Flagellum formation and the shift to non-motile sperm
Unlike most extant gymnosperms and all angiosperms, cycads retain motile sperm cells with flagella similar to more ancestral land plants (Fig. 1b; see Offer et al., 2023 for a comparative study of reproductive structures between cycads and Ginkgo ), providing an excellent opportunity to understand the evolutionary transition to non-motile sperm as well as the intricate network of genes and signaling pathways involved in the development and functionality of flagella. Previous research has identified several genes associated with flagellum assembly and maintenance, such as dynein genes and intraflagellar transport (IFT) proteins (Hodges et al ., 2011). Dyneins are molecular motors that play a vital role in flagellar movement and IFT proteins are crucial for cilia and flagella assembly and maintenance. The C. panzhihuaensis genome (Lui et al ., 2022) contains intact genes encoding dynein proteins and IFTs as well as other flagellar components including outer dense fibers and some of the regulatory mechanisms governing gene expression and signal transduction pathways associated with flagellum formation and function (Arnaizet al., 2009). However, some genes encoding radial spoke proteins are missing (Fig. 5), suggesting partial flagellar defects compared to non-seed plants (Hodges et al ., 2011). Cycads represent a transitional state in the evolutionary shift toward non-motile sperm in gymnosperms and angiosperms and the identification of core flagellar genes present in the cycad genome but not in other seed plant lineages strengthens our knowledge of the genetic machinery behind flagellum formation and the shift to non-motile sperm.