♂  ♀  

Evolution of Sex-Specific Traits through Changes in HOX- Dependent

doublesex Expression "

Presenter: Peng, Yi-Hung"

Introduction�

“Sexual Dimorphism”性別二型性�

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The origin of sex- specific structures from sexually monomorphic

precursors implies the evolution of new, sexually dimorphic regulatory

pathways. �

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Drosophila species including the melanogaster and obscura species groups � 6�

“Sex Comb”on T1 leg, ta1 segment. (A sexually Dimorphic trait of Male D. melanogaster)�

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on T1 leg, ta1 segment. (A sexually Dimorphic trait of Male D. melanogaster)�

♂   Sex Comb"

♀   No Sex Comb"

dsx (doublesex)�

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dsx (doublesex gene) establishes and modifies sexually dimorphic traits of Drosophila melanogaster � 12�

DM domain genes share the same characteristic DNA binding domain; dsxM and dsxF both share a common binding domain�

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DSX M!Common DNA binding domain DSX F & DSX M"

Bind & Control Expression!

Gene’s binding site for DSX" DNA

Scr (sex comb reduced)�

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Sex comb development requires both dsx and the HOX gene Sex combs

reduced (Scr), suggesting that dsx and Scr cooperate to induce sex- and

segment-specific downstream targets �

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They affect each other…

But How?�

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Experiments & Results�

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What is the expression pattern of dsx

correlated with sex comb?�

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In the male, the most distal TBR is modified into the sex comb � 22�

TBR:  transverse  bristle  rows    

Sex  Comb

Male Female

late non-wandering L3 Stage, T1 leg:high levels of Scr; no Dsx expression�

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Green  :  Dsx  Red    :  Scr

wandering L3 and white prepupal stages, T1 leg:Dsx expression is apparent in both male and female �

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Green  :  Dsx  Red    :  Scr

wandering L3 and white prepupal stages, T1 leg:Dsx expression was not detected in T2 or T3 leg discs �

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Green:  Dsx  Red:  Scr

Prepupal legs at 5 h after pupariation (5 h AP):Dsx expression is clearly seen�

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M M M F

Green:  Dsx  Red:  Scr

The overlap with high Scr expression, which extends more proximally, is more extensive in males than in females � 27�

Male Female Green:  Dsx  Red:  Scr

In males, but not in females, Dsx expression is also seen in clusters of cells in ta2–ta5 segments� 28�

Male Male Green:  Dsx  Red:  Scr

Dsx expression becomes sexually dimorphic at the

prepupal stage�

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Prepupal legs at 16 h after pupariation (16 h AP):Males" ": Dsx is expressed strongly in and around the " " " presumptive sex comb

Female ": expression is consistently lower �

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presump=ve    sex  comb  

Male Female

Male

Male-specific expression of Dsx in ta2–ta5 disappears by this time " 31�

Male Female

Male

24 h AP male : Dsx and Scr develop complementary expression patterns� 32�

Dsx,  Sex  comb  

Scr    

36 h AP male T1 leg:This pattern is maintained at later stages � 33�

Both Dsx and Scr are expressed in restricted and sex-specific patterns in the sex comb at the critical time in its

development.

“dsx Expression Is Regulated in a Spatio-Temporal and Sex-Specific Manner” �

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Is dsx regulation transcriptional ?�

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RNA probe, in situ hybridization (L3 and white prepupal): dsxM transcript is the same as the Dsx protein � 37�

Male  T1  leg

This transcript is undetectable either in the male T2 and T3 discs or in the female T1 � 38�

At 24 h AP:dsxM transcript is similar to the protein distribution � 39�

Male  T1  leg

Ectopic UAS-dsxM expression, rn-Gal4 driver:no evidence for a post-transcriptional mechanism �

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Male Female

Quantitative rt-PCR:not reveal any differences�

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Spatially restricted expression of dsx is caused by transcriptional regulation.

“dsx Regulation Is Transcriptional”�

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What are the roles of dsx and scr in sex comb development?�

We examined the effects of loss and ectopic expression of dsxM

in different cell types. �

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First thing first, you should know the “Gal4-UAS system”�

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tub-Gal80ts; rn-Gal4/UAS-ScrRNAi �

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Prmoter Target

(接下來文中省略)�

Different -Gal4 line"

♂  

UAS-Bline"

X!♀  

Collect F1 (First offspring generation) Only!

F1 (First offspring generation) Genotype"

Bind and activate UAS!

