Can Cloned Plants Live Forever?
ScienceDaily (Aug. 18, 2010) — Despite the many cosmetic products, surgical treatments, food supplements, and drugs designed specifically to reverse the biological effects of aging in humans, long-lived aspen clones aren't so lucky. Researchers at the University of British Columbia have shown that as long-lived male aspen clones age, their sexual performance declines. Dilara Ally, who conducted this research for her Ph.D., also showed that with that loss of sex and sexual fitness, ultimately the lineage could go extinct.
Stand of autumn Aspen trees (Populus tremuloides). (Credit: iStockphoto/Ken Canning)
The findings will be published next week in the online, open access journal PLoS Biology.
A clone is a group of genetically identical individuals that originate from a single ancestor without the need of sex (for instance cuttings from a plant). Although many organisms can propagate clonally, this feature is most common in plants. In aspen, asexual reproduction or clonality is achieved via underground lateral roots that eventually produce new clone members/ramets.
Although a clone can produce new members asexually and avoid meiosis (the stage where parental genomes recombine), it still continues to undergo cell division over the years. As the clone spreads and new trees replace old trees, the number of mitotic cell divisions increases, resulting in an accumulation of mutations along the way. Ally and colleagues used a molecular clock to estimate the age of individual clones. To do this they measured the number of accumulated mutations at microsatellite markers and calibrated the clock using an independent, geological estimate of time. By coupling estimates of clone age with a measure of male fertility, they found that long-lived aspen clones do indeed suffer reduced sexual fitness with age.
"One reason the evidence for aging in trees is scarce is because it is very difficult to obtain long-term demographic data. Imagine trying to follow cohorts of plants that live on average 100 years of age and don't start reproducing until they are 25 yrs; its impossible within the timeframe of a Ph.D. or even over an entire career," says lead author Ally, who will be taking a postdoctoral position at San Diego State University in the Fall.
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Read more here: Science Daily
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Aging in a Long-Lived Clonal Tree
Dilara Ally1,2*, Kermit Ritland3, Sarah P. Otto2
1 Department of Biological Sciences, University of Idaho, Moscow, Idaho, United States of America, 2 Department of Zoology, University of British Columbia, Vancouver, British Columbia, Canada, 3 Department of Forest Sciences, University of British Columbia, Vancouver, British Columbia, Canada
Abstract
From bacteria to multicellular animals, most organisms exhibit declines in survivorship or reproductive performance with increasing age (“senescence”) [1],[2]. Evidence for senescence in clonal plants, however, is scant [3],[4]. During asexual growth, we expect that somatic mutations, which negatively impact sexual fitness, should accumulate and contribute to senescence, especially among long-lived clonal plants [5],[6]. We tested whether older clones of Populus tremuloides (trembling aspen) from natural stands in British Columbia exhibited significantly reduced reproductive performance. Coupling molecular-based estimates of clone age with male fertility data, we observed a significant decline in the average number of viable pollen grains per catkin per ramet with increasing clone age in trembling aspen. We found that mutations reduced relative male fertility in clonal aspen populations by about 5.8×10−5 to 1.6×10−3 per year, leading to an 8% reduction in the number of viable pollen grains, on average, among the clones studied. The probability that an aspen lineage ultimately goes extinct rises as its male sexual fitness declines, suggesting that even long-lived clonal organisms are vulnerable to senescence.
Author Summary
Aging has been demonstrated in many animals and even in bacteria, but there is little empirical work showing that clonal plants age. Evidence for aging in long-lived perennials is scarce because it typically requires survivorship or fecundity schedules from long-term demographic data. Given the extreme lifespan of many long-lived perennials, it is difficult to follow cohorts of individual clones to collect late-life survivorship or fertility. Our work offers a novel approach for obtaining late-life demographic data on a clonal species by using genetic data to estimate the age of individual clones. We studied plant clones in a natural population of trembling aspen, which grows clonally via lateral root suckers. By coupling estimates of each clone's age with a measure of its male reproductive performance, we show that long-lived plant clones do senesce. Although clonal plants have the capacity for continued growth and reproduction even late in life, mutations that reduce fertility can accumulate because selection on sexual fitness is absent during clonal growth, potentially explaining senescence in this species.
Citation: Ally D, Ritland K, Otto SP (2010) Aging in a Long-Lived Clonal Tree. PLoS Biol 8(8): e1000454. doi:10.1371/journal.pbio.1000454
Academic Editor: Thomas B. L. Kirkwood, University of Newcastle upon Tyne, United Kingdom
Received: January 18, 2010; Accepted: July 2, 2010; Published: August 17, 2010
Copyright: © 2010 Ally et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: Funding for this project was provided to DA by NSERC, Brink/McLean Grassland Conservation Fund, UBC Zoology Computing Unit, PEO Educational Fund, and Waterton Lakes National Parks; to SPO and KR by NSERC Discovery grants. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
* E-mail: dilara.ally@gmail.com
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