In 1994, hikers discovered a group of strange trees growing in a canyon in Wollemi National Park, about 100 kilometers west of Sydney, Australia. A hiker informed a park service naturalist, who then showed leaf specimens to a botanist. It was eventually determined that they represented an ancient species that was essentially frozen in time since dinosaurs roamed the Earth.
Described as a “living fossil” by some, the Wollemi pine (Wollemie nobilis) is almost identical to the preserved remains dating from the Cretaceous period (145 million to 66 million years ago). Only 60 of these trees exist in the wild today – and these tenacious survivors are under threat from bushfires in the region. It was thought to have gone extinct around 2 million years ago.
Today, scientists from Australia, the United States and Italy have decoded its genome, highlighting its unique evolution and breeding habits, while contributing to conservation efforts. The article was published in the preprint database bioRxiv on August 24 and has not been peer-reviewed.
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The pine has 26 chromosomes – containing a staggering 12.2 billion base pairs. By comparison, humans only have about 3 billion base pairs. Despite their genome size, Wollemi pines have extremely low genetic diversity, suggesting a bottleneck (when the population was significantly reduced) around 10,000 to 26,000 years ago.
This is because plants do not exchange a lot of genetic material. The remaining trees appear to reproduce primarily by coppicing, in which suckers emerge from the base and grow into new trees.
Their rarity may be partly due to the high number of transposons, or “jumping genes” – segments of DNA that can change position in the genome. These elements also explain the size of the genome. “The smallest plant genome and the largest plant genome have almost the same number of genes. The big size differences usually come from transposons,” said Gerald Schoenknecht, director of the National Science Foundation’s Plant Genome Research Program. . Schoenknecht was not involved in the research, but the NSF provided the funding.
As let’s transpose jumping to new locations, they can change the sequence of “letters” in a DNA molecule, thereby causing or reversing mutations in genes. They can carry functional DNA with them or modify DNA at the insertion site, and thus have a substantial impact on the evolution of an organism.
If the transposons induced harmful mutations, they could have contributed to population declines precipitated by climate change and other factors, the researchers said. These stressful conditions could have led the plant to switch to clonal reproduction. Since increases in transposons correlate with sexual reproduction, the shift to asexual reproduction may have reduced the potential introduction of damaging mutations. Paradoxically, while trees still relied on sexual reproduction, transposons may have played a role in increasing genetic diversity and thus making them, at least temporarily, more resilient to changing conditions.
“In 99 percent of cases, mutations are probably not a good idea,” Schoenknecht said. “But over millions of years, the 1 percent who helps can advance the species. In this case, it may have been a small advantage.”
Decoding the genome also revealed why the Wollemi pine appears to be susceptible to diseases, particularly Phytophthora cinnamomi, a pathogenic water mold that causes dieback. The tree’s disease resistance genes are suppressed by a type of its own RNA associated with the development of wider leaves. Wollemi pines, unlike most conifers, have broad needles.
Thus, the evolution of wider leaves may have led to the removal of disease resistance and exposed the species to pathogenic threats – which may have been inadvertently spotted by hikers illegally visiting the protected spot. P. cinnamomi is common in cultivated plants.
Although only four small populations remain in the wild, pines have been widely propagated by botanical gardens and other institutions in an effort to conserve them and study their unique biology. The species is considered critically endangered by IUCN.
Thus, the analysis of the Wollemi pine genome is not a simple academic curiosity: it has serious implications for the survival of the species.
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