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.