BG15 Flashcards
tetrapod limb homology
clearly homologous to pelvic fins of fish
ray homology to digits
teleost rays are made of dermal bone, tetrapod fingers are made of endochrondral bone.
therefore not homologous
Panderichthys
385mya - late devonian
- series of endochrondrial bones called distal radials
until recently thought not to be homologous to digits
- recent reconstruction homologizes proximal bones to radius and ulna and humerus and distal radials to tetrapod digits.
tiktaalik
375mya
late devonian
has elabortate dermal skeletion of rays but also endochondral bones
icthyostega and acanthostega
365mya late devonian
acanthostega: forelimbs have good radius, ulna, humerous and digits (7-8) rather than canonical 5.
doesnt have wrist ankle bones that modern tetrapods do.
icthyostega = similar, fish like tails.
trend in fossils
distal endochondral skeleton of stem group tetrapods became increasaingly elaborate and articulated
radial bones are probs homologous to tetrapod digits.
Pandericthys -> tiktallik -> acanthostega
hox genes which specify limb elements
only hox clusters hox a and hoxd
and paralogy groups 5’ 9-13
hox gene expression in mouse limbs phases
phase 1: hoxd9 and hoxd10 expressed across bud when pd bones are forming
phase 2: hoxd11 and hoxd12 and hoxd13 are expressed around zpa
phase 3: all hox genes expressed in autopod to specify digits
hox gene mutation humans
lof hoxa13
- brachydactyly - short digits.
- also affects genetical, fauly ureters and vulvas in woman and micropenis in men.
zebrafish
ray finned fish actinopterygii
distantly related to tetrapods
zebrafish hox expression phases
have phase 1 and 2 but not phase 3
suggests hoxd12/13 expression is an innovation associated with evo of tetrapod digits.
catshark
cartilaginous fish
complicated endochondral skeleton with radial cartilages and polygonal plates.
catshark hox expression phase
have phase 3 hoxd12/13 expression but no digits
evovled twice?
does phase 3 define digit homologues
turns out phase 3 is found widely doesnt seem to define digit homologues but is an ancient expansion of endochondral skeleton.
paddlefish
polydon
ray finned fish actinopterygii
lots of cartilage
paddlefish hox phase expression
has phase 3 expression implies ray finned fish ancestry had phase 3 expression lost in zebrafish.
lunch fish
sarcoptergyian fish living sister group to tetrapods lobe finned fish
elaborate endochondral skeleto with radials going off the sides
lung fish hox phase expression
still has phase 3 hoxd13 expression
still being used here to pattern the radials which as fossils suggest are homologous to tetrapod digits.
mechanism causing change in hoxd phase 3 expression
sequencing and alignment of hox clusters from different vertebrates identified several enhancers upstream of hoxd cluster.
* identified as non-coding regions that failed to diverge.
important enhancer in autopod expression
CsB
CsB experiment
take CsB from flat fish (skate) zebra fish adn chicken and make it drive reporter in mice.
- zebrafish enhancer = low hoxd expression in mice - no autopod expression.
- skate and chicken enchancer gave high hoxd expression.
- zebrafish showed more limited expression that skate and both showed much more limited expression than mouses own.
evolution of phase 3 expression
change in CsB structure seems to be atleast partly responsible for evo of phase 3 expression.
late phase digit enhancers in mice
located in gene desert proximal to hoxd cluster.
* among the six cis-acting reg sequences in this area - 3 are restricted to tetrapods.
2 are pan-gnathostome (inc. CsB)
1 is shared by tetrapods and coelocanth, but not by telosts.
shared enhancer with tetrapod and coelocanth
if transfected into mouse its limb specific suggesting its a lobe fin developmental enhancer coopted into autopod enhancer of modern tetrapods.
