Cryptomonad algae are evolutionary chimaeras of two phylogenetically distinct unicellular eukaryotes

Area of Science:

• * Evolutionary biology
• * Cell biology
• * Phycology (study of algae)

Background:

• * Plastid origins in plants and algae are widely accepted as endosymbiotic events, but details remain unclear.
• * Organisms with plastids enclosed by more than two membranes may have engulfed a eukaryotic alga, not a prokaryote.
• * The nucleomorph in cryptomonad algae is hypothesized to be the remnant nucleus of an algal endosymbiont.

Purpose of the Study:

• * To investigate the evolutionary origins of cryptomonad algae as chimeric entities.
• * To determine the phylogenetic ancestry of the host and symbiont components within Cryptomonas phi.
• * To test the hypothesis that chromophyte algae evolved directly from a cryptomonad-like ancestor.

Main Methods:

• * Analysis of small-subunit ribosomal RNA (rRNA) genes within Cryptomonas phi.
• * Localization of eukaryotic-type rRNA sequences to the nucleomorph's nucleolus-like structure.
• * Construction of phylogenetic trees incorporating rRNA sequence data.

Main Results:

• * Cryptomonas phi possesses two transcriptionally active, phylogenetically distinct nuclear-type small-subunit rRNA genes.
• * Eukaryotic-type rRNA sequences were confirmed within the nucleomorph.
• * Phylogenetic analyses did not support the direct evolution of chromophyte algae from a cryptomonad-like ancestor.

Conclusions:

• * Cryptomonad algae are confirmed as evolutionary chimeras, resulting from the fusion of two distinct eukaryotic cells.
• * The presence of two separate rRNA gene types provides strong evidence for a eukaryotic endosymbiont.
• * The evolutionary trajectory of chromophyte algae likely does not stem directly from cryptomonads.