Primate vision

Question 1

What is colour vision?

Answer 1

Ability to distinguish object based wavelength of the light

Question 2

What two things are needed to generate a colour image?

Answer 2

1. Cone cells - photocepetor cells
2. Neural mechanisms - Generates colour signal

Question 3

Photopigments: Opsins

Answer 3

• At least 4 types of opsins in the common ancestor to sauropsids and synapsids
• Sensitive to different types of wavelengths (longer/shorter)

Question 4

Evolutionary changes of the Opsins

Answer 4

• The synapsids lost some of the opsins
• LWS is retained in all mammals
• RH2 lost in all mammals
• Convergent loss of colour vision in different taxanomic groups

Question 5

Which Opsins were lost and retains in mammal lineages?

Answer 5

• All mammals lost RH2
• All mammals retained LWS
• Marsupials & eutherians lost SWS2
• Monotremes lost SWS1?

Question 6

What happens when an animal loses all opsins (Apart from LWS)?

Answer 6

Monochromatic vision (B&W)

Question 7

What happens when there is a mutation on the opsin SWS1?

Answer 7

• UV sensitive
• Some mammals and marsupials have this (very rare)

Question 8

What vision do most primates have?

Answer 8

• Trichromacy
• Derived condition

Question 9

Comparison between primate and non-primate vision (cones etc)

Answer 9

• Non-primate: Signals are pooled. Groups of cones are attached to 1 photoreceptor.
• Primate: Signals are seperate. Each photoreceptor is connected to a nerve (has its own innovation)

Question 10

Dichromacy is also fairly common in primates what is it?

Answer 10

• Similar to red/green colour blindness
• Found in most new world monkeys
• Ancestral feature

Question 11

How did trichromacy occur?

Answer 11

• L opsin gene on the X chromosome
• Underwent duplication event - 2 copies of gene
• Gradual evolution to the variants that are sensitive to green and red wavelengths

Question 12

What primates is trichromacy seen in?

Answer 12

• Apes, humans, Old world monkeys, NWM howler monkeys
• Howler monkeys have undergone convergent evolution more recently

Question 13

What is trichromacy polymorphism?

Answer 13

• The ability to detect colour vision (trichromacy) is heterozygote
• Homozygous females and all males are dichromats

Question 14

What proportion of females in a trichromatic polymorphic population are trichromatic?

Answer 14

50-66%

Question 15

Why has polymorphic trichromacy persisted?

Answer 15

• Thought to be a benefit to the group for both vision types (variation)
• Frequency dependent selection

Question 16

Why has trichromacy persisted?

Answer 16

• Ripe fruits can be distinguished by dichromats so must have another benefit?
• Frruits in old world are seasonal - must eat young leaves during off season
• Allows detection of young leaves
• Howlers are the most folivorous monkeys in the New World, enforcing their need for trichromacy

Question 17

What else could trichromacy be adapted for?

Answer 17

• Callosities
• Nose - face colouration
• Young with a different coloured coat

Question 18

Summary of primate vision lecture

Answer 18

1. Ancestral amniotes were tetrachromatic and mammals lost two cones so most dichromatic
2. 3 types of “colour” vision in primates:

• Dichromacy (red/green colour blind)
• Trichromacy (Old World + howlers)
• Convergent evolution (OW/NW)
• Polymorphic trichromacy (New World)
• Individuals of same sex and species can differ
• Frequency dependent advantage

3. Potential advantages of trichromacy