Stéphanie Jacquet

Colonization of the Mediterranean Basin by the vector biting midge species #Culicoides imicola#: an old story

Understanding the demographic history and genetic make‐up of colonizing species is critical for inferring population sources and colonization routes. This is of main interest for designing accurate control measures in areas newly colonized by vector species of economically important pathogens. The biting midge Culicoides imicola is a major vector of orbiviruses to livestock. Historically, the distribution of this species was limited to the Afrotropical region. Entomological surveys first revealed the presence of C. imicola in the south of the Mediterranean basin by the 1970s. Following recurrent reports of massive bluetongue outbreaks since the 1990s, the presence of the species was confirmed in northern areas. In this study, we addressed the chronology and processes of C. imicola colonization in the Mediterranean basin. We characterized the genetic structure of its populations across Mediterranean and African regions using both mitochondrial and nuclear markers, and combined phylogeographical analyses with population genetics and approximate Bayesian computation. We found a west/east genetic differentiation between populations, occurring both within Africa and within the Mediterranean basin. We demonstrated that three of these groups had experienced demographic expansions in the Pleistocene, probably because of climate changes during this period. Finally, we showed that C. imicola could have colonized the Mediterranean basin in the Late Pleistocene or Early Holocene through a single event of introduction; however, we cannot exclude the hypothesis involving two routes of colonization. Thus, the recent bluetongue outbreaks are not linked to C. imicola colonization event, but rather to biological changes in the vector or the virus.

Range expansion of the Bluetongue vector, Culicoides imicola , in continental France likely due to rare wind-transport events

The role of the northward expansion of Culicoides imicola Kieffer in recent and unprecedented outbreaks of Culicoides-borne arboviruses in southern Europe has been a significant point of contention. We combined entomological surveys, movement simulations of air-borne particles, and population genetics to reconstruct the chain of events that led to a newly colonized French area nestled at the northern foot of the Pyrenees. Simulating the movement of air-borne particles evidenced frequent wind-transport events allowing, within at most 36 hours, the immigration of midges from north-eastern Spain and Balearic Islands, and, as rare events, their immigration from Corsica. Completing the puzzle, population genetic analyses discriminated Corsica as the origin of the new population and identified two successive colonization events within west-Mediterranean basin. Our findings are of considerable importance when trying to understand the invasion of new territories by expanding species.

New data regarding distribution of cattle ticks in the south-western Indian Ocean islands

Recent studies have produced new insight into the origin and distribution of some cattle ticks in the south-western Indian Ocean islands. Rhipicephalus appendiculatus, introduced from Tanzania in 2002, is now well established on Grande Comore but has not yet reached the other islands of the archipelago (Mohéli, Anjouan and Mayotte). Only one of the two clades identified in Africa has settled so far. Amblyomma variegatum, which was not supposed to be able to persist in the Antananarivo region (1300 m) nor in other Malagasy regions of high altitude without regular introductions of ticks by infested cattle, is now endemic as a general rule up to 1600 m although other regions of lower altitude (1400 m) are still free of the tick. This species remains confined in a small area of the west coast on La Reunion Island. On the contrary, Hyalomma dromedarii could not settle on Madagascar where it was introduced in 2008 and Rhipicephalus evertsi evertsi is not yet present in Grande Comore despite regular introductions by infested cattle from Tanzania. A phylogeographic approach has been carried out at an intra-specific level for A. variegatum. This study has led to the identification of two main lineages, one covering all species distribution and one restricted to East Africa and the Indian Ocean area. These two lineages are in sympatry in Madagascar where a high genetic diversity has been described, whereas a lower genetic diversity is observed on other islands. These results seem to agree with the historical data concerning the introduction of the tick in the Indian Ocean area.

Low effective population sizes in Amblyomma variegatum, the tropical bont tick

Effective population sizes have rarely been estimated in ticks despite the importance of this parameter for evaluating the evolutionary and adaptive potential of tick populations. The present study was aimed at evaluating the effective population sizes of Amblyomma variegatum, the tropical bont tick, in three villages in Burkina Faso. For this purpose, microsatellites markers were developed. Eight out of 19 assessed markers provided good amplification results with 4 to 24 alleles recorded per marker on 216 genotyped ticks. The within-samples polymorphism was congruent with Hardy-Weinberg expectations at four markers while sex linkage and/or null alleles were observed at the others. As sampling involved two tick generations, effective population sizes were independently estimated by two methods insensitive to heterozygosity: the first one is based on linkage disequilibrium analysis within a single cohort while the second uses the changes in allele frequencies across generations. Both methods estimated the number of reproducing ticks ranging from two to a few tens reproductive adults per village and cohort. Such small estimates are congruent with the rarity of records of acaricide resistance in A. variegatum.

#Culicoides imicola#: Phylogeography, population genetics, and invasive status in the Mediterranean basin. [3166]

Introduction: Biological invasions are of major concern because of their environmental, economic, and health consequences. Determining and understanding the factors underlying invasion success of species allows predicting potential other biological invasions and developing vector control strategies. Culicoides imicola is a major vector species of Orbivirus, including the bluetongue virus (BTV) which affects domestic ruminants. Following BT emergence in the Mediterranean basin, C. imicola populations were recorded in territories where the species was considered to be absent, and consequently was described as expanding its distribution range on a short period. This work aimed at understanding the colonization history of the Mediterranean basin by C. imicola and determining the factors underlying the current distribution of the species. Methods: We used a multi-loci approach and combined population genetics analyses, Approximate Bayesian Computation (ABC) methods and mathematical simulations of the atmospheric dispersion of the species. Results/Conclusion: This approach enabled us to (i) demonstrate that C. imicola Mediterranean populations originated from the northern region of sub-Saharan Africa and are established in the Mediterranean basin since the Pleistocene/Holocene period, (ii) confirm a North African origin of southwestern European populations and reveal two routes of colonization: starting from Morocco to the Iberian Peninsula and from Algeria to France and Italy, and (iii) highlight the major role of wind-mediated dispersal and population abundances in the range expansion success of the species. Altogether, our study shed a new light on the evolutionary and demographic history of C. imicola. (Texte integral)