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Abstract Thirty-three cases of sex reversals in six cycad genera and seventeen species are summarized. Unidirectional male-to-female and female-to-male changes have been observed and are typically associated with gross physical damage or severe environmental changes. We propose that sex expression in dioecious plants is governed by the presence or absence of methylation of DNA cytosine residues on genes that regulate production of gametes and/or sex hormones..