The Eyes of the Tree

1. What function do simple light gathering pigment cells serve for the organisms that currently have them?

The pigment cells are responsible for blocking entry of light to the eye from certain directions by separating each unit in the eye.

2. What would be the advantage to a tubellerian worm to be able to sense light?

Considering that its eye does not form an image, sensing light would be advantageous as it would protect it from its predators and desiccation. Sensing light means exposure hence it will be able to avoid light and move towards darkness.

SIDE TRIP: HOW YOUR EYE WORKS

3. What feature of human eyes do we share with more primitive vertebrates such as fish? Why does it cause problems for humans?

Features: photoreceptors cells

It causes problems in humans because of the backwards orientation of the photoreceptors. The orientation implies that nerves carrying visual signals to the brains must run across the retina. They then have to converge at a spot on the back of eye before they exit the eye to reach the brain. This spot is known as optic disc. This exit point is crammed full of nerves meaning that there is no room for photoreceptors. Due to lack of photoreceptors in the area, sensing of light by the optic disc is hindered thus forming a blind spot.

INTRODUCING THE VISIONARIES

FINDING HOMOLOGIES

4. Click on each organism to view information about their eyes. Fill in the table below as you go. Leave the gene information until later.

Squid

Flatworms

Mouse

Flies

Sharks

How do they use their eyes?

By adjusting their eyes to different levels of light by widening or narrowing pupils. They can also focus objects at different distances

they use their eyes to sense the direction of light and the amount of light in the environment

Since their eyes are located on the side of the head, they detect predators coming from different directions. The eyes are adapted to seeing at night despite having poor color vision.

Its compound eyes makes it great at detecting quick motion. The different facets of the eye work together to produce a whole image

Relies on eyes for close range vision after locating prey using other senses.

Eye position

Photoreceptors oriented towards light

Photoreceptor oriented away from light

Photoreceptors oriented away from light

Photoreceptors oriented toward light

Photoreceptors oriented away from light

Composition of the eye

Iris, lens alternating pigment and photoreceptor cells

Pigment cells and photoreceptor cells

Iris, lens, pigment cells and retina (photoreceptors)

Lens, photoreceptor cells and pigment cells

Retina (photoreceptors), iris, lens and pigment cells

Development

Inpocketing of ectoderm

Regenerated from different cells in the body

Inpocketing of nearby ectoderm forms lens and outpocketing of soon-to-develop brains from optic nerve and retina

from disc of specialized ectoderm tissue

Inpocketing of nearby ectoderm forms lens and outpocketing of soon-to-develop brains from optic nerve and retina

Gene involved in eye formation

Squid eye control gene

Flatworm eye control gene

Mouse pax6 gene

Fly eyeless gene

Shark eye control gene

5. Based on your table, take the quiz. Which eyes are homologous?

Shark’s and mouse’s eyes. They have similar development, composition and eye position.

6. Which eyes are analogous?

Squid’s and shark’s eyes.

MAPPING THE EYES ON THE TREE

7. What do genes do?

Genes encodes the directions for synthesis of proteins used as raw materials to develop specific traits.

8. How do genes control genes to build complex structures like eyes?

They do this by encoding proteins that regulates other genes. These proteins might trigger events that turn other genes off or on, they may directly interact with other DNA pieces or they may bind to other proteins that affects these processes.

9. What is the name of the control gene involved in eye development? What kind of gene is it?

 Pax6 is the gene involved in eye development. In humans it’s called Aniridia.  It is a regulatory gene.

SURPRISINGLY SIMILAR

10. How similar is the sequence DNA sequence of the eye control genes in flies and in mice?

The gene sequences are 76.66% similar.

11. How similar are the proteins made by the eye control genes in flies and mice?

The proteins corresponding to the DNA regions are 100% similar in these two animals.

12. What is the relationship between the eyeless gene in flies, the pax6 gene in mice, and the Aniridia gene in humans?

The eyeless gene in flies has an important role in directing eye development in flies , just like aniridia

in humans and pax6 in mice.

13. What do these genes tell the organism to do?

They turn on all the genes required in building of a working eye.

HOMOLOGOUS GENES

Go back to your table and add the information about genes.

14. Are the genes for the formation of eyes homologous or analogous? What is the evidence that supports your answer?

They are homologous. This is because all of these gene versions have related functions, are remarkably similar in sequence and are incredibly widespread across the tree of life.

EVOLVING TWO EYES WITH ONE GENE & HOMOLOGIES AT DIFFERENT LEVELS

15. Read through the material. Write a paragraph explaining how eye development in these animals can contain both homologous and analogous components.

            More than 500 million years ago, the animal’s ancestors evolved a “switch” gene which aided in building of simplest type of an eye. This basic gene was then passed to its descendants. These descendants ended up living in different environments where natural selection resulted into use of gene to build more complex and sensitive eyes as a way of adaptation. This complex eye was then switched to descendant of original gene. Through this, different lineages evolved analogous complex eyes that are controlled by homologous genes (eye-building genes)

TAKE A SIDE TRIP: EVOLVING COMPLEXITY

16. Identify how natural selection could favor steps in the evolution of a complex structure such as the eye.

It could favor steps in evolution because most of the traits are adaptive. Therefore, different environments resulted into different adaptations. This can be explained by slight improvement and complexity of ancestral version of eye over time.

17. Do we have examples of living organisms that still survive with some of these ”intermediate forms”? Give an example.

Yes, there are living organisms that still survive with some intermediate forms.

A good example is the flatworm with a simple eye with few components that only helps in sensing light but not vision.