Species of the genus Tabanus Linnaeus (horse flies) are significant pests and vectors transmitting pathogens to economically important animals including cattle. Therefore, knowledge about genetic diversity; a necessity for understanding disease epidemiology, is of paramount importance. However, such information is limited in Thailand, particularly in the northeastern region despite it being a major cattle production area. We examined genetic diversity of three Tabanus species, T. striatus Fabricius and T. megalops Walker, and T. rubidus Wiedemann based on mitochondrial cytochrome oxidase I (COI) sequences. There was relatively high genetic diversity (max. 2.14%) within populations of T. rubidus from northeastern Thailand but no indication of genetic structuring. Phylogenetic relationships between Thai populations and those recorded from Malaysia, India and Bangladesh involved three genetically divergent lineages which possibly represent different but morphologically similar species. High levels of genetic diversity (max. 4.43% and 5.20%) were found within northeastern populations of T. megalops and T. striatus due to the existence of three genetically divergent lineages within these species. However, there was no signal of geographical association of lineages suggesting panmixia within populations. Phylogenetic relationships between specimens obtained in the present study and those recorded from other countries revealed cryptic genetic diversity with three divergent clades. Further study is required to solve this taxonomic complexity of the T. striatus-complex.

Culicoides oxystoma Kieffer is a vector of viruses, filarial nematodes and protozoa of the genus Leishmania transmitted to humans and other animals. Understanding genetic diversity, genetic structure and genetic relationships among geographically widespread populations will provide important information related to disease epidemiology. In this study, genetic diversity, genetic structure and genetic relationships between Thai C. oxystoma and those reported from other countries were inferred based on mitochondrial cytochrome oxidase I (COI) and nuclear internal transcribed spacer 1 (ITS-1) sequences. A high level of genetic diversity was found in C. oxystoma from Thailand. The maximum K2P intraspecific genetic divergence for COI gene and ITS-1 sequences were 4.29% and 6.55%, respectively. Despite high genetic diversity, no significant genetic differentiation was found within the 13 Thai populations. This could be a result of unspecialized habitat requirement of the larval habitat, abundance and continuous distribution of host blood sources, potential for long distance movement with host via trading. Mitochondrial genealogy analysis of the global population of C. oxystoma revealed three (A, B and C) genetically divergent lineages. Specimens from Thailand were included in the main lineage (A) with those from all other countries except those from Senegal that formed lineage B and those of Lineage C that was exclusively found in Bangladesh. The nuclear (ITS-1) genetic markers genealogy indicated that Thai C. oxystoma belong to the same genetic lineage with those from East, South and Southeast Asia which presumably the true C. oxystoma.