Introduction
One of the most important properties characterizing a parasite taxon is its host specificity. It is indicative of intrinsic biological characteristics of both host and parasite and an emergent property of their ecological and evolutionary relationship (Dick & Patterson, Reference Dick and Patterson2007). Host specificity can be defined as the extent to which a parasite taxon is restricted in the number of host species used at a given stage in the life cycle (Poulin, Reference Poulin2007). Thus, highly host-specific parasites have one host species, and specificity declines as the number of suitable host species increases (Poulin, Reference Poulin2007). The parasite-host association has been focused from two perspectives, the host centred view (the focus of coevolution is the host species) and the parasite-centred view (the resources for parasites are attributes of the host species, not the host species themselves) (Brooks & McLennan, Reference Brooks and McLennan1993; Fenton & Pedersen, Reference Fenton and Pedersen2005; Brooks & Hoberg, Reference Brooks and Hoberg2007). The biological significance of these two visions lies in the frequency of host-switch events, which determines the host range and potential distribution of a parasite species. Host switching results in low host specificity because a parasite colonizes host lineages related or unrelated to its original host (Poulin, Reference Poulin2007). A parasite interacts with its host (physiology, immunity, ecology, phylogeny, size) but also with the environment. The external environment in which parasite and host interact can affect the strength and the evolution of the host-parasite interaction (Krasnov et al., Reference Krasnov, Korallo-Vinarskaya, Vinarski, Shenbrot, Mouillot and Poulin2008, Reference Krasnov, Mouillot, Shenbrot, Khokhlova, Vinarski, Korallo-Vinarskaya and Poulin2010; Wolinska & King, Reference Wolinska and King2009). Therefore, the degree of host specificity of a parasite and its variation in time and space not only are functions of intrinsic properties of host and parasite but also depends on external environmental conditions.
Hard ticks (Acari: Ixodidae) are haematophagous ectoparasites of amphibians, reptiles, birds and mammals, and they are divided into Prostriata (genus Ixodes) and Metastriata (genera Amblyomma, Anomalohimalaya, Bothriocroton, Compluriscutula†, Cornupalpatum†, Cosmiomma, Dermacentor, Haemaphysalis, Hyalomma, Margaropus, Nosomma, Rhipicentor, Rhipicephalus) (genera are listed according to Guglielmone et al., Reference Guglielmone, Robbins, Apanaskevich, Petney, Estrada-Peña, Horak, Shao and Barker2010). The life cycle of hard ticks encompasses parasitic and non-parasitic phases. In the former, larvae, nymphs and adults feed or mate on the host (members of Prostriata and a few species of Amblyomma and Bothriocroton have adults with ability to mate off host). However, hard ticks spend most of their life cycle off host, exposed to environmental conditions. In three-host cycle species, the non-parasitic phase includes important biological processes, such as moulting of engorged larvae and nymphs, oviposition of engorged females, incubation of eggs and host-seeking of all parasitic stages. Some ticks have a two-host or a one-host life cycle (i.e. species of the genera Rhipicephalus, Hyalomma, Dermacentor and Margaropus), where larvae or both nymphs and larvae moult on the host, respectively. Overall, it is evident that the life cycle of hard ticks is strongly influenced by both host and environmental factors.
