Ticks are the second vector disease globally and potentially one of the most critical organisms to be studied in every concept. Establishing a solid evolutionary framework of Amblyomma within a Metastriata context may be key to understanding the origin of the diversification of the genus. We used the leafier tree (in number of species) published recently [26], to select the species representatives of each main lineage. Thus, the molecular dataset generated here is described and, for the first time at the mitogenomic level, used to reconstruct the Amblyomma phylogenetic relationships to assess previous hypotheses related to the origin and patterns of diversification of the genus.
Mitogenomic features
A total of 17 mt genomes (three partial) were newly sequenced in this study, increasing the mt genome catalog available for this tick group, from 22 to 39. The sequencing methods applied here to determine the mt genomes (amplicon and WGS) allowed us to obtain a similar sequencing depth reached in related articles recently published ([16, 28]; see depth coverage in Table S1). The features of the new complete mt genomes described here fit with those already reported for other mt genomes of Metastriata [58], including the gene order (except Africaniella tranversale [27]), length of the genomes/genes, and the two characteristic control regions. Also, a general feature of the 37 genes (13 protein-coding, 22 tRNAs, and two ribosomal RNAs), fit with the most metazoan mt genomes [41].
Phylogeny of Amblyomma
To reconstruct the internal relationships of Amblyomma, twelve phylogenetic trees were inferred in this study, six at the nucleotide and six at the amino-acid codification. All trees were summarized in Fig. 1. where the nodes that were not recovered at least in half of the trees with significative statistical support (node ≥ 0.95 PP/80 PB) were collapsed (see all topologies in Figure S1a–l). Also, data set features, best-fit partition schemes and models, and log-likelihood values of all phylogenetic analyses are provided in (Table S3). Amblyomma was recovered as monophyletic in all analyses (Fig. 1, node 1), and sister to a clade that includes (Dermacentor (Rhipicentor (Hyalomma + Rhipicephalus), as has been widely reported in previous phylogenies [27, 58].
All topologies (but two, Topology 7 and 11) are congruent in the early cladogenetic events of Amblyomma. Topology 7 and 11 (see Figures S1g and k) recovered the Am. maculatum complex as a sister clade of Amblyomma boeroi with significative and moderated support. The rest of the trees recovered endemic Australian species as a sister clade of Am. boeroi (southern cone of South America) + the rest of Amblyomma lineages. No compatible results have been reached so far in the phylogenetic analyses (Fig. 1, node 2). Although Seabolt, [23], Beati and Klompen. [24], and Santodomingo et al., [26] recovered three endemic species from the southern cone of South America (Am. boeroi, Am. parvitarsum, and Am. neumanni) and Australian endemic species as the first divergences of Amblyomma, these relationships differ between them. Beati and Klompen, [24] recovered a sister-relationship between Am. boeroi and Am. parvitarsum, while Santodomigo et al., [26] and Seabolt, [23] recovered Am. neumanni + Am. parvitarsum as a sister lineage of Am. boeroi and endemic Australian clade in an early phylogenetic position, but without statistical support. In our analyses, Am. neumanni and Am. parvitarsum are recovered as sister species but in a derivate position with strong support (Fig. 1, node 4). The close phylogenetic relationship between Am. parvitarsum and Am. neumanni was previously reported [59]. Amblyomma boeroi was recovered as an independent phylogenetic lineage within the genus Amblyomma, but it shares some morphological characters with both Am. parvitarsum and Am. neumanni [19]. On the other hand, some endemic species of Australia were recovered as the first divergence, sister clade of all other species of Amblyomma in all phylogenetic trees (Fig. 1). Our results are compatible with previous phylogenetic analyses (although they differ in some included endemic species of Australian) recovering Am. fimbriatum and Am. postoculatum as the sister clade that includes Am. moreliae, Am. albolimbatum, and Am. limbatum [24]. The species clustered in this Australian clade are mostly associated with reptile hosts, except Am. postoculatum, which parasitize the wallaby banded hare [60].
In the node 3 (Fig. 1) a tripartite polytomy exist between: i. Am. maculatum group that does not appear to be closely related to any other species included in our analyses (except in Topology 8, see above). ii. (Am. parvum + Am. auricularium) + (Am. cajennense group + Am. americanum) relationships are well supported. These relationships have been recovered in previous studies (clades G, H, I in [26]); the internal relationships within Am. cajennense complex as well as its sister relationships with Am. americanum are congruent with previous phylogenetic analyses [20, 26, 28]; however, the related species to Am. americanum should be included in a mitogenomic framework to be discussed in futures studies. iii. and, (Am. neumanni + Am. parvitarsum) + the remaining species, including Neotropical, Neotropical-Nearctic (as Am. dissimile), and IAA species.
