Apr 26, 2013
Pollen trapped in glaciers could reveal plant species as well as family
Currently researchers look at the shapes of fossil pollen grains to determine the genus or family of plants that used to grow nearby. But it is hard to break down this information to a species level. And while studying DNA could reveal species type, tests of ancient pollen found in sediments have had a low success rate. Now a team from Japan has used DNA to identify pollen grains found in Russia's Belukha glacier to section level – a classification that lies between genus and species – in the first published DNA study of glacier-preserved pollen.
"Although our research is still ongoing for species identification, we demonstrated the persistence of DNA in the generative nucleus of a pollen grain in the ice core layer of 1965," Fumio Nakazawa of Japan's National Institute of Polar Research and Transdisciplinary Research Integration Center told environmentalresearchweb. "Due to low-temperature conditions, pollen grains in glaciers are less affected by diagenesis [changes to sediments caused by temperature and pressure], and their DNA is therefore more likely to be preserved in the older layers."
Nakazawa and colleagues from the National Institute of Polar Research, Transdisciplinary Research Integration Center, Tohoku University, University of Tokyo, Chiba University and Nagoya University, examined pollen grains from the Pinus genus in surface snow taken from the Belukha glacier in summer 2003 and in an ice-core sample from roughly 24 m deep, dated to 1965.
Fluorescent staining and microscopy indicated that the DNA was well preserved, although there may have been some fragmentation and degradation. Snow is a good light insulator and so could have protected the pollen from ultraviolet damage, the researchers said.
"We have analysed pollen grains in ice cores and snow pits from mid- and low-latitude glaciers to date the ice core and pit layers," said Nakazawa. "In our previous research, including unpublished studies, we have found that the pollen grains contained intact cytoplasm. It is observed even with 1000-year-old pollen grains. Hence, the pollen may be utilized for DNA analysis."
The researchers also extracted, amplified and sequenced DNA from the pollen grains from 2003; they will use the 1965 samples in future work as they were limited in number. Eight of 105 pollen grains tested produced DNA sequences, a success rate of 7.6%. Similar studies of pollen grains found in sediments such as peat and lake deposits had a success rate of 0–3.2%. These grains were older, however, so the researchers "cannot make a simple comparison of the success rates," they wrote in Environmental Research Letters (ERL). "Further investigation of older pollen from glaciers is necessary."
While the team could not identify the species of the plant producing the pollen, the scientists were able to "identify single Pinus pollen grains at the section level that traditional pollen analysis cannot do by using obtained sequence data". It is hard to classify Pinus pollen by morphology to any more detail than the genus level. The Pinus genus has two subgenera, four sections, 17 subsections and approximately 111 recognized species. In this case, DNA analysis indicated that the pollen grains belonged to the section Quinquefoliae. Trees of the species Pinus sibirica, a member of this section, are currently found around the glacier, suggesting that the pollen originated from this species.
"Identification of the pollen from glaciers at lower taxonomic levels may allow definition of pollen provenance transported to the polar regions from distribution of the pollen source plant, detailed reconstruction of past vegetation, and discussion of genetic diversity and environment adaptability with climate change by additionally examining intraspecific variation in the same pollen species," said Nakazawa.
The researchers said that they now need to improve the success rate of pollen DNA analysis using Multiplex PCR or WGA, and to identify pollen grains down to the species level. "Moreover, we plan to obtain sequences using older pollen grains," said Nakazawa.
- DNA analysis for section identification of individual Pinus pollen grains from Belukha glacier, Altai Mountains, Russia Fumio Nakazawa, Jun Uetake, Yoshihisa Suyama, Ryo Kaneko, Nozomu Takeuchi, Koji Fujita, Hideaki Motoyama, Satoshi Imura and Hiroshi Kanda 2013 Environ. Res. Lett. 8 014032
- National Institute of Polar Research, Japan
About the author
Liz Kalaugher is editor of environmentalresearchweb.