Earlier this week, scientists announced the successful propagation of a 32,000-year-old seed, discovered in a burrow made by an Siberian Arctic ground squirrel during the last ice age. The placental tissue of the Silene stenophylla seed was used to cultivate flowering, reproductively viable adults of narrow-leafed campion, which is still found in the Kolyma River region today. Sharon Levy has a really nice write-up of the story at Scientific American, and you can read the original study by Russian researcher Svetlana Yashiva and colleagues here.
What makes this study really interesting in my mind is not just the fact that we’ve pushed back the date of the oldest cultivated plant by some 30,000 years (the previous record-holder was a 2000-year-old date palm from Iraq). This experiment yeilded plants with phenotypic differences from modern versions of Silene stenophylla, including longer and more widely-spaced petals and slower-growing root systems than their modern counterparts. This indicates that the ice age population of these plants is genetically different than their interglacial descendants, which gives us the opportunity to study evolutionary change at the glacial-interglacial scale. The researchers hope that their methods may be used to bring back extinct plant species where seeds are available.
This study suggests that– in cold and arid environments, at least– some seeds may be able to survive long enough to withstand adverse conditions for thousands of years. I wonder what the implications are of these finding to what we know about the migration and establishment of plants after the ice age? Were small mammal burrows or permafrost effectively seed banks for the re-establishment of plants after the ice melted? Why migrate, when you can essentially hibernate?
The paleoecological record suggests much more rapid rates of plant dispersal than we see in modern trees– so much so that it’s been called a paradox (Clark et al. 1998). Very little is known about whether and to what extent animals may have facilitated the dispersal of trees as they migrated northwards after the last ice age. Mark Vellend and colleagues speculated in this 2003 paper that deer browsing would have facilitated long-distance dispersal of Trillium seeds; Trillium are generally thought to be ant-
dispersed, but given that ants typically move seeds <10 m from the parent plant, Trillium wouldn’t have made it very far from its ice age refuge locations relying on ants alone. Johnson & Webb (1989) suggested that the nut-caching habits of blue jays may help explain the rapid expansion of oaks during the Holocene. Still, ideas of animal dispersal haven’t been widely integrated into models of the post-glacial spread of plants. Work on modern population genetics of American beech and sugar maple by McLachlan and colleagues suggests that small, “cryptic” (i.e., not easily detectable) populations of trees may have been growing quite close to the ice sheets in populations too small to show up in the pollen record. In that case, trees may not have needed to migrate very far, or very quickly, to track their suitable climates.
“How fast can species migrate?” is more than an academic question. Conservationists and ecologists often look to the paleoecological record for a sense of how well plants can migrate in response to climate change. Depending on the evidence you look at, you might decide “very fast indeed!” or “not very fast, unless they have help from animals.” Silene stenophylla may well be adding a new hypothesis– seed banking– to the study of how plants spread so quickly after the last ice age. What remains to be seen is how many more of those 600,000 seeds and fruits found in the permafrost are viable, and whether or not there are similar caches to be found in other regions. I’m hoping to pursue these questions more in the future, and I welcome your thoughts!
Categories: Commentary Ideas Papers I Wish I'd Written
Very nice write-up. You say that animal dispersal isn’t widely integrated into models of post-glacial plant dispersal. Are cryptic caches the main idea then?
On a lighter note, your title definitely brought to mind that animated movie from some years ago. 🙂
Paleoecologists have really struggled to come up with a dispersal kernel– the distribution of seed dispersal distances– in part because the migration rates using the observed kernels are too slow to fit the rates we see in the pollen record. So, one assumption is that long-distance dispersal, difficult to observe in modern studies, plays a big role. Animals aren’t necessarily thought to be the vector. Cryptic northern populations are another idea, but it’s hard to assess with the data. The McLachlan paper is cool, but I think needs some adjustment; beech trees weren’t growing in permafrost, or where their climates weren’t suitable. That’s a project I’d really love to pursue in the near future.
“This study suggests that– in cold and arid environments, at least– some seeds may be able to survive long enough to withstand adverse conditions for thousands of years.”
If I understand the paper, they didn’t just take seeds and plant them. They took tissue samples of the placenta (not even the seeds) and cultured those in the lab, then grew plants from those.
While this result is remarkable, I don’t think these experiments tell us a lot about whether seeds could be dormant for tens of thousands of years, and still be viable under natural conditions. The date palm, as far as I can tell, is still the record for growth from a seed. That would mean that about 2,000 years is the upper boundary for ecologically relevant plant dormancy.
Good point. The original seeds in the initial stage of the experiment did germinate, they just didn’t thrive. Who knows whether one may have made it eventually? Also, the ice-margin seeds in late glacial North America wouldn’t have had to wait 32,000 years, so seed banks are still within the realm of possibility, I think.