Genetic diversity of Salp15 in the Ixodes ricinus complex (Acari: Ixodidae)

Abstract

Salp15, a 15-kDa tick salivary gland protein, is both essential for ticks to successfully obtain host blood and also facilitates transmission of Lyme borreliosis. To determine whether the Salp15 gene is expressed in Ixodes persulcatus and Ixodes sinensis, principle vectors of Lyme borreliosis in China, we studied transcriptions of this gene in semi-engorged larvae, nymph and adults of these two species. A total of eight Salp15 homologues, five in I. persulcatus and three in I. sinensis, were identified by reverse transcriptase-polymerase chain reaction (RT-PCR). Interestingly, the intra-species similarity of Salp15 is approximately equal to its interspecies similarity and more than one Salp15 protein is expressed in a certain tick developmental stage. Comparison of DNA and proteins with other available tick Salp15 homologues suggests that the Salp15 superfamily is genetically conserved and diverse in the Ixodes ricinus complex. These findings indicate that Salp15 proteins in the I. ricinus complex may play an essential role in interacting with the host immune system and transmission of Borrelia genospecies.

Figure 1. Comparison of Salp15 animo acid sequences from Ixode persulcatus and Ixodes sinensis with selected Salp15 sequences from other Ixodes species.

Footnote: I. persulcatus Ipers-1 (JX500419), I. persulcatus Ipers-2 (JX094824), I. persulcatus Ipers-3 (JX094825), I. persulcatus Ipers-4(JX094826) from I. persulcatus (northern China), I. sinensis Is-1 (JX094821), I. sinensis Is-2 (JX094822), I. sinensis Is-3 (JX500420) from I. sinensis (southern China), I. ricinus iric1(ABU93613), I. ricinus iric2(ABU93614), I. ricinus iric3(ABU93615) from I. ricinus (Europe), I. persulcatus iper1(BAH09310), I. persulcatus iper2(BAH09311) from I. persulcatus in Japan, I. persulcatus iper3(ACV32167) from I. persulcatus (Russia), I. pacificus Ipac-1(ACV32166) from I. pacificus and I. scapularis Salp15(AAK97817) (North America). Positions of amino acid sequence identity, amino acid residue similarity and highly conserved amino acid substitutions are marked by an asterisk, full stop and colon, respectively. Signal Peptides of each sequence are in bold type. CD4 binding residues are underlined.

Figure 2. Phylogenetic tree of Salp15 homologues from members of Ixodes ricinus complex constructed with Mega 4 Tree Explorer.

Footnote: The developmental stage of ticks from which sequences were obtained is in bold type. ▴ indicates Salp15 homologues from I. sinensis collected in southern China; • indicates the Salp15 homologues from I. persulcatus collected in northern China. *indicates the Salp15 homologues from I. persulcatus collected in Russia; ⧫ indicates Salp15 homologues from I. persulcatus collected in Japan; ▪ indicates Salp15 homologues from I. ricinus collected in Europe; ★ indicates the Salp15 from I. scapularis collected in easternNorth America. ▾ indicates the Salp15 from I.pacificus collected in western North America.