Genome sequencing of Prototheca zopfii genotypes 1 and 2 provides evidence of a severe reduction in organellar genomes

TLDR

Prototheca zopfii is a non‑photosynthetic alga that can infect vertebrates, causing systemic infections such as mastitis in cows. The study sequenced and assembled the mitochondrial and plastid genomes of genotypes 1 and 2, along with nuclear drafts, to elucidate Prototheca evolution and host‑pathogen interactions. The genomes are the smallest known in Trebouxiophyceae, highly reduced and lacking photosynthetic and RNA‑polymerase genes, yet plastid genes are actively transcribed via nuclear‑encoded paralogs, and their architecture aligns closely with P.

Abstract

Abstract Prototheca zopfii ( P. zopfii , class Trebouxiophyceae, order Chlorellales, family Chlorellaceae), a non-photosynthetic predominantly free-living unicellular alga, is one of the few pathogens belonging to the plant kingdom. This alga can affect many vertebrate hosts, sustaining systemic infections and diseases such as mastitis in cows. The aim of our work was to sequence and assemble the P. zopfii genotype 1 and genotype 2 mitochondrial and plastid genomes. Remarkably, the P. zopfii mitochondrial (38 Kb) and plastid (28 Kb) genomes are models of compaction and the smallest known in the Trebouxiophyceae. As expected, the P. zopfii genotype 1 and 2 plastid genomes lack all the genes involved in photosynthesis, but, surprisingly, they also lack those coding for RNA polymerases. Our results showed that plastid genes are actively transcribed in P. zopfii , which suggests that the missing RNA polymerases are substituted by nuclear-encoded paralogs. The simplified architecture and highly-reduced gene complement of the P. zopfii mitochondrial and plastid genomes are closer to those of P. stagnora and the achlorophyllous obligate parasite Helicosporidium than to those of P. wickerhamii or P. cutis . This similarity is also supported by maximum likelihood phylogenetic analyses inferences. Overall, the P. zopfii sequences reported here, which include nuclear genome drafts for both genotypes, will help provide both a deeper understanding of the evolution of Prototheca spp. and insights into the corresponding host/pathogen interactions.