Multigene phylogenetics of euglenids based on single-cell transcriptomics of diverse phagotrophs

Area of Science:

• Protistology
• Evolutionary Biology
• Molecular Phylogenetics

Background:

• Euglenids are diverse single-celled eukaryotes with varied feeding strategies (phototrophy, osmotrophy, phagotrophy).
• The evolutionary relationships among euglenid feeding types, particularly phagotrophs, and the placement of symbiontids are poorly understood.
• Previous phylogenetic studies relied heavily on SSU rDNA, which can be highly divergent, and many phagotrophs are uncultured, limiting molecular data.

Purpose of the Study:

• To conduct the first multigene phylogenetic analyses of euglenids incorporating phagotrophs and symbiontids.
• To resolve the evolutionary relationships within the Euglenozoa, focusing on the Euglenida.
• To establish robust phylogenetic frameworks for understanding feeding strategy evolution in euglenids.

Main Methods:

• Generation of transcriptome data for 28 euglenid taxa, primarily using a single-cell approach.
• Multigene phylogenetic analyses incorporating diverse feeding types and symbiontids.
• Comparative analysis with existing phylogenetic data and inferences.

Main Results:

• Euglenids are confirmed as monophyletic, with symbiontids forming a distinct lineage within Euglenozoa.
• The clade Spirocuta (phototrophic Euglenophyceae and osmotrophic Aphagea) is robustly resolved, with Olkasia forming the new taxon Olkaspira.
• New taxa Alistosa and Olkaspira are defined; Petalomonadida is sister to other euglenids; Euglenophyceae and Anisonemia are well-supported clades; relationships among some phagotrophs remain ambiguous.
• Results contradict previous hypotheses regarding Entosiphon's pivotal phylogenetic position.

Conclusions:

• The study provides a robust multigene phylogeny for euglenids, clarifying major evolutionary relationships and defining new taxa.
• The findings resolve the monophyly of euglenids and the placement of symbiontids, offering insights into the evolution of feeding strategies.
• At least three transitions in flagellar gliding mechanisms occurred within euglenids, though their precise phylogenetic context remains to be fully elucidated.