Evolution of Animal Body Axes

Question 1

2 aspects of HOX gene function

Answer 1

-spatial control of where HOX genes are turned on (providing TF in that area)

-consequences of HOX gene activity (whsat can the TFs do through control of target genes)

Question 2

HOX genes and arthropod legs:

Answer 2

other arthropods (crustacea, velvet worm [not arthropod, onycophoran])
-have distal-less expressed all the way down the body
-ALSO have Ubx expressed all the way down thorax and abdomen
-there is co-expression of both D and U in abdomen cells

Question 3

drosophila vs artemia Ubx repression ability

Answer 3

drosophila Ubx expression throughout whole drosophila embryo
-embryo lethal, all segments had abdominal transformation

then took artemia Ubx and expressed it thtoughout drosophila embryo
Distal-less was able to turn on in thorax (though still some transformation)

so in crustacean (artemia) Ubx does not repress Distal-less
change happened in divergence of crustaceans and insects

chimeric artemia Ubx gene containing mostly artemia sequence but SMALL section of drosophila Ubx 3’ end
was able to repress distal-less

Question 4

evolution of legless abdomen in insects

Answer 4

ancestral arthropod form:
legs all down abdomen

in insects to change to no leg abdomen:
-Ubx change to be able to repress distal-less
-change in where it is expressed - moved posterior to just abdomen as if it was expressed throughout whole insect it would repress ALL leg development

-Ubx would have needed to move back FIRST before repression ability gained

Question 5

problem with artemia Ubx in drosophila experiment

Answer 5

-artemia Ubx is challenging drosophila distal-less
can you conclude drosophila Ubx gaining repressive mechanism by using proteins from diverged groups?
-change may have been in in nature of D-less enhancer
-i.e. couldve been change in distal-less
-(from hypothesis that states regulatory changes more common than changes in protein coding seq)

Question 6

Hox genes and vertebrae
chicken v ostrich

Answer 6

ostrich longer neck
because more neck vertebrae

14 in chick (thoracic starts somite 15)
17 in ostrich (T starts somite 18)
vertebrae arise from somite HOX genes expressed in them
Hox 3-5 cervical
Hox 6-8 for thoracic

border of expression between them shifted posterior in ostrich
also changes forelimb position (develop form spanning border between neck and thorax)

Question 7

vertberate Hox genes and forelimbs

Answer 7

backward shunted hox border in ostrich
Hox4/Hox9 border at somite 20 in chick
somite 25 in ostrich

Tbx5 expressed in tissue where forelimb will grow
Hox4 activator of Tbx5
Hox9 repressor
Tbx5 expression is balance between these two

Question 8

experimentally shifting forelimb location

Answer 8

retinoic acid soaked bead
mimics Hox4???
tbx5 expression shifted forward in embryo to where bead is
limb grows anteriorly sifted

Question 9

vertebrates- hox genes and vertebrae

Answer 9

in mouse (and other verts)
-Hox4 - cervical identity
-Hox6&8 - promotes ribs - thoracic identity
-Hox10-inhibits ribs - lumbar identity

Hox promoters and inhibitors of ribs bind rib development Myf5 gene’s
H1 enhancer

Question 10

snakes - hox genes and vertebrae

Answer 10

divergence in Hox expression in snakes:
Hox C6&C8 expressed all the way down body (all the way up to head - only small region of Hox4 behind head)

Python:
end up with very long trunk (100s somites) with ribs all the way down
no forelimbs
very vestigial hindlimbs in some cases
-change in rib activating hox gene expression correlates with ribs all the way down
-no ribless neck region as Hox 4 is not expressed where 6 and 8 are not)

(mouse:

• neck
• limb at neck-thorax junction
• ribs at thorax
• lumbar)

Question 11

corn snake ribs

Answer 11

Hox10 (rib repressor in python)
doesnt work same way in corn snake
in CS - Hox10 turns on further down trunk to the end
Hox 10 and rib development in same place
Hox10 relation to ribs has changed somehow

CS hox 10 expressed in mouse
inhibited rib development
not property of 10 itself that has changed

changed of H1 enhancer of rib development gene Myf5 in snakes changed in c10 binding sequence
same specific change in ALL snake species looked at
likely that this is reason for change in c10 rib relation

