Can genomics replace electron microscopy?
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| next-generation sequencing |
This might be suggested by the rise of rapid sequencing and the ensuing increased availability of wholly sequenced virus genomes. It is indeed advocated in discussions by unconditional partisans of genomics. The answer is round ‘‘no.’’ Genomics gives us the genome and genes, thus the elementary building blocks of a virus. It also provides gene order and direction of transcription. It identifies genes coding for proteins with homology to known enzymes or virion components, restriction-modification enzymes, capsid protein size, or the length of tape measure proteins. Further, genomics indicates horizontal gene transfer or gene swapping, may reveal relationships between virus groups and individual viruses, and allow quantifying connections and constructing phylogenetic trees. This provides unprecedented insights into virus evolution and is a precious help in phage classification.
However, electron microscopy provides information on virion structure. At the same time, genomics does not show the whole virus, gives no single dimension, provides no information on virus structure and physicochemical properties, identifies unusual bases such as 5-hydroxymethylcytosine, and predicts only some biological properties, such as a lysogenic nature. No sequence can indicate simple things such as the size of phage capsids, their geometry, or the number of capsomers. If, as likely, the length of phage tails depends on the length of ruler protein genes (Katsura and Hendrix, 1984; Pedulla et al., 2003), this must be ascertained by measuring many phage tails under strict magnification control. Unfortunately, this has not been the case. If, as pretended, a genome contains all information on a virus, we have not yet found the instruction manual to read it.
Concerning virus identification, genomics generally does not indicate to which virus family a tailed phage belongs; for example, there are no sequences specific to Myo-, Sipho-, or Podoviridae. Only in the case of small polyhedral or filamentous phages (Micro-, Levi, and Inoviridae) does genomics allow for identifying virus families (Ackermann and Kropinski, 2007). Similarly, a Bacillus tectivirus from the earthworm gut was identified by genomics alone without the benefit of electron microscopy (Schuch et al., 2010). However, in general, investigation of a complete virus sequence may take months and is infinitely slower and more labor-intensive than electron microscopy.
Can metagenomics replace electron microscopy?
The answer is ‘‘no’’ again. For virus identification, metagenomics relies totally on known and identified genes and genomes, which, in turn, belong to viruses known and characterized by electron microscopy. In other terms, the vast majority of numerous genes detected by metagenomics can be identified only to the extent as they belong to known sequences from known viruses. Further, metagenomics will not tell whether any detected lines belong to complete, infectious virions or not.
Can electron microscopy replace genomics?
The answer is ‘‘yes,’’ but only when it comes to identifying high-level taxonomic categories. Clearly, electron microscopy and genomics (or metagenomics) are not alternatives but complementary. Both of them answer different questions and appear as other fingers of the same hand.
(Read more about bacteriophage here)
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