Phaeovirus Infections in Kelp
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The latent dsDNA viruses of the genus Phaeovirus (family Phycodnaviridae, clade Nucleo-cytoplasmic Large DNA Viruses; NCLDVs) employ genome integration in their brown algae hosts (class Phaeophyceae). The only phaeoviruses described in detail infected the order Ectocarpales, though Phaeovirus major capsid protein (MCP) occurs in 4 kelp (order Laminariales) species. Phaeoviruses are a major knowledge gap because brown algae are ecologically and economically important and have independently evolved complex multicellularity. This study aimed to investigate kelp Phaeovirus morphology, evolution, host range, distribution, host impacts, and genomics. Microscopy of Laminaria digitata gametophytes revealed particles and cell morphology typical of Phaeovirus infections. This putative Laminaria digitata virus 1 (LdV-1) infection, unlike the Ectocarpales phaeoviruses, often occurred in vegetative cells. L. digitata Phaeovirus symptoms were ~3 times more common in 18 versus 15 oC culture, but overall were uncommon and highly variable. No impact on gametophyte reproduction was observed. Broad-scale MCP PCRs and subsequent phylogeny identified 4 novel kelp phaeoviruses, placing the phaeoviruses of Ecklonia maxima, Ecklonia radiata, and Undaria pinnatifida in subgroup A, a Macrocystis pyrifera Phaeovirus in subgroup C, and a Saccharina japonica Phaeovirus in the novel subgroup D. Kelp phaeoviruses may follow the Ectocarpales Phaeovirus evolutionary trend of genome reduction (in subgroups B, C, and D versus A). Combined with all available data, 26 % of kelp were Phaeovirus MCP-positive. Genomic data from LdV-1 and 3 available kelp genomes contained Phaeovirus orthologs from the following putative, integrated phaeoviruses: LdV-1, Ecklonia radicosa virus (ErcV), Saccharina japonica virus (SjV), and Undaria pinnatifida virus (UpV). Subsequent phylogeny of 9 Phaeovirus core genes showed similar subgroups as before and non-core orthologs had implications for Phaeovirus evolution. For kelp phaeoviruses, this study has revealed a partial infection cycle, preliminary observations of viral symptoms, a broader distribution and host range, and evolutionary insights for both viruses and hosts.