Ancestral loss of short wave-sensitive cone visual pigment in lorisiform prosimians, contrasting with its strict conservation in other prosimians.

Mammals are basically dichromatic in color vision, possessing middle to long wave-sensitive (M/LWS) and the short wave-sensitive (SWS) cone opsins in the retina, whereas some nocturnal mammals lack functional SWS opsins. Prosimians, primitive primates consisting of three extant groups (Lorisiformes, Lemuriformes, and Tarsiiformes), include many nocturnal species. Among nocturnal prosimians, a species of lorisiforms, the greater galago (Otolemur crassicaudatus), is known to lack a functional SWS opsin gene, while lemuriforms and tarsiiforms appear to retain SWS opsins in the retina. It has not been established, however, whether the loss of SWS opsin is a universal phenomenon among lorisiforms and whether the functional SWS opsin genes of lemuriforms and tarsiiforms are under strict or relaxed selective constraint. To gain better insight into an association between nocturnality and loss of SWS function, we isolated and sequenced the SWS opsin genes from two species of lorisiforms, the slow loris (Nycticebus coucang; nocturnal) and the lesser galago (Galago senegalensis; nocturnal), and one species each of lemuriforms and tarsiiforms, the brown lemur (Eulemur fulvus; cathemeral) and the western tarsier ( Tarsius bancanus; nocturnal), respectively. Our sequence analysis revealed that (1) the SWS opsin gene was disrupted in the common ancestor of galagids and lorisids and (2) the rate of nonsynonymous nucleotide substitution has been kept significantly lower than that of synonymous substitution in tarsier and lemur, demonstrating the presence of strict selective constraint on the SWS opsin genes in tarsiiforms and lemuriforms.