Host specificity and phylogeography of the prochlorophyte Prochloron sp., an obligate symbiont in didemnid ascidians

Abstract

Prochloron is an oxygenic photosynthetic bacterium that lives in obligate symbiosis with didemnid ascidians, such as Diplosoma spp., Lissoclinum spp. and Trididemnum spp. This study investigated the genetic diversity of the genus Prochloron by constructing a phylogenetic tree based on the 16S rRNA gene sequences of 27 isolates from 11 species of didemnid ascidians collected from Japan, Australia and the USA. The 27 isolates formed three phylogenetic groups: 22 of the samples were identified to be closely related members of Prochloron. Two samples, isolated from Trididemnum nubilum and Trididemnum clinides, were found to belong to the species Synechocystis trididemni, the closest relative of Prochloron. Three isolates formed a separate group from both Prochloron sp. and S. trididemni, potentially indicating a new symbiotic phylotype. Genomic polymorphism analysis, employing cyanobacterium-specific highly iterative palindrome 1 repeats, could not delineate the isolates further. For the Prochloron sp. isolates, the phylogenetic outcome was independent of host species and geographic origin of the sample indicating a low level of host specificity, low genetic variation within the taxon and possibly a lack of a host-symbiont relationship during reproductive dispersal. This study contributes significantly to the understanding of Prochloron diversity and phylogeny, and implications for the evolutionary relationship of prochlorophytes, cyanobacteria and chloroplasts are also discussed.

Complex microbiome underlying secondary and primary metabolism in the tunicate- Prochloron symbiosis

“…This includes the apparent absence of genome reduction, the presence of a full set of primary metabolic genes, and a G+C content relatively standard for free-living cyanobacteria (i.e., 41-42%). This is consistent with previous studies showing that vertical transmission is not the only way that hosts can acquire P. didemni (39)(40)(41)(42)(43). Although some host ascidians appear to require P. didemni for survival, our genome-sequencing data indicate that this relationship may be metabolically facultative for the bacteria.…”

“…We interpret this multiplication of hli genes in P. didemni, which is reminiscent of that previously reported for high light-adapted Prochlorococcus (54) as a photoprotective mechanism. As Prochlorococcus and Prochloron are taxonomically unrelated but share some photosynthetic features (39), this probably reflects functional convergence.…”

Down under the tunic: bacterial biodiversity hotspots and widespread ammonia-oxidizing archaea in coral reef ascidians

“…Even when inhabiting the colonial tunic, the symbionts are mostly extracellular, with only a few instances of intracellular associations (Hirose et al, 1996;Moss et al, 2003;Kojima and Hirose, 2010). However, few studies to date have employed the molecular approaches required to accurately assess microbial biodiversity in ascidians (Martínez-García et al, 2007, 2011Mü nchhoff et al, 2007;Tait et al, 2007;Ló pez-Legentil et al, 2011;Behrendt et al, 2012;Erwin et al, 2013). For example, DNA sequence analysis and fluorescence in situ hybridization techniques only recently revealed the first archaeal symbionts in the ascidian tunic, indicating that Thaumarchaeota may be involved in nitrification inside host tissues (Martínez-García et al, 2008).…”

Ultrastructural and microspectrophotometric characterization of multiple species of cyanobacterial photosymbionts coexisting in the colonial ascidianTrididemnum clinides(Tunicata, Ascidiacea, Didemnidae)

“…Partensky & Garczarek, 2003). A molecular phylogenic analysis, based on 16S rRNA, indicated that in 22 strains of Prochloron isolated from different species of didemnid ascidians around the Pacific Ocean both host specificity and biogeographic variation were unexpectedly low (Mu¨nchhoff et al, 2007). However, an analysis of the gene encoding the patellamide synthetic enzyme showed that Prochloron spp.…”

“…from Heron Island (Great Barrier Reef, Australia) also contain similar unicellular coccoid cyanobacteria (Parry, 1984;Cox et al, 1985;Kott, 2001). 16S rRNA sequencing was used to identify photosymbionts similar to those of S. trididemni in T. clinides and T. nubilum (Shimada et al, 2003;Mu¨nchhoff et al, 2007). These reddish-coloured symbiotic cyanobacteria commonly have R-phycoerythrin (the characteristic photosynthetic pigment of almost all red algae) as a major pigment instead of the C-phycoerythrin commonly observed in cyanobacteria (Cox et al, 1985, Larkum et al, 1987.…”