Abstract
BackgroundAmblyomma variegatum and A. hebraeum are two ticks of veterinary and human health importance in south-east Africa. In Zimbabwe they occupy parapatric (marginally overlapping and juxtaposed) distributions. Understanding the mechanisms behind this parapatry is essential for predicting the spatio-temporal dynamics of Amblyomma spp. and the impacts of associated diseases. It has been hypothesized that exclusive competition between these species results from competition at the levels of male signal reception (attraction-aggregation-attachment pheromones) or sexual competition for mates. This hypothesis predicts that the parapatry described in Zimbabwe could also be present in other countries in the region.MethodsTo explore this competitive exclusion hypothesis we conducted field surveys at the two species’ range limits in Mozambique to identify areas of sympatry (overlapping areas) and to study potential interactions (communicative and reproductive interference effects) in those areas. At sympatric sites, hetero-specific mating pairs were collected and inter-specific attractiveness/repellent effects acting at long and short distances were assessed by analyzing species co-occurrences on co-infested herds and co-infested hosts.ResultsCo-occurrences of both species at sampling sites were infrequent and localized in areas where both tick and host densities were low. At sympatric sites, high percentages of individuals of both species shared attachment sites on hosts and inter-specific mating rates were high. Although cross-mating rates were not significantly different for A. variegatum and A. hebraeum females, attraction towards hetero-specific males was greater for A. hebraeum females than for A. variegatum females and we observed small asymmetrical repellent effects between males at attachment sites.ConclusionsOur observations suggest near-symmetrical reproductive interference between A. variegatum and A. hebraeum, despite between-species differences in the strength of reproductive isolation barriers acting at the aggregation, fixation and partner contact levels. Theoretical models predict that sexual competition coupled with hybrid inviability, greatly reduces the probability of one species becoming established in an otherwise suitable location when the other species is already established. This mechanism can explain why the parapatric boundary in Mozambique has formed within an area of low tick densities and relatively infrequent host-mediated dispersal events.Electronic supplementary materialThe online version of this article (doi:10.1186/s13071-015-1116-7) contains supplementary material, which is available to authorized users.
Highlights
Amblyomma variegatum and A. hebraeum are two ticks of veterinary and human health importance in south-east Africa
Whereas A. variegatum is widely distributed in Africa, its southern limit (Mozambique, Zimbabwe and Botswana) corresponds to the northern limit of A. hebraeum’s geographical range, which extends into South Africa and across Swaziland [6]
We examined the morphology of 30 ticks per species per study site under a stereomicroscope to search for phenotypical patterns that differed from those known for A. variegatum or A. hebraeum and may represent hypothetical co-dominant or incomplete dominant hybridization
Summary
Amblyomma variegatum and A. hebraeum are two ticks of veterinary and human health importance in south-east Africa. Exclusive competition between the two species has been hypothesised as a mechanism explaining the parapatry seen in Zimbabwe [7, 9] This hypothesis predicts that similar parapatric range limits could exist in Mozambique and Botswana. In Mozambique, tick presence data arising from occasional sampling between 1940 and 1975 was only recorded at the district level giving rise to a geographical uncertainty of some hundreds of kilometres [10, 11 & Travassos Santos Dias J,(unpublished data)] These data suggested the contact zone between these two species was located somewhere near the Save river (19th-23rd southern parallels): in the east of the country, A. variegatum was found south of the river together with A. hebraeum (Govuro and Vilankulo districts) whereas in the west it was the only species recorded to the north of the river and was never found south of the river (Additional file 1: Figure S1). These data indicated the presence of A. hebraeum to the north-west of the Save river at a distance of 150–200 km from the nearest A. variegatum records (Additional file 1: Figure S1)
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