Activate!+  

+  Activate!

5’! 3’!Gal4"

5’! 3’!UAS" B!

Gal4"Produce Gal4!

A  

So……�

Knocking down dsx in rn-Gal4/UAS-dsxRNAi (M&F):Intersex phenotype, similar to the dsx null phenotype� 51�

Reference

à dsxM promotes sex comb development in males while dsxF actively blocks it in females " 52�

Reference

Overexpression dsx - neur-Gal4/ UAS-dsxM flies, restrictive temperature (M&F):Resulted in the transformation of all or most TBR bristles into sex comb teeth �

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None of the bristles outside of the TBRs showed any signs of transformation. àDsxM need to express with Scr together to induce sex comb �

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These results confirm that sex comb rotation is driven by the surrounding epidermal cells and that these cells

require high levels of both Dsx and Scr �

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Overexpression dsx - rn-Gal4/UAS-dsxM flies (F&M):Transformed two to four distal TBRs into small sex combs that underwent rotation.� 56�

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Knock down Scr: 100% loss of the sex comb However, If KD specifically in bristle precursor cells: 50% loss of the sex comb "

It is possible that Scr levels determine the number of bristle precursors during larval or prepupal stages " 58�

Thus, many functions of Scr in sex comb development may be mediated by the activation of dsx expression �

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Knock down "Scr: No Dsx Overexpress "Scr: Dsx express� 60�

Major role of Scr in sex comb development is to initiate a sex-

specific developmental program by turning on dsx expression. �

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Scr expression in the T1 leg is sexually dimorphic

Does Dsx modulate Scr expression in

the sex comb region? �

Knock down "dsxM: medium level Scr Overexpress "dsxM: high level Scr� 64�

"DsxM "up-regulates "DsxF "down-regulates Scr

à"both isoforms are involved in "sexually dimorphic development. �

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The spatial correlation of Dsx and Scr expression is maintained in all species

and reflects sex comb morphology rather than phylogenetic history

How to say it?�

Distantly related species have evolved similar sex combs independently (in melanogaster species group )�

Bigger picture!�

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Cross-regulatory relationship between Dsx and Scr is conserved throughout the melanogaster species group and may contribute to the rapid evolution

of sex comb morphology. �

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The sex comb is a recent evolutionary innovation that is absent in most Drosophila species.� 75�

Dsx expression domain was already present in the last common ancestor of both species groups.� 76�

dsx is expressed in temporally dynamic, rapidly evolving� 77�

However, dsx expression in the

presumptive sex comb region appears to be an evolutionary innovation that

coincides with the origin of the sex comb �

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Sex combs are only one example of sex-specific structures�

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Example 1: D. immigrans with smaller but much more numerous and densely packed bristles � 81�

Male   Female  

Example 2: genus Zaprionus TBRs on the ta1 of the T1 leg are replaced with thinner and numerous bristles� 82�

Male   Female  

Phylogenetic analysis suggests that male-specific morphological structures

originated independently in the immigrans species group, Zaprionus, and the melanogaster+obscura clade �

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Evolutionary gain of new dsx expression domains through a

regulatory link between Scr and dsx has been a key step in the origin of

novel sexually dimorphic structures. �

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Discussion �

Traditional models: sex determination pathway functions ubiquitously Recent work: has shown that dsx is expressed in tightly restricted spatial patterns�

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Sexually dimorphic development may also be regulated through localized

deployment of dsx. �

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Recent Discovery:Upstream regulator of dsx transcription, the HOX gene Scr�

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Positive Feedback Loop?�

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the HOX and sex determination genes establish a positive autoregulatory loop� 92�

No ‘‘master regulators,’’ both dsx and Scr are intimately integrated into the middle of this network � 93�

The positive feedback loop between dsx and Scr may play a major role in

generating sex comb and its diversity across species. �

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Why is this idea important?

To our knowledge, the sex comb is the first example of an evolutionary

change of this kind�

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Sex- specific traits:Established by a dsx-Scr positive autoregulatory loop� 96�

Moreover, there is a constant turnover of sex- specific traits.

Is it possible that the proximate cause

of this turnover of sex- specific traits lies in the acquisition and loss of new spatial expression

domains of dsx? �

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Sex- specific traits lies in the acquisition and loss of new spatial expression domains of dsx� 98�

DSX M!

Scr!Other!

New Sex- specific traits "

So we believe that it is a general role for dsx in evolutionary

innovations.�

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Thank you�

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