Organellar Genomes of var. Provide Insight into the Characteristics of Phaeophyceae.

var. , a prevalent seaweed along the Chinese coast, has economic and ecological significance. However, systematic positions within and among the three orders of Phaeophyceae, Fucales, Ectocarpales, and Laminariales are in debate. Here, we reported the organellar genomes of var. (34,686-bp mitogenome with 65 genes and 124,323 bp plastome with 173 genes) and the investigation of comparative genomics and systematics of 37 mitogenomes and 22 plastomes of Fucales (including var. ), Ectocarpales, and Laminariales in Phaeophyceae. Whole genome collinearity analysis showed gene number, type, and arrangement were consistent in organellar genomes of with 360 SNP loci identified as var. and two genes (7 and 2) identified as intrageneric classifications of . Comparative genomics of the three orders of Phaeophyceae exhibited the same content and different types (L was only found in plastomes of the order Fucales and Ectocarpales) and arrangements (most plastomes were rearranged, but A and D in the mitogenome represented different orders) in genes. We quantified the frequency of RNA-editing (canonical C-to-U) in both organellar genomes; the proportion of edited sites corresponded to 0.02% of the plastome and 0.23% of the mitogenome (in reference to the total genome) of var. . The repetition ratio of Fucales was relatively low, with scattered and tandem repeats (nine tandem repeats of 14-24 bp) dominating, while most protein-coding genes underwent negative selection (Ka/Ks < 1). Collectively, these findings provide valuable insights to guide future species identification and evolutionary status of three important Phaeophyceae order species.