Lecture 12 Flashcards

1
Q

Jaws in vertebrates

A
  • first vertebrates in Cambrian had no jaws and earliest vertebrates had no appendages
  • first fish with jaws were placoderms
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2
Q

Graph Balloon Diagram

A
  • 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
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3
Q

Jawless fish phylogney

A
  • start acquiring appendages–>tetrapods then acquire jaws
  • ray fin fish have paired appendages
  • 1/2 of all living vertebrates are ray fin fish
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4
Q

Why no more that 4 appendages

A
  • 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

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5
Q

Ray-fins

A

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

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6
Q

Lobe fin

A

-appendages built like ours

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7
Q

Rhipidistian and Labyrinthodont

A
  • 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
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8
Q

Evolution of the pelvic girdle

A
  • lobefin fish–>tetrapods

- happened early in the Devonian

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9
Q

Acanthostega

A
  • 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
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10
Q

Gills and Lungs

A
  • 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
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11
Q

Primitive number of digits for tetrapods

A

-7-8 (not 5)

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12
Q

Why did fish need legs?

A

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

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13
Q

In Utero

A
  • 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
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14
Q

Patterning of limb buds

A
  • 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
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15
Q

Limb buds

A
  • grow from closest to body out
  • grow along axes
  • correct budding depends on correct specification of axes
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16
Q

Hox genes

A
  • 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
17
Q

Hox-d

A
  • 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)
18
Q

Patterning of digits

A
  • 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
19
Q

Why is arm different from leg?

A
  • 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
20
Q

Ecology of first tetrapods

A
  • 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
21
Q

Control of digit number

A
  • 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