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Genetic analysis

A deceptive plant

A study questions the origin of pine trees

Eduardo Cesar

Cycads: the current species descend from ancestors that lived 12 million years agoEduardo Cesar

No landscape with dinosaurs would be complete without cycads. During the same period in which these reptiles became so enormous and spread across the entire earth, these plants – which are physically similar to palm trees, but related to pine trees – dominated the planet’s landscape. Fossil records show that the cycads first appeared about 270 million years ago and are still in existence today. Since their appearance has hardly changed at all over time, the cycads are looked upon as living fossils. Or to be more accurate, they were. A study carried out by an international team, including one Brazilian researcher, has just radically altered the direction of this story.

There is nothing like DNA analysis to shed new light on the study of the evolution of life. Based on the morphology (i.e., on plants’ visible aspects), biologists did not see many differences between fossils tens of millions of years old and their living counterparts. The only major difference was the number of species. Apparently, there was a much greater variety in ancient times (the peak was during the Jurassic Period, between 201 million and 146 million years ago), which made many researchers assume that it was the disappearance of dinosaurs that caused the decrease in the biodiversity of the cycads.

This new study, headed up by Sarah Matthews and Nathalie Nagalingum, from Harvard University, in the United States, was based on the species that are currently out there (roughly 300, a rather modest number) to outline their phylogeny – a type of genealogical tree reconstructed using DNA. In order to do this, they analyzed a specific gene, the P phytochrome (PHYP). The idea was to use the variations of this gene, together with the dates of fossil records, to determine when the common ancestor of the species lived.

The concept of dating based on genetic differences stems from a very simple assumption: random mutations occur in DNA at a more or less homogeneous rate – with larger or smaller variations between distinct groups, which are taken into account by the scientists. Establishing a correlation between the number of differences and the time that it would take for this divergence to occur, one can estimate when the common ancestor lived. For instance, this is how researchers confirmed that we are more closely related to chimpanzees (96% of whose DNA is the same as ours) than to mice (90%).

Well then. Upon analyzing the PHYP gene of 199 species (two thirds of all those that exist today), as well as two other genes (rbcL and matK) of a smaller number of plants, they discovered that the common ancestor of each of the genuses that together make up the current species lived 12 million years ago. This is a very substantial amount of time, but not so radical as to suggest that these species have been more or less unchanged over the last 200 million years.

“This result shows that the species which are alive today first appeared in the last 10 million years, which in geological terms is very recent,” states Tiago Quental, a biologist from the University of São Paulo (USP) who took part in the study, which was published in the American scientific journal Science. “This indicates that these species cannot be regarded as living fossils and that the species that are currently alive were most certainly not in existence at the time of the dinosaurs, which became extinct 65 million years ago.”

New with an old look
Though only indirectly, this result casts a shadow over the entire concept of living fossils. Given that the paleontologists can only assess the morphology of the fossils – and if it has been discovered this is not a completely safe technique for identifying the emergence of new species (a phenomenon known as speciation) and of important genetic modifications – who is to say that other living fossils are not simply new species that look like old ones?

In addition to telling us what the current cycads are not, the analysis also helps to reconstruct their evolutionary story. Cycads are gymnospermic plants, which means that they present naked seeds, without any flowers. During the time of the dinosaurs, the giant herbivores ate the seeds and spread them in other places. However, because of the phylogenetic analysis, it is now known that those cycads which co-existed with these giant reptiles are not the same species as those living today.

Actually, instead of being well-adapted survivors from the distant past, the cycads almost disappeared once and for all in the middle of the path up to the present. Their reemergence, which has now been documented by phylogenetic analysis, occurred about 10 million years ago. “What is curious is that this reemergence took place in a synchronized way across our entire planet, which suggests that a global effect may have caused this pattern,” states Quental.

The researchers know this because they analyzed species from different parts of the world. The greatest variety in terms of species is found in Australia, but there are also cycads in both hot and in temperate regions of Africa, Asia and Central America. Additionally, it seems that the local variety of the species increased significantly at more or less the same time in all of these different regions.

Therefore, the scientists speculate that it was something that happened across the entire planet that gave the cycads a second chance – possibly climate change. What happened at that time was that there was global cooling.

Thus, it comes as no great surprise that the period we are living in right now is not a very good one for these plants. Moreover, the rising increase of the earth’s average temperatures, partly as a result of anthropogenic activities, is unlikely to help them flourish. “The current diversification of the cycads seems to be diminishing, and its recent evolution will probably not guarantee its survival through the next wave of extinctions,” is the assessment of Susanne Renner, a biologist at the University of Munich in Germany, who did not take part in the study but was invited by Science magazine to comment upon it.

Scientific article
NAGALINGUM, N.S. et al. Recent synchronous radiation of a living fossil. Science. v. 334. 11 Nov. 2011.