Abstract
Bombyx mandarina is generally thought to be the wild ancestor nearest to the domesticated silkworm, B. mori. Here, we sequenced and assembled a complete mitochondrial genome (mitogenome) of B. mandarina. The mitogenome contains 13 protein-coding genes (PCGs), 22 tRNA genes, 2 rRNA genes, and one A + T-rich region. Phylogenetic analysis confirmed that B. mori is more closely related to the Chinese B. mandarina than the Japanese B. mandarina. The new mitogenome provides useful information to further explore the origin and domestication of this species.
The wild silkworm, Bombyx mandarina, is believed to be the ancestor nearest to the domesticated silkworm B. mori, which is an economically important lepidopteran insect (Goldsmith et al. Citation2005; Xia et al. Citation2014). Extant in Asian mulberry fields, B. mandarina remains an important source of genetic variation and has two main types, which differ in chromosome number. B. mandarina living in Japan has 27 chromosomes per haploid genome, while B. mandarina in China carries 28 chromosomes in its haploid genome, equal to those of B. mori (Nakamura et al. Citation1999; Banno et al. Citation2004). To examine their phylogenetic relationship, we determined the mitochondrial genome of B. mandarina in this study.
B. mandarina was collected from Haiyang, China (36°54′57″, 121°21′13″). After morphological identification, the specimens were stored at Sericulture Research Institute, Zhejiang Academy of Agricultural Sciences. Total genomic DNA was extracted from a single individual using a DNeasy Tissue Kit (Qiagen). The mitochondrial genome was determined by Sanger sequencing. Genome annotation was performed using MITOS (Bernt et al. Citation2013) and tRNAscan-SE server (Schattner et al. Citation2005).
The complete mitogenome of B. mandarina is 15,768 bp in length (GenBank accession no. MK246423), and contains 13 protein-coding genes (PCGs), 22 tRNA genes, 2 rRNA genes, and 1 A + T-rich region. The overall base composition was estimated to be 43.04% A, 38.41% T, 11.25% C, and 7.30% G, with a high A + T content of 81.45%, which is similar to that of B. mori (Zhang et al. Citation2016). All PCGs begin with ATN, except for COX1 gene starting with CGA. Eleven PCGs use TAA as the stop codon, whereas COX1 and COX2 end with a single T. All of the 22 tRNA genes, ranging in size from 64 to 75 bp, have a typical cloverleaf secondary structure. The gene order and orientation of B. mandarina were consistent with those of B. mori (Li et al. Citation2016; Zhang et al. Citation2016).
The phylogenetic relationship was recovered by the Maximum Likelihood method in MEGA 7.0 (Kumar et al. Citation2016), based on complete mitochondrial genomes. Bootstrap analysis was performed with 1000 replications. Phylogenetic analysis revealed that B. mori is more closely related to Chinese B. mandarina than Japanese B. mandarina (), consistent with their chromosome number data.
Figure 1. Maximum-likelihood phylogenetic tree based on mitochondrial genome sequences. All the bootstrap values are indicated at the nodes. Antheraea pernyi was used as an outgroup. GenBank accession numbers of each species were listed in the tree.
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The authors report no conflict of interest and are responsible for the content and writing of the article.
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References
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