BG10 Flashcards

1
Q

Diversity of segment identity

A

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

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2
Q

segment identity drosophila

- following hypothesise

A

controlled by hox genes

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

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3
Q

evolution of wing loss diptera

A

change in ubx downstream response

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4
Q

wing loss in diptera

A

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

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5
Q

Ed lewis

A

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

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6
Q

rebuttle to ed lewis

A

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

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7
Q

drosophila thorax code

A

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

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8
Q

hypothesis for T3 halteres

A

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

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

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9
Q

test for ubx t3 repressive activity

and result

A

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

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10
Q

evo of abdominal leg loss in hexapoda

A

change in ubx sequence

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11
Q

hexapoda leg loss

A

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.

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12
Q

ubx and abdominal leg loss observations and conclusion

A
  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.
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13
Q

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

A
  • 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.
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14
Q

test for ubx differences in crustaceans and diptera

A

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

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15
Q

insect ubx protein sequence

A

different form crusaceans

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

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16
Q

importance of QA motif in repressing abdominal limbs

A

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

17
Q

re-evo of abdominal legs in lepidoptera larvae

A

change in ubx expression

18
Q

abdominal leg evo in lepidoptera larvae

A

butterfly larvae have re-evolved adominal legs

  • although it has evolved hexapod c-terminus with QA motif
  • but has gone back to crustacean condition
19
Q

DII

A

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

20
Q

DII expression in insects

A

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

21
Q

why does ubx not repress limbs in buttefly larvae

hypotheses

A

ubx restricted?

has ubx protein lost its insect specific limb repressing function?

22
Q

butterfly embryogenesis and limb formation

A

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

23
Q

tree hopper

A

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

24
Q

2011 tree hopper claim

A

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.
25
further evidence for helmet wing homology
during development helmet primordium expresses homologs of drosophila wing genes DII nubbin homothorax
26
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
27
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.
28
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
29
overexpression of treehopper scr in drosophila thorax
same effect causing wing loss - scr hasnt changed in protein structure function must be downstream genes
30
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
31
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
32
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
33
paths of segment identity in hox
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
34
are evo of hox genes important in arthropod seg identity?
appaz not | contrary to lewises theory.