Evolution of highly diverse gene orders in mitochondrial genomes of Syllidae (Annelida).

Mitochondrial genomes are extensively utilized in phylogenetic and gene order analyses. While earlier studies suggested a conserved mitochondrial gene order within Pleistoannelida, recent findings indicate notable rearrangements, particularly among lineages in the Syllidae within Errantia. This study investigates 13 newly sequenced mitochondrial genomes (9 complete, 4 partial), uncovering complex gene order variations and offering deeper insights into their evolutionary relationships. Phylogenetic analyses confirm well-supported groups previously identified through single-gene studies, additionally, the genera Brachysyllis and Parasphaerosyllis are included in a molecular phylogenetic analysis for the first time. The phylogenetic relationships of these two genera within Syllidae are finally clarified. A total of eight different patterns other than Pleistoannelida gene order pattern are represented in Syllidae. Within Syllidae, Syllinae is the group where this varies more often and more drastically from the Pleistoannelida ground pattern. When excluding tRNAs, which tend to have higher variability in location, Streptosyllis sp. shows the greatest similarity to the Pleistoannelida ground pattern. In contrast, Syllis okadai and Megasyllis nipponica exhibit the most divergent gene order, followed by Syllis sp. The presented dataset enabled to conduct phylogenetic analyses involving a smaller number of taxa compared to earlier studies but with a substantially larger dataset containing 17 genes and spanning 13,000 to 20,000 base pairs. The findings provide further evidence on the phylogenetic relationships within the family Syllidae. The represented subfamilies are confirmed to be monophyletic and well-supported, and a newly identified, robust clade comprising Amblyosyllis and Brachysyllis is unveiled.