Comparison of and spp. Chloroplast Genomes: Evidence for Endosymbiosis and Horizontal Virus-like Gene Transfer.

Resequencing of the chloroplast genome (cpDNA) of UTEX 25 was completed (GenBank Accession no. KC631634.1), revealing a genome size of 84,576 base pairs and 30.8% GC content, consistent with features reported for the previously sequenced 0710, (GenBank Accession no. KC843975). The UTEX 25 cpDNA encoded 78 predicted open reading frames, 32 tRNAs, and 4 rRNAs, making it smaller and more compact than the cpDNA genome of (124,579 bp) and (150,613 bp). By comparison, the compact genome size of was attributable primarily to a lower intergenic sequence content. The cpDNA coding regions of all known species were found to be organized in conserved colinear blocks, with some rearrangements. The and species genome structure and composition were similar, and of particular interest were genes influencing photosynthetic efficiency, i.e., chlorophyll synthesis and photosystem subunit I and II genes, consistent with other biofuel species of interest. Phylogenetic analysis revealed that is the closest known relative, followed by members of the genus . The cpDNA of encodes 37 genes that are highly homologous to representative cyanobacteria species, including , , and , corroborating a well-recognized symbiosis. Several putative coding regions were identified that shared high nucleotide sequence identity with virus-like sequences, suggestive of horizontal gene transfer. Despite these predictions, no corresponding transcripts were obtained by RT-PCR amplification, indicating they are unlikely to be expressed in the extant lineage.