Lecture 12

Question 1

Jaws in vertebrates

Answer 1

• first vertebrates in Cambrian had no jaws and earliest vertebrates had no appendages
• first fish with jaws were placoderms

Question 2

Graph Balloon Diagram

Answer 2

• where balloon starts out at bottom is time when it originated and where it peters out at top is when they went extinct
• width corresponds to how many species there are

Question 3

Jawless fish phylogney

Answer 3

• start acquiring appendages–>tetrapods then acquire jaws
• ray fin fish have paired appendages
• 1/2 of all living vertebrates are ray fin fish

Question 4

Why no more that 4 appendages

Answer 4

• it’s possible some early on had multiple paired appendages

- accident of evolutionary history–>fish that gave rise to tetrapods only had two sets of paired appendages

Question 5

Ray-fins

Answer 5

-long bony (but not actual bone so probably cartilaginous) structures that the fin ray attaches to

Question 6

Lobe fin

Answer 6

-appendages built like ours

Question 7

Rhipidistian and Labyrinthodont

Answer 7

• what lobe fin looks like
• shoulder girdle like ours and have bones in them
• compare to early tetrapod like Labyrinthodont (amphibian) have same structures but also has a hand
• somehow Rhipidistian transformed into Labyrinthodont
• transformation is easy and hard at same time
• turned swimming organ into a land organ
• underlying structure has a lot in common
• key change in anatomy is that you evolve fingers instead of having fin structures
• homologous limb bones, increased strength of attachment of humerus to shoulder girdle

Question 8

Evolution of the pelvic girdle

Answer 8

• lobefin fish–>tetrapods

- happened early in the Devonian

Question 9

Acanthostega

Answer 9

• first tetrapod
• fish like: has internal gills, vertebral column, tail fin like fish
• tetrapod like: bones of skull, legs
• unique: limb girdles built for swimming, not walking, 7-8 toes rather than 5 (used for swimming through weeds)
• transitional fossil between fish and tetrapods
• basal ancestors of amphibians and amniotes

Question 10

Gills and Lungs

Answer 10

• lobe fin fish had both
• lived in water that would become stagnant or in shallow streams
• breathed with gills when oxygen level in water was high but if it wasn’t they’d use there lungs
• we have lungs because fish had lungs

Question 11

Primitive number of digits for tetrapods

Answer 11

-7-8 (not 5)

Question 12

Why did fish need legs?

Answer 12

-end of every season ponds would dry out so in order to survive they needed legs to get to next pond

Question 13

In Utero

Answer 13

• have gill slits
• fore limb buds and hind limb buds give rise to hands, legs, and feet
• how do you build these?–>patterning of limb buds–>condensation of cartilage cell–>bone as bud–>limb

Question 14

Patterning of limb buds

Answer 14

• Zone of polarizing activity (in mesenchyme) contains signals that instruct developing limb bud to develop using A/P axis
• FGF-8 signals shh expression in posterior mesoderm and shh then stimulates FGF-4 to be expressed in posterior part of Apical Ectodermal Ridge
• these events means there is a co-dependence between FGF-4 and Shh for subsequent expression and maintenance
• Cells on AER have specific fate in forming limbs
• Shh and Zone of PA induces AER to produce growth factor which stimulates mesenchymal cells to multiply
• can interrupt it by removing apical epidermis ridge-how they determined the entities were sending molecular signals

Question 15

Limb buds

Answer 15

• grow from closest to body out
• grow along axes
• correct budding depends on correct specification of axes

Question 16

Hox genes

Answer 16

• members of a transcription factor family
• clustered
• sites of expression determine anterior to posterior identities along the body axis
• co-linear expression; 1 more anterior than 2, etc
• when appendages started to evolve from fish without the Hox genes evolved to help in development of body limbs
• if you knock them out you find each gene is responsible for development in a different part of the limbs

Question 17

Hox-d

Answer 17

• bands of expression in limb bud
• each one responsible for different sets of bone in limbs–>determined from knocking out each and seeing what’s affected (shoulder girdle, humerus, etc)
• fingers also controlled by Hox; not all are alike (not a single gene for each finger-polarity comes from early signaling–sonic hedgehog responsible for pinky side)

Question 18

Patterning of digits

Answer 18

• patterning and identites of fingers probably arises from Hox genes and there interactions with other regulatory genes
• when you disturb these genes you get more digits
• Hoxd, Hoxa, and Gli3R

Question 19

Why is arm different from leg?

Answer 19

• two regulatory genes
• Tbox genes
• under the control of some pattern regulator that decides which Tbox gene is expressed anterior and which is expressed further down
• activated somewhere in the body trunk
• Tbox 5 produces forelimb identity
• Tbox 4 produces hind limb identity

Question 20

Ecology of first tetrapods

Answer 20

• not fish seeking to walk on land, either for food or to escape drying ponds
• rather, were aquatic forms with gills as well as lungs and evolved legs that served to pull themselves through vegetation in swamps
• legs did not initially evolve for walking, but were preadapted so an early tetrapod could move on land if needed
• ability opened new niches for tetrapods
• carboniferous tetrapods were both aquatic and terrestrial

Question 21

Control of digit number

Answer 21

• Hox13a and distal Hoxd genes creates a situation where gene products set up a reaction-diffusion system that, a digital field, resulting in digit numbers.
• patterning mechanism where there is not an individual gene for each element
• once digits had evolved, what distinguishes Acanthostega’s digit count from later tetrapods is the evolution of the digit field