Variations in the Mitogenomes of the Widespread Invasive Marine Alga Gracilaria vermiculophylla (Gracilariales, Rhodophyta) in the Temperate Northern Hemisphere

Intraspecific variation in mitogenomes has provided the resolution to distinguish between morphologically indistinguishable species and highlight cryptic speciation. This level of variation is underexplored in introduced marine algae. This study evaluated the mitogenomic variability of Gracilaria vermiculophylla, one of the most invasive red algae in the Northern Hemisphere. Nine mitogenomes, comprising seven newly generated complete mitogenomes from Korea, Japan, and Morocco, plus two publicly available mitogenomes, were included in our analysis. The mitogenomes were highly conserved, ranged from 26,064 to 26,178 bp in length, and contained 52 genes comprising 25 protein-coding genes, three rRNAs, and 24 tRNAs. Nucleotide composition showed a strong AT bias (71.6%), with AT skew and GC skew values of 0.050 and 0.014–0.021, respectively. Pairwise divergences of cox2, rnl and rns rRNA were remarkably low, in a range of 0–0.06%, suggesting alternate markers for determining recent introductions. The phylogenetic relationship based on protein-coding genes revealed three shallow groups in G. vermiculophylla, which were correlated with distribution: two C groups from warm temperate sites and one T group from cold temperate sites. This result highlights mitogenomics as a new tool for the deeper understanding of phylogeography and invasion genetics of marine red algae.