title: Cophylogenetic interactions between marine viruses and eukaryotic picophytoplankton.
abstract: BACKGROUND
  Numerous studies have investigated cospeciation (or cophylogeny) in various host-symbiont systems, and different patterns were inferred, from strict cospeciation where symbiont phylogeny mirrors host phylogeny, to complete absence of correspondence between trees. The degree of cospeciation is generally linked to the level of host specificity in the symbiont species and the opportunity they have to switch hosts. In this study, we investigated cophylogeny for the first time in a microalgae-virus association in the open sea, where symbionts are believed to be highly host-specific but have wide opportunities to switch hosts. We studied prasinovirus-Mamiellales associations using 51 different viral strains infecting 22 host strains, selected from the characterisation and experimental testing of the specificities of 313 virus strains on 26 host strains.
  
  
  RESULTS
  All virus strains were restricted to their host genus, and most were species-specific, but some of them were able to infect different host species within a genus. Phylogenetic trees were reconstructed for viruses and their hosts, and their congruence was assessed based on these trees and the specificity data using different cophylogenetic methods, a topology-based approach, Jane, and a global congruence method, ParaFit. We found significant congruence between virus and host trees, but with a putatively complex evolutionary history.
  
  
  CONCLUSIONS
  Mechanisms other than true cospeciation, such as host-switching, might explain a part of the data. It has been observed in a previous study on the same taxa that the genomic divergence between host pairs is larger than between their viruses. It implies that if cospeciation predominates in this algae-virus system, this would support the hypothesis that prasinoviruses evolve more slowly than their microalgal hosts, whereas host switching would imply that these viruses speciated more recently than the divergence of their host genera.
authors.forename: Laure
authors.initials: L
authors.lastname: Bellec
authors.forename: Camille
authors.initials: C
authors.lastname: Clerissi
authors.forename: Roseline
authors.initials: R
authors.lastname: Edern
authors.forename: Elodie
authors.initials: E
authors.lastname: Foulon
authors.forename: Nathalie
authors.initials: N
authors.lastname: Simon
authors.forename: Nigel
authors.initials: N
authors.lastname: Grimsley
authors.affiliation: Integrative Biology of Marine Organisms, Observatoire Océanologique, Sorbonne Universités, UPMC Univ Paris 06, UMR 7232, F-66650 Banyuls-sur-Mer, France. desdevises@obs-banyuls.fr.
authors.forename: Yves
authors.initials: Y
authors.lastname: Desdevises
mesh_terms: D004279:DNA, Viral / Q000235:genetics; D058507:Host Specificity; D010802:Phylogeny; D010839:Phytoplankton / Q000235:genetics*; D014780:Viruses / Q000235:genetics*
chemical_list: D004279:DNA, Viral