Publication Date:
2015-03-23
Description:
Understanding the evolution of the free-living, cyanobacterial, diazotrophTrichodesmiumis of great importance because of its critical role in oceanic biogeochemistry and primary production. Unlike the other 〉150 available genomes of free-living cyanobacteria, only 63.8% of theTrichodesmium erythraeum(strain IMS101) genome is predicted to encode protein, which is 20–25% less than the average for other cyanobacteria and nonpathogenic, free-living bacteria. We use distinctive isolates and metagenomic data to show that low coding density observed in IMS101 is a common feature of theTrichodesmiumgenus, both in culture and in situ. Transcriptome analysis indicates that 86% of the noncoding space is expressed, although the function of these transcripts is unclear. The density of noncoding, possible regulatory elements predicted inTrichodesmium, when normalized per intergenic kilobase, was comparable and twofold higher than that found in the gene-dense genomes of the sympatric cyanobacterial generaSynechococcusandProchlorococcus, respectively. ConservedTrichodesmiumnoncoding RNA secondary structures were predicted between most culture and metagenomic sequences, lending support to the structural conservation. Conservation of these intergenic regions in spatiotemporally separatedTrichodesmiumpopulations suggests possible genus-wide selection for their maintenance. These large intergenic spacers may have developed during intervals of strong genetic drift caused by periodic blooms of a subset of genotypes, which may have reduced effective population size. Our data suggest that transposition of selfish DNA, low effective population size, and high-fidelity replication allowed the unusual “inflation” of noncoding sequence observed inTrichodesmiumdespite its oligotrophic lifestyle.
Print ISSN:
0027-8424
Electronic ISSN:
1091-6490
Topics:
Biology
,
Medicine
,
Natural Sciences in General
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