Science Update – Origins of Hosta Virus X




Hosta Virus X, first discovered in 1996, has been responsible for much damage to hosta plants. While more is gradually becoming known about ways to identify the virus and decrease transmission, relatively little is known about the virus itself, and particularly little about its origins.

Those wishing to know more about how this virus originated will be heartened by a very recent (2009) scientific paper by Fajolu and co-authors, published in the prestigious Archives of Virology. This article showcases the most comprehensive effort to study and classify Hosta Virus X since its discovery back in 1996, and shows that there are many different forms of this virus but that they are all closely related; probably via a common ancestor.

Slightly different types of Hosta Virus X were found

In this paper (Genetic variability and phylogenetic analysis of hosta virus X, Fajolou et al. 2009, Archives of Virology 154:1909-1916), the authors begin at a very practical standpoint; they isolated Hosta Virus X from a range of 30 different hosta plants throughout Tennessee. Most of the cultivars were known, but some were from plants for which the cultivar was unknown.

Although most of the plants contained one main type of Hosta Virus X, some differences were found in this virus isolated from a few of these plants. The differences were evident in two parts of the virus: the Coat Protein which, put simply, refers to how the virus ‘looks’ from the outside, and a different bit called TGB1, which reflects how the virus is able to move from cell to cell. So in other words, different types of Hosta Virus X, while not common, do exist. However, virtually all viruses also exist in more than one type, so Hosta Virus X is certainly not unique in this regard.

The possibility that different types of Hosta Virus X may act differently

Should we be worried about these different types of Hosta Virus X? It is reasonable to be a bit concerned about the important parts of the virus in which the differences were found: in Coat Protein and TBG1. These affect the way the virus looks (in the case of Coat Protein) and functions (in the case of TGB1). So it's entirely possible that these different types of Hosta Virus X may each act a little bit differently (e.g. in how it infects the hosta, how it gets around in the hosta, etc.) However the authors are careful to say that this idea of different types acting differently is so far just a possibility, and it is outside the scope of their paper to prove whether or not this is indeed the case (although it would certainly be an interesting follow-up study).

Now that we have seen that differences in Hosta Virus X do exist, we will look at the question of whether these different types are vastly different from the main type, or only a little bit different. The answer is: only a little bit different. Indeed, for Hosta Virus X the Iargest of the differences observed in Coat Protein or TGB1 reflected only a 2% change from the main form. This tells us that the majority of Hosta Virus X out there is present in one main form and that the many different versions that exist are only subtly different. This makes Hosta Virus X a fairly uniform virus. Other viruses out there tend to have much more obvious differences between different types.

To make this clearer, let's use an analogy about cars. If we had different types of cars and we were told they had differences in important parts, such as their engines, we would expect these cars might act (drive) differently (as opposed to if differences were in something incidental, such as the windows). These hosta viruses did indeed have differences in important parts of the virus. However (going back to the cars) if we were told the differences in the car engines were all small differences, we would not expect that the cars would drive very differently. They might drive a little differently, but not vastly differently. So the different types of hosta virus X are a bit like cars which have small differences in their engines.

Let's get back to the original question: could the different types of Hosta Virus X act differently? The answer is that in theory, they could act differently toward the hosta plant (because of the important parts of the virus in which differences are seen), but in practice (because they are only subtle differences) we would not expect to see some radically different action by different forms of the virus. So, none of these different versions is likely to turn out to be some kind of extra-strength super-virus.

A common ancestor for the different types of Hosta Virus X

From the slightly different forms found of the Hosta Virus X, the authors deduced that these various forms of the virus did not tend to be found in a particular geographic area or a particular cultivar. They said that the various types of Hosta Virus X are likely to have originated from a single common ancestor. The results of this study also confirmed the previous placement of Hosta Virus X in the family Flexiviridae, a virus family containing a wide range of plant viruses. The authors believe that the virus is currently heading toward even more diversity through further natural mutations. Does this mean that there will be a whole lot of different types of Hosta Virus X popping up out there in the next several years? No - because Hosta Virus X (when compared to other viruses) does not appear to be under much selective pressure to keep new mutations when they do occur. This means that there is no advantage for Hosta Virus X to change, so in the future we expect most of the Hosta Virus X to be still in one main form with some not-so-common different versions around (as it is now).

What does this all mean?

This recent publication shows that the various forms of Hosta Virus X originated from a single common ancestor. In your garden, there is no reason to treat different infected plants differently from each other, because the different types of Hosta Virus X were found not to be restricted to specific cultivars or geographic regions. These different forms of Hosta Virus X are theoretically capable of acting differently toward the hosta plant, but because the differences are subtle, there is no reason to attempt to determine which variant of Hosta Virus X you have.

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