Cophylogenetic interactions between marine viruses and eukaryotic picophytoplankton

Laure Bellec,Elodie Foulon,Camille Clerissi,Roseline Edern,Nathalie Simon,Nigel Grimsley,Yves Desdevises

doi:10.1186/1471-2148-14-59

Abstract

BackgroundNumerous 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.ResultsAll 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.ConclusionsMechanisms 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.

Highlights

Numerous studies have investigated cospeciation 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
In order to gain some understanding of oceanic ecosystems, it is important to analyse how viruses are transmitted from a host to another: are they mostly vertically transmitted, from ancestor to descendant, globally coevolving and cospeciating with their hosts with limited possibilities to switch to other host species, or can they colonize different host species, even phylogenetically distantly related? An understanding of the pattern of hostvirus cospeciation, or cophylogeny is needed for comparing evolutionary rates based on molecular divergences (e.g. [11]), because it relies on the assumption of simultaneous speciation
Specificity Experimental tests of host-specificity suggest that virus strains are specific to their host genus (Table 1), and infect in majority host species from the same clade (Table 1 and Figure 1)

Summary

Introduction

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. 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. All of the Bathycoccus strains recorded so far form a more homogeneous evolutionary group [16], but recent work based on metagenomic analyses of natural samples suggested that this genus includes 2 to 3 different genotypes [26]. Many genetically different strains have been characterized and form distinct clades according to the host species from which they were isolated [13,27] These associations include numerous host and viral strains, but nothing is known to date about their joint macroevolutionary history, that is does the evolution of the viruses follow that of their hosts, which would be reflected by congruent phylogenetic trees?

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Discussion

Conclusion