Phytolites' DNA

Specialists of the V.L. Komarov Botanical Institute, Russian Academy of Sciences (St. Petersburg ) – Igor Ozerov, Nadezhda Zhinkina, Alexander Rodionov, Alexander Efimov and Eduard Machs have discovered DNA in the remains of leaves and fruit of fossil trees which got the names of Paramyrtaciphyllum and Paramyrtacicarpus. They used to grow in the territory of Yakutia in the early eocene, i.e. about 55 million years ago. Fossil leaves and fruit resemble respective organs of plants of the Eugenia (Myrtaceae gen.) family, where eucalyptus and feijoa belong. Future investigations will prove if the eocene trees really belong to this family: after all, resemblance may be deceptive.

Fortunately, despite such considerable age, the these plants’ cells are surprisingly well preserved: nuclei can be seen in them under a light microscope. Moreover, these nuclei get the typical pink tint after being processed by special reagents, which have been used by cytologists for more than 80 years in order to reveal DNAs.

DNA molecules passed from parents to their posterity bring us traces of events in evolutionary history. By comparing certain DNA sections’ structure of different living creatures, scientists reveal kinship between them. Contemporary biological taxonomy is already impossible to imagine without evolutionary kinship schemes of organisms (phylogenetic trees) based on molecular data. The DNA structure determination has now become a routine laboratory procedure, and it is already difficult to find such a plant or an animal, whereof individual sections of these gigantic molecules have not been studied. So far, only fossil organisms are exceptions.

Some researchers managed to educe DNAs from remains of plants and animals aged several dozens of million years, however, such finds always provoked deep scepticism on behalf of their colleagues. Scepticism is understandable: it seems incredible that macromoluecules could be preserved for so long. It is easier to assume that educed DNAs belonged not to the ancient organism but got into its remains together with some organic contaminations.

The find by Igor Ozerov and his colleagues is above such suspicion. They have managed to see fossil DNA with their own eyes under the microscope. Thereby they have proved that they deal with “native” macromoluecules of fossil trees, and not with the results of later contamination.

Such find is great luck: fossil remains of plants are very rarely preserved so well. But, on the other hand, there is nothing extraordinary about the find. Examples have long been known of excellent preservation of dead cells structure and their macromolecules for thousands of years (for example, in tissues of mammoths' dead bodies), and analysis of DNA from herbarium specimen collected more than 200 years ago has become a commonplace for botanists. The eocene trees’ remains are in a way a “herbarium” as well, which remained intact due to a very rare combination of natural conditions.

Decoding structures of DNAs from ancient cells is still ahead. This effort is connected with new difficulties: within millions of years, large molecules fell into small fragments, which do not always allow to work with them by commom methods. The St. Petersburg researchers, however, hope that DNA analysis will allow to identify exactly to what contemporary plants Paramyrtaciphyllum and Paramyrtacicarpus relate.