Rhipicephalus sanguineus Complex in the Americas: Systematic, Genetic Diversity, and Geographic Insights

Abstract

The Rhipicephalus sanguineus group encompasses at least 12 validated species of Palearctic and Afrotropical hard ticks, which are relevant in veterinary medicine and public health. The taxonomy of R. sanguineus s.s., has been particularly intensely debated, due to its wide geographic distribution, morphological variants, parasite-host associations, and its capacity and vectorial competence for the transmission of several pathogens. By sequencing mitochondrial markers, it was possible to identify the existence of multiple lineages, among which the Tropical and the Temperate lineages stand out, particularly in America. However, the northern limit between these lineages is not clear due to the lack of extensive sampling across Mexico. For this reason, the aim of the present study was to determine the genetic diversity and structure of the R. sanguineus group in Mexico and to compare it with the populations reported in the Americas, in order to propose the northern limit of the R. sanguineus Tropical lineage and the potential regions of sympatry with R. sanguineus s.s. The findings of this study now confirm the presence of R. sanguineus s.s. in Mexico, showing a subtle genetic structure and high genetic diversity throughout its distribution in the Americas. In contrast, the Tropical lineage seems to be genetically less diverse in its overall distribution in the Americas. The genetic diversity of these two independent lineages could have important epidemiological implications in the transmission of tick pathogens.

Keywords: Tropical linage; brown dog tick; genetic diversity; vector.

Figure 1

Distribution of Rhipicephalus sanguineus s.s. and R. sanguineus s.l. in the Americas. (A) Records of R. sanguineus s.l. in the Americas based on previously reports; shaded countries have at least one record. (B) Sampling done in this study throughout Mexico. Blue circles correspond to R. sanguineus s.s. and orange circles to R. sanguineus s.l. Tropical Lineage based on records of the 16S rDNA gene. Red hexagons correspond to R. sanguineus s.s. records with records from the literature using the 12S-rDNA gene sequences available in GenBank; similarly, yellow triangles correspond to R. sanguineus s.l. Tropical Lineage. A layer from ESRI Terrain was used to construct the figure (https://server.arcgisonline.com/ArcGIS/rest/services/World_Terrain_Base/MapServer/tile/{z}/{y}/{x}, accessed on 30 August 2021). A detailed list of localities is available in Supplementary File 1.

Figure 2

Bayesian Inference phylogenetic compress analysis of the 16S rDNA partial gene for Rhipicephalus sanguineus s.s. and Rhipicephalus sanguineus s.l. Tropical lineage from the Nearctic and Neotropical regions, as well as of a few selected genes from the Afrotropical, Australasian, and Oriental regions. The phylogenetic relationships were inferred based on the Hasegawa-Kishino-Yano (HKY85) with gamma distribution for a total of 372 bp. Branch labels show branch support at each node according to posterior probabilities/non-parametric bootstrap, from the BI and ML analyses, respectively. Orange and blue sequences correspond to sequences obtained in this study.

Figure 3

PCoA plot of the first and second coordinates based on the eigenvalue scale and Euclidean distances for the 191 sequences of Rhipicephalus sanguineus and Rhipicephalus sanguineus s.l. Tropical Lineage from the Americas used in this study. The proportion of total variance along the two coordinates were 84.094% and 7.0986%, the eigenvalues were 3435.9 and 290.04, respectively. Orange sequences correspond to R. sanguineus s.l. Tropical Lineage, and blue sequences to the R. sanguineus. We used the ISO 3166-1 alfa-3 code for the countries.

Figure 4

Haplotype network for the Rhipicephalus sanguineus s.l. Tropical Lineage and Rhipicephalus sanguineus s.s. from the Americas. The colours correspond to the country of origin of each haplotype. Black lines represent the mutational steps between each haplotype. Black dots represent a putative haplotype that must exist to link the network.

Figure 5

Heatmap of Nei’s genetic distances for Rhipicephalus sanguineus s.s. and Rhipicephalus sanguineus s.l. Tropical lineage from the Americas. Colour gradient represents the percentage of differentiation when comparing the 191 sequences according to the scale bar in the right.