Trends in Parasitology
Volume 34, Issue 12, December 2018, Pages 1017-1026
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Opinion
Species Concepts: What about Ticks?

https://doi.org/10.1016/j.pt.2018.09.009Get rights and content

Highlights

The issue of the species concept in tick taxonomy is addressed.

The typological species concept is the most widely used in tick taxonomy.

The phylogenetic species concept is being increasingly used, but caution is needed to avoid excessive splitting and taxonomic inflation.

Information from genetics, morphology, reproduction, ecology, and/or geography may be valuable while assessing conspecificity.

Studies on ticks should report voucher sequences to enable assignment of the gained data to a given species.

Since ancient times, philosophers and taxonomists have tried to classify forms of life. This is what taxonomy is about: the science of identifying, naming, classifying, and describing organisms. In this article I address the issue of the species concept in tick taxonomy. While the typological species concept is still the most widely used, the biological and phylogenetic species concepts are growing in popularity among tick taxonomists. The integrative approach is increasingly being used, but the question is how to define a tick species when using this approach, particularly if data are incongruent. The adoption of an integrative species concept is discussed, in light of recent advances in our understanding of the genetics, morphology, and biology of ticks.

Section snippets

The Species Problem

For a long time, philosophers and taxonomists have tried to classify different forms of life. This is what taxonomy is about: the science (and practice) of identifying, naming, classifying, and describing organisms. For instance, as early as the 4th century BC, the Greek philosopher Aristotle, in his book series Ton peri ta zoia historion (translated into English as ‘History of Animals’), grouped animals according to their morphology: for example, birds have feathers, wings, beaks, and two bony

The ‘Tic-Tac’: Tick Evolution

Ticks are arthropods belonging to the order Ixodida, which is traditionally subdivided into three families: Argasidae, Ixodidae, and Nuttalliellidae [4]. Recently, a new, extinct family, namely Deinocrotonidae, was proposed based on adult ticks found in a Burmese amber from the Cretaceous (circa 99 million years ago) [5].

Ticks diverged from other Acari millions of years ago, probably as parasites of the ancestors of modern reptiles, amphibians, or both. An early hypothesis placed the origin of

Tick Taxonomy: From Linnaeus to Present Days

Ticks have been known since ancient times [12]. Linnaeus’ first endeavour to describe and classify ticks appeared in 1746, in his Fauna Svecicaiii. Nonetheless, the International Commission on Zoological Nomenclature arbitrarily fixed the date January 1, 1758 as the starting point of zoological nomenclature. Consequently, tick taxonomy is considered to be born in 1758, with the 10th edition of Systema Naturae, when Linnaeus described different tick species, including the lone star tick

Species Concepts in Ticks

The oldest and the newest valid tick species descriptions were based on morphology. In fact, morphology was, and still is, the most widely used criterion for species delineation, nowadays with much more refined descriptions [16] – with some exceptions [17]. High-quality light microscopy and, eventually, scanning electron microscopy have been used in recent species descriptions, providing additional morphological details previously overlooked 18, 19, 20. Furthermore, new methodological

Okay, Let Us Talk Biology and Genetics

Biological and genetic studies are becoming more widely used in modern tick taxonomy, shedding new light on old taxonomic issues, but also uncovering novel taxonomic problems to be solved. In fact, results from biological and genetic analyses may not always be congruent.

According to the BSC, species are groups of natural populations that actually or potentially interbreed and that are reproductively isolated from other such groups. Reproductive isolation may occur due to prezygotic barriers

Concluding Remarks

While several criteria have been used to delimit and delineate tick species, the typological approach is still the most widely used in tick taxonomy. When assessing whether a tick (or group of ticks) represents a new species on morphological grounds, one should keep in mind that intraspecific variability does exist and that threshold between intraspecific and interspecific variabilities is still poorly understood, especially when dealing with species complexes 41, 43. Thus, cautiousness is

Note added in Proof

One of the outstanding questions raised in this article is: ‘Can we reach a consensus about the genus-level classification of the family Argasidae?’ (see Outstanding Questions). After this article was written, a new study dealing with this question was published [68]. The authors sequenced the mitochondrial genomes of several soft tick species and proposed a revised genus-level classification for the family Argasidae. The authors also presented an updated list of soft tick species, which

Acknowledgments

Thanks to Professor Chris Arme and Professor Andrey José de Andrade for their critical review of an early draft of this article. Thanks also to my research group (Vector-Borne Diseases: A One Health Approach) for the scientific discussions during the preparation of this article and for their suggestions on Figure 1.

Disclaimer Statement

The content of this article is the sole responsibility of the author.

Glossary

Basal lineage
a genetic lineage that connects a variant allele (type) possessed by a more common ancestor that evolves into two descendant variants possessed by a branch ancestor.
Crossbreeding
the mating of organisms of different breeds, varieties, or species.
Cryptic species
two or more distinct species that are morphologically similar or identical.
Diagnosability
the quality of being diagnosable.
Evolutionary lineage
a temporal series of organisms, populations, cells, or genes connected by a

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      We note further, for perspective, that the surface area of South Africa is approximately 13% that of the U.S., and yet nationwide South African tick diversity is surprisingly equal to the number of tick species living in the U.S. (i.e., 96 species, JWM unpublished records). Disregarding extralimital tick species documented only once in the Fynbos region on non-native hosts, introduced/established exotic species, and also other species now locally extirpated and absent (Horak et al., 2018), the present resident diversity of ticks in the Fynbos comprises 30 ixodid species and 4 argasids (3.5% of the world diversity) (Walker, 1991; Horak et al., 2018; Dantas-Torres, 2018; Mans et al., 2019, 2021). Of the overall South African tick diversity, no soft ticks, but seven species (6.4%) of the hard ticks are endemic (or nearly so) to the Fynbos, and R. capensis is one of the two endemic Rhipicephalus spp. ticks.

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