Sex Determination in Drosophila

What is the Role of Sex

Chromosomes in Drosophila?

• Humans: XX = female; XY = male

• Flies: XX = female; XY = male

• Humans: XXY = male; XO = female

• Flies: XXY = female; XO = male

• The number of X chromosomes is important?

– Absolute or relative to the number of autosome sets (two)?

X:Autosome Ratio is Critical

• XX = female;

XY = male

• XXY = female;

XO = male

• RULE:

– 2X : 2A --> female

– 1X : 2A --> male

How to Measure the X:A Ratio?

• Numerator proteins are encoded by X

– Sis-a, sis-b, sis-c, runt, dpn

• Denominator proteins encoded by autosomes

– Da, her, emc, gro

• Numerator proteins are transcription factors

• Denominator proteins block numerators, probably by forming inactive dimers

Several Mutations Affect Drosophila

Sex Determination

• sex-lethal (Sxl) male (dead) male

• transformer (tra) male male

• transformer 2 (tra2) male male

• doublesex (dsx) intersex intersex

XX XY

Sxl

• Master regulatory protein that controls all

aspects of sexual development

• X:A ratio controls Sxl transcription

– Brief window: early promoter

What Does Sxl Do?

• Sxl = RNA-binding protein

– Binds pre-mRNAs: sxl, tra, dsx

– Causes “female-specific” mode of splicing

Sex-specific RNA Splicing Cascade

Sex-specific RNA Splicing Cascade

Sex-specific RNA Splicing Cascade

Doublesex is Needed for Both Sexes

• Sex-lethal (Sxl) male(dead) male

• transformer (tra) male male

• transformer 2 (tra2) male male

• doublesex (dsx) intersex intersex

XX XY

Regulation of Somatic Sex in

Drosophila

• Female Male

Summary Drosophila

• Calculation of X/A

• Importance of splicing controls

• Cascade of interactions

C. elegans

Worm Choice

• Somatic sex

differentiation

– hermaphrodite or

male body?

Hermaphrodite

Male

Initial Trigger is X:A ratio

Chromosomes Autosomes X:A ratio Sex

XX 2 2/2 (1) herm

X 2 1/2 (0.5) male

XX 3 2/3 (0.67) male

XXX 4 3/4 (0.75) herm

Reading of X:A Ratio is Similar to

Fly

• Numerator proteins = X-linked genes

– fox-1 and sex-1 and others

– Prevent expression of master regulatory gene:

xol-1

• Denominator proteins (autosome-linked) still

unknown

What Are Key Downstream Genes?

• her-1,

• tra-2

• fem-1, 2, 3

• tra-1

XX XO

Tra-1 is Critical

• Last gene is tra-1

• If Tra-1 active herm

• If not male

• All others regulate

Genetic Analysis

• Tra-1 mutations suppress her-1 mutations

– Why?

• Her-1 is upstream

• Her-1 is on in males

her-1 --| tra-2 --| fem-1,2,3 --| tra-1 herm

--| male

Problems

• Herm needs to execute two programs after

gonad is made

– Make sperm

– Make eggs

Tra-2 Repression Needed for

Sperm Production

Tra-2 mRNA translation needs to be repressed in germ line cells to get sperm

• 3’ region binds inhibitor of translation

– If delete 3’ region - herm -> female

– Escapes repression, no sperm production

• Translational repression of tra-2 by laf-1

• Laf-1 opposed by tra-3 in hermaphrodites

– Laf-1 expressed in embryo to allow construction of ovary and again past L3 after sperm are made to allow egg production

Fem-3 Also Needed for Sperm

• Fem-3 needed for male somatic cells and germ line differentiation – But need to turn off to make eggs

• Fem-3 transcription inhibited by tra-2 – But leftover fem-3 mRNAs from spermatogenesis need to be

inhibited

– by binding of inhibitors (nanos, pumilio, FBF) to 3’ region to turn off protein synthesis during egg formation in herm (L3) after sperm are produced

C. elegans Summary

• Calculation of X/A

• Importance of translational controls

• Cascade of interactions

• Problem in hermaphrodite of forming both

sperm and egg as opposed to just the sperm

forming program