Cycads look like palms but split from them hundreds of millions of years ago
With stout trunks and crowns of stiff feathery leaves, cycads pass easily for palms. They are not even close relatives. These slow-growing seed plants date back roughly 300 million years, borrowed an insect-killing gene from a microbe, and host root bacteria that make a toxin once linked to a mysterious disease on Guam.
Cycads are gymnosperms, plants whose seeds sit exposed rather than enclosed in fruit. Each plant is either male or female, and both sexes grow cone-like strobili holding pollen or seeds. Trunks range from a few centimetres to several metres tall, growth is slow and lives are long. Pollination is usually handled by one particular beetle species, occasionally by thrips or moths. The resemblance to palms is convergent evolution: palms are flowering plants that bear fruit, and new cycad leaves unfurl like fern fiddleheads.
The oldest probable cycad leaves come from rocks around 300 million years old in South Korea and China, with unambiguous fossils from the Permian onward. The Mesozoic is sometimes nicknamed the Age of Cycads, but other extinct plant groups with similar foliage, such as the Bennettitales, were far more common; true cycads were minor players. Genetic studies put them closer to Ginkgo than to other living gymnosperms. They have changed relatively little since the Jurassic, yet they are not living fossils, because they kept evolving.
Their biology holds surprises. Special coralloid roots push up through leaf litter and house nitrogen-fixing cyanobacteria of the genus Nostoc, which produce a neurotoxin called BMAA that builds up in the seeds, possibly deterring grazers. Cycads also picked up a family of genes, probably from a fungus, through horizontal gene transfer, letting them make an insecticidal toxin.
People have eaten cycads with care. The Yolngu of Arnhem Land soak Cycas orientis nuts overnight before grinding and baking them. On Guam, the Chamorro used Cycas micronesica seed flour as a staple, and chronic BMAA exposure, including through flying foxes that feed on the seeds, is now considered a cause of a neurological disease there. Cycads worldwide are in decline: four species are on the brink of extinction and seven have fewer than 100 wild plants, partly because of collecting for the drug trade. In Vanuatu the plant symbolises peace and appears on the national flag.
Source: Cycad
