Our results agree with previous studies in showing that gulls have Cys90 in their SWS1 opsins, characteristic of the UVS class of colour vision. This trait has been conserved during gull evolution, as revealed by the denser taxon sampling of this study compared with previous ones (Håstad et al. 2005
). Another new finding is that gulls are not unique among shorebirds in being UVS, as the terns A. minutus
, A. tenuirostris
and G. alba
and the skimmer R. niger
share the UVS opsin forming Ser90Cys substitution.
The evolutionary interpretation of our results hinges on the correct inference of the phylogeny. As the relationships in the Laridae/Sternidae/Rynchopidae clade are somewhat uncertain, different interpretations are possible. However, the most parsimonious reconstruction suggests a Ser90Cys substitution in the common ancestor to Laridae/Sternidae/Rynchopidae, and a subsequent reversal to Ser in the Sternidae lineage (except Anous and Gygis), in all formed by two consecutive point mutations in the codon at aa position 90, from AGC to TGC to TCC. This is true even if the uncertainty in the gene tree is taken into account by considering the Sternidae branch (except Anous and Gygis) in alternative positions. For example, the posterior probability that Sterna is outside the clade comprising Laridae, Rynchops, Anous and Gygis—which would support a single Ser90Cys substitution but no subsequent reversal—is lesser than 0.06. Accordingly, our favoured hypothesis based on the available data is a single shift from VS to UVS in the common ancestor to Laridae/Sternidae/Rynchopidae and a subsequent reversal in the Sternidae lineage (except Anous and Gygis).
All key aas we report in positions 86, 90 and 93 have already been described (Wilkie et al. 2000
; Yokoyama et al. 2000
; Ödeen & Håstad 2003
), but the spectral tuning effects of some of them are still unknown. The effect of Cys86 in birds is probably negligible (Ödeen et al. 2009
), but neither the effects of Thr86 or Ile86 nor Ile93 has been investigated in vitro
. However, the λmax
of SWS1 pigment of Anas platyrhynchos
, which carries Ile93, is not significantly different from that of species with aas of known effect in this position (Ödeen et al. 2009
In one aspect, avian colour vision is under simple genetic control: the VS/UVS pigment opsin character depends on the Ser90Cys substitution, which can be caused by a single nucleotide mutation. Still, this character is surprisingly conserved among birds, indicating strong stabilizing selection. This is a rare example of a system where two very similar and well-understood variants of the same gene, which have a significantly different effect on the phenotype and ecology of the carrier, have been equally successful in a large taxon.