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There are several sex determining systems in animals.

In mammals XY = male(except in rare cases where SRY crosses with genes on the X so that it is the reverse) and XX = female.

In some birds it is just the opposite. That is XY(actually ZW) = fertile female and XX(actually ZZ) = fertile male.

In some reptiles it is determined by temperature and has no genetic determination.

In lots of insects there is XX = female and X0 = male. This is usually accompanied by males being haploid and females being diploid like it is in bees.

In some insects and reptiles there is also the ZW/ZZ system but they are not dependent on it. In other words ZZW in some insects and reptiles can give rise to either sex but there is no temperature determination either.

Why are there so many sex determining systems? Why did animals evolve to not all have the familiar XY system? It must be so confusing as to which species has which sex determining system when biologists are trying to determine the gender of an animal.

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Could not fit in a comment…

I don't quite know how to answer. I think that the only answer one could give is why not?. Various mechanisms may evolve under different life-history, different environment, different ecological strategy, different mutations occuring,..

In some species (including some amphibians) the evolution of sex determination system is highly dynamic and switch back and forth between one system and another.

I highly recommend the book: The Evolution of sex determination system (2014) that will give you a very good theoretical understanding of how sexual determination system evolve and make a complete review of empirical studies on the evolution and mechanisms of sex determination system. I think it is the only book existing on the subject.

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  • $\begingroup$ I felt the same way ,thought of writing a comment but this will do. $\endgroup$ – Tyto alba Sep 29 '16 at 7:22
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Here's just the paper for you: Sex Determination: Why So Many Ways of Doing It? (Bachtrog et al, 2014.)

(And, it's open access!)

One important point is that although the initial "triggers" are diverse, the downstream pathways can be highly evolutionarily conserved.

Some tidbits from the paper:

  • ESD [Environmental Sex Determination] is favored over GSD [Genotypic Sex Determination] when specific environments are more beneficial to one sex, selecting for sex-determining mechanisms that match each sex to its best environment...
  • The reverse transition, from ESD to GSD, is thought to be favored when the environment is unpredictable or not variable enough, in which case ESD could produce strongly skewed sex ratios or intersex individuals.
  • In species with genotypic sex determination, the chromosome pair that determines sex can change rapidly over time... Sexually antagonistic selection, which occurs when a mutation is beneficial to one sex but detrimental to the other, can... drive transitions between sex determination by different pairs of chromosomes. For example, if an allele of an autosomal gene is beneficial to males but harmful to females and becomes linked to a dominant masculinizing mutation, then chromosomes that carry both the male-beneficial and male-dominant alleles create a novel Y that can replace the ancestral mechanisms. Conversely, alleles that benefit females and harm males can create novel W chromosomes when linked to feminizing mutations...

And more. Check it out!

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