title: Algal genomes reveal evolutionary mosaicism and the fate of nucleomorphs.
abstract: Cryptophyte and chlorarachniophyte algae are transitional forms in the widespread secondary endosymbiotic acquisition of photosynthesis by engulfment of eukaryotic algae. Unlike most secondary plastid-bearing algae, miniaturized versions of the endosymbiont nuclei (nucleomorphs) persist in cryptophytes and chlorarachniophytes. To determine why, and to address other fundamental questions about eukaryote-eukaryote endosymbiosis, we sequenced the nuclear genomes of the cryptophyte Guillardia theta and the chlorarachniophyte Bigelowiella natans. Both genomes have >21,000 protein genes and are intron rich, and B. natans exhibits unprecedented alternative splicing for a single-celled organism. Phylogenomic analyses and subcellular targeting predictions reveal extensive genetic and biochemical mosaicism, with both host- and endosymbiont-derived genes servicing the mitochondrion, the host cell cytosol, the plastid and the remnant endosymbiont cytosol of both algae. Mitochondrion-to-nucleus gene transfer still occurs in both organisms but plastid-to-nucleus and nucleomorph-to-nucleus transfers do not, which explains why a small residue of essential genes remains locked in each nucleomorph.
authors.affiliation: Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada.
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mesh_terms: D020418:Algal Proteins / Q000235:genetics / Q000378:metabolism; D017398:Alternative Splicing / Q000235:genetics; D002467:Cell Nucleus / Q000235:genetics*; D056907:Cercozoa / Q000166:cytology / Q000235:genetics* / Q000378:metabolism; D044785:Cryptophyta / Q000166:cytology / Q000235:genetics* / Q000378:metabolism; D003600:Cytosol / Q000378:metabolism; D019143:Evolution, Molecular; D020440:Gene Duplication / Q000235:genetics; D022761:Gene Transfer, Horizontal / Q000235:genetics; D020043:Genes, Essential / Q000235:genetics; D016678:Genome / Q000235:genetics*; D054629:Genome, Mitochondrial / Q000235:genetics; D018745:Genome, Plant / Q000235:genetics; D054627:Genome, Plastid / Q000235:genetics; D008969:Molecular Sequence Data; D009030:Mosaicism; D010802:Phylogeny; D021381:Protein Transport; D020543:Proteome / Q000235:genetics / Q000378:metabolism; D013559:Symbiosis / Q000235:genetics*; D059467:Transcriptome / Q000235:genetics
chemical_list: D020418:Algal Proteins; D020543:Proteome