BG9 Flashcards
hox genes origin
all metazoans but sponges have hox genes
- discovered by DNA radioactive probes
GRNs
animal development has a heirachy of GRNs postgastrula axiel patterning GRN Body part specificaiton GRN Body pattern formation GRN cell fate specification GRN
AP axis
all bilateria have ap axis - head and tail
potential homology?
specification of regions in drosophila
syncitum = TFs specifiy
- other organisms use small molecule morphogens like RA that bind to CSRs
drosoph
maternal effect, hb protein gradient, gapt genes, pair rule genes, segmeented polarity genes nad homeotic genes in that order
drosophila - what specifies the posterior axis
caudal = maternal effect TF posterior expression of caudal is ancient found in vertebrata and nematoda homologs in c. elegans = Pal-1 homologs in xenopus = xcad2
specification of anteriror axis in bilateria
expression domains of tf six3/6 and optx mark the head region in many biilateria
ecdysozoa, lophotrochozoa, vertebrates etc.
anterior effect gene in drosophila
bicoid
ap axis homology in bilateria?
seems to be homolloogous in animas
but many genes are not ancient, e.g. bicoid
cnidarian ap axis
cniderians ahve oral/aboral axis
Nematostella anenome has six3/6 expressed at aboral pole of larvae and foxq2 which is also expressed anteriorlly in bilateria
- suggests aboral is homologous to anterior.
scenario for aboral to anterior
gastrea like creature flopped on side so aboral became anteiror
radially symmetrical creatures dorsal ventral axis?
dont have, would imply were radial is had both axis.
nematostella radial symmetry
has a ciliate groove off to one side - the siphonoglyph - creates currents of water in the pharynx
- must be positional info for this, so cannot be radial.
** homologs of a variety of genes import in axis formation in vertebrates are expressed assymmetrically in nematostella larvae
noggin, chordin and two hox genes
what is dorsal and ventral?
hard to decide
geoggrey st hillaire first noted organs are dorsal in some animals and ventral in others.
lobster upside down = CNS - nerve chord is found ventrally, where ours is dorsal
dv axis specifcation in drosophila
works by a series of signals along dv axis
cell terrioties are marked by two interacting gradients of signalling molecules
* dorsal gradient of Dpp
ventral one of sog. which antagonise
dv axis specfication in xenopus
vertebrates dorsal side forms the blastopore lip = organiser
dorsal chordin is one of the key signalling molecules of the organised, antagonised by ventral bmp4
homology of dv axis
ventral sog is homologous to dorsal chordin
dorsal dpp is homologous to ventral bmp4
- inverted
dorsal and ventral definitons are redundant?
hemichordates acorn worm
potential LCA of deuterostomes.
closely related too echinoderms, dont have laters dervied radial geometry.
however both have similar larvae - trochophore.
Saccoglossus ventral axis
no cns no nerve net
mouth is ventral
chordin is expressed ventrally
bmp is expressed dorsally
like insects rather than vertebrates
suggests inversion happens after split from hemichordates
potentially in response to specification of a new CNS?
CNS of animals
DEL have very diff CNS.
verebrates form CNS via neurulation (dorsal neuroectoderm folds into a tube) - E and L dont do this.
- thought therefore CNS = convergent in superphyla. vertebrates and invertebrates.
as LCA of vertebrates didnt have a CNS free to form dorsally or ventrally. - however CNS appears homolgous
Homology of CNS drosophila and xenopus
- both drosophila and xenopus CNS arises from 3 coloumns of neuroblasts
- both these neuroblasts are specified by particular TFs
- from out to in msh/Msx -> ind/Gsh-1 - insects and vertebrate both have a tripartite brain - regions of brain specfied in the same way.
drosophila and xenopus and platyneris annelis the tfs incolved in ap patterning are similar.
**otd/otx, optic/six etc.
homology in brain structure
in mammals basal ganglia and in drosophila central complex are both rich in dopaminergic neurons and both responsible for motor control
thought to be homologous.
proposed homologies in brain structure
mammalian cudate nucleus (found in basal ganglia) and plutamen correspond to insect fan shaped boy and protocerebral bridge (found in central complex).
mammalian external and internal globus pallidus (found in basal ganglia) correspond to ellipsoid body (found in central complex)
gut and mouth homology?
protostomes - actually gives rise to mouth and anus simultaneosuly.
deuterstomes - blastopore gives rise o anus and mouth forms later.
mouth and anus thought not to be homologous
however brachyury TF is expressed in fore and hind gut in similar way - suggesting homology
what must a model for evo of metazoan geometry account for
- bilaterian AP axis homology
- bilaterian DV axis specification homology
- and inversion - bilaterian fore and hind gut homology
- bilaterian cns homology
- bilaterian tripartite brain homology
what does a model taking all major things into account for metazoan geometry sugesst
- acoels and xenoturbella separated before rest of bilateria.
- LCA of ec/lop/duet was a swimming creature like a modern trochophore larvae and had a brain - CNS and through gut
- creature setlled either on one side or other giving rise to ec and lop and or else higher deuterstomes and hemichordates setttled seperately and lost their cns.