Question 12

limb development in snakes

Answer 12

python - vestigial hind limb
some other snakes - no sign at all

Hox d13 expressed in vertebrate limbs at most posterior of limb bud in embryo
one of its functions - turn on Shh signalling
Shh gives positional cues to cellls along A-P axis of LB

Hoxd13 binds ZRS enhancer of Shh in birds/mammals
Hox d13 expression in early hind limb of python too
so later on in python would expect Shh activation
only get weak transient Shh - never goes on to pattern hind limb

in all snakes ZRS enhancer very degraded - many different mutations - even wholesale deletions pf parts
v abnormal looking compared to other vertebrates
so Hoxd13 doesnt recognise this ZRS enhancer in snakes
so no/v little Shh expression
so no hindlimb patterning

swap out mouse ZRS for snake ZRS
SERPENTISED
limbs dont develop
so its not change in Hoxd13 in snake but in the ZRS enhancer of Shh

Question 13

regulatory vs coding changes of hox Genes

Answer 13

regulatory change:
affect how that Hox gene is expressed

change in coding sequence:
affects how that Hox gene interacts with enhancers of its target genes
(or can be a change in target regulators)
finction of Hox gene expression has changes (like snake leg/rib development)

Question 14

camera eyes vs compound eyes

Answer 14

both “complex eyes”
large
physiologically complez
v different in structure and how they work

surprising commonality - all photoreception done by Rhodopsin protein
(opsin complexed with retinal- a Vit A derivative)
and are also shielded by shielding pigments

Question 15

simple eyes:

Answer 15

groups of cells with only these two components
groups of cells that contain both photoreceptors and pigment shielding

Question 16

complex eye requirements:

Answer 16

-a lens - refract light and produce image
-crystallins in that lens (transparent and refract light)

Question 17

how could the basic similarity that opsin proteins always do photoreception

Answer 17

opsin proteins - large family in genome
many functions in family outside of photoreception

subfamilies distinguished by sequence:
-C opsins
-R ospsins
difference in how they are localised (rabdomes vs cilia) and how they induce nerve impulse:
R opsins lodged on membrane sheets called rabdomes?
- stimulation leads to membrane De-polarisation

C opsins lodged on cilia - change in conformation when struck by photon:
- leads to sudden membrane polarisation that will lead to nerve impulse eventually

Question 17

how could scattered (seemingly random) distribution of eyes have evolved?

Answer 17

• theory that very many types of eyes have been evolved throughout kingdom
• types of eyes have been selected for performance
• so have CONVERGED on fewer types of eye structure as they work best in competition
• Hence similar convergent eye types throughout taxa

Question 17

camera complex eyes in animal kingdom

Answer 17

vertebrates

molluscs (cephalopods - similar structure to vertebrate)

cnidaria -v similar structures in some jellyfish

scattered throughout animalia

Question 17

complex compound eyes in animalia

Answer 17

artropods
mollusca
annelida

scattered around animalia

differeing coimplex eye types all scattered through animalia

Question 17

vertebrate photoreception opsin type?

Answer 17

C opsin
(be it camera or chamber eyes)

invertebrates (rest of deuterostomes and all other groups) all use R opsin for photoreception

Question 18

Pax6 in eye development

Answer 18

eyeless phenotype in drosophila caused by Pax6 mutation
just cuticle where eye should be -clean absence of compound eye

in mammals -same gene expressed in developing eye and mutation in mammalian pax6 also give eyeless phenotype

this orthologous gene is necessary for eye development
forced ectopic expression of Pax6 can also cause formation of compound eyes in other regions of drosophila - look exactly how eye should just in wrong place
with JUST pax6 expression
even Pax6 from another animal will do this

Pax6 = master control gene of eye development?

Question 19

Pax6 = master control gene of eye development?

Answer 19

sort of eye you get depends on the genes that RESPOND to pax6 expression
however not as simple:
group of genes in deosophiila
-Pax6
-Sine Oculus
-OTD
all 3 of these genes turn each other on and work together in animals

Question 20

Pax 6 and photoreceptors (opsin proteins)

Answer 20

R opsin turned on in drosopjila
C ospin in mouse
each gene in eye regulator group directly activate opsins

can argue that even though eyes present in different forms now - eye development mechanisms can trace back to common ancestor

Question 21

Shielding pigments in animalia

Answer 21

variety used

vertebrates - melanin (the 3 genes also turn on melanin synthesis pathway)
melanin also in planarians, annelids, nematodes

-no melanin in arthropod eyes
instead Pterins and Ommochromes

Question 22

crystallins?

