Prasinoviruses of the Marine Green Alga Ostreococcus tauri Are Mainly Species Specific

Prasinoviruses infecting unicellular green algae in the order Mamiellales (class Mamiellophyceae) are commonly found in coastal marine waters where their host species frequently abound. We tested 40Ostreococcus tauri viruses on 13 independently isolated wild-typeO. tauri strains, 4 wild-typeO. lucimarinus strains, 1Ostreococcus sp. (“Ostreococcus mediterraneus ”) clade D strain, and 1 representative species of each of two other related species of Mamiellales,Bathycoccus prasinos andMicromonas pusilla . Thirty-four out of 40 viruses infected onlyO. tauri , 5 could infect one other species of theOstreococcus genus, and 1 infected two otherOstreococcus spp., but none of them infected the other genera. We observed that the overall susceptibility pattern ofOstreococcus strains to viruses was related to the size of two host chromosomes known to show intraspecific size variations, that genetically related viruses tended to infect the same host strains, and that viruses carrying inteins were strictly strain specific. Comparison of two completeO. tauri virus proteomes revealed at least three predicted proteins to be candidate viral specificity determinants.