The molecular biology of frog virus 3 and other iridoviruses infecting cold-blooded vertebrates

Abstract

Frog virus 3 (FV3) is the best characterized member of the family Iridoviridae. FV3 study has provided insights into the replication of other family members, and has served as a model of viral transcription, genome replication, and virus-mediated host-shutoff. Although the broad outlines of FV3 replication have been elucidated, the precise roles of most viral proteins remain unknown. Current studies using knock down (KD) mediated by antisense morpholino oligonucleotides (asMO) and small, interfering RNAs (siRNA), knock out (KO) following replacement of the targeted gene with a selectable marker by homologous recombination, ectopic viral gene expression, and recombinant viral proteins have enabled researchers to systematically ascertain replicative- and virulence-related gene functions. In addition, the application of molecular tools to ecological studies is providing novel ways for field biologists to identify potential pathogens, quantify infections, and trace the evolution of ecologically important viral species. In this review, we summarize current studies using not only FV3, but also other iridoviruses infecting ectotherms. As described below, general principles ascertained using FV3 served as a model for the family, and studies utilizing other ranaviruses and megalocytiviruses have confirmed and extended our understanding of iridovirus replication. Collectively, these and future efforts will elucidate molecular events in viral replication, intrinsic and extrinsic factors that contribute to disease outbreaks, and the role of the host immune system in protection from disease.

Keywords: Iridoviridae; antisense morpholino oligonucleotides; ectothermic vertebrates; knock out mutants; ranavirus; viral gene function; virus replication.

Figure 1.

Ultrastructural analysis of FV3-infected FHM cells. (A) An FV3-infected cell displaying virions budding from the plasma membrane (long arrow) or present within a paracrystalline array (short arrow), a viral assembly site (star), and the nucleus (N) showing chromatin condensation; (B) an enlargement of a viral assembly site (star) showing both full and empty virions and possible assembly intermediates. The assembly site is surrounded by mitochondria (M) and membraneous structures, possibly elements of the endoplasmic reticulum (ER).

Figure 2.

Life cycle of Frog virus 3 (FV3). The life cycle of FV3 is shown in schematic form. See text for details [54]. Used with permission.

Figure 3.

Phylogenetic analysis of iridovirus genomes. A concatenated iridovirus phylogeny using 26 conserved open reading frames from completely sequenced members of the family Iridoviridae. The tree was created using Mega4 with the Neighbor-Joining method, bootstrap (500 replicates) and pairwise deletion options [89]. Members of the genus Ranavirus are highlighted in red.