Answer 22

fomr transparent focusing lens structure
crystallins used are very diverse and are very distinct among groups
though commonality - their production are directly turned on by specifically Pax6 (pax6 activates different crystallins depending on species)

Question 23

recruitment of crystallins

Answer 23

diverse between groups
NOT a gene family - not related by sequence
BUT when the prodicts of these genes are produced in high concentrations in part of eye

all have been recruited to eye development but have very different functions in other areas:
-metabolic enzymes
-heat shock proteins
(sometimes not exact same gene as the outside function one but arose as duplicate of it)

so array of disparate genes RECRUITED into function of beinf prodiced in High Conc in eye to act as lens

Question 24

camera eye structure:

Answer 24

-large lens
-retina behind
-pigmented epithelium behind retina
-cornea at front
development in camera eyes for this anatomy in different ways in molluscs, vertebrates…

Question 25

compound eye structure

Answer 25

hundreds of similar components called ommatidia
-have rhabdomeres with R opsins(in arthropod) sitting on them (8 in each component)
-pigment cells make shielding components

in annelids and arthropods (though again develpoment likely

Question 26

vertebrate camera eye signalling pathway recruitment

Answer 26

stalks grow out side of brain
form optic cup
optic cup signalling to head epidermis above eye causing it to form lens
lens then signals to other cells to form cornea
optic cup and lens both signal with BMP (FGF too)

Question 27

drosophila eye formation signalling pathway

Answer 27

adult eye forms in late pupa not larva embryo
imaginal disc forms in head epidermis
uses Hedgehog and BMP
(BMP for forming ommatidia - hedgehog maturing)

one in common with vert - BMP
one different (hh instead of fgf)

these signalling pathways have other functions (both drosophila and mouse examples)
were recruited to have different finction in eye too

Question 28

complexity of Ur-bilateria

Answer 28

argument for Ur-bilateria being complex
based on how similar complex systems are between bilateria branches
(eyes, heart, limbs, body segmentation, CNS)

however can also argue that a simpler system existed in Ur bilateria was used as basis and was built upon separately in different branches of bilateria
e.g. eyes
- very different pigments used for shielding or proteins used for lens formation
- also very different development in compound and camera eyes in different groups

Question 29

eye evolution - origin and assembly:

Answer 29

cnidarians (metazoa but not bilateria) no pax6 regulation of eye development??? idk

so
between common metazoan ancestor and Ur-bilateria - Pax 6 gene became linked to photoreception genes

assembled a sort of module for photoreception

Ur-bilateria diverged:

• to give protostome groups R-opsins in eye development (even though C opsins are also used in other mechanisms
• to give deuterostome branch C opsins in eye development (even thoug R opsins exist in other systems/mechanisms)
• then other different genes were recruited into eye development finctions (e.g. melanins used as eye shielding pigments in vertebrates instead of ommochromes)
• different genes performing different functions in different parts of teh body recruited into different eye development mechanisms

this didn;t just happen in the descendents of Ur-bilateria, but also in the outgroups like cnidaria

Question 30

pax and eyes in cnidaria

Answer 30

no eyes in porifera

but cnidaria (jellies, hydra…) have eyes

jellyfish can have complex eyes

box jellyfish medusa:

4 structures around bell called rophalium

rhopalium have collections of simple eyes and a recognisable CAMERA eye

(lens in front of retina which has photoreceptor cells and pigment behind it,)

this camera eye is able to form an image

has all of the components you’d expect

retina has opsin

opsin expression driven by pax gene (related pax family gene, related to pax 6)

if this related pax gene is put in drosophila - eye development

also has crystallins directly activated by pax that are used in Cnidaria, but not in other taxa - different crystallins recruited

these crystallins are also expressed in simple eye - involved in some way there

Question 31

assembly of cnidarian camera eye from vertebrate like components

Answer 31

eye type - camera - convergent
Pax control - ancestral derived
components - convergent - C opsin, melanin

-crystallins - novel