|Cycas circinalis with old and new male cones.|
They were very common in the Mesozoic era, the age of the dinosaurs, and are still living, but are now much less common. Only 9 genera (about 100 species) of cycads are found to be extant nowadays. Cycads are thought to have evolved from order Pteridospermae (seed ferns) of angiosperms, now totally extinct.
Cycads are gymnosperms (naked seeded). Their unfertilized seeds are open to the air to be directly fertilized by pollination. This contrasts with angiosperms, which have enclosed seeds and more complex fertilization arrangements. Cycads have very specialized pollinators, usually a specific species of beetle.
These photosynthetic cyanobacteria produce a neurotoxin called BMAA that is found in the seeds of cycads. This neurotoxin may enter a human food chain if the cycad seeds are eaten, and may cause some neurological diseases.
- Willis K.J & McElwain J.C. 2002. The evolution of plants. Chapter 5 Major emergence of the seed plants, and specifically p133/4.
- Pinnate: feather-like or multi-divided features arising from both sides of a common axis.
- Rai, A.N.; Soderback, E.; Bergman, B. (2000), "Tansley Review No. 116. Cyanobacterium-Plant Symbioses", The New Phytologist, 147 (3): 449–481, doi:10.1046/j.1469-8137.2000.00720.x, JSTOR 2588831CS1 maint: uses authors parameter (link)
- Holtcamp, W. (2012). "The emerging science of BMAA: do cyanobacteria contribute to neurodegenerative disease?". Environmental Health Perspectives. 120 (3): a110–a116. doi:10.1289/ehp.120-a110. PMC 3295368. PMID 22382274.
- Cox, PA, Davis, DA, Mash, DC, Metcalf, JS, Banack, SA. (2015). "Dietary exposure to an environmental toxin triggers neurofibrillary tangles and amyloid deposits in the brain". Proceedings of the Royal Society B. 283 (1823): 20152397. doi:10.1098/rspb.2015.2397. PMC 4795023. PMID 26791617.CS1 maint: uses authors parameter (link)