Complete mitochondrial genome of the spotted alfalfa aphid, Therioaphis trifolii (Hemipera: Aphididae).

The complete mitochondrial genome (mitogenome) of Therioaphis trifolii is sequenced and annotated based on high-throughput sequencing method in the present study. It is a typical circular DNA molecule of 16,068 bp, with 37 genes and 82.6% A + T content. The gene order is the same as the putative ancestral arrangement of insects. Thirteen protein-coding genes (PCGs) start with an ATN codon and terminate with a TAA codon or a single T residue. All the 22 tRNAs, ranging from 64 to 73 bp, have the typical clover-leaf structure except for trnS1. The length of rrnL and rrnS genes are 1265 bp and 779 bp respectively. The control region is 856 bp in length with 85.2% A + T content. The phylogenetic tree supports the monophyly of Aphidinae and the sister relationship between T. trifolii and seven species of subfamily Aphidinae.

Aphids (Hemiptera: Aphidoidea) are an important insect group with complicated life cycles, elaborate polyphenisms, gall formation, and harboring diverse endosymbionts (Li et al. 2017). Therioaphis trifolii originating in North Africa was first recorded in 1882 (Monell 1882). It is one of the most damaging insect pests of alfalfa spreading worldwide. In this study, we firstly present the complete mitogenome of T. trifolii from the aphid family Aphididae. The samples were collected in Maosheng, Yinchuan, Ningxia province, China (38°28′18.3″N 106°27′02.4″E). Voucher specimen (NO. VAph-0045) was deposited in the Entomological Museum of China Agricultural University (CAU), Beijing, China. The complete mitogenome of T. trifolii was obtained by high-throughput sequencing method and submitted in GenBank (accession number: MK766411).

The complete mitogenome is 16,068 bp in length with the typical insect complement of 13 protein-coding genes (PCGs), 2 rRNA genes, 22 tRNA genes, and a control region (Boore 1999). Twenty-three genes are transcribed on the J-strand, the remaining being oriented on the N-strand. The arrangement of these genes is identical to the putative ancestral arrangement of insect (Clary and Wolstenholme 1985; Cameron 2014; Song et al. 2016), no rearrangement was found in this mitogenome. There are a total of 44 overlapped nucleotides between adjacent genes in 9 locations, ranging from 1 to 20 bp in length. A total of 701 bp intergenic spacers are present at 15 positions, ranging from 1 to 494 bp in length.

The overall nucleotide composition is 44.7% A, 37.9% T, 11.4% C, and 5.9% G, with a strong bias towards A + T (82.6%). It is significantly biased towards A + T with positive AT-skew (+0.08) and negative GC-skew (–0.49). All PCGs initiate with typical ATN as the start codon (six with ATA, five with ATT, one with ATC, and one with ATG) and terminate with conventional stop codons (TAA), whereas ND4 uses a single T residue which is commonly in aphids and other Hemiptera mitogenomes (Li et al. 2016; Wang et al. 2016; Li et al. 2017).

All 22 tRNA genes, ranging from 64 to 73 bp, can be folded into typical clover-leaf secondary structure, except for trnS1 that the dihydrouridine (DHU) arm forms a simple loop. The length of rrnL and rrnS genes are 1265 bp and 779 bp, with 85.0% and 84.0% A + T content, respectively. The 856 bp control region is located between srRNA and trnI with 85.2% A + T content. Two 61 bp repeat sequences are detected in this region.

Phylogenetic relationship was constructed by the maximum-likelihood (ML) method. The result supports the monophyly of Aphidinae, which is congruent with previous hypotheses (Li et al. 2017; Choi et al. 2018). It also demonstrates the sister relationship between T. trifolii and seven species of subfamily Aphidinae (Figure 1).

Figure 1.

Phylogenetic relationship resulted from ML analysis based on 13 PCGs. Phylogenetic tree was generated by IQ-TREE 1.6.5 (Trifinopoulos et al. 2016) under the GTR + I + G model. The nodal values indicate the bootstrap percentages obtained with 1000 replicates. Accession numbers were shown in the right of the species name.