Study reveals ancient tymoviruses spread before the last Ice Age

A new study indicates that a major group of plant viruses was circulating among wild Eurasian plants well before agriculture began, before international trade existed and even prior to the last Ice Age.

The research examines tymoviruses, a virus family that now infects wild plants as well as key crop species.

Its findings suggest these viruses have an evolutionary history extending back tens of thousands of years, whereas their current global distribution appears to be far more recent and strongly shaped by human activity.

The study was headed by Adrian J. Gibbs, emeritus professor at the Australian National University.

Working with an international team, Professor Gibbs applied genetic sequencing and evolutionary “family tree” analysis to assess when the viruses first appeared and how they travelled between regions over time.

From wild plants to cultivated crops

Tymoviruses predominantly infect dicotyledonous plants. Although leaf-feeding beetles commonly transmit them, they may also be spread in seeds or when plants come into physical contact.

This combination of transmission methods helps account for their presence in both natural habitats and agricultural fields.

Across Eurasia and the Americas, tymoviruses infect wild plants and crops and can sometimes cause severe disease.

According to the study, they affect economically significant crops including cultivated brassicas, covering oilseed and vegetable species, and solanaceous plants such as potato, tomato, tobacco and aubergine.

The viruses also infect legumes in areas including Africa, South-East Asia and Australia. Their shared presence in wild and cultivated hosts makes them difficult to manage.

A virus may circulate unnoticed in wild vegetation before spilling over into crops, or it may move in the opposite direction. Either pathway can create problems for farming and ecosystems.

Reconstructing the tymovirus family tree

To investigate the long-term history of tymoviruses, the researchers sequenced and analysed 109 distinct tymoviruses.

Many of the newly sequenced specimens came from historical virus culture collections. This allowed the scientists to obtain valuable genetic data from older archived material, rather than depending solely on newly gathered field samples.

The researchers then carried out phylogenetic analysis - in effect, constructing a family tree from genetic similarities - to establish the viruses’ relationships and estimate their origins in time and place.

Their findings indicate that the most recent common ancestor of all known tymoviruses lived before the last Ice Age. They further suggest that certain tymovirus lineages may have arrived in the Americas about 15,000 years ago.

This dates their movement to a period well before modern agriculture or commercial shipping, probably through natural ecological pathways, shifting habitats and spread assisted by animals.

Globalisation appears to be recent

One particularly relevant finding is that the small number of tymoviruses now found on multiple continents do not appear to have had a global presence for very long.

The study indicates that most intercontinental transmission occurred chiefly over the past two centuries.

That period closely matches the modern expansion of global trade, seed transport and agricultural exchange.

Put simply, the viruses may be ancient, but people seem to have enabled their far more efficient movement around the world in recent history.

It is a helpful reminder that biosecurity threats are not always caused by newly emerged pathogens. In some cases, they involve very old viruses gaining fresh opportunities to travel.

Some genes remain stable while others evolve quickly

The researchers also investigated how different sections of the tymovirus genome alter over time. They found strong evidence of stabilising evolutionary pressure on genes linked to replication and the virus’s protective structure.

This generally indicates that these functions are vital and cannot alter greatly without damaging the virus.

In contrast, genes responsible for movement between plant cells appeared to change more rapidly. Such adaptability may be particularly important because movement from one plant cell to another is a crucial stage in successfully infecting a host.

If these genes are able to adapt faster, they could help tymoviruses move into new plant hosts, including crop species.

Thus, while the virus’s “core machinery” remains conservative, the components enabling it to spread through plants may be more adaptable.

A study spanning continents and decades

The science also has a human dimension. Researchers from South America, Europe, the Middle East and Australasia contributed to the team, bringing together contemporary genomic techniques and substantial historical expertise.

Professor Gibbs published one of the first studies to describe an Andean tymovirus in 1966. Other team members have researched Andean potato viruses since the 1970s.

This is important because evolutionary reconstruction of this kind often relies on long-running collections and knowledge accumulated gradually over time.

Without decades of specimens and thorough records, tracing a virus lineage through history becomes considerably more difficult.

Why this matters for the future

The practical significance goes beyond simply recognising that viruses are old. Knowing where plant viruses originated and how they spread can help researchers forecast future developments, especially as plants, seeds and agricultural products routinely cross national borders.

The study presents a two-part account: tymoviruses have deep evolutionary origins shaped by ancient ecosystems, while human activity during the past couple of centuries has had a major influence on where they occur today.

This broader perspective is relevant to plant-health surveillance, crop breeding and biosecurity planning. It also changes how “emerging disease” can be understood.

At times, the threat is not a new virus at all, but an ancient one pushed into unfamiliar places and hosts by modern transport and agriculture.