The Candidatus Glomeribacter gigasporarum genome sequencing project

Symbiotic associations between endocellular bacteria and eukaryotic cells are widespread in nature, but only a few examples have been described in the fungal kingdom. Arbuscular mycorrhizal species, belonging to the Gigasporaceae family, represent a specialized niche for a homogeneous population of rod shaped bacteria, named Candidatus Glomeribacter gigasporarum. Previous molecular studies on the Gigaspora margarita BEG34 endosymbionts, closely related to the Burkholderiaceae beta-proteobacterial family, have estimated a genome size of ~1.4 MB. A genomic library was constructed from G. margarita BEG34 spores into the fosmid vector pCC1Fos. The library contains ~36K clones with an average insert size of 35 kb. Fungal and bacterial clones were sorted by database sequence homology searches using fosmid ends as a query. A preliminary analysis of 140 clones showed that ~35% of inserts are of prokaryotic origin. Bacterial clones, showing high similarity with Burkholderiaceae genomic sequences, have been validated by PCR and sequenced using a transposon tagging strategy. Here we show a preliminary gene annotation based on a draft genome assembly. Moreover, a new isolation protocol of Ca. G. gigasporarum cells from its host allowed the whole genome amplification (WGA) of the bacterial DNA. A preliminary comparative genomics experiment using the WGA DNA on the Ralstonia solanacearum microarrays demonstrated a high degree of of sequence divergence between the two beta-proteobacteria.