to the mechanisms in primate pigments.
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