In the clade that includes the remaining species of Amblyomma, successive relationships with strong support were recovered between (nodes 5, 6, 7, Fig. 1), i. ((Am. aureolatum, Am. ovale), Am. naponense) (Fig. 1). The close relationship of Am. aureolatum and Am. ovale was obtained in previous works [17, 25], as well as these with Am. naponense (clade E, [26, 61]). Amblyomma aureolatum and Am. ovale normally have Carnivora as hosts for adult stages (despite both species have a more widespread range of hosts) and are morphologically similar, which have caused several confusions between them, being Am. ovale more widely distributed (Nearctic and Neotropic) than Am. aureolatum (Neotropic) [8].
ii. ((Am. calcaratum, Am. nodosum), Am. dubitatum) (Fig. 1). These relationships were previously recovered [26, 62–63], however, in our analyses the relationship of Am. calcaratum and Am. nodosum is not recovered in four out of 12 trees. It happens likely because of insufficient taxon sampling of the species related to this clade, as Am. yucumense (normally associated to Am. dubitatum), Am. hadanii, and Am. coelebs [26, 63], which are missing in our analyses. Note that adults of Am. nodosum and Am. calcaratum have been often confused because they are found normally in Pilosa hosts, regardless both have other hosts throughout their life cycles [8].
iii. ((Am. calcaratum, Am. nodosum), Am. dubitatum) was recovered as sister linage of ((Am. argentinae + Am. dissimile) plus the IAA species) with confident support in all topologies (node 6 and 7, Fig. 1). But, (Am. argentinae + Am. dissimile) as a sister clade of the IAA species was recovered just in the analyses with nucleotide codification (six of out 12, Fig. 1). While in amino acids codification, (Am. argentinae + Am. dissimile) appears in a wide polytomy with IAA species (see inset, Fig. 1). Am. argentinae and Am. dissimile were recovered closely related to IAA species before but without statistical support [26]. The lack of taxonomic sampling may be the cause of the loss of the phylogenetic signal in this phylogenetic assembly.
In relation of the IAA species, three well supported clades are recovered in our analyses, i. (Am. javanense + Am. testudinarium), as was also recovered in Uribe et al., [25]. ii. ((Am. breviscutatum + Am. geoemydae) Am. nitidum) clade recovered for the first time with strong support in all topologies. Am. geoemydae and Am. nitidum have a Indomalayan distribution associate mostly to reptiles [64–65], while Am. breviscutatum is endemic of Australia, mostly found in the southwestern Pacific on feral pigs and rats (as nymphs) [60, 66]. iii. ((((Am. nutallli + Am. sparsum) Am. marmoreum) Am. hebraeum) Am. tholloni) clade that is well supported in seven out of 12 topologies (Fig. 1). This clade has also been reported in mitogenomic analyses [27, 67], it has an Afrotropical distribution and includes ticks with public health importance, as are the species of the marmoreum complex [67]. Finally, the increasing taxon sampling of the “typical” Aponomma forms (Am. gervaisi, Am. latum, and Am. fimbriatum) corroborates the inclusion of these species in Amblyomma (see [13]) with strong statistical support, despite that their phylogenetic positions remained unclear so far (Fig. 1).
The current work, in agreement with previous analysis [16–18, 31], shows that most of the Amblyomma subgenera sensu Camicas et al, [31] and Santos Dias, [29] are not monophyletic, as in the cases of Cernyomma, Anastosiella, Xiphiastor, Adenopleura, Aponomma, and Dermiomma. Amblyomma (as in [26, 28]) and Walkeriana were the only monophyletic subgenera.
Finally, the nuclear tree (Figure S2) recovered an Amblyomma monophyletic but a polytomy in its early relationships, in which are involved: Am. boeroi, Am. postoculatum, Am. neumanni + Am. parvitarsum, and a clade with the remaining Amblyomma species.
Origin of diversification and biogeographical history
Within Ixodidae, the recent incorporation to molecular phylogenetic trees of Af. transversale and some species associated to previous “Aponomma” have been demonstrated that the diversity of Amblyomma is retaining plesiomorphic characters that mislead the circumscription of its living fauna [13–15] and potentially those extinct [69]. Thus, with the aim to evaluate an independent hypothesis of the origin and diversification of Amblyomma, not linked to a potential bias related to the assignation of not phylogenetic corroborated lineages, we use geographic dating from well-established phylogenetic frameworks of Am. cajennense complex [20, 28] and Am. parvum [22] as an alternative to the taxonomic and phylogenetic unresolved questions of the genus.
The higher diversity of Amblyomma is displayed in the Neotropic, which constitutes a biogeographic ecoregion extending from the center of Mexico to the southernmost point of South America, including the Caribbean Islands [70]. This region has a geological and paleoclimatic complex history given that mountain ranges with different origins converge with several climatic fluctuations plus associated glaciations that have occurred [71]. These circumstances have given rise to intricate scenarios that could explain the hyper diversity of the Neotropical region [72]. Additionally, this diversity could have been boosted by geological events through time, such as i. continental drift [73–74], ii. the intermittent connection through bridges, such as the Antarctic bridge that connected the southern cone of South America with the Australian Region [75–76] and Berigian bridge with a northern connection to with Palearctic Region [77], iii. and the complex geological dynamics in the northern Neotropical, e.g., formation of the Isthmus of Tehuantepec and Panama [78–80].
