The apicomplexans are a group of obligate animal pathogens that include Plasmodium (malaria), Toxoplasma (toxoplasmosis), and Cryptosporidium (cryptosporidiosis) [1]. They are an extremely diverse and specious group but are nevertheless united by a distinctive suite of cytoskeletal and secretory structures related to infection, called the apical complex, which is used to recognize and gain entry into animal host cells. The apicomplexans are also known to have evolved from free-living photosynthetic ancestors and retain a relict plastid (the apicoplast), which is non-photosynthetic but houses a number of other essential metabolic pathways [2]. Their closest relatives include a mix of both photosynthetic algae (chromerids) and non-photosynthetic microbial predators (colpodellids) [3]. Genomic analyses of these free-living relatives have revealed a great deal about how the alga-parasite transition may have taken place, as well as origins of parasitism more generally [4]. Here, we show that, despite the surprisingly complex origin of apicomplexans from algae, this transition actually occurred at least three times independently. Using single-cell genomics and transcriptomics from diverse uncultivated parasites, we find that two genera previously classified within the Apicomplexa, Piridium and Platyproteum, form separately branching lineages in phylogenomic analyses. Both retain cryptic plastids with genomic and metabolic features convergent with apicomplexans. These findings suggest a predilection in this lineage for both the convergent loss of photosynthesis and transition to parasitism, resulting in multiple lineages of superficially similar animal parasites.

Department of Botany University of British Columbia Vancouver BC V6T 1Z4 Canada

Department of Botany University of British Columbia Vancouver BC V6T 1Z4 Canada; Department of Zoology University of British Columbia Vancouver BC V6T 1Z4 Canada

Department of Botany University of British Columbia Vancouver BC V6T 1Z4 Canada; GEOMAR Helmholtz Centre for Ocean Research Duesternbrooker Weg 20 24105 Kiel Germany

Department of Botany University of British Columbia Vancouver BC V6T 1Z4 Canada; Institute of Parasitology Biology Centre Czech Acad Sci Branišovská 31 České Budějovice 370 05 Czech Republic

Institute for Experimental Pathology University of Iceland Keldur Keldnavegur 3 112 Reykjavík Iceland

Ross University School of Veterinary Medicine PO Box 334 Basseterre St Kitts West Indies

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Curr Biol. 2019 Sep 9;29(17):R836-R839. doi: 10.1016/j.cub.2019.07.047. PubMed

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