Two contrasting hypotheses have been developed about host specificity in ticks. The first assumes that host specialisation was instrumental in the evolution of ticks and of their morphological characters (Hoogstraal, Reference Hoogstraal1978; Hoogstraal & Aeschlimann, Reference Hoogstraal and Aeschlimann1982; Hoogstraal & Kim, Reference Hoogstraal, Kim and Kim1985) and is based on the idea of coevolution between ticks and terrestrial tetrapods. These authors state that phenotypic variations in mouthparts and coxae are the result of adaptation to a particular group of hosts, which lead to a high host specificity. Furthermore, Hoogstraal & Aeschlimann (Reference Hoogstraal and Aeschlimann1982) presented a classification system with six categories characterized by decreasing levels of host-specificity, namely: (i) strict-total, (ii) moderate-total, (iii) strict-stage-stage, (iv) strict/moderate-stage-stage, (v) moderate-stage-stage and (vi) nonparticular. Klompen et al. (Reference Klompen, Black, Keirans and Oliver1996) raised an alternative hypothesis that stresses the importance of ecological specificity. According to this opinion, adaptation to a particular habitat is more relevant for tick evolution than adaptation to a particular host. Klompen et al. (Reference Klompen, Black, Keirans and Oliver1996) found a strong positive correlation between the total number of collections and the number of hosts, through an analysis using data from ixodid (Ixodes) and argasid (Carios) collections, and they concluded that “much of the distinction between strict-total and less-specific categories might be the difference between rarely and frequently collected species”. Additionally, Klompen et al. (Reference Klompen, Black, Keirans and Oliver1996) criticized the lack of differentiation between the processes of adaptation and speciation in the Hoogstraal′s studies. Klompen et al. (Reference Klompen, Black, Keirans and Oliver1996) stated that “although the presence of host adaptations may lead to host specificity, observed host specificity is not necessarily an indicator of host adaptation. Host specificity may also arise because of no opportunity to transfer to alternative hosts (which secondarily may result in cospeciation) or as a secondary effect of adaptation to off-host habitat”. In the same way, Balashov (Reference Balashov2004) rejected the idea of coevolution as a key factor in the evolution of ticks affirming that “phylogenetic parallelism between ticks and their hosts is absent or limited to short evolutionary periods”. After the analysis of a large data set of African ticks, Cumming (Reference Cumming1998) reached a conclusion that can be interpreted as intermediate between the two aforementioned hypotheses. This author suggested that both host specificity and ecological specificity could be important and that it depends on each particular tick species. However, in a subsequent analysis on limiting factors for species ranges of African ticks, Cumming (Reference Cumming2002) concluded that the distribution of ticks over a wide spatial scale is mainly determined by direct climatic effects, while hosts only generate heterogeneity in tick distribution at a smaller geographical scale.
In a broad sense, the host specificity of a parasite is quantified as the number of hosts species, or host range. The number of host species in itself is not informative because it implies that a parasite uses each host species equally. However, the utilization of a spectrum of host species by a given parasite is typically uneven. In fact, from an ecological perspective, some host species are used more intensively than others; and, from a phylogenetic perspective, some host species used by a parasite can be phylogenetically more closely related than others (Poulin et al., Reference Poulin, Krasnov, Morand, Morand, Krasnov and Poulin2006; Poulin, Reference Poulin2007). Therefore, an attempt at evaluating the importance of host specificity must take into account all these aspects. In this work, a meta-analysis of host specificity in Neotropical hard ticks was performed. The number of hosts for each tick species and the index of host specificity (index STD*) proposed by Poulin & Mouillot (Reference Poulin and Mouillot2005), which integrates phylogenetic and ecological information, were applied to the analysis. In addition, the two values were generated separately for adult and immature stages in order to investigate possible ontogenetic changes in host specificity, and the role of domestic animals in determining host specificity was also analyzed.
Materials and methods
The data set used in this study was obtained from a list of host records for all species of Neotropical ticks compiled from the literature (scientific papers, book sections and selected meeting proceedings) by one of us (AAG). The list is available under request. A record is defined as the finding of a tick species, on a determined host, and at a given locality, regardless of the number of host sampled and of ticks collected on a particular host. Findings on the same species of host and at the same locality, but on different dates, are each considered separate records. Only references that include tick stage, locality and scientific name of the host were considered, and tick species with less than 15 records were not included in the analysis. Records of immature stages made before the description of larva and nymph of the corresponding tick species became available were excluded. Exceptions were made for those references where the taxonomic determination method was explained (e.g. immature ticks identified to species after rearing the ticks to the adult stage in the laboratory or by using molecular tools). Records from humans, from hosts kept in captivity (laboratories, zoos, etc.) and from imported animals were not considered. When necessary, scientific names of mammal hosts have been updated following Wilson & Reeder (Reference Wilson and Reeder2005), Weksler et al. (Reference Weksler, Percequillo and Voss2006) (for oryzomyne rodents), Voss & Jansa (Reference Voss and Jansa2009) (for marsupials), Barquez et al. Reference Barquez, Díaz and Ojeda2006 (for Felidae and Tayassuidae) and Francés & D'Elía (Reference Francés and D'Elia2006) and D'Elía et al. (Reference D'Elía, Pardiñas, Jayat and Salazar-Bravo2008) for some synonymized sigmodontine rodents. Scientific names of birds were taken from Clements (Reference Clements2007), and the nomenclature of amphibians and reptiles was the same chosen in Guglielmone & Nava (Reference Guglielmone and Nava2010). Rhipicephalus (Boophilus) microplus and the Rhipicephalus sanguineus species group were not considered in this work because they have a wide world distribution, and both taxa were recently introduced in the Neotropical region (approximately 400 years ago); therefore, an accurate analysis of their host-association should include records of other biogeographic regions. The list of host species recorded for each tick species is shown in the supplementary appendix 1.
