BG15

Question 1

tetrapod limb homology

Answer 1

clearly homologous to pelvic fins of fish

Question 2

ray homology to digits

Answer 2

teleost rays are made of dermal bone, tetrapod fingers are made of endochrondral bone.
therefore not homologous

Question 3

Panderichthys

Answer 3

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.

Question 4

tiktaalik

Answer 4

375mya
late devonian
has elabortate dermal skeletion of rays but also endochondral bones

Question 5

icthyostega and acanthostega

Answer 5

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.

Question 6

trend in fossils

Answer 6

distal endochondral skeleton of stem group tetrapods became increasaingly elaborate and articulated
radial bones are probs homologous to tetrapod digits.
Pandericthys -> tiktallik -> acanthostega

Question 7

hox genes which specify limb elements

Answer 7

only hox clusters hox a and hoxd

and paralogy groups 5’ 9-13

Question 8

hox gene expression in mouse limbs phases

Answer 8

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

Question 9

hox gene mutation humans

Answer 9

lof hoxa13

• brachydactyly - short digits.
• also affects genetical, fauly ureters and vulvas in woman and micropenis in men.

Question 10

zebrafish

Answer 10

ray finned fish actinopterygii

distantly related to tetrapods

Question 11

zebrafish hox expression phases

Answer 11

have phase 1 and 2 but not phase 3

suggests hoxd12/13 expression is an innovation associated with evo of tetrapod digits.

Question 12

catshark

Answer 12

cartilaginous fish

complicated endochondral skeleton with radial cartilages and polygonal plates.

Question 13

catshark hox expression phase

Answer 13

have phase 3 hoxd12/13 expression but no digits

evovled twice?

Question 14

does phase 3 define digit homologues

Answer 14

turns out phase 3 is found widely doesnt seem to define digit homologues but is an ancient expansion of endochondral skeleton.

Question 15

paddlefish

Answer 15

polydon
ray finned fish actinopterygii
lots of cartilage

Question 16

paddlefish hox phase expression

Answer 16

has phase 3 expression implies ray finned fish ancestry had phase 3 expression lost in zebrafish.

Question 17

lunch fish

Answer 17

sarcoptergyian fish living sister group to tetrapods lobe finned fish
elaborate endochondral skeleto with radials going off the sides

Question 18

lung fish hox phase expression

Answer 18

still has phase 3 hoxd13 expression

still being used here to pattern the radials which as fossils suggest are homologous to tetrapod digits.

Question 19

mechanism causing change in hoxd phase 3 expression

Answer 19

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.

Question 20

important enhancer in autopod expression

Answer 20

CsB

Question 21

CsB experiment

Answer 21

take CsB from flat fish (skate) zebra fish adn chicken and make it drive reporter in mice.

1. zebrafish enhancer = low hoxd expression in mice - no autopod expression.
2. skate and chicken enchancer gave high hoxd expression.
3. zebrafish showed more limited expression that skate and both showed much more limited expression than mouses own.

Question 22

evolution of phase 3 expression

Answer 22

change in CsB structure seems to be atleast partly responsible for evo of phase 3 expression.

Question 23

late phase digit enhancers in mice

Answer 23

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.

Question 24

shared enhancer with tetrapod and coelocanth

Answer 24

if transfected into mouse its limb specific suggesting its a lobe fin developmental enhancer coopted into autopod enhancer of modern tetrapods.