Phylogenetic patterns of gene rearrangements in four mitochondrial genomes from the green algal family Hydrodictyaceae (Sphaeropleales, Chlorophyceae)

Background: The variability in gene organization and architecture of green algal mitochondrial genomes is only recently being studied on a finer taxonomic scale. Sequenced mt genomes from the chlorophycean orders Volvocales and Sphaeropleales exhibit considerable variation in size, content, and structure, even among closely related genera. However, sampling of mt genomes on a within-family scale is still poor and the sparsity of information precludes a thorough understanding of genome evolution in the green algae. Methods: Genomic DNA of representative taxa were sequenced on an Illumina HiSeq2500 to produce 2x100 bp paired reads, and mitochondrial genomes were assembled and annotated using Geneious v. 6.1.5. Phylogenetic analysis of 13 protein-coding mitochondrial genes spanning the Sphaeropleales was performed. Results: This study presents one of the first within-family comparisons of mt genome diversity, and is the first to report complete mt genomes for the family Hydrodictyaceae (order Sphaeropleales). Four complete mt genomes representing three taxa and four phylogenetic groups, Stauridium tetras, Pseudopediastrum boryanum, and Pediastrum duplex, range in size from 37,723 to 53,560 bp. The size variability is primarily due to intergenic region expansion, and intron content is generally low compared with other mt genomes of Sphaeropleales. Conclusions: Certain gene rearrangements appear to follow a phylogenetic pattern, and with a more thorough taxon sampling genome-level sequence may be useful in resolving systematic conundrums that plague this morphologically diverse family.