Genomic and metabolic surveys reveal diverse consequences of loss of photosynthesis to plastids of free-living nonphotosynthetic algae in the order Chlamydomonadales

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University of New Brunswick


Photosynthesis has numerous advantages, but many algae and land plants have lost that ability in favor of heterotrophic lifestyles. There are diverse and numerous investigations about the loss of photosynthesis in parasitic algae, but our knowledge about free-living nonphotosynthetic cases is very limited. In my thesis, I set out to investigate genomic and functional consequences at the plastid level following the loss of photosynthesis in free-living species of the green algal order Chlamydomonadales. Recent studies have revealed different trajectories of plastid genome (ptDNA) evolution in this group, including outright losses, inflation and compaction in lineages that lost photosynthesis independently. The two novel ptDNAs I present here continue to showcase variety. The ptDNA of Hyalogonium fusiforme is inflated and has an almost intact gene repertoire, whereas the ptDNA of Polytoma oviforme is compact and lacks all the genes involved in photosynthesis. My analysis of transcriptomic data of Polytoma uvella indicates that the colorless plastid of this alga is heavily involved in the biosynthesis of most essential amino acids, tetrapyrrole, starch, nucleotides, and carotenoids. My comparative study identified key metabolic functions that have been retained in nonphotosynthetic plastids regardless of the algal trophic lifestyle.