Genetic Diversity of Algal Viruses Which Lyse the Photosynthetic Picoflagellate Micromonas pusilla (Prasinophyceae).

The genetic similarity among eight clones of Micromonas pusilla virus (MpV) isolated from five geographic locations was measured by DNA hybridization. Our objective was to explore the existence of genetically distinct populations of MpV by comparing the similarity among MpVs isolated from a single water sample to the similarity among viruses isolated from geographically distant locations. The highest and lowest similarities we observed were 70% (plusmn) 1.1% (mean (plusmn) standard error [SE], n = 3) for virus strains SP1 and SP2 isolated from a California coastal water sample and 13% (plusmn) 1.9% for strains SP2 and PB6; the latter was isolated from New York estuarine water. However, the similarity between MpV isolated from a single water sample was not always greater than the similarity between viruses isolated from different locations. Viruses PB7 and PB8 were isolated from a single New York estuarine sample but were only 16% (plusmn) 0.5% similar, whereas PB7 was quite similar (43% (plusmn) 2.9%) to PL1, a virus from Texas coastal water. Overall, the similarity among MpVs isolated from a single geographic location, 34% (plusmn) 12.6% (mean (plusmn) SE, n = 4), was not significantly different from the similarity among MpVs isolated from geographically distant locations, 26.6% (plusmn) 2.7% (mean (plusmn) SE, n = 24) (P = 0.92, Mann-Whitney U test). Clones of MpV were more similar to each other than they were to the related algal virus PBCV-1, and three groups of MpVs consisting of (i) PL1, SG1, PB6, and PB7, (ii) PB8, and (iii) GM1, SP1, and SP2 were resolved. The genetic variation among MpVs isolated from a single water sample was as large as the variation between viruses isolated from different oceans. If MpVs within a geographic location share genetic characteristics not shared with MpVs from geographically distant locations, this was not reflected in the overall similarity of their genomes.