Changes in the morphology and gene expression of developing zebrafish gonads

•Current zebrafish laboratory strains do not have a sex chromosome.•The number of germ cells controls zebrafish sex determination.•Transcripts in premeiotic germ cells control zebrafish sex differentiation.•Zebrafish ovaries differentiate first, and some can revert to testes later.•Zebrafish testes differentiate from indifferent gonads or transient female gonads.