A survey of sex‐determining systems in amphibians revealed widespread variation in the heterogametic sex and the extent of sex chromosome differentiation. Systems of sex determination have now been resolved in 63 species of salamanders and frogs. Species with ZW female heterogamety, OW female heterogamety, and XY male heterogamety have all been reported. A phylogenetic analysis suggests that the ancestral state for amphibians was female heterogamety. XY/XX sex determination has evolved independently at least seven times, whereas there is only one case in which the data suggest that male heterogamety has subsequently reversed to female heterogamety. Differentiation of heteromorphic sex chromosomes in Amphibia has occured repeatedly. A model of an ancestral two‐locus sex‐determination mechanism, based on the findings of the phylogenic analysis and the condition found in the primitive frog Leiopelma hochstetteri , allows a recessive mutation to result in the eventual acquisition of male heterogamety. Reversal to female heterogamety requires de novo evolution of a functional control locus under the model. The model is consistent with all information on sex determination in amphibians, and explains the observed bias in evolution from ZW/ZZ to XY/XX systems.
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Hillis et al. (1990) studied this question.