Lecture 24

Question 1

Eyes

Answer 1

• optical entity that follow specific laws of physics
• only few hundred of years is enough to go from patch of skin with photoreceptors in it to an eye with lenses
• doesn’t take that many steps-fairly simple structure
• evolved into diverse kinds in the cambrian radiation
• many different kinds among animals
• master regulatory genes shared by eyes in a different taxa
• ongoing evolution of eye lens proteins (flexibility of genome in evolution and selection of old genes for new functions)
• forms as an extension of the nervous system

Question 2

Cambrian Radiation

Answer 2

• apperance of large, fossilizable bilaterian animals
• novel body plans and ways of development evolved
• stabilization of body plans
• life history evolution, including dispersible larvae
• eyes arose many different times independently

Question 3

Jellyfish

Answer 3

• have eyes that point inward and can be either slits or pinholes or even lenses
• transparent so they can see through their insides out to the other side
• what sort of processing can they do if they don’t have a brain only a neural net
• uses rhythm sense organs to guide themselves in moving

Question 4

Pigment Cells

Answer 4

-keep stray light from hitting the photoreceptors

Question 5

Pax6

Answer 5

• everyone has this
• important in eye development as well as nose development
• involved in development of all known animal eyes and light sensing organs, so although distinct eyes have evolved many times vision systems are homologous at the level of basic regulatory genes

Question 6

Embryo Cells

Answer 6

-when lens cells finally form a lens they die and for the rest of your life those cells provide the clear things you see through–>lens

Question 7

Human Eyes vs. Squid Eyes

Answer 7

• squid eye behind retina are photoreceptor cells and a nerve coming out of those cells and go back to the brain
• mammal brain retina and photoreceptor cells are backward so nerves coming out go through the retina and back out the other side again and then back to the brain
• the blind spot is wehre thenerves go back through the retina

Question 8

Independent Evolution of an Eye

Answer 8

• like receptor on squid

- don’t change gene but change regulatory element

Question 9

Cnidaria

Answer 9

• group where eyes originated
• modern jellyfish have eyes and balance organs
• eyes seem to look up into the bell but may help jellyfish avoid obstacles in crowded places
• box jellies have fish-like hunting behavior and have complex eyes and other sensory organs

Question 10

Brittle Stars

Answer 10

• echinoderms with whole body eyes
• entire skeleton is studded with calcite-based light-detectors
• light sensors are part of the skeletal spicules

Question 11

Fish Eyes

Answer 11

• anableps can look up into air and down into water

- different properties for each set of eyes to work in a different medium

Question 12

Specialized Eyes

Answer 12

• can be in unexpected parts of anatomy and can have unexpected functions
• butterfly example with eyes on genitals used for searching and copulating

Question 13

Macropinna

Answer 13

• upward directed tubular eyes and tiny, terminal mouth
• visual field of highly specialized eyes does not include mouth-feeding?
• eyes can change position from dorsally directed to rostrally directed
• eyes contained within transparent shield that covers top of head and may provide protection for eyes from tentacles of cnidarians

Question 14

Spider Eyes

Answer 14

• major eyes are as good as eyes of small rodents

- high resolution

Question 15

Vertebrate Eye

Answer 15

• embryonic development: molecular signals from the neuroectoderm induce lens development in ectoderm
• Pax6 function sin other functions in addition to eyes
• Pax6 loss of function mutations are lethal in homozygotes
• Pax6 minus has no eye or nose
• Pax6 essential in mammals and is haplo-insufficient (# of gene copies matters)
• heterozygotes for Pax6 have defects in eye development

Question 16

Insect Compound Eyes

Answer 16

• critical to hunting and avoid being caught
• greater number of lenses, greater clarity of vision
• dragon flies are the most effective hunters on earth; visual pursuit hunter

Question 17

Inverted Eye

Answer 17

-squid light organ built like an eye

Question 18

Genes to Make Eyes

Answer 18

• regulatory genes in development
• genes that play functional roles in the eye
• the ancient genetic regulatory genes that underlie the diversity of eye evolution: Pax-6

Question 19

Pax6 In Flies

Answer 19

• Pax6 minus eyes have reduced or missing eyes
• turns on eye development anywhere in the body of the fly where the gene is expressed
• individual ommatidia of ectopic eyes are normally formed but the eyes are often small and oddly shaped, reflecting absence of normal cues for eye size and shape
• ectopic eyes don’t usually work because photoreceptor axons from misplaced eyes can’t find brain (too far to go and normal directional cues absent)

Question 20

Special Features of Some Vertebrate Eyes

Answer 20

• improvement light gathering in the dark-role of light reflector in some eyes
• like organs that don’t gater light, but produce it like living flash lights
• convergent on real eyes-not evolved from eyes

Question 21

Three Properties Under Selection

Answer 21

• properties of proteins co-opted into expression in the eye lens as “taxon-specific crystallins”
• globular: efficient packing
• clear: light transmission
• stable: long term survival in lens that makes no new proteins

Question 22

Vertebrate Taxon-Specific Lens Crystallins

Answer 22

• recruited enzymes
• arise in many taxa
• they represent ordinary old enzymes used at low levels of expression in a variety of metabolic processes

Question 23

Alpha Crystallin

Answer 23

• universal in vertebrates
• critical function in acting as a chaperonin to prevent protein degradation
• strong selection on this one

Question 24

Beta and Gamma Crystallins

Answer 24

-widespread and may function in protein stability

Question 25

Delta Crystallin

Answer 25

- and others are taxon specific and function in light transmission - tell informative stories about molecular evolution - weak selection on particular identities of these

Question 26

Gene Level

Answer 26

- genes first get duplicated then acquire new expression patterns - still used as enzymes in original tissues but now expressed at very high levels in eye lens - work fine as crystallins and are selected for new second function - gene duplication followed by mutations that allow expression at a new site at high level-->requires changes in upstream gene regulatory sites, not changes in the coding region - transparent - stable - efficiently packed

Question 27

Delta Crystallin Gene Regulatory Evolution

Answer 27

- shows that more than a single specific mutational change can lead to a common gene expression pattern in two organisms - flexibility in gene regulatory evolution associated with gene duplication - this can happen without loss of original gene function and regulation of expression - look at diagram

Question 28

Epsilon Crystallins

Answer 28

- LDH (a major enzyme in glycolysis) has second role as epsilon crystallin in some eyes - present in some birds but not other - likely that frequent gene co-option events occur