AME 45:1-14 (2006)  -  doi:10.3354/ame045001

ABSTRACT: Two viruses infecting Micromonas pusilla (Prasinophyceae) were isolated from the Gulf of Naples (Mediterranean Sea). MpVN1 and MpVN2 had comparable sizes (100 to 130 nm) and a typical icosahedral shape. However, MpVN1 attached to the host with a tail of about 0.2 µm, which was absent in MpVN2. Infectivity varied between the 2 viruses tested on 11 M. pusilla isolates, with no relationships to the geographic provenance of the host strains, nor to their phylogenetic diversity. MpVN2 DNA was amplified using the AVS1/2 primers that are specific for the pol gene in the family Phycodnaviridae, whereas MpVN1 DNA was only amplified using specifically designed primers, which were found not to amplify MpVN2 DNA. AVS1/2-amplifiable viruses were present in ca. ¹Ú₃ of 33 infective M. pusilla virus samples collected over the spring of 1996, whereas the primers designed for MpVN1 DNA were effective only in 1 case. After the infection, host cultures recovered and became resistant to infection from the same virus, but MpVN1-resistant cultures were still susceptible to infection from MpVN2 and from natural virus populations. Immunity to viral infection was apparently not associated with chronic infection (pseudolysogeny) or with viral latency. It is proposed that acquired immunity could partially account for the intraspecific variability in susceptibility to viral infection and have important implications for the outcomes of host–virus interactions at sea.

KEY WORDS: Algal virus · Micromonas pusilla · Ultrastructure · Host range · Resistance