Abstract
BackgroundBathycoccus prasinos is an extremely small cosmopolitan marine green alga whose cells are covered with intricate spider's web patterned scales that develop within the Golgi cisternae before their transport to the cell surface. The objective of this work is to sequence and analyze its genome, and to present a comparative analysis with other known genomes of the green lineage.ResearchIts small genome of 15 Mb consists of 19 chromosomes and lacks transposons. Although 70% of all B. prasinos genes share similarities with other Viridiplantae genes, up to 428 genes were probably acquired by horizontal gene transfer, mainly from other eukaryotes. Two chromosomes, one big and one small, are atypical, an unusual synapomorphic feature within the Mamiellales. Genes on these atypical outlier chromosomes show lower GC content and a significant fraction of putative horizontal gene transfer genes. Whereas the small outlier chromosome lacks colinearity with other Mamiellales and contains many unknown genes without homologs in other species, the big outlier shows a higher intron content, increased expression levels and a unique clustering pattern of housekeeping functionalities. Four gene families are highly expanded in B. prasinos, including sialyltransferases, sialidases, ankyrin repeats and zinc ion-binding genes, and we hypothesize that these genes are associated with the process of scale biogenesis.ConclusionThe minimal genomes of the Mamiellophyceae provide a baseline for evolutionary and functional analyses of metabolic processes in green plants.
Highlights
Bathycoccus prasinos is an extremely small cosmopolitan marine green alga whose cells are covered with intricate spider’s web patterned scales that develop within the Golgi cisternae before their transport to the cell surface
Contrary to the type strain described as Bathycoccus prasinos [8], which was isolated at a depth of 100 meters, RCC1105 was isolated from surface water (5 m)
The strain RCC1105 has a typical Bathycoccus morphology with scales covering the cell (Figure 1) and we confirmed its taxonomic affiliation by PCR amplification of its 18S ribosomal gene
Summary
Bathycoccus prasinos is an extremely small cosmopolitan marine green alga whose cells are covered with intricate spider’s web patterned scales that develop within the Golgi cisternae before their transport to the cell surface. The eukaryotic fraction of picophytoplankton accounts for a modest part of the oceanic biomass, but contributes an important part to primary production in many oceanic waters [3,4] Among these picoeukaryotes, environmental diversity studies based on ribosomal gene sequences showed that small green algae, and notably the three genera Bathycoccus, Micromonas and Ostreococcus, are distributed worldwide and are numerically important in coastal areas. The three genera belong to the order Mamiellales, in the class Mamiellophyceae [12,13], a monophyletic group in the phylum Chlorophyta The ancestors of these micro-organisms emerged at the base of the green lineage and knowledge about them provides a baseline for exploring the evolution of this lineage, which gave rise to terrestrial plants. Given their small cellular and genome sizes, they may reveal the ‘bare limits’ of life as a free-living photosynthetic eukaryotes, presenting a simple organization with very little non-coding sequences [14]
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