Shivering Viromes Despite its icy reputation, freshwater ponds and lakes do occur in Antarctica, and open freshwater can be found for a few brief weeks during the austral summer. The ecology of these lakes is, as expected, rather specialized to cope with the extreme seasonal conditions. In a metagenomic study, López-Bueno et al. (p. 858 ) inspected the virus community of Lake Limnopolar on Livingston Island and found an unexpectedly rich genetic diversity. A dominant group of previously unidentified single-stranded DNA viruses was found, and a striking shift after ice-melt in spring from single-stranded to double-stranded DNA viruses was observed, probably as their algal hosts started to bloom with increasing daylight hours. The diverse viruses may donate specialized genes that host organisms can also exploit to aid their survival under winter extremes of heat and light deprivation.

The role of receptor recognition in the emergence of virulent viruses was investigated in the infection of severe combined immunodeficient (SCID) mice by the apathogenic prototype strain of the parvovirus minute virus of mice (MVMp). Genetic analysis of isolated MVMp viral clones (n ‫؍‬ 48) emerging in mice, including lethal variants, showed only one of three single changes (V325M, I362S, or K368R) in the common sequence of the two capsid proteins. As was found for the parental isolates, the constructed recombinant viruses harboring the I362S or the K368R single substitutions in the capsid sequence, or mutations at both sites, showed a large-plaque phenotype and lower avidity than the wild type for cells in the cytotoxic interaction with two permissive fibroblast cell lines in vitro and caused a lethal disease in SCID mice when inoculated by the natural oronasal route. Significantly, the productive adsorption of MVMp variants carrying any of the three mutations selected through parallel evolution in mice showed higher sensitivity to the treatment of cells by neuraminidase than that of the wild type, indicating a lower affinity of the viral particle for the sialic acid component of the receptor. Consistent with this, the X-ray crystal structure of the MVMp capsids soaked with sialic acid (N-acetyl neuraminic acid) showed the sugar allocated in the depression at the twofold axis of symmetry (termed the dimple), immediately adjacent to residues I362 and K368, which are located on the wall of the dimple, and approximately 22 Å away from V325 in a threefold-related monomer. This is the first reported crystal structure identifying an infectious receptor attachment site on a parvovirus capsid. We conclude that the affinity of the interactions of sialic-acid-containing receptors with residues at or surrounding the dimple can evolutionarily regulate parvovirus pathogenicity and adaptation to new hosts.The recognition of cell surface components acting as receptors for viruses is a major event in infection and a key parameter of viral tropism and pathogenesis. Proteins, carbohydrates, and glycolipids may serve as virus receptors, and in many examples, attachment to cells is facilitated by the interaction of the viral particle with a sugar component (reviewed in reference 90). The 25-nm-diameter nonenveloped capsid of the Parvoviridae (55), a large family of viruses with a 5-kb single-stranded DNA genome, offers a simple genetic and structural model to define the pathogenic consequences of the recognition of cellular receptors by icosahedral viruses. Molecules with specific binding properties or functional activity as parvovirus receptors have been identified for some members of the family, such as the ABP protein for the Aleutian disease virus (28); some globosides and the ␣5␤1 integrin for the human parvovirus B19 (12,40,88); transferrin receptors for canine (CPV) and feline (FPV) parvoviruses and mink enteritis virus (60, 59); and heparan sulfate, ␣v␤5 integrin, and growth factor receptors for adeno-associated viruses (A...