Genetic structure and population dynamics of the biting midgesCulicoides obsoletusandCulicoides scoticus: implications for the transmission and maintenance of bluetongue

Abstract

Abstract. Culicoides species belonging to the Obsoletus complex (Diptera: Ceratopogonidae) have been indicated as primary bluetongue (BT) vectors in many European countries and their possible involvement in the maintenance and overwintering of BT viruses has been suggested, even in regions where Culicoides imicola Keiffer is the main vector. The Obsoletus complex includes two predominant taxa, Culicoides obsoletus (Meigen) and Culicoides scoticus Downes & Kettle. However, the role played by each species in the epidemiology of BT is still unknown. Taxonomic identification is mainly based on the morphology of male genitalia and the lack of other reliable diagnostic features makes the screening of trap-collected vector populations, mainly females, particularly difficult. Although molecular markers have facilitated species identification, little information is yet available on the biology, abundance and population dynamics of the two taxa. The aim of this work was to investigate the genetic profile and temporal distribution of C. obsoletus and C. scoticus by using isozyme electrophoresis applied to adult midges, collected weekly at two selected farms in southern Sardinia. A total of nine enzyme loci were analysed and five of them provided diagnostic allozyme markers (Hk, Mdh, Pgi, Idh-1 and Idh-2 ). Nei's genetic distance between the two taxa was in the range of other well-separated taxa (D = 1.792), supporting their status as true species. Culicoides scoticus represented almost 61% of the 562 specimens analysed; its genetic structure was characterized by a very low level of intra-population variation (mean heterozygosity H e = 0.019) and higher genetic divergence between populations (F ST = 0.0016) than in C. obsoletus. The latter species had significantly more heterozygotes (H e = 0.123), a higher percentage of polymorphic loci, and no inter-population differentiation (F ST ∼ = 0). We suggest that different biological and ecological constraints, such as breeding habitat requirements, may contribute to shaping the genetic profiles of C. scoticus and C. obsoletus. However, enough gene flow was maintained between populations of each species as no spatial and temporal structuring was sustained by Fisher's exact probability test (P > 0.5). The seasonal distributions of C. scoticus and C. obsoletus only partially overlapped: both species were mainly found early in the year, when the main vector, C. 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 months and increased again in winter, whereas C. obsoletus decreased more slowly and was still present in early summer. Consequently, C. scoticus may be a good candidate for playing a role in the transmission and maintenance of BT virus in Sardinia,

Collection ofCulicoides(Diptera: Ceratopogonidae) Using CO2and Enantiomers of 1-Octen-3-ol in the United Kingdom

“…Physiological status CDC (CO 2 and (R)-1-octen-3-ol) a The evidence for involvement of members of the Avaritia subgenus in BTV transmission in both southern and northern Europe is far stronger than that for C. nubeculosus, although our understanding of the role of individual species within this group during epizootics remains limited , Pili et al 2010. As expected, numbers of Culicoides collected directly from a live host substantially exceeded those collected using semiochemical traps indicating the fact that host location involves multiple and complex visual, thermal and olfactory stimuli, which are difÞ-cult to replicate artiÞcially (Logan and Birkett 2007).…”

Bluetongue: control, surveillance and safe movement of animals

“…In Sardinia, populations of C. imicola appeared only from April to December, meanwhile C. obsoletus, C. newsteadi and C. pulicaris were captured all year around (Foxi and Delrio, 2010;Foxi et al, 2011). Pili et al (2010) confirmed that C. scoticus was most abundant from late winter to early spring, whereas C. obsoletus was prevalent in early summer. A similar pattern was also found in Corsica (Venail et al 2012).…”

“…Pili et al. () confirmed that C. scoticus was most abundant from late winter to early spring, whereas C. obsoletus was prevalent in early summer. A similar pattern was also found in Corsica (Venail et al.…”

Scientific Opinion on bluetongue monitoring and surveillance

“…Three principal routes of virus introduction into Europe have been identified, all of them situated in areas where multiple vector species occur (Calvete et al, 2008;Pili et al, 2010). At a distance, Europe would seem to consist of two BT sub-episystems: southern, with C. imicola, and northern, without C. imicola ).…”

“…The existence of zones with a higher risk of virus introduction is, however, recognised. Three principal routes of virus introduction into Europe have been identified, all of them situated in areas where multiple vector species occur (Calvete et al, 2008;Pili et al, 2010). These routes are: i) Morocco -Spain, ii) North-Africa -Western and Central Mediterranean islands and Italy, iii) Turkey -Greece (Wilson and Mellor, 2008).…”