21 October 2011

Cycads are not ‘dinosaur plants’ after all

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A group of extremely rare plants called cycads, thought to have survived since the age of the dinosaurs, are not as ancient as we thought, according to new genetic analysis.
Cycas taitungensis

A pollen cone borne on the species Cycas taitungensis. This particular plant is growing in the Royal Botanic Garden in Sydney. Credit: Nathalie Nagalingum/Science/AAAS/

SYDNEY: A group of extremely rare plants called cycads, thought to have survived since the age of the dinosaurs, are not as ancient as we thought, according to new genetic analysis.

Cycads are stout, non-flowering palm-like plants that have been nicknamed ‘dinosaur plants’ because scientists believed they originated 270 million years ago, survived the extinction of the dinosaurs around 65 million years ago and have remained unchanged since.

There are around 300 species of cycads living today, found everywhere from South and Central America and Africa to China and Southeast Asia. There are 70 species of cycads found in Australia alone.

New genetic analyses examining a worldwide spread of two-thirds of extant cycad species has revealed that while they might look the same as their ancient counterparts, they are all drastically different on a genetic level, and evolved as recently as 10 million years ago.

“The result was absolutely surprising,” said lead author Nathalie Nagalingum from the National Herbarium of New South Wales at the Royal Botanic Gardens in Sydney. “It goes against what we think about cycads – we thought that there were some new and some old ones, but to find out that they’re all new is astonishing. It shows we shouldn’t take things for granted.”

Not ‘living fossils’

For years, scientists classed the cycads with the horseshoe crab, the Ginko tree and the coelacanth (a large prehistoric fish) as examples of groups that exist virtually unchanged today from the time they flourished around 145 million years to 65 million years ago.

Due to the fact that morphologically, cycads in the fossil record from the Jurassic to the Cretaceous period from are so similar to modern cycads, it was thought that cycads had their last major episode of genetic diversification during this time before experiencing a steady decline in numbers due to competition with flowering plants and no dinosaurs to disperse their seeds.

Now a three-year study based on computer-automated analyses and nuclear DNA sequences of 199 cycad species from the 11 currently recognised genera (or groups) has indicated that they appear to have undergone another major speciation – the evolutionary process by which new biological species arise – just 10 million years ago.

Constructing timetrees

“We used DNA sequences combined with fossil data and used a molecular clock to figure out when the species originated,” said Nagalingum. Nagalingum and her colleagues used two different types of molecular clocks – strict and relaxed – based on two different algorithms to construct a ‘timetree’ for the cycads. Both algorithms bought up the same result.

“It turns out that they all evolved 10 million years ago, and there’s a huge gap from when we thought they originated,” said Nagalingum.

Palaeobotany expert, Robert Hill from the University of Adelaide in South Australia cautioned that analyses like this are only as accurate as the available fossil data. “From what I can see, the known fossil record of living genera lines up OK with this, so it is internally consistent – that doesn’t mean it is right, but it certainly means I can’t say it is wrong.”

Was it weevils?

Exactly what triggered a sudden radiation of these plants around 10 million years ago to produce the new, current range of species remains a mystery that researchers will now try to solve.

“The molecular evidence provided generates further scope for exploring hypotheses of what may have been the cause of either the recent diversification or extinction of taxa,” commented Thomas Wallenius from the Australian National University in Canberra. “Global climate shifting during the late Miocene (10.4 to 5 million years ago) from globally warm to cooler, seasonal climates is cited as a possible trigger for near-simultaneous diversification.”

Wallenius also added that weevils, an insect species that are the modern cycads’ main pollinator, could also have been involved. “Whether it was climate change alone or adoption of weevil pollination that was the trigger for cycad diversification remains unanswered.”

Cycads are now listed as the most endangered plants in the world, due to a number of factors such as land clearing, poaching and climate change. They are also extremely slow to grow, which means a loss of a generation through seed poaching could set plants’ numbers back for many years. “The situation is really dire,” said Nagalingum. “We use DNA to find out how many species we actually have and how we can conserve them.”

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