Drosophila Sex Determination: Every Cell for Itself Mechanism

Phenotypic Consequences of Different Ratios of X Chromosomes to Autosomes

In Drosophila n=4, where there is 1 sex chromosome and 3 autosomes.
Call each autosome set A, therefore there are 3 autosmes in a diploid fly (A=2)
Sex determination is due to the ratio of X chromosomes to the sets of autosomes (A)

Therefore:

XXAA (diploid)	X:A ratio = 1	Female
XYAA (diploid)	X:A ratio – 0.5	Male
XOAA (diploid)	X:A ratio = 0.5	STERILE Male
XXXAAA (aneuploid)	X:A ratio = 1	Female
XYYAAA (aneuploid)	X:A ratio = 0.33	Male
XXYAAA (aneuploid)	X:A ratio = 0.67	Intersex

Regulatory Pathway

The phenotype is carried out by the ‘master regulatory switch’ and downstream specific sex genes.

Sxl gene (Sex lethal gene): choice of pathway is initiated by differential transcription of gene as well as tra (transformer) gene.

The direction of the switch is maintained by an autofeedback loop

The desistion is propograted along the developmental pathway by differential RNA splicing of dsx (doublesex) gene.

The default is a male phenotype.

Regulatory Switch (Sxl gene)

If X:A ratio = 1, then SXL protein is synthesized and a phenotypically female is the result. If X:A ratio = 0.5, then a non-functional SXL protein is synthesized and a phenotypically male fly is the result.

Mechanism

The X:A ratio set in motion the sex determination pathway by interaction between numerator genes ( X-chromosomal and zygotically expressed) and denominator genes (autosomal, maternally and zygotically expressed).

These genes encode for transcription factors in a basic helix-loop-helix (bHLH): NUM, the X-encoded bHLH and DEM, the autosomal encoded bHLH transcription factor.

The transcription factors have a very short time window, 2-3 hours of fertilization, to determine the sex of the individual via the Sxl gene regulatory switch.

If the gene is turned on, then the level of active X:A NUM transcription factor must be high. They bind to enhancers of Sxl gene, activating protein transcription from the early promoter (P_E) resulting in the fully functional SXL protein.

If the gene is turned off, the level of NUM is lower than the threshold, and it is insufficient to promote transcription.

The A:X ratio is measured by NUM proteins competing for dimmer formation with DEM proteins. The transcription factor is only active if NUM forms a dimeric protein complex.

NUM monomers have sequence specific DNA binding sites, DEM does not.

Binding site recognition enhancer sequences regulates transcript from promoter of Sxl regulator gene switch

NUM and DEM polypeptides synthesized at level proportionate to number of copies of each bHLH encoding numerator or denominator gene

All possible combinations of dimmers can form, but to be active both subunits have to have sequence specific DNA binding sites.

Maintenance of condition

Sxl gene has 2 promoters, P_E and P_L

P_E is activated by NUM:NUM and is active only in early embryogenesis

P_L promoter transcribes the gene regardless of X:A ratio and is active from mid-embryogenesis to adulthood

The transcript from P_L is larger and is subject to mRNA splicing. The splicing depends on the presence or absence of the pre-existing SXL protein. The SXL protein is a RNA binding protein and alters the splicing of the mRNA. If the SXL protein is present, the mRNA is spliced to encode for more SXL protein (female), and if not a nonsense protein is translated (male).

Propagation of the design

SXL protein has autoregulatory maintenance function and is capable of activating the shunt pathway leading to the female condition via mRNA splicing.

In the presence of SXL, the primary tra (transformer) gene transcript is spliced to result in the TRA protein, which like SXL is a RNA binding protein. It bind to mRNA to produce female specific splicing of dsx (doublesex) nacent mRNA resulting in the female DSX protein. The DSX protein is a global repressor of male specific gene expression.