Characterisation of the Swarming Behavior of An. coluzzii and An. Gambiae Populations from a Hybrid Zone of Senegal.

Abstract Background Anopheles gambiae s.s. and An. coluzzii, two major malaria vectors in Africa, exist in the nature as two incipient, sympatric or allopatric species. In most of their sympatric areas, the reproductive isolation between these two distinct species is thought to be the main barrier to hybridization. However, in Senegal, barriers to the gene flow seems to be leaky in some areas with relatively higher than expected hybridization rates. Here, we characterized the swarming behavior of these two species to investigate its role in the observed high hybridization.Methods The study was carried out in the south and center of Senegal during the 2018 rainy season. Swarms were surveyed at sunset towards the lightest part of the sky, about 0.5–4 m above the ground. Once located, swarm were collected using a net. Indoor resting populations were also collected during the same period from each sentinel village by pyrethrum spray catch earlier the morning. All specimens collected were identified morphologically followed by PCR to estimate the frequency of the two species and female hybrids.Results Results showed that An. gambiae swarmed mainly over bare ground whereas An. coluzzii swarms over various objects forming a dark-light contrast with the ground. The height of swarms varied between 0.5 to 2.5 meters and the swarming duration was about 10 minutes. Start of swarming was mainly correlated with sunset and no mixed swarms were found in areas of sympatry despite the high level of hybridization rate (2.4% − 4.4%).Conclusion As found elsewhere in West Africa, swarming site segregation is an important pre-mating reproductive barrier between An. coluzzii and An. gambiae in Senegal. No link was found between swarming behavior and hybridization, but the lack of mixed swarms may be the result of low number of samples obtained in the sympatric area.