Generalists or Specialists? Testing Genetic Specificity of Leucocytozoon Lineages and Black Fly Vectors in Thailand

The diversity, genetic differentiation, and cophylogeny of avian protozoa of the genus <i>Leucocytozoon</i> and their black fly vectors (Diptera: Simuliidae) in Thailand were investigated. <i>Leucocytozoon</i> infections were detected in 46 (11.2%) of 410 black flies representing six species: <i>Simulium asakoae</i>, <i>S. chumpornense</i>, <i>S. khelangense</i>, <i>S. siamense</i> complex, <i>S. yvonneae</i>, and <i>S. gombakense</i>. The latter three species represent the first reports of having <i>Leucocytozoon</i> infections, suggesting their potential role in parasite transmission. A total of 12 <i>Leucocytozoon</i> lineages was identified, including novel lineages with low sequence similarity (92%) to previously known records in the <i>S. siamense</i> complex and <i>S. yvonneae</i>. These findings indicate uncharacterized parasite diversity in wild birds of Thailand. Genetic differentiation among <i>Leucocytozoon</i> populations was minimal across different geographic populations, but highly significant among parasites in different species of black flies, even when collected from the same location. Cophylogeny analyses revealed significant co-phylogenetic relationships between <i>Leucocytozoon</i> lineages and their potential black fly vectors, despite similarities in host-blood sources. These findings emphasize that parasite-vector specificity is shaped not only by host preference, but also by coadaptation mechanisms between black flies and <i>Leucocytozoon</i> species. Further studies on avian hosts, bloodmeal sources, and parasite-vector interactions are necessary for understanding <i>Leucocytozoon</i> transmission dynamics in Southeast Asia.