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.