Evolution of alternative sex‐determining mechanisms in teleost fishes

We compiled information from the literature on the taxonomic distributions in extant teleost fishes of alternative sex‐determination systems: male‐heterogametic (XY) gonochorism, female‐heterogametic (ZW) gonochorism, hermaphroditism, unisexuality, and environmental dependency. Then, using recently published molecular phylogenies based on whole‐genomic or partial mitochondrial DNA sequences, we inferred the histories and evolutionary transitions between these reproductive modes by employing maximum parsimony and maximum likelihood methods of phylogenetic character mapping. Across a broad teleost phylogeny involving 25 taxonomic orders, a highly patchy distribution of different sex‐determination mechanisms was uncovered, implying numerous transitions between alternative modes, but this heterogeneity also precluded definitive statements about ancestral states for most clades. Closer inspection of family‐level and genus‐level phylogenies within each of four orders further bolstered the conclusion that shifts in sex‐determining modes are evolutionarily frequent and involve a variety of distinct ancestral‐descendant pathways. For possible reasons discussed herein, the evolutionary lability of sex‐determining modes in fishes contrasts strikingly with the evolutionary conservatism of sex determination within both mammals and birds. © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 87 , 83–93.