Nucleotide sequence variation in mitochondrial COI gene among 147 silkworm (Bombyx mori) strainsfrom Japanese, Chinese, European and moltinism classes

Abstract

We characterized the nucleotide sequences of PCR- amplified mitochondrial COI fragments of 147 silkworm (Bombyx mori) strains that have been maintained in the National Institute of Agro biological Sciences. Coding sequences (714 bp) of the 147 COI fragments were classified into eight haplo types based on nucleotide differences at eight segregating sites. No length variation was identified in this region. The 5'-noncoding region showed different features, wherein changes in the number of Ts in the T-stretch, together with two base substitutions, were observed. As a result, the 147 COI non coding sequences were classified into six haplo types. Combining the coding and non coding regions, we identified 14 haplo types. One of the 14 haplo types, Hap1A was exclusively abundant in the Japanese native strain class, while this haplo type was less frequent in the other three native strain classes. This finding suggests that the Japanese strain class underwent significant genetic differentiation from the Chinese, European, and moltinism classes, when the each class is regarded as a population. Comparison of the nucleotide sequences to those of B. mandarina (which inhabits Japan) revealed changes that are significantly larger than those within either B. mori or B. mandarina. Furthermore, we detected no common haplotypes between them, which suggests the concept of suppressed gene flow between the two species.