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

## TL;DR

This study explores how Leucocytozoon parasites and black flies in Thailand are genetically linked, revealing new parasite lineages and evidence of co-evolution.

## Contribution

The study identifies three novel Leucocytozoon lineages and provides evidence of co-speciation between black fly species and parasites.

## Key findings

- 12 Leucocytozoon lineages were detected, including three novel and genetically distinct ones.
- Phylogenetic analysis showed that closely related black fly species host genetically similar Leucocytozoon lineages.
- Cophylogeny analysis revealed significant co-evolution between Leucocytozoon and black fly species.

## Abstract

Understanding host–vector–parasite relationships is essential for effective disease control. Black flies are blood-sucking insects that transmit a variety of pathogens causing diseases in humans and other animals. Avian blood parasites of the genus Leucocytozoon are transmitted exclusively by black flies, with only one known exception. In this study, we examined the diversity and genetic specificity between Leucocytozoon parasites and black fly species from Thailand. Six species of black flies were genetically screened for Leucocytozoon infection. In total, 12 Leucocytozoon lineages were detected, three of which are novel and genetically distinct from previously recorded lineages. Phylogenetic analyses revealed evidence of cospeciation between black fly species and Leucocytozoon lineages. Lineages infecting closely related species of black flies were genetically more similar, suggesting that coadaptation between Leucocytozoon parasites and their black fly vectors might be an important mechanism driving vector–parasite specificity.

The diversity, genetic differentiation, and cophylogeny of avian protozoa of the genus Leucocytozoon and their black fly vectors (Diptera: Simuliidae) in Thailand were investigated. Leucocytozoon infections were detected in 46 (11.2%) of 410 black flies representing six species: Simulium asakoae, S. chumpornense, S. khelangense, S. siamense complex, S. yvonneae, and S. gombakense. The latter three species represent the first reports of having Leucocytozoon infections, suggesting their potential role in parasite transmission. A total of 12 Leucocytozoon lineages was identified, including novel lineages with low sequence similarity (92%) to previously known records in the S. siamense complex and S. yvonneae. These findings indicate uncharacterized parasite diversity in wild birds of Thailand. Genetic differentiation among Leucocytozoon 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 Leucocytozoon 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 Leucocytozoon species. Further studies on avian hosts, bloodmeal sources, and parasite–vector interactions are necessary for understanding Leucocytozoon transmission dynamics in Southeast Asia.