The specificity index STD* proposed by Poulin & Mouillot (Reference Poulin and Mouillot2005) was calculated for each hard tick species. This index measures the average taxonomic distinctness of all host species used by a parasite, weighted by the prevalence of the parasite in the different hosts (Poulin & Mouillot, Reference Poulin and Mouillot2005). Because of the lack of data on prevalence in most tick-host associations cited in the literature, we calculated instead, for each tick species, the proportion of records corresponding to each host species. It is obvious that the proportion of records is less informative than prevalence because it increases the risk of biases associated with differential sampling efforts. Another caveat to keep in mind is that presence of a representative of a given tick species on a given host does not mean this host is competent to sustain tick development and reproduction. The index STD* places hosts within the Linnean taxonomic hierarchy (phylum, class, order, family, genera, species). Taxonomic distinctness between two host species represents the mean number of steps up the taxonomic hierarchy that must be taken to reach a taxon common to both (for example: when two host species are congeners, one step species-to-genus is necessary to reach a common node; if two host species belong to different genera but are included in the same family, two steps species-to-genus and genus-to-family are necessary) (Poulin & Mouillot, Reference Poulin and Mouillot2005). This value is weighted by the product of the parasite prevalence (in this work, proportion of records). The value of this index is inversely proportional to specificity. A high index value means that the host species more frequently used by a parasite are not closely related (Poulin & Mouillot, Reference Poulin and Mouillot2005). In absence of differences in the proportion values, the index STD* reaches its maximum value (5) when all host species belong to different classes, whereas it tends towards its minimum value (1) when all hosts belong to a same genus. Index STD* was calculated with a computer program developed by Poulin & Mouillot (Reference Poulin and Mouillot2005) using Borland C++ Builder 6.0 (available online at http://www.otago.ac.nz/zoology/downloads/poulin/TaxoBiodiv2). The classifications used to place each host species within the correct classes, orders and families were Wilson & Reeder (Reference Wilson and Reeder2005) for class Mammalia, Clements (Reference Clements2007) for class Aves and Guglielmone & Nava (Reference Guglielmone and Nava2010) for classes Reptilia and Amphibia.
A Pearson's correlation analysis was performed to assess the extent of co-variation between number of records of each tick species and number of host species and between number of records and STD* values. The statistical significance of the differences in STD* values between the samples which included domestic hosts and samples which excluded them was determined through a Student's t-test. The significance of the differences in number of host species between adult and immature stages was tested with a Mann-Whitney U test, while the significance of the differences in STD* values between adult and immature stages was evaluated with a Student's t-test.
Results
A total of 4172 records of hard ticks (species of the genera Amblyomma, Dermacentor, Haemaphysalis, and Ixodes) collected from wild and domestic tetrapods were obtained. The data included 41 tick species for adult specimens (3007 records) and 22 for immature stages (1165 records) (table 1). Of the 4172 records, 1012 (24.2%) were from domestic mammals (cattle, goat, sheep, pig, horse, donkey, mule, dog, cat) and corresponded to adults of 22 tick species and immature stages of nine tick species. Number of host species, number of records and values of STD* (calculated with and without domestic hosts) for each tick species are shown in table 1. In the cases where domestic hosts were excluded from the analysis, the difference in the values of STD* for each species was not significant (see table 1) when the sample was analyzed as a whole (P = 0.28, Student's t-test).
