Genetic structure and population dynamics of Culicoides obsoletus and Culicoides scoticus (Diptera: Ceratopogonidae): implications for the transmission and maintenance of bluetongue

摘要

Abstract. Culicoides species belonging to the Obsoletus complex (Diptera: Ceratopogonidae)\udhave been indicated as primary bluetongue (BT) vectors in many European\udcountries and their possible involvement in the maintenance and overwintering of BT\udviruses has been suggested, even in regions where Culicoides imicola Keiffer is the\udmain vector. The Obsoletus complex includes two predominant taxa, Culicoides obsoletus\ud(Meigen) and Culicoides scoticus Downes & Kettle. However, the role played\udby each species in the epidemiology of BT is still unknown. Taxonomic identification\udis mainly based on the morphology of male genitalia and the lack of other reliable\uddiagnostic features makes the screening of trap-collected vector populations, mainly\udfemales, particularly difficult. Although molecular markers have facilitated species\udidentification, little information is yet available on the biology, abundance and population\uddynamics of the two taxa. The aim of this work was to investigate the genetic\udprofile and temporal distribution of C. obsoletus and C. scoticus by using isozyme\udelectrophoresis applied to adult midges, collected weekly at two selected farms in\udsouthern Sardinia. A total of nine enzyme loci were analysed and five of them provided\uddiagnostic allozyme markers (Hk, Mdh, Pgi, Idh-1 and Idh-2 ). Nei’s genetic distance\udbetween the two taxa was in the range of other well-separated taxa (D = 1.792),\udsupporting their status as true species. Culicoides scoticus represented almost 61%\udof the 562 specimens analysed; its genetic structure was characterized by a very\udlow level of intra-population variation (mean heterozygosity He = 0.019) and higher\udgenetic divergence between populations (FST = 0.0016) than in C. obsoletus. The latter\udspecies had significantly more heterozygotes (He = 0.123), a higher percentage\udof polymorphic loci, and no inter-population differentiation (FST\ud∼=\ud0). We suggest\udthat different biological and ecological constraints, such as breeding habitat requirements,\udmay contribute to shaping the genetic profiles of C. scoticus and C. obsoletus.\udHowever, enough gene flow was maintained between populations of each species as\udno spatial and temporal structuring was sustained by Fisher’s exact probability test\ud(P > 0.5). The seasonal distributions of C. scoticus and C. obsoletus only partially\udoverlapped: both species were mainly found early in the year, when the main vector,\udC. imicola, was present in low numbers, and peaked in abundance in April and May.Culicoides scoticus was predominant until May, decreased rapidly in the following\udmonths and increased again in winter, whereas C. obsoletus decreased more slowly\udand was still present in early summer. Consequently, C. scoticus may be a good candidate\udfor playing a role in the transmission and maintenance of BT virus in Sardinia,as well as in other Mediterranean countries, during the months of late winter and early\udspring when the seroconversion of sentinel animals is still occurring in the absence of the main vector.