Abstract
The dinoflagellates have repeatedly replaced their ancestral peridinin-plastid by plastids derived from a variety of algal lineages ranging from green algae to diatoms. Here, we have characterized the genome of a dinoflagellate plastid of tertiary origin in order to understand the evolutionary processes that have shaped the organelle since it was acquired as a symbiont cell. To address this, the genome of the haptophyte-derived plastid in Karlodinium veneficum was analyzed by Sanger sequencing of library clones and 454 pyrosequencing of plastid enriched DNA fractions. The sequences were assembled into a single contig of 143 kb, encoding 70 proteins, 3 rRNAs and a nearly full set of tRNAs. Comparative genomics revealed massive rearrangements and gene losses compared to the haptophyte plastid; only a small fraction of the gene clusters usually found in haptophytes as well as other types of plastids are present in K. veneficum. Despite the reduced number of genes, the K. veneficum plastid genome has retained a large size due to expanded intergenic regions. Some of the plastid genes are highly diverged and may be pseudogenes or subject to RNA editing. Gene losses and rearrangements are also features of the genomes of the peridinin-containing plastids, apicomplexa and Chromera, suggesting that the evolutionary processes that once shaped these plastids have occurred at multiple independent occasions over the history of the Alveolata.
Highlights
The primary endosymbiosis that gave rise to photosynthetic eukaryotes probably occurred only once and represents a major and critical event in evolutionary history
Almost half of the extant dinoflagellate species contains either this peridinin plastid, considered to be ancestral for the lineage, or have replaced it by a plastid derived from a wide range of algal lineages [4,5,6,7,8,9]
A significant gene reduction is seen in the tertiary endosymbiont genome The plastid genome of K. veneficum contains only 70 proteincoding genes, whereas the haptophyte Emiliana huxleyi has 110 plastid-encoded protein genes [16]
Summary
The primary endosymbiosis that gave rise to photosynthetic eukaryotes probably occurred only once and represents a major and critical event in evolutionary history This endosymbiosis between a eukaryote and a cyanobacterium resulted in the primary plastids found in plants, green algae, red algae and glaucophytes. The primary plastids were laterally transferred to several eukaryotic lineages as result of a eukaryote engulfing another eukaryote carrying a primary plastid, giving rise to secondary plastids, or even engulfed eukaryotes harboring secondary plastids (a tertiary plastid) These complex plastids are characterized by increased number of membranes (three-four) and subsequent modifications of the intracellular transport [1,2,3]. There are four major groups comprising a secondary plastid of likely red algal origin: dinoflagellates, haptophytes, cryptophytes and heterokonts Among these groups, dinoflagellates are unique in featuring secondary and even tertiary plastids that have independently been acquired from several algal lineages. The variety of plastids makes the dinoflagellates the most dynamic system for acquiring and loosing plastids as well as nuclear-encoded plastid-targeted genes among eukaryotes [9,10,11,12] It is a suitable model for studying the processes involved in plastid evolution
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
