sex determination - fly lab - homethe critical gene in drosophila sex determination, the autosomal...
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Sex Determination BIOS 0802
2018
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Drosophila
• Drosophila sex is generally determined by ‘X’ chromosome
• One copy in a diploid cell = male
• Two copies in a diploid cell = female
• ‘Y’ chromosome plays no part in determination of sex
• Interestingly, each cell in Drosophila (and many other insects) determines its own sex
• In one fly one cell can be male (XO) while the neighbouring cell female (XX)
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Gynandromorphs
• Morgan and Bridges (1919) concluded, "Male and female parts and their sex-linked characters are strictly self-determining, each developing according to its own aspiration,"
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X-to-autosome ratio
• Although it had long been thought that a fruit fly's sex was determined by the X-to-autosome (X:A) ratio (Bridges 1925), this assessment was based largely on the fact that flies have aberrant numbers of chromosomes.
• But recent molecular analyses suggest that X chromosome number alone is the primary sex determinant in normal diploid insects (Erickson and Quintero 2007).
• Lets see how
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Sex-lethal (Sxl) gene
• X chromosome contains genes encoding transcription factors that activate the critical gene in Drosophila sex determination, the autosomal locus Sex-lethal (Sxl)
• The Sxl gene has two promoters. The early promoter is active only in XX cells; the later one is active in both XX and XY cells.
• The X chromosome appears to encode four protein factors that activate the early promoter of Sxl.
• Three of these proteins are transcription factors
• SisA, Scute, and Runt—which bind to the early promoter to activate transcription
• Unpaired, reinforces other 3 by JAK/STAT pathway
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Sxl mRNA
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Re
gu
lato
ry c
asca
de
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Doublesex function
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Behaviour and fruitless
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Dosage compensation
• Sxl binds to the 5’ UTR of msl2 mRNA
• That inhibits msl2 attachment with ribosome
• Therefore no msl2 protein
• Without Sxl msl2 proteins are made
• msl2 proteins (it’s part of a complex) acelylates histone 4 and unwinds ‘X’ chromosome
• Therefore hypertranscription of ‘X’ in males
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Sex determination in bees
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In mammals
• XX = female
• XY = males
Define this individual
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Its all starts with
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If o
ne
ha
s ‘Y
’
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If o
ne
ha
s n
o ‘Y
’
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Make ‘A’ and don’t make ‘B’
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Sex determination in mammals
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Dosage compensation • each mammalian somatic cell, whether male or female, has only one functioning X
chromosome. This phenomenon is called X chromosome inactivation.
• Inactive chromosome converted into heterochromatin