An Investigation of the Variations in Complete Mitochondrial Genomes of Lingula anatina in the Western Pacific Region

SIMPLE SUMMARY: Lingula anatina has been attracted researchers because its morphological characteristics show limited changes compared to the ancestor. Even though L. anatina is a common brachiopod in the western Pacific region, a few studies were performed to investigate genetic variations of this species. To understand the mitogenomic diversity of L. anatina in the region, the present study conducted comparative analyses of mitogenome sequences from Korea, Vietnam, and Japan. The sequencing results indicated that L. anatina mitogenomes are extraordinarily longer than the typical animal mitogenome. Besides, the gene orders of L. anatina mitogenomes are variable among localities. The calculation based on protein-coding genes revealed relatively low substitution rates among mitogenome sequences from Buan (Korea), Doson (Vietnam) and Yanagawa (Japan). The phylogenetic analyses indicated the divergence of L. anatina in the western Pacific region. This investigation provides new information on the molecular systematics of L. anatina and could be helpful in exploring the diversity and evolution of mitogenomes in brachipod. ABSTRACT: Lingula anatina is a brachiopod widely distributed in the western Pacific region. Even though L. anatina has been targeted for a number of biological studies, there is still limited information on intraspecific genetic variations of L. anatina. In this study, L. anatina specimens were collected from Korea and Vietnam, and complete mitochondrial genome (mitogenome) sequences were analyzed and compared with previous records. The total mitogenomes of L. anatina were 24,875 bp and 25,305 bp in size for Korean and Vietnamese specimens, respectively. Those mitogenomes are extraordinarily longer than the typical mitogenome size for an animal but shorter than the previous record from Yanagawa (Japan) for this species. The gene orders and the sizes of the protein-coding genes are also different from those for the Japanese specimen. Furthermore, the nonsynonymous (Ka) and synonymous (Ks) substitution rates in protein-coding genes (PCGs) were calculated to test the idea of evolutionary rate differences in mitochondrial genomes. The analyses showed relatively low Ka and Ks for the complete mitogenomes from Buan (Korea), Doson (Vietnam) and Yanagawa (Japan). The Ka/Ks ratio was less than 1 in comparisons of three localities, indicating the existence of purifying selection in this species. The phylogenetic analyses showed that L. anatina diverged among localities in the western Pacific region.