The complete mitochondrial genome of a balanced sex-linked lethal silkworm and its phylogenetic analysis

Abstract

Male-only rearing has long been desirable in sericulture because males have superior economic characteristics. To this end, a balanced sex-linked lethal (BSL) system has been developed. Here, we report the first complete mitochondrial genome of the BSL silkworm (Bombyx mori). Phylogenetic analysis revealed that the BSL silkworm is more closely related to Japanese B. mori than Chinese B. mori. The new mitochondrial genome provides valuable information to further explore genetic diversity and origin of this species.

Keywords:

• Mitogenome
• sex-linked lethal system
• phylogenetic relationship

As the foundation of sericulture, the mulberry silkworm, Bombyx mori, has been used for silk production for thousands of years (Goldsmith et al. 2005). Male-only rearing has long been sought in sericulture because males have superior economic characteristics, such as higher silk yield, higher silk quality, and lower food consumption (Xia and Tang 1980). To this end, Strunnikov (1975) first established a balanced sex-linked lethal (BSL) system by classic genetics. The BSL system was then introduced to our institute in the last century and applied in Chinese sericulture. For a better understanding of its genetic diversity and phylogenetic position, we analyzed the complete mitochondrial genome of the BSL silkworm, called as B. mori Ping1 in China.

B. mori Ping1 was collected from Hangzhou City, Zhejiang Province in China (30°18′ N, 120°11′ E). The specimens were stored in the Silkworm Germplasm Resources Repository at Sericulture Research Institute, Zhejiang Academy of Agricultural Sciences. Total genomic DNA was extracted from a single pupa using a DNeasy Tissue Kit (QIAGEN, Hilden, Germany). Whole mitochondrial genome was determined using Sanger sequencing. The DNA sequence was analyzed using MITOS (Bernt et al. 2013) and tRNAscan-SE server (Schattner et al. 2005). The complete mitogenome is 15,675 bp in length (GenBank accession no. MK246423) and contains 13 protein-coding genes, 22 tRNA genes, 2 rRNA genes, and 1 non-coding control region. The overall base composition was 43.10% A, 38.31% T, 11.30% C, and 7.29% G, with a high A + T content of 81.41%.

To validate the phylogenetic position of B. mori Ping1, we used MEGA 7.0 (Kumar et al. 2016) to construct a maximum-likelihood tree based on the complete mitochondrial DNA sequence. Bombyx mandarina and Antheraea pernyi were used as outgroups. As shown in the phylogenetic tree (Figure 1), B. mori Ping1 is more closely related to Japanese B. mori J106 than Chinese B. mori, inconsistent with the result of our observation on their cocoon shapes. B. mori Ping1 had been thought to be Chinese silkworm strain because they both spin a globular or spherical cocoon, in contrast with a peanut-shaped cocoon by Japanese silkworm strain (Kiyosawa et al. 1999). Our finding suggests that the maternal ancestor of B. mori Ping1 comes from Japanese silkworm strain, while its paternal ancestor originates from Chinese silkworm strain. More work will be needed to further confirm this finding.

Figure 1. Maximum-likelihood phylogenetic tree based on mitochondrial genome sequences. The bootstrap values with 1000 replicates are indicated at the nodes. Antheraea pernyi and Bombyx mandarina were used as outgroups. GenBank accession numbers of each species were listed in the tree.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The study was supported by supported by Zhejiang Provincial Natural Science Foundation of China (LY16C060004), and Scientific Research Funds of Zhejiang Academy of Agricultural Sciences.