Colonisation history of a vector species of veterinary interest: phylogeography of Culicoides imicola (Diptera: Ceratopogonidae) in the Indian Ocean revealing low genetic structure.

Culicoides imicola (Diptera: Ceratopogonidae) is the main historical vector of livestock viruses such as bluetongue virus (BTV), Epizootic Haemorrhagic Disease virus (EHDV) and African Horse Sickness virus (AHSV), in the Afrotropical region. Despite its wide distribution across Africa, the Mediterranean basin and parts of Asia, the genetic structure and colonisation history of this species in the South-West Indian Ocean (SWIO) territories remain poorly understood. The aim of this study was to investigate how passive wind-driven dispersal and recent animal movements associated with human colonisation may have shaped the genetic structure of this vector. We used two mitochondrial markers (cytochrome oxidase 1 and cytochrome oxidase 2) and characterized genetic structure of populations from 4 territories. In addition, phylogenetic analyses were conducted with COI sequences from 11 territories bordering the Indian Ocean retrieved from GenBank database. Phylogenetic analyses on COI sequences suggest no clear lineage or genetic grouping among the 11 territories. In the SWIO, concatenated COI and COII gene analyses show significant genetic differentiation between the four territories, especially between Mauritius and Reunion where populations exhibit signs of demographic expansion. The haplotype network suggests multiple events or potential colonisation pathways, supporting the hypothesis that animal trade is the primary means of introduction from the continent or Madagascar to Mauritius and Reunion. This work serves as a basis for understanding the risk of introduction of viruses transmitted by Culicoides into the island territories of the SWIO and highlights the need to strengthen practices in animal trade in the region.