Ovule development and pollen tube growth in Tsuga chinensis: insights into the evolution of siphonogamy

Confirmation of the correlation between archegonium development and pollen tube guidance and identification of putative genes implicated in male‒female interactions in gymnosperms. Studying angiosperm-like siphonogamy in gymnosperms will increase our understanding of seed plant evolution. This study involved an exploration of pollination and ovule development in Tsuga chinensis, which is an interesting gymnosperm species endemic to East Asia that exhibits angiosperm-like siphonogamy. Using comprehensive morphological approaches, we determined that the mechanism underlying T. chinensis pollination involves the germination of pollen grains outside the ovule and the growth of pollen tubes into the micropyle. Furthermore, a correlation between the timing of archegonial appearance and pollen tube growth was confirmed. Additionally, transcriptome sequencing revealed 19,643 DEGs that are involved in ovule development. Through bioinformatics analysis, we identified several putative genes that are involved in male‒female interactions in gymnosperms, and further validation of the functions of these DEGs is worthwhile. These findings offer valuable insights into the progression of the complex evolution of siphonogamy across seed plants and lay a foundation for understanding the molecular mechanisms underlying siphonogamy in gymnosperms.