L19- Colour Vision in Primates

Colour Vision→

• the ability to distinguish an object based on wavelength of light

• to see colour, you need both:

  • photoreceptor cells, called cones, in the retina

  • neural mechanisms in the brain (translates signals into info)

Cones→

• within the cones are photopigments called opsins

• there are 4 types of opsins:→ are all sensitive to different wavelengths of light

• all 4 types of opsins were present in the amniote ancestor:

  • synapsids lost RH2, sauropsids did not lose any

  • monotremes lost SWS1

  • marsupials lost SWS2

    → there was a convergent loss of colour vision in different taxonomic groups, suggesting the common ancestor had both these opsins
    → basic mammals were dichromatic as they were nocturnal

Evolution of colour vision:

• monochromacy→ 1 opsin, some placental mammals have lost the SWS1 opsin too so are only left with LWS

• dichromacy→ 2 opsins, most mammals, limited ability to perceive colours, some have a mutation on SWS1 so can see UV e.g. fat-tailed dunnart

• trichromacy→ 3 opsins, most primates, is a derived condition, some marsupials have convergently evolved this too e.g. honey possums

→ the more opsins you have, the more colours you can detect

e.g. chickens have 4 opsins and can see UV (mutation on SWS1 gene)

Neural Mechanisms:

Non-Primates vs Primates→

• non-primates→ each cone cell is combined to a single nerve to give a combined signal, better at detecting light differences but not colour

• primates→ each cone cell is connected individually to a nerve, is an own innovation, can detect colour

Dichromacy-

• in most new world monkeys

• have the SWS opsin on chromosome 7

• have the L opsin on the X chromosome→ can see colours to an extent, similar to red-green colour blindness

Trichromacy-

• in all old world monkeys and the new world howler monkeys

• have the SWS opsin on chromosome 7

• have a duplication of the L opsin on the X chromosome→ are now two copies of the L opsins that have evolved to two different variant→ one variant is sensitive to red light, one variant is sensitive to green light

• genetic dating has shown the duplication has occurred when the new world and old world monkeys split (30-40,000mya), but is much more recent in howler monkeys→ there has been a convergence of colour vision in the howlers

Polymorphic Trichromacy-

• seen in some new world monkeys and some old world monkeys:

• have the S opsin on chromosome 7

• have 1 L opsin on the X chromosome that has multiple alleles

  • heterozygous females can see colour (trichromats)

  • homozygous females and all males cannot see colour (dichromats)

    → the ability to detect colour depends on if you are heterozygous (colour) or homozygous (no colour)

• e.g. in new world monkeys, there are 3 L gene alleles

  • 50-66% of females in a group are trichromatic

• possible benefit of having a mixture of dichromats and trichromats in a group:

  • variation

  • dichromats can see shades better so may be able to detect predators better

  • may be frequency dependent selection/heterozygote advantage

• e.g. white faced capuchins-

  • trichromatic females were better at picking red fruits

    BUT

  • this has no fitness benefits (fertility rate, offspring survival, maternal survival)

    → must be maintained due to frequency dependent selection

• there must be a selective advantage to trichromacy for it to have been maintained:

  • one possible suggestion is that it allows ripe fruit to be distinguished from the green foliage background

    → but the previous study disproved this

  • another suggestion is that fruits are seasonal so monkeys need to eat leaves when there are no fruits

    • young leaves are softer, more nutritious and red

    • dichromats cannot see young leaves but trichromats can

    • trichromats can eat the young leaves → advantage

    • can be seen as howlers are the most leaf-specialist of new world monkeys

• after evolving, trichromacy could have other purposes:

  • detecting young (orange coat of young)

  • detecting sexual status of partners (red rumps of ovulating individuals)

  • finding a mate (red nose and red rump)→ trichromacy initially evolved for foraging and has now been recruited for other purposes