Only six kingdoms of life

Area of Science:

• Microbial ecology
• Eukaryotic phylogeny
• Molecular evolution

Background:

• Most microbial biodiversity remains uncharacterized due to difficulties in culturing.
• Phylogenetic analysis of environmental DNA (eDNA) offers insights into microbial diversity.
• Previous studies claimed novel 'kingdom-level' lineages within anaerobic eukaryotes based on eDNA.

Purpose of the Study:

• To re-evaluate claims of novel kingdom-level lineages in eukaryotes using a more comprehensive dataset.
• To assess the accuracy of phylogenetic analyses based on ribosomal RNA (rRNA) genes from environmental samples.
• To determine if uncharacterized kingdoms of life exist.

Main Methods:

• Phylogenetic analysis of ribosomal DNA (rDNA) sequences.
• Re-analysis of existing eDNA sequence data with an expanded reference database (167 species).
• Comparative analysis of eukaryotic and prokaryotic phylogenetic signals.

Main Results:

• All previously identified 'mysterious' eukaryotic lineages were placed within five recognized phyla (Amoebozoa, Cercozoa, Apusozoa, Myzozoa, Loukozoa).
• These lineages primarily belong to known classes, orders, families, or genera within the Protozoa kingdom.
• The study suggests that claims of novel bacterial divisions or kingdoms may stem from limitations in rRNA gene resolution and evolution, not true biological disparity.
• The ancestral eukaryote is likely a mitochondrial aerobe.

Conclusions:

• There are no uncharacterized kingdoms of life.
• The current understanding of microbial biodiversity at the phylum or division level is largely comprehensive.
• Significant uncharacterized diversity exists at lower taxonomic levels, and the root of the eukaryotic tree of life requires further investigation.