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Molecular evolutionary studies on the chlorarachniophyte : Bigelowiella natans Rogers, Matthew Brian


Chlorarachniophytes are marine cercozoan amoeboflagellates with plastids derived from a secondary endosymbiotic event involving a green alga. The retention of a vestigial eukaryotic nucleus, or 'nucleomorph' in the plastid of chlorarachniophytes makes chlorarachniophytes ideal organisms for the study of secondary endosymbiosis. Among chlorarachniophytes, the majority of sequence data are from a single species, Bigelowiella natans. Thousands of expressed sequence tags and a complete nucleomorph and chloroplast genome from Bigelowiella natans provide a detailed picture of genes encoding proteins with function in the plastid of chlorarachniophytes. The phylogeny of three plastid-targeted Calvin cycle enzymes with complicated distributions and multiple recompatmentalisation events are described. These three enzymes are also the only known Calvin cycle enzymes in B. natans that show any evidence of having been acquired through endosymbiotic gene transfer. Previous studies have detailed the contribution of lateral gene transfer to the plastid function of B. natans. Several specific examples of these events are expanded on and discussed. Plastid-targeting peptides from published sequences are characterized and a heterologous targeting experiment in to the apicoplast of Toxoplasma gondii is described. The complete plastid genome of Bigelowiella natans has been sequenced. This plastid genome appears to be the smallest among all known photosynthetic plastid genomes, though it nevertheless retains most of the photosynthesis related genes present in chlorophytes. Phylogenetic analysis of concatenated chloroplast protein coding genes indicates that the endosymbiont of B. natans may be a derived green algae, and argues strongly against a single origin of chlorarachniophyte and euglenids plastids predicted by the cabozoa hypothesis.

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