3.1. General features of the mitochondrial genome of two
parasites
The complete mt genomes of
the
cysticerci collected from Qinghai vole and Plateau pika, named asT. caixuepengi larva (GenBank ID: MT882036) and T.
tianguangfui larva (GenBank ID: MT882037), were 13,747 bp and 13,522 bp
in length,
respectively.
Both of them contain 2 rRNA genes (the small [rrn S] and large
[rrn L] subunits of rRNA), 12 protein-encoding genes
(atp 6, cyt b, nad 4L, cox 1-3 andnad 1-6) and 22 tRNA genes, but lack atp 8 gene, which are
typical of cestode mt genomes (Fig. 1). The inferred gene boundaries and
their lengths are shown in Table 1.
In accordance with other mtDNAs of flatworms sequenced to date (Liu et
al., 2011; Jia et al., 2010), the nucleotide compositions are mostly
biased toward T, while least favored toward C. AT-richness of mtDNAs inT. caixuepengi and T. tianguangfui are 71.96% (45.00% T,
26.97% A, 19.17% G, 8.87% C) and 73.48% (46.35% T, 27.13% A,
18.61% G, 7.91% C), respectively.
Flatworms use an unusual mt code to exert protein translation (Nakao et
al., 2000; Telford et al., 2000). GTG was used as an alternative
initiation codon in cox 3 and nad 3 genes ofT.
caixuepengi and nad 5 gene of T. tianguangfui .
Furthermore, the codon ATT was inferred as a more unusual start codon ofatp 6 gene in T. caixuepengi .
The
termination codon was mostly TAA, and the ending codon TGA was
deprecated (Table 1).