When it comes to genomes, most mammals don't stint on their sense of smell. A substantial proportion of the mammalian genome is devoted to olfaction, and conservative estimates indicate that odorant receptor genes number in the hundreds. However, the significance of the size of an animal's odorant receptor genome is not clear because the receptors are not very narrowly tuned, and one receptor can be stimulated by several different odorants. Nevertheless, in humans, many of the odorant receptor genes have become pseudogenes and are nonfunctional, which fits nicely with our impression that we do not rely as heavily on our sense of smell as some other mammals do. A new study by Yoav Gilad and colleagues examines the occurrence of odorant receptor pseudogenes across primates and demonstrates an interesting correlation between odorant receptor pseudogenes and color vision.
First, Gilad and his coworkers demonstrated that the proportion of odorant receptor genes that are pseudogenes varies considerably across primates. The authors examined the sequences of 100 odorant receptor genes from 19 primate species and found that about 20% of the odorant receptor genes are pseudogenes in six species of New World monkeys and in lemurs. This proportion jumps to around 30% in the New World black howler monkeys, in six species of Old World monkeys and in four species of non-human apes that were examined. In humans,more than 50% of the odorant receptor genes examined were pseudogenes. Curiously, the species in which many of the odorant receptor genes are pseudogenes also have enhanced color vision.
How has enhanced color vision evolved in primates? In most mammals, the retina contains rod cells, which are extremely sensitive and allow for vision at low light levels, and cone cells, which are less sensitive and allow for daytime vision. Humans have three classes of cone cells containing slightly different opsin molecules that are maximally sensitive to light at short,medium or long wavelengths, creating the perception of blue, green and red light. When stimulated in different proportions, these opsins give us the ability to perceive fine gradations in color across the spectrum of visible light. By contrast, most mammals have dichromatic vision: their cone cells have only two opsin genes, one that makes an opsin sensitive to short (blue)wavelengths and another that makes one sensitive to longer (red or green)wavelengths. Many New World monkeys have two different alleles at this second locus, one of which makes an opsin that is most sensitive at green wavelengths, and the other at red wavelengths. These two alleles are believed to have evolved into separate loci two times, once in the New World howler monkeys and once again in Old World monkeys and apes, giving us trichromatic vision.
Gilad and his co-workers suggest that this improved color vision led to decreased reliance on olfaction, which in turn may have led to relaxed selection pressure to maintain functional odorant receptor genes. If so, their work provides a mechanistic explanation for the long-held belief in neurobiology that an ecological change that increases use of one sensory system tends to lead to decreased function in other senses.