More About Plant Virus Evolution: Past, Present, and Future

ABSTRACT

Gene sequencing was invented in the 1980s, enabling the evolutionary relationships of organisms to be studied in detail. The ways in which these studies provide the intellectual framework for research into the life of viruses continues to expand.

Plants, animals and other organisms present viruses with very different environments, both structurally and biochemically, and this may be the reason why so few virus groups span host kingdoms, but a few do, and studies of these reveal the shared and unique constraints and opportunities provided by different types of host, and also the diverse ways that viruses overcome the constraints. The RNA-silencing system seems to provide the primary plant defense against viruses, and although RNA-silencing mechanisms are present in all eukaryotes, they are most developed in plants where they also modulate the expression of plant genes. Plants have a rigid cellular structure with the cells connected by plasmodesmata too narrow for virions to pass through. This has required viruses to adopt specific mechanisms to aid their systemic spread within plants. The diverse measures adopted by viruses to suppress RNA silencing and to aid their spread through plants indicate that such mechanisms have evolved independently on several occasions.

Likewise a great range of symbiotic, commensal, and satellite relationships are found among plant viruses, and again the diversity of the relationships, of the virus groups involved, and of the resulting phenotypes, emphasizes that viruses of plants are polyphyletic.

Studies of mutations in model experimental systems, and of gene sequence variation in natural viral populations, are clarifying the mechanisms that produce “quasispecies,” even though the concept seems to be still largely misunderstood. The relative contribution of different evolutionary processes, including mutation, drift, recombination, and selection, to viral population change is becoming better understood.

The taxonomies of tobamoviruses and of their principle hosts seem to be congruent, indicating that they have probably co-evolved, and hence may be of the same age, around 100 million years. However potyviruses and their hosts show no such relationships, indeed gene sequence differences in viral populations, of which the history is known, indicate that the genus Potyvirus may be only a few thousand years old. Our understanding of more distant relationships remains very speculative as it depends on comparisons of “molecular phenotypic” characters (e.g. structure and function) rather than of gene sequences.

Viruses have been studied for more than a century, their molecules are well known, but our understanding of the molecular basis of plant virus biology is still in its infancy, and we have little idea of how viruses will respond to “climate change” and “transgene pollution.”