title: A plastid without a genome: evidence from the nonphotosynthetic green algal genus Polytomella.
abstract: Polytomella spp. are free-living, nonphotosynthetic green algae closely related to the model organism Chlamydomonas reinhardtii. Although colorless, Polytomella spp. have a plastid, but it is still unknown whether they harbor a plastid genome. We took a next generation sequencing approach, along with transcriptome sequencing, to search for a plastid genome and an associated gene expression system in Polytomella spp. Illumina sequencing of total DNA from four Polytomella spp. did not produce any recognizable plastid-derived reads but did generate a large number of mitochondrial DNA sequences. Transcriptomic analysis of Polytomella parva uncovered hundreds of putative nuclear-encoded, plastid-targeted proteins, which support the presence of plastid-based metabolic functions, similar to those observed in the plastids of other nonphotosynthetic algae. Conspicuously absent, however, were any plastid-targeted proteins involved in the expression, replication, or repair of plastid DNA. Based on these findings and earlier findings, we argue that the Polytomella genus represents the first well-supported example, to our knowledge, of a primary plastid-bearing lineage without a plastid genome.
authors.affiliation: Department of Biology, University of Western Ontario, London, Ontario, Canada N6A 5B7.
authors.forename: David Roy
authors.initials: DR
authors.lastname: Smith
authors.forename: Robert W
authors.initials: RW
authors.lastname: Lee
mesh_terms: D002467:Cell Nucleus / Q000235:genetics; D000460:Chlorophyta / Q000235:genetics*; D018744:DNA, Plant / Q000235:genetics; D018506:Gene Expression Regulation, Plant; D017343:Genes, Plant; D054629:Genome, Mitochondrial / Q000235:genetics; D054627:Genome, Plastid / Q000235:genetics*; D010788:Photosynthesis / Q000235:genetics*; D010802:Phylogeny; D018087:Plastids / Q000235:genetics*; D017422:Sequence Analysis, DNA
chemical_list: D018744:DNA, Plant