Date Published: October 27, 2016
Publisher: Public Library of Science
Author(s): Sheree Yau, Claire Hemon, Evelyne Derelle, Hervé Moreau, Gwenaël Piganeau, Nigel Grimsley, Shou-Wei Ding.
Micro-algae of the genus Ostreococcus and related species of the order Mamiellales are globally distributed in the photic zone of world’s oceans where they contribute to fixation of atmospheric carbon and production of oxygen, besides providing a primary source of nutrition in the food web. Their tiny size, simple cells, ease of culture, compact genomes and susceptibility to the most abundant large DNA viruses in the sea render them attractive as models for integrative marine biology. In culture, spontaneous resistance to viruses occurs frequently. Here, we show that virus-producing resistant cell lines arise in many independent cell lines during lytic infections, but over two years, more and more of these lines stop producing viruses. We observed sweeping over-expression of all genes in more than half of chromosome 19 in resistant lines, and karyotypic analyses showed physical rearrangements of this chromosome. Chromosome 19 has an unusual genetic structure whose equivalent is found in all of the sequenced genomes in this ecologically important group of green algae.
Eukaryotic micro-algae of the genus Ostreococcus and related species of the order Mamiellales are globally distributed in the photic zone of the world’s oceans where they contribute to fixation of atmospheric carbon and production of oxygen, besides providing a primary source of nutrition in the food web [1–3]. Their tiny size (1–3 μm), simple cells (one chloroplast, one mitochondrion), ease of laboratory culture and extremely small genomes, several of which have been completely sequenced [4–8], render them attractive as models for marine ecology [9–11], cell biology  and evolution in the green lineage [4,13]. Typical features of the highly streamlined genomes of the Mamiellales include a higher GC content (48–64% [7,14], 59% for O. tauri) than higher plants (41%, ) and two unusual “outlier” lower GC chromosomes that can carry higher proportions of transposons and genes predicted to originate from prokaryotes (see  for a recent review). In the type species O. tauri, these two chromosomes are chromosome 2 (the big outlier chromosome or BOC) and chromosome 19 (the small outlier or SOC), both of which greatly vary in size between strains isolated from the environment .