Virus or not? Phylogenetics of polydnaviruses and their wasp carriers.

Our current, still limited, understanding of the comparative biology and evolution of polydnaviruses (PDVs) is reviewed, especially in the context of the possible origins of these parasitoid viruses and of their coevolution with carrier wasps. A hypothetical scenario of evolution of PDVs from ascovirus (or ascovirus-like) ancestors is presented, with examples of apparent extant transitional forms. PDVs appear, in the case of bracoviruses, to show phylogenetic relationships that mirror those of their wasp carriers: with ichnoviruses, the picture is less clear. Ongoing sequencing studies of entire PDV genomes from diverse wasp species are likely to greatly contribute to our understanding of PDV evolution.