Abstract
In gymnosperms dioecy occurs in 64.6%, and sex chromosomes have been identified in 0.6% of the total species. All the Cycadidae, Gnetidae, Ginkgo and some species in Pinidae are dioecious. Heteromorphic sex pairs have been studied in six species where the heteromorphic sex may either be male giving rise to XX/XY system or female resulting in ZZ/WZ system. The chromosome determining the male sex may also be formed by the fusion of two telocentrics resulting in X1X2X1X2/X1X2Y system as in some Podocarpus species. In other cases where chromosomal hetromorphism is undetectable, the molecular markers and functional genomics have been applied to facilitate early sex detection while DNA methylation has also been indicated to be associated with sex determination. The multiple origins of dioecy is evident by its presence in evolutionarily distinct taxa, which provides an opportunity to explore diverse types and their mechanisms of sex determination in this group of plants.
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