Characterization of the complete mitochondrial genome of Sphaerirostris picae (Rudolphi, 1819) (Acanthocephala: Centrorhynchidae), representative of the genus Sphaerirostris.

The Centrorhynchidae (Acanthocephala: Palaeacanthocephala) is a cosmopolitan family commonly found in various avian and mammalian hosts. Within Centrorhynchidae, species of the genus Sphaerirostris Golvan, 1956 are usually parasitic in the digestive tract of various passerine birds. In the present study, adult specimens of Sphaerirostris picae (Rudolphi, 1819), the type species of this genus, were recovered from the small intestine of Acridotheres tristis (Sturnidae) and Dendrocitta vagabunda (Corvidae) in Pakistan. Molecular data from the nuclear or mitochondrial genome is either very limited or completely absent from this phylogenetically understudied group of acanthocephalans. To fill this knowledge gap, we sequenced and determined the internal transcribed spacers of ribosomal DNA (ITS rDNA) and the complete mitochondrial (mt) genome of S. picae. The ITS rDNA of S. picae was 95.2% similar to that of Sphaerirostris lanceoides which is the only member of the Centrorhynchidae whose ITS rDNA is available in GenBank. The phylogenetic tree based on the amino acid sequences of 12 mt protein-coding genes (PCGs) placed S. picae close to Centrorhynchus aluconis in a monophyletic clade of Polymorphida which also contain members of the families Polymorphidae and Plagiorhynchidae on separate branches. The mt gene arrangement, nucleotide composition and codon usage of 12 PCGs were discussed and compared with those of other acanthocephalan mt genomes. Within the Centrorhynchidae, S. picae and C. aluconis showed 67.7-86.8% similarity in the nucleotide sequences of 12 PCGs and 2 rRNAs, where nad4L is the most conserved gene while atp6 is the least conserved. The similarity in amino acid sequences ranged from 68.1 to 91.8%, where cox1 was recorded as the most conserved gene, while atp6 is highly variable among 12 PCGs. This novel mt genome of S. picae provides genetic resources for further studies of phylogenetics and molecular epidemiology of acanthocephalans.