This study provides experimental evidence that L. turanica develops late-stage infections in P. papatasi. According to the criteria of Killick-Kendrick, this finding, together with the well-known anthropophily of this sand fly species and repeated isolation of L. turanica from wild-caught P. papatasi in Central Asia (reviewed by) and Iran[9, 10], incriminate P. papatasi as the vector of L. turanica.
Comparing the development of L. turanica and L. major in P. papatasi revealed high similarity and found that statistical differences are not significant from a biological point of view. Although the differences in intensities between L. turanica and L. major infections were statistically significant (p = 0.03), the rates of infections were at the same high level in both species (around 80% of analyzed females were positive on all examined days) and the development of location and stomodeal valve colonization ability were the same in both species without significant differences.
The ability of L. major to develop in the specific (selective) vector P. papatasi depends on the presence of species-specific modifications of the major surface glycoconjugate of promastigotes, lipophosphoglycan (LPG), which controls the parasite attachment to the sand fly gut[12, 27]. The successful development of L. turanica in P. papatasi suggests that the LPG of this species must be identical or very similar to that of L. major.
Experiments on the development of L. turanica in two other sand flies distributed in the Middle East revealed that L. turanica develop late stage infections in P. (Adlerius) arabicus but not in P. (Paraphlebotomus) sergenti. In all P. sergenti examined, infection did not persist after digestion and defecation of the bloodmeal. This result is in agreement with previous findings that P. arabicus is a permissive vector susceptible to various Leishmania species, including L. major[23, 28], while P. sergenti is the specific vector of L. tropica and is refractory to L. major[29, 30]. Reports of L. turanica from other Paraphlebotomus species[1, 9], however, suggest that vector competence for Leishmania may differ between members of this subgenus.
Using GFP and RFP-marked parasites we have experimentally demonstrated that L. turanica and L. major are able to develop in P. papatasi together, without any visible sign of competition. First, we used an initial infection dose of 1x107 (resp. 5x106L. major + 5x106L. turanica) promastigotes/ml. Parasites did not differ in intensities or localization on all examined days, and the infections developed well with high rates of heavy infections for both species. Next, we decided to decrease the infection dose to better simulate natural conditions. Again, no significant differences were observed in the localization of infections and both Leishmania species colonized the stomodeal valve during mixed infections. Minor differences found in the infection intensities seem to be biologically unimportant, as other parameters, like colonization of the stomodeal valve, are more important for the vector status of the sand fly than the exact parasite number found in the midgut.
Despite the fact that both Leishmania species coexisted in the P. papatasi and P. duboscqi midgut for the relatively long period of two weeks (until the end of the experiment), the presence of dual fluorescing L. major / L. turanica hybrids was not demonstrated. Sadlova et al. speculate that there may be species-specific differences among Leishmania species in their capacity for sexual reproduction and it should be noted that although L. turanica and L. major belong to the subgenus Leishmania, they are not closely related to each other. Considering this, the similarity of LPG in L. turanica and L. major proposed here is likely to be a result of co-evolutionary convergence due to the adaptation of these two Leishmania species to the same sand fly vector.