Single gene circles in dinoflagellate chloroplast genomes

Photosynthetic dinoflagellates are important aquatic primary producers and notorious causes of toxic 'red tides'. Typical dinoflagellate chloroplasts differ from all other plastids in having a combination of three envelope membranes and peridinin-chlorophyll a/c light-harvesting pigments. Despite evidence of a dinoflagellete satellite DNA containing chloroplast genes, previous attempts to obtain chloroplast gene sequences have been uniformly unsuccessful. Here we show that the dinoflagellate chloroplast DNA genome structure is unique. Complete sequences of chloroplast ribosomal RNA genes and seven chloroplast protein genes from the dinoflagellate Heterocapsa triquetra reveal that each is located alone on a separate minicircular chromosome: 'one gene-one circle'. The genes are the most divergent known from chloroplast genomes. Each circle has an unusual tripartite non-coding region (putative replicon origin), which is highly conserved among the nine circles through extensive gene conversion, but is very divergent between species. Several other dinoflagellate species have minicircular chloroplast genes, indicating that this type of genomic organization may have evolved in ancestral peridinean dinoflagellates. Phylogenetic analysis indicates that dinoflagellate chloroplasts are related to chromistan and red algal chloroplasts and supports their origin by secondary symbiogenesis.