A tertiary plastid uses genes from two endosymbionts

Area of Science:

• * Evolutionary biology
• * Molecular biology
• * Cell biology

Background:

• * Plastid endosymbiosis significantly impacted eukaryotic evolution and biodiversity.
• * Dinoflagellates possess unique tertiary plastids, resulting from a secondary plastid replacement by a haptophyte-derived plastid.
• * Nuclear genes encode most plastid proteins, with gene transfer assumed during endosymbiosis.

Purpose of the Study:

• * To investigate the proteome of a tertiary plastid in Karlodinium micrum.
• * To determine the origin of plastid-targeted proteins in K. micrum.
• * To analyze the evolutionary history and protein trafficking mechanisms of tertiary plastids.

Main Methods:

• * Large-scale expressed sequence tag (EST) surveys were performed on Karlodinium micrum, Isochrysis galbana, and Pavlova lutheri.
• * Plastid-targeted proteins were identified and their phylogenetic origins analyzed.
• * Plastid-targeting transit peptides were compared across different species.

Main Results:

• * Many plastid-targeted genes in K. micrum are of haptophyte origin, consistent with tertiary endosymbiosis.
• * Some genes from the original secondary plastid were retained, suggesting transient co-existence.
• * Multiple gene isoforms of different evolutionary origins were identified.
• * Transit peptides in K. micrum differ from those in other dinoflagellates and haptophytes, indicating significant protein trafficking remodeling.

Conclusions:

• * The tertiary plastid of K. micrum possesses an evolutionarily chimeric proteome.
• * Plastid replacement involved massive remodeling of protein import and trafficking pathways.
• * This study provides insights into the dynamic evolution of plastids and their associated gene transfer mechanisms.