Lessons from magnolias about past climatic events and plant evolution

Life on Earth is rapidly changing because of the climate crisis. Some species are adopting to new environments, others are going extinct. By learning how plants were impacted by past climatic events, we can prepare for the future. Scientists can “time-travel” by piecing puzzles together that consists of fossils, historical plant and climatic records, and genetics. 

Magnolias (Magnoliaceae family) are one of the earliest surviving flowering plants, containing about 210-340 species that mostly grow in north temperate to tropical and subtropical regions of Asia and America. Their complex classification system consists of subfamilies, genera, subgenuses, sections and subsections. This complexity reflects the magnolias’ morphological and genetic diversity that enabled them to grow in many different parts of the world. 

The recent study by Nan Zhao, Suhyeon Park and colleagues suggest that a group of magnolia species (subgenus Magnolia, section Michelia) has gone through two key evolutionary shifts, 8 and 3 million years ago (Mya) potentially due to drastic climatic changes (Asian monsoons, global cooling). This group of 37 species might have moved from tropical areas to sub-tropical areas first, followed by spreading from sub-tropical areas into the tropics. Whilst these movements took millions of years to evolve, it is an interesting evolutionary story.

Before diving into the details of this study, let’s start with a bit of a reminder of the geological time scales. The Phanerozoic (540 Mya to today) is divided into the Paleozoic (“early life”), the Mesozoic (“middle life”), and the Cenozoic (“new life”) era. Dinosaurs became extinct at the start of the Cenozoic (65 Mya). This era is further divided into 3 periods (Paleogene, Neogene, and Quaternary) and 7 epochs (e.g., Paleocene, Eocene to Holocene). There were many tectonic and drastic climatic events, including a global cooling trend.

In Asia, the monsoon systems are said to had started to form between the Eocene to Early Miocene and varied in intensity, having intensified again since approx. 3 Mya. The rise of the Himalaya-Tibetan plateau and dynamics of monsoon systems probably led to rapid diversification rates of animals and plants. Southern Asia (<32°N) has tropical and sub-tropical regions, numerous biodiversity hotspots and is home to many magnolias. 

As mentioned before, magnolias have a complicated classification system. The family Magnoliaceae has two subfamilies, Liriodendroideae and Magnolioideae. The latter consists of subgenera Yulania, Gynopodium, and Magnolia. The latter has 9 sections, including Magnolia and Michela. The section Michelia is characterised by evergreen leaves and has about 73 species. The subsection Michelia has another 4 subsections (Elmerrillia, Maingola, Aromadendron and Michelia). Yes, plant taxonomists like to group species into nice and tidy boxes but then confuse everyone what a plant species is and how they all relate to each other! 

Sir David Attenborough himself highlighted the interesting story of the magnolia trees that grew from 2,000 year old seeds and had more petals than the trees growing today in the Private Life of Plants series. There are countless questions about the evolution of these plants. 

Nan Zhao, Suhyeon Park and colleagues set out to study the section Michelia and ask questions about their evolutionary dynamics between tropical and sub-tropical regions in Southern Asia. Specifically, the researchers were interested in whether the evolutionary patterns (diversification and dispersal) of these plants coincided with past climatic events. 

They gathered 96 magnolia samples, out of which 42 belonged to the Michelia section Michelia subsection. Based on plastid genomes, they constructed a phylogenetic tree and estimated when the different species separated (diverged) from each other. This molecular clock was estimated using magnolia fossils. The oldest fossil seed was from Liriodendroidea alata found in Kazakhstan, and was approximately 100.5–93.9 million years old (Myo).

The ancestral distribution, diversification and dispersal events of these plants were estimated based on their current distribution, phylogenetics and various macroevolutionary models. 

The scientists found that family Magnoliaceae was 94.9 Myo, and the subfamily Magnolioideae evolved 36  Mya. The Magnolia sections Michelia and Yulania diverged around 25.5 Mya. Then, the section Michelia might have originated in the tropics and split 16.3 Mya, in the middle of the Miocene.

Next, the subsection Michelia separated 10.8 Mya that also originated in the tropics. Within this subsection, the diversification increased until 8 Mya in the tropics and then, declined slowly. In the sub-tropics however, this peak happened approx. 3 Mya. These “peaks” coincide with the abrupt weakening and then variability of the East Asian monsoon approx 7.5 Mya and 3 Mya, and also when these plants might have evolved drought-resistant traits. 

“[T]he Oligocene origin of M. section Michelia suggests its diverged adaptation to a wet/warm monsoon environment in southern Asia, while its sister section Yulania would have adapted to an arid/cool environment in North-east Asia”, Zhao and colleagues wrote. 

The researchers concluded that Asian monsoons were fundamental for the origin of M. section Michelia in the tropics. The team also found that the dispersal and diversification events happened in the tropics before the sub-tropics, in an asymmetrical pattern that may have been enabled by a cooling period. 

These findings by Zhao and colleagues show how only one group of Magnolia species have been previously impacted by climatic events. The latest IPPC report suggests that it is “now or never” for the world to prevent the 1.5°C global warming above pre-industrial levels.  Almost half of all Magnolia species were threatened with extinction in 2007 and Asian species are more vulnerable to climate change. 

“Overall, we provided a typical example of plants whose evolution reflects past climatic changes in southern Asia. This information might provide unique insights into the effects of ongoing global warming (including biodiversity loss) and suggest possible methods to tackle them.”

READ THE ARTICLE 

Zhao, N., Park, S., Zhang, Y.Q., Nie, Z.L., Ge, X.J., Kim, S. and Yan, H.F., 2022. Fingerprints of climatic changes through the late Cenozoic in southern Asian flora: Magnolia section Michelia (Magnoliaceae). Annals of Botany. https://doi.org/10.1093/aob/mcac057