2016 Volume 91 Issue suppl Pages 120-127
A male Ginkgo tree at Kami Yagisawa, Minobu-cho, Yamanashi Prefecture, Japan, is shown to possess a small, localized, branch that produces ovules that mature into viable seeds. This tree is recognized as an Ohatsuki Icho because of the occasional production of pollen sacs on otherwise normal vegetative leaves, but most of the abundant male cones that it produces are of normal morphology. Localized sex conversion, such as that seen in the Kami Yagisawa tree, by which part of otherwise male Ginkgo tree switches to producing seeds, may be more common than has previously been noted. Dioecy in extant Ginkgo biloba most likely evolved from monoecious ancestors that had ovulate and pollen cones on different parts of the plant. This change from monoecy to dioecy has also been accompanied by the differentiation of a ZW system of sex chromosomes (heterogametic females ZW, homogametic males ZZ) in which the W chromosome is slightly larger. The molecular developmental basis for sex conversion in Ginkgo is unknown, but in certain angiosperms a specific transcription factor regulates the expression of the female phenotype. A microRNA, which seems to be male specific, negatively affects this transcriptional regulator, resulting in the suppression of female characters and expression of the male phenotype. Further studies are needed to determine whether a similar or different mechanism operates in Ginkgo, and whether localized disruption of the male specific microRNA is responsible for ovule production on a plant that is otherwise morphologically male.