Table 1. Number of host species and values of the specificity index STD* proposed by Poulin & Mouillot (Reference Poulin and Mouillot2005) (calculated with and without domestic hosts) for the species of Neotropical hard ticks included in this study.
NA, not applicable.
* Recent studies suggested that Amblyomma cajennense is a species complex (Labruna et al.; Reference Labruna, Soares, Martins, Soares and Cabrera2011, Mastropaolo et al., Reference Mastropaolo, Nava, Guglielmone and Mangold2011).
** Because 89% of the records of adults of D. nitens were made on domestic animals, the analysis excluding domestic animals is unjustified.
*** Because 87% of the records of immature stages of D. nitens were made on domestic animals, the analysis excluding domestic animals is unjustified.
The distribution of number of host species was skewed for adults (fig. 1a), for immature ticks (fig. 1b) and also when immature and adults were grouped (fig. 1c). The figures clearly show that most tick species in this study parasitize between three and 20 different host species. No tick species has been associated either with a single species or with a single host genus. The mean number of hosts differed significantly (P = 0.002, Mann-Whitney U test) between adults (mean = 12.56; median = 8; range = 3–49) and immatures (mean = 21.72; median = 15; range = 5–90). The frequency distribution of STD* values are shown in fig. 2a for adults, in fig. 2b for immatures, and in fig. 2c for adults and immatures together. The most frequent values of STD* were between 2.5 and 3.5. The average values of STD* for adults and immatures were 2.78 (median = 2.71; range = 1.13–4.58) and 3.32 (median = 3.31; range = 2.02–4.39), respectively (P = 0.001, Student's t-test). A significant positive correlation between number of host species and number of records was found (r=0.64; P ≤ 0.0001) (fig. 3a). Conversely, there was no correlation of STD* (r=0.19; P = 0.12) with the number of records (fig. 3b). This absence of covariation indicates that STD*, unlike the number of host species, is far less sensitive to sampling effort.
Fig. 1. Frequency distribution of number of host species among species of Neotropical hard ticks: (a) adults, (b) immature and (c) adults and immature grouped together.
Fig. 2. Frequency distribution of STD* values among species of Neotropical hard ticks: (a) adults, (b) immature and (c) adults and immature grouped together.
Fig. 3. Relationship between the number of records of each tick species included in this study and the number of (a) host species and (b) STD* value per tick species (Pearson's correlation analysis).
Discussion
This is the first attempt to describe general patterns of host specificity in Neotropical hard ticks through a quantitative approach. Although sometimes limited by incomplete data or lack of samples, in this study we generated a large database. We found a positive correlation between number of host species and number of records, which is in agreement with the findings reported by other authors for ticks (Klompen et al., Reference Klompen, Black, Keirans and Oliver1996) and for other parasites (Poulin, Reference Poulin1992; Walther et al., Reference Walther, Cotgreave, Price, Gregory and Clayton1995; Guégan & Kennedy, Reference Guégan and Kennedy1996; Walther & Morand, Reference Walther and Morand1998). Therefore, the number of host species used by a particular tick species cannot be used as indicator of host specificity, because it can be an artefact caused by different sampling efforts. The index STD*, however, appears to be largely independent from sampling effort. Most tick species showed values of STD* ranging between 2.5 and 3.5, with an average close to 3 (table 1, fig. 2). These results show that, in general, an elevated proportion of tick species feeds on hosts that belong to different families or different orders. Naturally, some tick species are characterized by higher host specificity (STD* values between 1.13 and 2) (see table 1). However, none of them parasitize a single host species. When the analyses of number of hosts species and STD* in adult and immature specimens were performed separately, both measures were significantly higher for immatures than for adults, which suggests that immature stages tend to use a broader taxonomic range of hosts than adults. Two hypotheses can be stated for these results. One of them is related to a physiological involvement, where larvae and nymphs may have greater adaptive plasticity than adults to feed on different host species. The other hypothesis is based on the number of host species available. If adult stages generally feed on hosts with larger body size (a fairly well-established trend), the number of host species available for those instars should be lower, given that there are relatively more small-bodied than large-bodied host species in most environments.
