The Cretaceous-Paleogene (K-Pg) extinction event is famous for the deaths of three-quarters of plant and animal species on Earth. Now, research by Zecca and colleagues from the University of Milan-Bicocca shows that the same asteroid that wiped out the dinosaurs also led to dramatic changes in the grape family. Their paper, published in the Annals of Botany, found evidence of multiple positive selection events in grape plant DNA.
The DNA studied by the scientists was not in the plant cells’ nucleus but in the plastids. These are organelles, like chloroplasts, that live within cells but have their own distinctive DNA. The team examined DNA from 91 different accessions, studying 70 candidate plastid genes. The research team found that some plastome genes related to photosynthesis, self-replication and metabolism may have undergone positive selection. One of the most interesting genes in this group is the one encoding for the large subunit of Rubisco, a key photosynthesis enzyme. This selection process may have improved the performance of Rubisco activity in response to changing environmental conditions.
Looking at the timing of these changes, Zecca and colleagues found evidence of steps in the rate of evolution. Many of these changes occurred between 50 and 20 million years ago, but the first change dates to 65 or 64 million years ago. This era coincides with the time of the mass extinction at the end of the Cretaceous period.
“For the last 100 million years, genomes have been widely shaped by environmental perturbations… In particular, ancient whole-genome duplication (WGD) events seem to be related to the Cretaceous-Paleogene (K/Pg) mass extinction (66 Ma) caused by the impact of an asteroid in Mexico or volcanic activity,” write Zecca and colleagues. “Although the K/Pg mass extinction is not considered the main cause of extinction of plants, it may have influenced the evolution and diversification of taxa… Dust clouds and sulfur aerosols are thought to have prevented solar radiation, producing a rapid global cooling, the so-called “impact winter”. Polyploidization events appear to have contributed to the adaptation of several plants to changing environments… Apart from chromosomal duplications that show important indications in favour of an adaptive process…, single genes could also have been affected by environment perturbations and new arising mutations might have been selected.”
In the aftermath of the asteroid impact at the Chicxulub crater, plants would undoubtedly have had challenges to meet. Improving photosynthesis would give species a competitive advantage. But there would have also been opportunities, with plants able to move into vacated evolutionary niches. In this environment, the Vitaceae family diversified, eventually leading to the evolution of Vitis vinifera, the grape that produces so much of the world’s wine.
But it’s not just at the dinner table where these molecular changes have an effect. Positive selection events have an impact on botanists trying to produce evolutionary histories of species, say Zecca and colleagues. “High levels of positive selection may affect the reconstructive ability of phylogenetic methods and their ability to estimate divergence times between lineages correctly. Normally, positive selection should be tested beforehand, and positively selected markers should be avoided in phylogenetic studies. We recommend that at least codons evolving under positive selection should be excluded to improve the analysis when coding DNA is used. Our work offers a first step in this direction, providing useful information on positively selected plastid genes and codons.”
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Zecca, G., Panzeri, D. and Grassi, F. (2022) “Detecting signals of adaptive evolution in grape plastomes with a focus on the Cretaceous-Paleogene (K/Pg) transition,” Annals of Botany. Available at: https://doi.org/10.1093/aob/mcac128