Persistent virus integration into the genome of its algal host, Ectocarpus siliculosus (Phaeophyceae).

Abstract

The brown alga Ectocarpus siliculosus frequently carries an endogenous virus, E. siliculosus virus (EsV-1), the genome of which is a circular, doublestranded DNA molecule of about 320 kbp. After infection, which occurs in the unicellular spores or gametes, the virus is present latently in all somatic cells of the host. Virus multiplication is restricted to cells of the reproductive organs. It has been an open question whether the latent viral DNA occurs as a free episome or becomes integrated into the host genome. PCR studies showed that viral DNA co-migrates with high molecular mass DNA in pulsed-field gel electrophoresis, which confirms that latent viral DNA is integrated into the host genome.Marine filamentous brown algae of the order Ectocarpales frequently carry endogenous viruses with large doublestranded DNA genomes (Mu$ ller et al., 1998). A well-studied example is Ectocarpus siliculosus virus-1 (EsV-1), the icosahedral capsid of which encloses a circular genome of about 320 kbp (Mu$ ller et al., 1990 ; Lanka et al., 1993). Like other brown-algal viruses, EsV-1 exclusively infects cell wall-free zoospores or gametes. Infected cells develop into mature thalli which can produce pathological symptoms in their sporangia or gametangia. These organs become densely packed with viral particles that are eventually released into the surrounding sea water (Fig. 1 a). It is not unusual, however, for infected algae to appear normal and to produce viable spores (Fig. 1 d ) containing the viral genome ( Fig. 1 e, lane 4). In fact, genetic studies and PCR analyses have shown that the latent viral genome is transmitted vertically through meiosis as a Mendelian trait Bra$ utigam et al., 1995).These earlier reports could not distinguish between an episomal free state of the EsV-1 DNA and integration into the Author for correspondence : Nicolas Delaroque.Fax j49 7531 88 2966. e-mail Nicolas.Delaroque!uni-konstanz.de host genome. We report here the results of experiments that were designed to distinguish between these possibilities. We prepared high molecular mass DNA from zoospores and gametes of infected and uninfected algae by using a modification of the protocol of Liu & Whittier (1994). The following clonal isolates of E. siliculosus were used : strain PAr 10n (Fig. 1 b), a healthy female gametophyte (Mu$ ller, 1979 ;Sengco et al., 1996) ; NZVicZ14 ( Fig. 1 a, c), which is infected by EsV-1 and produces virions as well as zoospores carrying a latent viral genome Sengco et al., 1996) ; and strain Nap R-B1 (Fig. 1 d ), which is a phenotypically normal male gametophyte that is latently infected by EsV-1 and does not produce virions . The cultures of PAr 10n and NZVicZ14 were axenic, Nap R-B1 was unialgal. EsV-1 DNA was prepared from virions released by strain NZVicZ14 as described by Lanka et al. (1993). Culture conditions were as described by Mu$ ller (1991 a, b). Mature algae were stored in the dark at 2 mC overnight. Mass release of reproductive cells within 20 min was induced by rapid transfer into a small volume of f...

Virus adsorption process determines virus susceptibility in Heterosigma akashiwo (Raphidophyceae)

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