Figure 5. Distribution of flagellar genes across embryophyte genomes reveals key genes in the evolution of non-motile sperm. a. Illustration of Cycas sperm cell, scale = 40 µm (reproduced from The Biology of the Cycads, Norstog & Nicholls 1997 Cornell University Press). Each sperm cell possesses between 40,000-50,000 flagella arranged in 5-10 sinistral coils (Offer et al ., 2023). b. Cladogram illustrating flagellum loss in green plants. c. Distribution pattern of outer dense fiber protein and other crucial flagellar proteins among representative embryophytes and green algae. Each lineage is color-coded. Protein nomenclature follows Liu et al ., (2022)
Conclusion
Here we have reviewed the last decade’s worth of research on cycad phylogenetics, genomics, and symbioses. These exciting recent discoveries position cycads as integral to our understanding of seed plant evolution and ecology, with far-reaching implications for the biological sciences. Phylogenetic advances continue to shed light on the early diversification of seed plants, while investigations into cycads’ diverse associations with insects has directed new attention to a broader range of signaling methodologies and provided fertile ground for investigations into adaptation and coevolution. Studies on coralloid root symbioses have highlighted the importance of multi-species bacterial communities in cycad-cyanobacteria symbiosis and opened the door for studies on the ecology of specialized metabolites. Similarly, investigations of plant phyllosphere microbiota have identified previously overlooked putative symbionts. Finally, recent genomic advances have uncovered a wealth of information highlighting important aspects of land plant evolution from the shift to non-motile sperm to ancient horizontal gene transfer across biological kingdoms. Overall, cycads and their biotic associates provide a rich study system for investigating ancient co-evolution and microevolutionary and ecological interactions.
Much of the current research on cycads can be unified through the lens of chemical ecology, as we become increasingly aware of the myriad ways that cycads’ chemodiversity drives their interactions with other organisms. Investigations into chemical trait evolution and ecology will shed light on the origins and maintenance of plant chemodiversity over long timescales and the implications for mutualistic and parasitic insect associations. Further investigations into the metabolic diversity and ecology of microbionts will improve our understanding of the mechanisms underlying plant-microbe symbioses. Just as cycads provide a bridge between extant and early seed plants, they will also help scientists to connect diverse research disciplines and continue to provide insights into the evolution and ecology of seed plants.

Data availability statement

Leaf microbiome data presented in Figure 4 from Sierra et al. (in review) are currently in review and are available at https://github.com/adrielmsierra/Zamia_Phyllosphere/. Raw data is found in the Genbank BioprojectPRJNA1061598.