Taxonomy is one of those wonderful subjects that at first seems very simple (and very boring). The word comes from Greek - taxis meaning 'order' and nomos meaning 'law', or 'science' and is at it's most basic the science of classification. Most people do varying amounts of taxonomy at school; dividing living things into plants, animals and fungi, dividing the vertebrates into birds, mammals, amphibians, fish and reptiles etc.
Putting the dubious accuracy value of these classification labels aside (particularly the bit about reptiles) taxonomy is, at it's most basic level, a simple system for ordering things and putting them in little boxes. Glorified stamp collecting, as it were. But there are still plenty of arguments and various feuds about the exact relationships of things, most often at the species level, and even whether the whole 'five kingdom' model is any use (the five kingdoms are animals, plants, fungi, prokaryotes - roughly bacteria, and protoctista - which are basically the equivalent of the filing draw marked misc.). In fact, the more you delve into the science of taxonomy, the more complications and problems you start to encounter. Even something that would seem fairly simple, such as what defines a 'species' is a matter of hot debate.
And when you get down to the level of single-celled organisms the whole system goes a bit haywire. The distinction between 'animals', 'plants' and 'fungi' completely breaks down, there are single-celled things that are motile and animal-like but photosynthesise, things that only photosynthesise at the right time of day, or are perfectly sessile and plant-like except they don't photosynthesise. Most protozoa (single celled thingys, more information under the link) are now broken up into a whole new set of labels, very few of which seem to relate to the larger multi-cellular organisms.
When it gets to things like bacteria and bacteriophages, of course, taxonomists just break down and cry. Because bacteria, unlike most other things, don't even maintain their genetic integrity. Bacteria can share bits of their genome quite happily, even with bacteria that are seemingly very unrelated, making the whole 'species' concept break down a bit anyway. Phages merrily incorporate various bits of bacteria into themselves, splice bits out, even splice themselves in to bacterial genomes and sit there for a while. It's a complete headache to try and organise the things.
Various attempts have been made, of course, since the first discovery of phages in 1915 by the wonderfully python-esquely named Frederick Twort. (to give all and full credit they were also discovered completely independently in 1917 by Felix d'Herelle). The first system was based on morphology, what the phage looked like, and was greatly helped by the electron microscope. Although most phages adopt the 'lunar landing module' look, there is plenty of variation within that. Length of tail fibres, size of capsid, symmetry of the capsule, alright, not very much variation, but still something for taxonomists to hang onto.
Size and shape are never good indications of relatedness, a dolphin is more related to a hyena than to a shark, however similar the two might look. Nucleic acid research during the 1960's shook up the whole discipline of taxonomy by providing lots of new exciting DNA information. Phages could now be classified by the amount and type of DNA, which, added to the morphological data, provided a system (albeit a slightly wobbly one).
One of the most currently most widely popular methods to classify things is to examine the genetic DNA that codes for the ribosomes. Ribosomes are complexes of RNA and protein that are used to turn the genetic code into proteins. They are very highly conserved and are therefore very useful in determining evolutionary relatedness and taxonomy.
The problem is, of course, that bacteriophages don't have any ribosomes. They use the bacterial ones; they harness the bacterial internal machinery for replicating DNA and making proteins and therefore don't need to carry any of their own. In view of this, one of the most recent attempts to organise phage taxonomy has focused on looking at their proteins. The relatedness of the proteins has been used to create clever sounding things like distance matrices and the highly impressively named 'phage proteome tree'. Of course there are several problems, possibly the main one being that phages, especially the ones that sit inside bacteria, have a distressing tendency to pick up bits of DNA that aren't theirs. Which translates into proteins that aren't theirs and makes the whole procedure just that little bit more awkward.
There's been some work comparing the genomes of just the structural components, in the hope that there won't be too much genetic exchange going on with the genes that are actually needed to build the phage. The people doing it seem fairly confident, and have managed to isolate about five separate genera. There's a paper from them here, hopefully should be accessible (in form if not in content).
The whole thing is really all a bit up in the air, with some fairly amusing piss-ups between the different schools of thought. Horizontal gene transfer can be a bitch sometimes :)
Narrow-minded, short-sighted university administrators
3 hours ago in The Phytophactor