BG10 Flashcards
Diversity of segment identity
all arthropods are segmented
segments have a variety of apendages
appear to have different identities: drosophila, butterfly centipede etc.
segment identity drosophila
- following hypothesise
controlled by hox genes
alterations in hox gene expression might give rise to all the morphological diversity in arthropods
evolution of wing loss diptera
change in ubx downstream response
wing loss in diptera
diptera, in hexapoda closely related to lepidoptera
most insects have two pairs of wings, T2 and T3 (dragonflies and butterflies) however the diptera, flies and mosquitos have only one (T2)
- lost third T3 - third thoracic segment
Ed lewis
1970
observed bithorax loss of function mutation in Ubx hox gene gives flies four wings.
- proposed evo of two winged condition occured when hox gene duplicated giving rise to ubx that represses wing formation in T3
rebuttle to ed lewis
1980s found ubx is ancient - all hox genes are atleast pan-bilaterian.
and not traditioanlly expressed in t3`
drosophila thorax code
ubx = on
all other posterior hox genes = off
-T3= halteres
hypothesis for T3 halteres
ubx is activated in T3 in diptera and represses wings here.
but not activated in t3 in four winged, like butterflies.
test for ubx t3 repressive activity
and result
lepidoptera
ubx found to be expressed in t3 imaginal disc, just as in drosophila
- therefore two winged condition cannot be due to change in ubx expression
- rather change in way downstream genes respond to ubx
evo of abdominal leg loss in hexapoda
change in ubx sequence
hexapoda leg loss
insects have three pairs of legs
all on their thoracic segments
however their ancestors had abdominal legs too
- drosophila larvae have small thoracic pro-legs kaelins organs but dont have abdominal legs.
ubx and abdominal leg loss observations and conclusion
- overexpression of ubx in thoracic segments cause loss of the pro-legs.
- loss of ubx expression causes extra abdominal legs to form
- -> therefore ub (and other abdomimal hox genes presumabaly) must repress limb formation.
hypothesis: ‘ ubx has expanded to repress limbs in hexapods and is not present in crustaceans which have abdominal limbs’
- porcellio isopod crustacean has ubx expression in all its limb bearing segments.
- therefore
either
1. ubx has evolved ability to repress legs in drosophila by a change in protein function
2. or insects have evolved to respond differently to ubx by changes in cis-acting reg elements of downstream limb formation genes.
test for ubx differences in crustaceans and diptera
transforming drosophila with artemia ubx
- failed to repress legs
therefore drosophila ubx protein must be differences
insect ubx protein sequence
different form crusaceans
-dipterans have a c-terminus string of glutamines and alanines (QA motif) not in crustaceans
importance of QA motif in repressing abdominal limbs
artemia- drosophila chimeric protein made
test them in drosophila
deleting the c terminus in the artemia ubx allows it to repress legs.
- found this was because the artemia c-terminus normally represses a limb repressor (DII) causing abdominal leg formation.
- QA domain in insects therefore allows ancient limb repressing domain to become active again causing no abdominal legs ubx represses limb formation by repressing DII
re-evo of abdominal legs in lepidoptera larvae
change in ubx expression
abdominal leg evo in lepidoptera larvae
butterfly larvae have re-evolved adominal legs
- although it has evolved hexapod c-terminus with QA motif
- but has gone back to crustacean condition
DII
distal-less TF expressed in appendage primordia (distal most part) of all insects and many metaxzoans
= deeply conserved
DII expression in insects
expressed in pro-legs of butterfly caterpillar
ubx represses limb formation in insects by repressing DII
DII causes abdominal limb formation
why does ubx not repress limbs in buttefly larvae
hypotheses
ubx restricted?
has ubx protein lost its insect specific limb repressing function?
butterfly embryogenesis and limb formation
butterfly embryos express ubx and abd-a in their abdominal segments
however both ubx and abd-a become repressed in local patches in the abdominal segments, allowing DII expression and so leg formation.
**ubx is switched off in small parts to evolved legs
therefoe must have changed the way ubx is regulated.
= local changes in hox expression
tree hopper
hemiptera with helmet like structures on their throaxes - first throacic segment
allows camoflage - looks like a leaf or a thorn etc.
2011 tree hopper claim
helmet is wing homolog
- microscopic examination showed it is paired, has a hinge, is scelortized and has veins.
- usually wings on t1/t2 - treehopper it seems is on t1.
further evidence for helmet wing homology
during development helmet primordium expresses homologs of drosophila wing genes
DII
nubbin
homothorax
wing repression in t1 in most insects
usually in most insects sex combs reduced SCR an anterior hox gene represses wing formation in t1
KO causes ectopic wing formation in t1 in tribolium beetle
scr expression treehopper
- hypothesis
not changed in treehopper t1
expressed here as in other insects and throughout helmet development
- either scr protein has changed or way in which genes respond to it has changed.
scr overexpression in treehopper thorax
represses wings in throax just as native scr does
therefore downstream response must have changed
hox gene is the same
overexpression of treehopper scr in drosophila thorax
same effect
causing wing loss
- scr hasnt changed in protein structure function
must be downstream genes
hypothesis for helmet and scr relationship in tree hopper
originally scr did not conttrol wings, but evolved to repress wings in t1
this repression was lost again by unresponsive downstream genes in treehopper t1
entomologist re-examination
claimed no evidence helmet was a wing
suggested was an outgrowth like dungbeetle horn
- if so wing genes have been co-opted to form a non-wing
explains why scr has no effect
explain why wing genes switched on in helmet if not wing
e.g.DII and nubbin
- DII switched on in many apendages possibly an outgrowth
not neccessarily wing specific
paths of segment identity in hox
- changes in hox protein function -= abdominal limb repression by ubx in hexapods
- upstream changes in hox gene expression - re-evo abdominal legs
- dowstream changes in response to hox genes - evo of wing loss diptera and helmet gain
are evo of hox genes important in arthropod seg identity?
appaz not
contrary to lewises theory.
