BG10

Question 1

Diversity of segment identity

Answer 1

all arthropods are segmented
segments have a variety of apendages
appear to have different identities: drosophila, butterfly centipede etc.

Question 2

segment identity drosophila

- following hypothesise

Answer 2

controlled by hox genes

alterations in hox gene expression might give rise to all the morphological diversity in arthropods

Question 3

evolution of wing loss diptera

Answer 3

change in ubx downstream response

Question 4

wing loss in diptera

Answer 4

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

Question 5

Ed lewis

Answer 5

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

Question 6

rebuttle to ed lewis

Answer 6

1980s found ubx is ancient - all hox genes are atleast pan-bilaterian.
and not traditioanlly expressed in t3`

Question 7

drosophila thorax code

Answer 7

ubx = on
all other posterior hox genes = off
-T3= halteres

Question 8

hypothesis for T3 halteres

Answer 8

ubx is activated in T3 in diptera and represses wings here.

but not activated in t3 in four winged, like butterflies.

Question 9

test for ubx t3 repressive activity

and result

Answer 9

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

Question 10

evo of abdominal leg loss in hexapoda

Answer 10

change in ubx sequence

Question 11

hexapoda leg loss

Answer 11

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.

Question 12

ubx and abdominal leg loss observations and conclusion

Answer 12

1. overexpression of ubx in thoracic segments cause loss of the pro-legs.
2. loss of ubx expression causes extra abdominal legs to form
   - -> therefore ub (and other abdomimal hox genes presumabaly) must repress limb formation.

Question 13

hypothesis: ‘ ubx has expanded to repress limbs in hexapods and is not present in crustaceans which have abdominal limbs’

Answer 13

• 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.

Question 14

test for ubx differences in crustaceans and diptera

Answer 14

transforming drosophila with artemia ubx
- failed to repress legs
therefore drosophila ubx protein must be differences

Question 15

insect ubx protein sequence

Answer 15

different form crusaceans

-dipterans have a c-terminus string of glutamines and alanines (QA motif) not in crustaceans

Question 16

importance of QA motif in repressing abdominal limbs

Answer 16

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

Question 17

re-evo of abdominal legs in lepidoptera larvae

Answer 17

change in ubx expression

Question 18

abdominal leg evo in lepidoptera larvae

Answer 18

butterfly larvae have re-evolved adominal legs

• although it has evolved hexapod c-terminus with QA motif
• but has gone back to crustacean condition

Question 19

DII

Answer 19

distal-less TF expressed in appendage primordia (distal most part) of all insects and many metaxzoans
= deeply conserved

Question 20

DII expression in insects

Answer 20

expressed in pro-legs of butterfly caterpillar
ubx represses limb formation in insects by repressing DII
DII causes abdominal limb formation

Question 21

why does ubx not repress limbs in buttefly larvae

hypotheses

Answer 21

ubx restricted?

has ubx protein lost its insect specific limb repressing function?

Question 22

butterfly embryogenesis and limb formation

Answer 22

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

Question 23

tree hopper

Answer 23

hemiptera with helmet like structures on their throaxes - first throacic segment
allows camoflage - looks like a leaf or a thorn etc.

Question 24

2011 tree hopper claim

Answer 24

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.

Question 25

further evidence for helmet wing homology

Answer 25

during development helmet primordium expresses homologs of drosophila wing genes
DII
nubbin
homothorax

Question 26

wing repression in t1 in most insects

Answer 26

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

Question 27

scr expression treehopper

• hypothesis

Answer 27

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.

Question 28

scr overexpression in treehopper thorax

Answer 28

represses wings in throax just as native scr does
therefore downstream response must have changed
hox gene is the same

Question 29

overexpression of treehopper scr in drosophila thorax

Answer 29

same effect
causing wing loss
- scr hasnt changed in protein structure function
must be downstream genes

Question 30

hypothesis for helmet and scr relationship in tree hopper

Answer 30

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

Question 31

entomologist re-examination

Answer 31

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

Question 32

explain why wing genes switched on in helmet if not wing

Answer 32

e.g.DII and nubbin
- DII switched on in many apendages possibly an outgrowth
not neccessarily wing specific

Question 33

paths of segment identity in hox

Answer 33

1. changes in hox protein function -= abdominal limb repression by ubx in hexapods
2. upstream changes in hox gene expression - re-evo abdominal legs
3. dowstream changes in response to hox genes - evo of wing loss diptera and helmet gain

Question 34

are evo of hox genes important in arthropod seg identity?

Answer 34

appaz not

contrary to lewises theory.