Mussel Unio douglasiae MG from the Chihe River: mitogenome description and phylogenetic analysis

Abstract The genus Unio is one of the widespread freshwater bivalves. To date, its intra-generic phylogeny remains controversial and therefore the mitochondrial genome data is needed. Here, we report the complete mitogenome of Unio douglasiae MG that is distributed in the Chihe River, a branch of Huaihe River, East China. This mitochondrial genome is 15,764 base pair in total length. It consists of 37 genes: 13 protein-coding genes, 22 tRNA genes, and 2 rRNA genes (12S and 16S). The base composition was 38.38% for A, 26.48% for T, 23.17% for C, and 11.98% for G, showing an obvious bias of higher A + T content (64.86%) than the G + C content (35.14%). Phylogenetic analysis showed that U. douglasiae MG is clustered with other Unio and Nodularia mollusks in the family Unionidae. These results showed that combine with morphological techniques, the mitogenome can provide useful information to further understanding of the genetics, systematics, and conservation of this endangered species.

In this study, the U. douglasiae MG specimen was collected from the Chihe River, a branch of Huaihe River in Mingguang city, Anhui province of China (Latitude 32.789726 and longitude 117.969780), and has been deposited at the Aquatic Service Platform of Shaoxing city (Voucher no. SXAF20200710).
The complete mitogenome sequence of U. douglasiae MG is obtained by PCR amplification and Sanger sequencing using an ABI3730 sequencer (Applied Biosystems, USA). It's 15,764 bp in total length (GenBank accession no. MT764726), including 37 genes: 13 protein-coding genes, 22 tRNA genes, and 2 rRNA genes (12S and 16S). The base composition was 38.38% for A, 26.48% for T, 23.17% for C, and 11.98% for G, showing an obvious bias of higher A þ T content (64.86%) than the G þ C content (35.14%).
The total length of two rRNA genes (12S and 16S) is 2152 bp, and they are separated by three tRNA genes including tRNA Lys , tRNA Thr , and tRNA Tyr , which is identical to the rRNA gene arrangement of other mussels (Breton et al. 2011;Wang et al. 2016aWang et al. , 2016b. Twenty-six complete mitogenome sequences were used for phylogenetic tree construction by the neighbor-joining method (Figure 1). The results showed that U. douglasiae MG is clustered with other Unio and Nodularia mollusks in the family Unionidae. The present study shows that, combine with morphological techniques (Wei et al. 1994;Wu et al. 1999), the mitogenome can provide useful information to further understanding of the genetics, systematics, and conservation of this endangered species.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Data availability statement
The data that support the findings of this study are freely available at NCBI GenBank database (https://www.ncbi.nlm.nih.gov) with a accession no. MT764726. And the data that support the findings of this study are also available from the corresponding author, Dr. Yang, upon reasonable request.