Are your tomatos looking dodgy?

Tomato yellow leaf curl Tomato yellow leaf curl virus (TYLCV) poses a serious threat to tomato production throughout the temperate regions of the world. A new analysis suggests that the virus probably arose somewhere in the Middle East between the 1930s and 1950s and that its global spread only began in the 1980s after the emergence of two strains – TYLCV-Mld and -IL. In agreement with other work, it finds that the highly invasive TYLCV-IL strain has jumped at least twice to the Americas – once from the Mediterranean basin in the early 1990s and once from Asia in the early 2000s. Although the results corroborate historical accounts of TYLCV-like symptoms in tomato crops in the Jordan Valley in the late 1920s, they indicate that the region around Iran is both the current center of TYLCV diversity and is the site where the most intensive ongoing TYLCV evolution is taking place. The analysis indicates that this region is epidemiologically isolated suggesting that novel TYLCV variants found there are probably not direct global threats, and identifies the Mediterranean basin as the main launch-pad of global TYLCV movements.

The Spread of Tomato Yellow Leaf Curl Virus from the Middle East to the World. (2010) PLoS Pathog 6(10): e1001164. doi:10.1371/journal.ppat.1001164
The ongoing global spread of Tomato yellow leaf curl virus (TYLCV; Genus Begomovirus, Family Geminiviridae) represents a serious looming threat to tomato production in all temperate parts of the world. Whereas determining where and when TYLCV movements have occurred could help curtail its spread and prevent future movements of related viruses, determining the consequences of past TYLCV movements could reveal the ecological and economic risks associated with similar viral invasions. Towards this end we applied Bayesian phylogeographic inference and recombination analyses to available TYLCV sequences (including those of 15 new Iranian full TYLCV genomes) and reconstructed a plausible history of TYLCV’s diversification and movements throughout the world. In agreement with historical accounts, our results suggest that the first TYLCVs most probably arose somewhere in the Middle East between the 1930s and 1950s (with 95% highest probability density intervals 1905–1972) and that the global spread of TYLCV only began in the 1980s after the evolution of the TYLCV-Mld and -IL strains. Despite the global distribution of TYLCV we found no convincing evidence anywhere other than the Middle East and the Western Mediterranean of epidemiologically relevant TYLCV variants arising through recombination. Although the region around Iran is both the center of present day TYLCV diversity and the site of the most intensive ongoing TYLCV evolution, the evidence indicates that the region is epidemiologically isolated, which suggests that novel TYLCV variants found there are probably not direct global threats. We instead identify the Mediterranean basin as the main launch-pad of global TYLCV movements.

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One Response to Are your tomatos looking dodgy?

  1. Ed Rybicki says:

    Ah, now…geminiviruses…meat and drink for the last 25 years, and hopefully also providing funding well into retirement…but we digress.

    Geminiviruses are really, really interesting, both from the point of view of rapid mutation giving rise to a frighteningly efficient and rapid adaptation to new hosts, and because they exemplify the “virushostvector—>humans” interaction scenario. That is, they stay where they are, safe in their little ecosystems, where insect (=whitefly in this case) vector is well adapted to feed on a narrow range of host organisms – until something perturbs their environment. Something being…oh, a new vector species, migrating around because of human intervention; new hosts, due to humans discovering intensive and large-scale agriculture; humans moving them around via infected plants – which is how TYLCV-Is got into the Caribbean first and then into the US, in the 1990s: via infected tomato seedlings being imported by growers.

    The phylogeography of geminiviruses is also fascinating, with natural centres of diversity in Africa (eg: cassava geminiviruses; about 8 species at last count; mastrevirus – many dozen cereal species alone and a couple new genera possibly) – and in the near East and Indian subcontinent and SE Asia, where huge numbers of begomoviruses continue to be discovered.

    Including the possible progenitor of all South American (aka New World) begomoviruses, in Vietnam. Whether by Chinese junk or floating log, there is a population bottleneck in begomoviruses that points to a very restricted set of genotypes giving rise to all “native” American begomoviruses.

    Which may not have been that long ago, given that the damn things mutate like retroviruses – MOST unlike sedate dsDNA viruses, which vary much more slowly.

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