Five new mitogenomes sequences of Calidridine sandpipers (Aves: Charadriiformes) and comparative mitogenomics of genus .

Background: The genus (Charadriiformes, Scolopacidae) includes shorebirds known as dunlin, knots, and sanderlings. The relationships between species nested within , including , and , are not well-resolved.

Methods: Samples were collected from Xiaoyangkou, Rudong County, Jiangsu Province, China. Mitogenomes were sequenced using the Illumina Novaseq 6000 platform for PE 2 × 150 bp sequencing, and then checked for PCR products. Protein-coding genes were determined using an Open Reading Frame Finder. tRNAscan-SE, MITOS, and ARWEN were used to confirm tRNA and rRNA annotations. Bioinformatic analyses were conducted using DnaSP 5.1 and MEGA X. Phylogenic trees were constructed using maximum likelihood and Bayesian analyses.

Results: We sequenced and annotated the mitogenome of five species and obtained four complete mitogenomes and one nearly complete mitogenome. Circular mitogenomes displayed moderate size variation, with a mean length of 16,747 bp, ranging from 16,642 to 16,791 bp. The mitogenome encoded a control region and a typical set of 37 genes containing two rRNA genes, 13 protein-coding genes, and 22 tRNA genes. There were four start codons, four stop codons, and one incomplete stop codon (T-). The nucleotide composition was consistently AT-biased. The average uncorrected pairwise distances revealed heterogeneity in the evolutionary rate for each gene; the COIII had a slow evolutionary rate, whereas the ATP8 gene had a fast rate. dN/dS analysis indicated that the protein-coding genes were under purifying selection. The genetic distances between species showed that the greatest genetic distance was between and (22.5%), and the shortest was between and (12.8%). Phylogenetic trees revealed that is not a monophyletic genus, as species from the genera and were nested within . The molecular data obtained in this study are valuable for research on the taxonomy, population genetics, and evolution of birds in the genus .