The tiny eukaryoteOstreococcusprovides genomic insights into the paradox of plankton speciation

Abstract

The smallest known eukaryotes, at ≈1-μm diameter, areOstreococcus tauriand related species of marine phytoplankton. The genome ofOstreococcus lucimarinushas been completed and compared with that ofO. tauri. This comparison reveals surprising differences across orthologous chromosomes in the two species from highly syntenic chromosomes in most cases to chromosomes with almost no similarity. Species divergence in these phytoplankton is occurring through multiple mechanisms acting differently on different chromosomes and likely including acquisition of new genes through horizontal gene transfer. We speculate that this latter process may be involved in altering the cell-surface characteristics of each species. In addition, the genome ofO. lucimarinusprovides insights into the unique metal metabolism of these organisms, which are predicted to have a large number of selenocysteine-containing proteins. Selenoenzymes are more catalytically active than similar enzymes lacking selenium, and thus the cell may require less of that protein. As reported here, selenoenzymes, novel fusion proteins, and loss of some major protein families including ones associated with chromatin are likely important adaptations for achieving a small cell size.