Lecture 12 Flashcards
Jaws in vertebrates
- first vertebrates in Cambrian had no jaws and earliest vertebrates had no appendages
- first fish with jaws were placoderms
Graph Balloon Diagram
- 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
Jawless fish phylogney
- start acquiring appendages–>tetrapods then acquire jaws
- ray fin fish have paired appendages
- 1/2 of all living vertebrates are ray fin fish
Why no more that 4 appendages
- 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
Ray-fins
-long bony (but not actual bone so probably cartilaginous) structures that the fin ray attaches to
Lobe fin
-appendages built like ours
Rhipidistian and Labyrinthodont
- 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
Evolution of the pelvic girdle
- lobefin fish–>tetrapods
- happened early in the Devonian
Acanthostega
- 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
Gills and Lungs
- 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
Primitive number of digits for tetrapods
-7-8 (not 5)
Why did fish need legs?
-end of every season ponds would dry out so in order to survive they needed legs to get to next pond
In Utero
- 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
Patterning of limb buds
- 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
Limb buds
- grow from closest to body out
- grow along axes
- correct budding depends on correct specification of axes
Hox genes
- 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
Hox-d
- 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)
Patterning of digits
- 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
Why is arm different from leg?
- 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
Ecology of first tetrapods
- 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
Control of digit number
- 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