Evidence of hybridization between Taenia saginata and Taenia asiatica

There has long been a debate as to the specific status of the cestode Taenia asiatica, with some people regarding it as a distinct species and some preferring to recognize it as a strain of Taenia saginata. The balance of current opinion seems to be that T. asiatica is a distinct species. In this study we performed an allelic analysis to explore the possibility of gene exchange between these closely related taxa. In total, 38 taeniid tapeworms were collected from humans living in many localities including Kanchanaburi Province, Thailand where the two species are sympatric. A mitochondrial DNA (mtDNA)-based multiplex PCR tentatively identified those parasites as T. asiatica (n=20) and T. saginata (n=18). Phylogenetic analyses of a mitochondrial cytochrome c oxidase subunit 1 (cox1) gene and two nuclear loci, for elongation factor-1 alpha (ef1) and ezrin-radixin-moesin (ERM)-like protein (elp), assigned all except two individual parasites to the species indicated by multiplex PCR. The two exceptional individuals, from Kanchanaburi Province, showed a discrepancy between the mtDNA and nuclear DNA phylogenies. In spite of their possession of sequences typical of the T. saginata cox1 gene, both were homozygous at the elp locus for one of the alleles found in T. asiatica. At the ef1 locus, one individual was homozygous for the allele found at high frequency in T. asiatica while the other was homozygous for the major allele in T. saginata. These findings are evidence of occasional hybridization between the two species, although the possibility of retention of ancestral polymorphism cannot be excluded.