Approximately 24% of the records were from domestic mammals, but the variation in STD* was statistically insignificant when domestic hosts were excluded from the analysis (see table 1). The capacity of some domestic mammals to totally or partially sustain the life cycle of endemic tick species is well known, as in the cases, for instance, of Amblyomma tigrinum, Amblyomma aureolatum, Amblyomma parvum, Amblyomma neumanni and Amblyomma triste (Guglielmone et al., Reference Guglielmone, Mangold, Luciani and Viñabal2000; Pinter et al., Reference Pinter, Dias, Gennari and Labruna2004; Nava et al., Reference Nava, Mangold and Guglielmone2008, Reference Nava, Estrada-Peña, Mangold and Guglielmone2009, Reference Nava, Mangold, Mastropaolo, Venzal, Fracassi and Guglielmone2011). Although this fact highlights the ability of some tick species for rapid adaptation to recently introduced host species, the impact of non-endemic hosts on the ecology of native ticks does not alter the patterns of host specificity.
The application of index STD* to quantify host specificity in Neotropical hard ticks shows that a high proportion of ticks feed on phylogenetically distant host species. Similarities must be sought in other host characteristics, such as morphology, physiology or habitat usage, which appear to be more relevant. It has been established for other parasite taxa (e.g. helminthes of fishes, fleas of small mammals) that ecological similarity among host species is more important than host phylogeny (Poulin, Reference Poulin2005; Krasnov et al., Reference Krasnov, Korine, Burdelova, Khokhlova and Pinshow2007) and that external environmental factors may play a key role in the evolution of host-parasite associations (Krasnov et al., Reference Krasnov, Mouillot, Shenbrot, Khokhlova, Vinarski, Korallo-Vinarskaya and Poulin2010). These conclusions are in agreement with the concept of ecological fitting, a process where organisms form a novel association with other species or use novel resources by ecological readjustment unrelated to previous evolutionary history, as a result of the suites of traits that they carry at the time they encounter the novel condition (Janzen, Reference Janzen1985; Agosta & Klemens, Reference Agosta and Klemens2008). Ecological fitting is applicable to parasites when these can exploit a specific type of resource that is distributed across different host species (Brooks et al., Reference Brooks, Leon-Regagnon, McLennan and Zelmer2006a,Reference Brooks, McLennan, Leon-Regagnon and Hobergb). In these cases, the main requirement for the parasite is the resource itself, not the way that resource is packaged (the host species) (Brooks et al., Reference Brooks, McLennan, Leon-Regagnon and Hoberg2006b). Support for ecological fitting in systems that involve parasites and hosts emerges from ecological and macroevolutionary evidence (Kethley & Johnston, Reference Kethley and Johnston1975; Hoberg & Brooks, Reference Hoberg and Brooks2008; Kelly et al., Reference Kelly, Paterson, Townsend, Poulin and Tompkins2009; Agosta et al., Reference Agosta, Janz and Brooks2010). Following this concept, the association of a tick species with its hosts may be primarily determined by the environment occupied by an assemblage of suitable hosts, regardless of host phylogenetic relatedness. Host associations of larvae and nymphs of Ixodes loricatus and Ixodes luciae clearly illustrate ecological fitting. Small rodents (Cricetidae: Sigmodontinae) and marsupials (Didelphidae) are the principal hosts for immature stages of these two ticks (Guglielmone & Nava, Reference Guglielmone and Nava2011). Sigmodontine rodents and didelphids occupy the same ecological niche, but they are not phylogenetically related and have different evolutionary histories. The diversification of marsupials of the family Didelphidae in South America probably occurred between the Oligocene and Miocene (Steiner et al., Reference Steiner, Tilak, Douzery and Catzeflis2005), while sigmodontines appeared on the continent only in the Pliocene (Pardiñas et al., Reference Pardiñas, Teta, D'Elia, Polop and Busch2010). This example, which could be applied to other tick species in this study, highlights the key role played by host ecological similarities in shaping host-parasite relationships in ticks. Also, our conclusions reject the hypothesis that coevolution was relevant in determining present tick-host associations, stressing the significance of host switching events during tick evolutionary history.
