2026-01-28 東京大学
ソテツ胚珠における花粉管、花粉管内の精子、鞭毛運動をする生きた精子
<関連情報>
現生裸子植物ソテツにおける運動性精子と花粉管の中間進化状態 Intermediate evolutionary state of motile sperm and pollen tubes in the extant gymnosperm Cycas revoluta
Yukiho Toyama, Satohiro Okuda, Takamasa Suzuki, and Tetsuya Higashiyama
Proceedings of the National Academy of Sciences Published:January 23, 2026
DOI:https://doi.org/10.1073/pnas.2506320123
Significance
Cycads and ginkgo, possessing primitive motile sperm and derived pollen tubes, are important for understanding the evolution of land plant fertilization from zooidogamy to siphonogamy. We explored gene expression in pollen tubes and capacitated motile sperm of Cycas revoluta in a tissue-specific transcriptome analysis. As in other land plants, transcriptional repression involving a male-specific histone variant has been suggested in these male cells. Cycad pollen tubes largely lacked genes related to rapid and guided growth as seen in angiosperms, while cycad sperm shared more distinctive orthologous genes with angiosperm sperm cells than with basal land plant motile sperm. A molecular intermediate state between zooidogamy and siphonogamy was revealed, providing important insights into the evolutionary process of land plant fertilization.
Abstract
Evolutionary transitions in land plant fertilization from zooidogamy to siphonogamy were characterized by transformations of male reproductive cells. Basal land plants such as bryophytes and pteridophytes have motile sperm, whereas most seed plants have nonmotile sperm, delivered by a pollen tube. Despite being seed plants, gymnosperm cycads and ginkgo uniquely form highly multiflagellated and large motile sperm within pollen tubes. However, the evolutionary state of these male reproductive cells remains unknown. We clarified the gene expression profiles of Cycas revoluta pollen tubes and motile sperm swimming toward female reproductive cells. Male cycad cells expressed fewer genes associated with transcription, translation, and related processes, which is consistent across land plants. We compared the distinctive orthologous groups (OGs) of the genes specifically expressed in sperm and pollen tubes with those in other plants. Cycad pollen tubes shared several OGs with angiosperms but possessed significantly fewer gene copies and lacked cell wall remodeling and plasma membrane-localized receptor genes that contribute to rapid and guided growth. The growth mechanism of cycad pollen tubes might be largely different from angiosperm pollen tubes. In contrast, despite their morphological uniqueness, cycad sperm shared representative OGs with angiosperm sperm cells to the same extent as egg cells. In addition, a sperm-specific histone variant may contribute to transcriptional regulation via chromatin condensation like other male gametes. As an extant gymnosperm that retains zooidogamy with pollen tubes, the cycad represents a molecular intermediate state in the transition from zooidogamy to siphonogamy, providing insight into the evolution of land plant fertilization.
