Rare evolutionary example of endosymbiosis undone

The evolution of intracellular organelles through endosymbiosis was a turning point in the history of life on Earth. Subsequent loss of endosymbiotic organelles, or plastids, has only been demonstrated in a single genus, but researchers have now discovered a second case of this remarkable evolutionary reversal.

Although plastids have become reduced in several lineages, the transfer of essential metabolic pathways from the host genome to the plastid makes their loss an extremely rare evolutionary event. 

An international team of researchers led by Ross Waller from the University of Cambridge and including Arnab Pain's group at KAUST, Saudi Arabia, used high-throughput sequencing to look for evidence of plastids in the parasitic dinoflagellate Hematodinium, which appears to lack plastids. They found no evidence of plastid-encoded genes, and showed that Hematodinium solved the metabolic dependency problem by remodeling its metabolic pathways. 

Analysing the sequenced transcripts, the team discovered evidence for biochemical alternatives to plastid functions. Some genes are retained as ancestral versions, while others seem to have been relocated from the plastid genome or by lateral gene transfer. 

One pathway was lost along with the plastid, but Hematodinium seems to make up for the absence by scavenging metabolites from its surroundings. 

According to Pain, comparing Hematodinium with organisms that retain reduced plasmids, such as the malaria parasite, will enable us to understand exactly why such relic plastids persist.

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