Evolution and speciation patterns of the world's largest tree genus Syzygium identified after a two-year study involving over 60 local and international collaborators

- Understanding the origin and relationships within this diverse tree genus will guide conservation and management efforts of the region’s rain forests

A study has confirmed the evolutionary relationships and speciation patterns of the world’s largest tree genus Syzygium. Native and widespread in tropical and subtropical rain forests, studying the origins and drivers of this hyperdiverse woody tree genus contributes to understanding of how plant species have emerged in the past in response to environmental changes. This knowledge, in turn, is valuable for predicting how plants might respond to environmental changes brought forth by climate change and thus guide conservation and management efforts for plant communities.

This study, published online in the journal Nature Communications[1] on 12 September 2022, was led by the Singapore Botanic Gardens of Singapore’s National Parks Board (NParks) in collaboration with 26 local and international research institutions including Nanyang Technological University in Singapore, the Royal Botanic Gardens, Kew in the United Kingdom, the University of Aberdeen in the United Kingdom and the University at Buffalo in the United States of America.

The significance of Syzygium

Syzygium, also known as the clove genus, is a hyperdiverse tree genus with about 1,200 named species and many more new species yet to be formally described by botanists. Among forest trees, Syzygium is one of the most important species-rich genera, and its diversity is highest in the Southeast Asian forests. For example, Syzygium is the most diverse tree genus in Singapore, with 54 species recorded from a variety of habitats and found mainly in nature reserves.

Syzygium species may be found growing together with other trees within the understorey and canopy layers of forests. Because of its large diversity, they play an inordinate role in the functioning of forest ecosystems. Many Syzygium species are also cultivated in tropical countries for their large edible fruits (Water Apple [Syzygium aqueum], Java Plum [S. cumini], Rose Apple [S. jambos], Malay Apple [S. malaccense] and Java Apple [S. samarangense]) and spices (clove [Syzygium aromaticum] and Indonesian bay leaf [Syzygium polyanthum]). Understanding how Syzygium species have evolved will help to advance our knowledge of the highly complex species-environment relationships in forest ecosystems and anticipate forest ecosystem changes under the impacts of climate change. This study has made two key discoveries that contribute to this.

(1)   Syzygium genome reveals a single ‘pan-Myrtales’ event for the order

The study compared the genome of the Sea Apple (Syzygium grande) to other species of its plant order, Myrtales, and discovered that all Myrtales (comprising nine families, approximately 400 genera and around 14000 known species) share a single whole genome duplication (WGD) event[2]. This overturns the results of an earlier study in 2019 which had reported the involvement of multiple WGD events. This is a key discovery, as a single WGD will imply that there was a single, major environmental trigger in the past which led to the ancestor of all species in the Myrtales gaining multiple copies of the genome and passing this duplication down through the following generations.  It is known that plants can contain several copies of their genome through WGD events. This can result in plants having advantages such as increased resistance to diseases and environmental stress. Scientists believe that WGD can also potentially lead to the formation of new species over time and WGD events are often associated with diversification in flowering plants, including in the Myrtales. Thus, the new discovery of a single WGD event in Myrtales helps scientists to better understand WGD events, a critical but poorly understood natural phenomenon. This, in turn, provides critical evidence for future studies on possible factors that might have triggered a single WGD, and how we might anticipate the consequences of climate change on species extinction and evolution.

Phylogeny of major lineages of Myrtales, depicting study findings of a single whole genome duplication event, named ‘pan-Myrtales’

(2)   Genomic analyses confirm the origin of Syzygium in Australia-New Guinea, later migrating westward to Southeast Asia multiple times

The study also confirmed that Syzygium species originated in Australia-New Guinea and later migrated eastward to the Pacific and westward to Southeast Asia before proceeding onward to India and Africa. The westward migration to Southeast Asia occurred multiple times. These repeated migrations to Southeast Asia correspond with many instances of geographic isolation of Syzygium populations. In turn, bursts of Syzygium speciation occurred, during which many new species were formed.

This helps us to understand the evolutionary history and complexity of Syzygium species, including why some can be distantly related yet morphologically[3] similar when comparing vegetative and floral features. For example, the previous understanding of the Sea Apple (Syzygium grande) is that it is one distinct species that can be found in a range of habitats in Southeast Asia, from lowland areas at sea level to the mountains. This assumption was made based on morphological characters and because these populations share similar leaf, fruit and flower characteristics. The study has now discovered that the lowland population occurring naturally in the coastal forests of Singapore is genetically distinct from the population occurring on Mount Kinabalu at 1700 m above sea level in Malaysia. It also found that these two lowland and highland populations descended from different lineages. Such discoveries are important in guiding our conservation efforts and ensuring that we do not lose our biodiversity.

Syzygium is one of the most difficult genera when it comes to distinguishing its species based on morphological characters. Using genomics tools, this study improves our ability to identify individual Syzygium species more accurately, thus allowing us to better assess and monitor their population sizes and extinction risk, as well as the factors that drive change in a region. This could enable a better understanding of how plant communities might respond to future climatic scenarios and predict resilience of plant populations in different areas. This is important to ensure that conservation and management efforts of our rain forests are effective in mitigating against the impacts of climate change.

This study’s findings were made possible through sampling of nearly 300 Syzygium individuals, including species from Africa, Sri Lanka, Malaysia, Singapore, Indonesia, Japan, Australia and the Pacific Islands. To date, this is the most extensive sampling of the genus, involving over 60 researchers from various institutions. It took two years to complete.

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[1] Article title: Genomic insights into rapid speciation within the world’s largest tree genus Syzygium
https://www.nature.com/articles/s41467-022-32637-x DOI: 10.1038/s41467-022-32637-x

[2] Whole genome duplication (WGD) event - A process where an organism’s entire genetic information is copied once or multiple times which can result in divergence and formation of new species over time.   

[3] morphologically – physically