Natural hybridization of the ticks Ixodes persulcatus and Ixodes pavlovskyi in their sympatric populations in Western Siberia

Hybridization of ticks of the genus Ixodes has been described for several species under laboratory conditions although no molecular genetics evidence confirming interspecific hybridization in nature is available. We have designed a real time PCR targeted on nuclear (ITS2) and mitochondrial (cox1) markers to accurately identify tick species and to detect interspecific hybrids of Ixodes persulcatus and Ixodes pavlovskyi in their sympatric populations in Western Siberia. A survey of 783 individual ticks from a suburb of Tomsk showed that 44.2% of ticks belong to I. pavlovskyi species and 55.8% to I. persulcatus, based on the mtDNA data. Results obtained with the nuclear marker were not consistent, indicating that approximately 10% of the ticks were hybrids and about 5% revealed mtDNA introgression. Both hybridization and introgression have been shown to occur bidirectionally but more efficiently in the mating pair female I. pavlovskyi × male I. persulcatus than vice versa. The existence of the first generation hybrids and backcrosses challenges the existing view about effective reproductive barriers between I. pavlovskyi and I. persulcatus. While using only mitochondrial markers can lead to errors in determining tick species, we propose to use nuclear or both markers instead. The results obtained in the present paper and published earlier suggest that hybridization between closely related tick species in their sympatric zones is common rather than exceptional. The role of hybrid populations of vectors in the evolution of transmitted pathogens is also discussed.