The platyfish has W, X, and Y sex chromosomes. Females are WY, WX, or XX and mal
ID: 19577 • Letter: T
Question
The platyfish has W, X, and Y sex chromosomes. Females are WY, WX, or XX and males are YY and XY. It is possible to generate WW fish in the lab, but in the environment they are presumably embryonic lethal. It is presently not known how sex is determined in these fish, but there are some theories consistent with how sex is determined in other systems. Propose a repression based mechanism to explain sex determination in these fish.My initial thoughts would be that W and X produces repressor proteins and that Y produces a protein which acts an an effector; this would lead to the result that lack of transcription results in the female condition and transcription results in the male condition. However, this theory doesn't account for the lack of WW genotype in the population. Any thoughts?
Explanation / Answer
According to the given data, the following can be concluded as an explanation for the sex determination in fish:
1> The main sex determining genes are probably X and Y. W is just a repressor over Y.
2> In the absence of W the default state is female with Y dominant over X so that YY as well as XY are male. If X would repress Y, XY would have been female. Thus Y is the male determining factor. Therefore in some sense Y is the repressor of X unlike suggested in the question.
3> The the presence of W, Y is repressed so that WY is female. This leaves XX and and WX to be female.
4> However the actual role of X is unclear since the fish is female without X (WY) and female with XX. Possibly the default state is female and X is just another activator/repressor and not the actual effector protein.
5> Since W is a repressor, WW is not possible naturally and is embryonic lethal without either X or Y effector proteins (sex determining factors). WW fish is possible in the lab probably because it is somehow able to bypass the the role of X or Y in activating some other cascade of protein synthesis down the line. The reason might have something to do with the natural versus artificial environment in which they develop.
So the mechanism goes like:
In presence of Wrepress YWY female (default state)
no action/activate XWX female (default state/absence of Y)
no action/activate XXX female (default state/absence of Y)
In absence of WY not repressedXY male
Y not repressed/absence of XYY male
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