The widespread use of laboratory animals in experimental studies on life cycles or reproductive barriers is another issue that reinforces the idea of ticks as parasites with low host specificity. Hosts, such as rabbits (Oryctolagus cuniculus), guinea pigs (Cavia porcellus), chickens (Gallus gallus), rats (Rattus norvegicus), mice (Mus musculus), dogs (Canis familiaris), domestic pigs (Sus scrofa), horses (Equus caballus), among others, were successfully utilized to feed a wide spectrum of ticks under laboratory conditions. Some examples of this type of study in Neotropical hard ticks include the life cycle of Amblyomma auricularium, Amblyomma incisum, A. triste, A. tigrinum, A. aureolatum, A. parvum, A. neumanni, Amblyomma brasiliense, Amblyomma pseudoconcolor, Haemaphysalis leporispalustris and I. loricatus (Guglielmone et al., Reference Guglielmone, Mangold and Garcia1991; Aguirre et al., Reference Aguirre, Viñabal and Guglielmone1999; Labruna et al., Reference Labruna, Cerqueira Leite, Faccini and Ferreira2000, Reference Labruna, Souza, Menezes, Horta, Pinter and Gennari2002, Reference Labruna, Fugisaki, Pinter, Duarte and Szabó2003; Schumaker et al., Reference Schumaker, Labruna, Dos Santos and Soares Clerici2000; Chacon et al., Reference Chacon, Faccini and Bittencourt2004; Pinter et al., Reference Pinter, Dias, Gennari and Labruna2004; Sanches et al., Reference Sanches, Bechara, Garcia, Labruna and Szabó2008; Szabó et al., Reference Szabó, Pereira, Castro, Garcia, Sanches and Labruna2009; Faccini et al., Reference Faccini, Cardoso, Onofrio, Labruna and Barros-Battesti2010; Olegário et al., Reference Olegário, Gerardi, Tsuruta and Szabó2011). Cross-mating trials to elucidate reproductive barriers between Amblyomma pseudoparvum and A. parvum, and among the members of the Amblyomma cajennense species complex, were carried out using rabbits as hosts (Guglielmone & Mangold, Reference Guglielmone and Mangold1993; Labruna et al., Reference Labruna, Soares, Martins, Soares and Cabrera2011; Mastropaolo et al., Reference Mastropaolo, Nava, Guglielmone and Mangold2011). These examples show the physiological plasticity exhibited by ticks to feed successfully when they are exposed to a novel host not related to the natural hosts.
The results of this investigation provide evidence that supports the hypothesis of Klompen et al. (Reference Klompen, Black, Keirans and Oliver1996) that ticks are habitat- rather than host-specialists. When one considers that hard ticks spend most of their life cycle off host, this conclusion is not unexpected. However, and regardless of habitat specificity, other variables, such as size, parasite generation time, phenology, time spent off host and the type of life-cycle (three-, two- or one-host), should be considered for future studies, as they are factors which can influence tick host associations.
By using two measures of host specificity, we describe a general pattern based on a restricted data set pertaining to specific region of the world (Neotropical region). We do not pretend that our conclusions will be applicable everywhere else. Local adaptations or the size of the geographical range of a parasite species may cause intraspecific variations in host specificity across its distribution (Kaltz & Shykoff, Reference Kaltz and Shykoff1998; Jackson & Tinsley, Reference Jackson and Tinsley2005; Krasnov et al., Reference Krasnov, Poulin, Shenbrot, Mouillot and Khokhlova2005, Reference Krasnov, Mouillot, Shenbrot, Khokhlova and Poulin2011; Morgan et al., Reference Morgan, Gandon and Buckling2005; Gandon et al., Reference Gandon, Buckling, Decaestecker and Day2008), and these variables should also be included in future analyses. Similarly, it would be interesting to evaluate the levels of fitness achieved by a particular tick species after feeding on different hosts species, through the measurement of biological parameters. Nevertheless, the evidence presented in this work shows that strict host specificity is not common among hard ticks.
Acknowledgements
We are very grateful to Lorenza Beati, Robert Poulin and Marcelo B. Labruna for their valuable comments on an earlier version of the manuscript. We thank the financial support of Instituto Nacional de Teconlogía Agropecuaria, Asociación Cooperadora of EEA-INTA Rafaela and Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina.
Supplementary material
The online appendix can be viewed at http://journals.cambridge.org/ber.
