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Abstract Sex determination in cycads"and most other plants and animals"is a function of cytosine methylation and/or chromatin formation downregulating genes that control female or male function. Sex change probably occasionally occurs in cycads because there is no genetic assimilation of these epigenetic signals. In plants, allopolyploidy is the only escape from such canalized dioecy, but allopolyploidy never occurs in cycads. Cycads never developed a pair of unequal length sex chromosomes because their haploid stages are large, complex, and long-lived.