Our new reconstructed time-calibrated tree dated the origin of Amblyomma at about 47.8 Mya, with a relatively short credibility interval (95% HPD, 57–39.6 Mya). This is the youngest origin estimated for Amblyomma so far, which controverts older estimations for the origin of the genus at 91 Mya [24], between late Jurassic and Early Cretaceous [81] or 89 − 77 Mya [23] as well as the Burmese amber fossils assigned to the genus (at 100 Mya; [69, 82]). The inconsistencies of our results with previous works could be related to the representativeness of taxa and the analysis exclusive of the mt genome. The absence of fossil calibrations may also result in an underestimation of divergence dates. Further work is needed to evaluate the time divergences of Amblyomma using solid topologies, perhaps including nuclear information and calibrations of endemic species of Islands (e.g., Galapagos, Hispaniola).
The origin of diversification of Amblyomma was estimated to have occurred 36.8 Mya (95% HPD, 43–31,6), an age that match with the end of the Antarctic bridge connection of the southern hemisphere, in the Late Eocene, at about 35 Mya [75–76]. The origin age of the diversification of the genus has been estimated at 74–60 Mya by Seabolt, [23], close ages to the starting connection of the southern hemisphere at 65 Mya [75–76]. Both scenarios do not rule out a Southern Hemisphere fauna connection. Additionally, our time-calibrated tree suggests an origin and posterior diversification of IAA lineages (positioned derivatives in Fig. 1) between 23,3 and 22,5 Mya (95% HPD, 27–19).
With the time divergence framework, we reconstructed the biogeographic patterns of Amblyomma. The ancestral area reconstructions under each evaluated biogeographical model were highly consistent (Fig. 3; Figure S3a-d; Table S5). Nevertheless, models with the founder-event speciation parameter (j parameter, alternative hypothesis) outperformed those without it (null hypothesis) (DEC vs DEC + J, p = 0.0011; DIVALIKE vs DIVALIKE + J, p = 0.007). According LnL and AICc values, the DEC + J model was the best selected model (Table S5) and therefore, we focus on reporting the results under this model. Our best biogeographic scenario supports the origin of the diversification of Amblyomma in the southern hemisphere at the end of the Eocene (Fig. 3), potentially associated with the faunistic flow in the final Antarctic Bridge connection [75–76]. This agrees with the South America and Australia lineages (here well represented) as the early divergent event in Amblyomma (Fig. 1) reached with high statistical support. This does not discard the hypothesis that argues that the origin of Amblyomma is in the Neotropic with a posterior colonization to Australia [24]. However, a solid phylogenomic framework of Metastriate (including main lineages of all genera) is necessary to evaluate the biogeographic patterns and ancestral distribution of the related lineages to Amblyomma, which could explain the Burmese amber fossil with a widespread ancestral distribution of the closest ancestors of Amblyomma [23, 69, 82]. Likewise, this could give sense also to older origin of diversification of other lineages restricted to the early divergences of Metastriata, which have a notorious gondwanic distribution, as is the case of Archaeocroton sphenodonti (New Zealand), Robertsicus elaphansis (Neartic), Bothriocroton (Australia) and Africaniella transversale (Afrotropic) [24].
While it is necessary to increase the representation of Amblyomma species in a phylogenomic framework, the biogeographic hypotheses here proposed evidence independent expansions along the Neocene to Quaternary era, as is the case of Am. dissimile, Am. ovale, Am. auricularium lineages. Also, our scenario supports a colonization event of Am. americanum and its related species (Fig. 3), despite that these are not included here.
There were hints in phylogenetic trees from mt genomes ([25] Fig. 4; [16]), from nuclear rRNA (eg., [16] Fig. 5; [26]; [27] Fig. 8;) and from combinations of mt and nuclear rRNA genes and morphology [83] that the Australasian Amblyomma or the Australasian Amblyomma plus the Amblyomma species from South America, might be the sister-group to the Amblyomma of the rest of the world. In the present paper, however, we found the strongest evidence yet that Australasian Amblyomma may indeed be the sister-group to the Amblyomma of the rest of the world (Figs. 1 & 3). The position of the Argentinian tick Am. boeroi, hints that the most recent common ancestor to the Amblyomma evolved in a region between South America and Australia, which would concur with the out-of-Antarctica hypothesis [24, 84]. And thus, on the one hand, the ancestor of the Australasian Amblyomma then dispersed from Antarctica into that part of Gondwana that became Australasia whereas on the other hand, the ancestor of the South American Amblyomma dispersed from Antarctic into that part of Gondwana that became South America. Our tree hypothesis also reveals that all Amblyomma subgenera included, but Walkeriana and Amblyomma, are not monophyletic. Our findings suggest an origin of Amblyomma and its posterior diversification more recent than the previous hypotheses (47.8 and 36.8 Mya, respectively). Finally, the biogeographic analyses let us see the colonization patterns of some neotropical Amblyomma species to the Nearctic.