The Chlorella variabilis NC64A Genome Reveals Adaptation to Photosymbiosis, Coevolution with Viruses, and Cryptic Sex  

Abstract

Chlorella variabilis NC64A, a unicellular photosynthetic green alga (Trebouxiophyceae), is an intracellular photobiont of Paramecium bursaria and a model system for studying virus/algal interactions. We sequenced its 46-Mb nuclear genome, revealing an expansion of protein families that could have participated in adaptation to symbiosis. NC64A exhibits variations in GC content across its genome that correlate with global expression level, average intron size, and codon usage bias. Although Chlorella species have been assumed to be asexual and nonmotile, the NC64A genome encodes all the known meiosis-specific proteins and a subset of proteins found in flagella. We hypothesize that Chlorella might have retained a flagella-derived structure that could be involved in sexual reproduction. Furthermore, a survey of phytohormone pathways in chlorophyte algae identified algal orthologs of Arabidopsis thaliana genes involved in hormone biosynthesis and signaling, suggesting that these functions were established prior to the evolution of land plants. We show that the ability of Chlorella to produce chitinous cell walls likely resulted from the capture of metabolic genes by horizontal gene transfer from algal viruses, prokaryotes, or fungi. Analysis of the NC64A genome substantially advances our understanding of the green lineage evolution, including the genomic interplay with viruses and symbiosis between eukaryotes.

Early eukaryotic origins for cilia-associated bioactive peptide-amidating activity

“…However, the occurrence of axonemal inner dynein arm heavy chain genes in Ostreococcus has been reported, raising the possibility that loss of ciliary components is incomplete and that remnants of a cilium might still exist (Wickstead and Gull, 2007). Alternatively, as suggested for Chlorella, which encodes nearly all ciliary outer dynein arm components (and a PAM gene), some green algae might have cilia at specific developmental stages that have not yet been observed; indeed, many cilia-related genes that are absent in land plants are conserved in these algae (Blanc et al, 2010).…”

Seaweed reproductive biology: environmental and genetic controls

“…is considered evidence for sexual reproduction (Schurko and Logsdon 2008). Also, in certain algae where sexual reproduction is very rarely observed (Trebouxiophyceae: Blanc et al 2010, Fučíková et al 2015, or has not yet been observed (Ostreococcus: Derelle et al 2006;Micromonas: Worden et al 2009), homologues of these core meiotic genes are present (Grimsley et al 2010).…”

Potassium Ion Channels: Could They Have Evolved from Viruses?

“…If the chlorovirus K + channel genes were acquired from their algal host, one would expect that coevolution occurred between the virus and host K + channel-encoded genes. This assumption of coevolution was recently tested experimentally because the genomes of two algal virus hosts, C. variabilis (Blanc et al, 2010) and E. siliculosus (Cock et al, 2010), have been sequenced and annotated. The coccal green alga Chlorella spp.…”