Ticks are blood-feeding external parasites. Truly disgusting creatures and I still say this even after I had studied them for four years. How did they become such repulsive organisms? To glimpse an answer, let’s take a walk down the earth’s memory lane and explore the ancestral fossils from which they came.

Fossils are used to establish evolutionary timescales. Ticks are soft-bodied and thus only a few fossils have ever been unearthed, most of which are held in private collection. However, many ticks have been preserved in amber (fossilized resin of conifers and other trees), but due to amber inclusion these are sometimes hard to examine and even harder to identify. Additionally, any genetic testing of these would mean irreversible damage to or the destruction of the precious sample, especially when the majority of samples constitute larval stages. Thus, most of the knowledge about prehistoric ticks comes from a tick preserved in the ear of a fossilized woolly-rhino no less (dating 2-5 million years ago)!

Ticks, mites and spiders are closely related under the Arachnida class. Non-parasitic mites (Acariformes) evolved prior to parasitic mites and ticks (Parasitiformes), where these two superorders are separated by Ricinulei (hooded tick-spiders) on an evolutionary time-scale (Figure 3, constructed from REF 1-7)*. Therefore, ticks and spiders have the same common ancestor (given a few million years), and ticks can be said to be spiders which have evolved to catch cattle. Under Parasitiformes ticks are categorised in the Ixodida order, which contains three tick families, Ixodidae (the hard ticks with large visible feeding apparatus known as hypostomes), Agrasidae (the soft ticks in which hypostomes are hidden beneath the body) and Nuttalleilidae (with a single species representative). In layman terms this means that hard ticks have large mouthparts and soft ticks have small mouthparts.

Several hypotheses explain the evolution of ticks, which were derived from the fossil record with regards to current tick-host ranges. Generally these suggest that tick evolution is based on host-associations due to co-speciation, and that ticks originated in the late Paleozoic-early Mesozoic, where they fed on slow moving, smooth-skinned reptiles approximately 250 Mya (REF 3-4). New analyses based on molecular and morphological data indicate that the main influence on tick evolution is biogeography, ecological specificity and host size (REF 3). This led to the new hypothesis that Ixodidae ticks have evolved from Argasidae-like ancestors, 120 Mya during the late Cretaceous, after the breakup of Gondwanaland when Australia became isolated (REF 3, 5-6). Due to the multiple host ranges of the ancestral ticks of recorded fossils, the original host animal is difficult to determine. The original host could have been reptiles, amphibians, mammals or birds, all of which were present during the Cretaceous period. However, the discovery of the Namaqua Tick had lead to an alternate hypothesis.

Nuttalliella namaqua (or the Namaqua tick for ease of use) is the only species in the Nuttalleilidae family and is endemic to South Africa. It is regarded as the ancestral tick lineage from which Ixodidae and Agrasidae ticks evolved about 270-260 Mya (Figure 3 and 4), since it has features of both the hard and soft ticks (REF 7). It was also postulated that N. namaqua  may have fed on therapsids during the this time (the Permian period). Therapsids evolved late during the reign of the dinosaurs and represent an evolutionary stage between mammals and reptiles. They move and appear this way as well and given their sparse body hair many of them look quite comical! After the extinction of the dinosaurs, therapsids dominated and later gave rise to mammals. Additional taxonomic and developmental studies on the Namaqua tick has lead to the speculation that it is a multiple host tick, which fed on lizards and mammals (likely from historically feeding on mammal-like reptiles when these were around). Given the fact that it is regarded as the evolutionary missing-link between the main tick families (REF 8), the ancestral tick could have been a generalist, which may have fed on multiple hosts.

The blue cattle tick, Rhipicephalus microplus, was the main focus of my master’s studies and is contained within the Rhipicephalus genus, under the Rhipicephalinae subfamily of Ixodidae. It is estimated that the Rhipicephalus lineage evolved and radiated (new species occurred) in Africa about 14 Mya (REF 9). Although the Rhipicephalus lineage has been well established for 14 million years, R. microplus as a species is only recently arrived in South Africa and has been genetically influencing the evolution of our native cattle tick species even since! I will expand on this in my next post…

*An example of tick cladistics and phylogeny as it is a understudied area – much still needs to be resolved.

References:

1. T. Oberholster (2014) Characterisation of the genetic diversity of the southern cattle tick, Rhipicepahlus microplus populations. 109 pages. Masters Dissertation, University of Pretoria.
2. Dunlop, J. A., 2010 Geological history and phylogeny of Chelicerata. Arthropod Struct & Dev 39: 124-142.
3. De la Fuente, J., 2003 The fossil record and the origin of ticks (Acari:Parasitiformes: Ixodida). Exp Appl Acarol 29: 331-344.
4. Hoogstaal, H., 1985 Argasid and nuttalliellid ticks as parasites and vectors. Adv Parasitol 24: 135-238.
5. Black, W. C., and J. Piesman, 1994 Phylogeny of hard- and soft-tick taxa (Acari: Ixodida) based on mitochondrial 16S rDNA sequences. Proc Natl Acad Sci USA 91: 10034-10038.
6. Klompen, J. S. H., W. C. B. IV, J. E. Keirans and D. E. Norris, 2000 Systematics and Biogeography of Hard Ticks,a Total Evidence Approach. Cladistics 16: 79-102.
7. Mans, B. J. , D. de Klerk, R. Pienaar, A. A. Latif 2011 Nuttalliella namaqua: A Living Fossil and Closest Relative to the Ancestral Tick Lineage: Implications for the Evolution of Blood-Feeding in Ticks. PLoS ONE 6(8): e23675. 
8. Latif, A. A., J. F. Putterill, D. G. d. Klerk, R. Pienaar and B. J. Mans, 2012 Nuttalliella namaqua (Ixodoidea: Nuttalliellidae): First Description of the Male, Immature Stages and Re-Description of the Female. Plos One 7: e41651.
9. Barker, S. C., and A. Murrell, 2004 Systematics and evolution of ticks with a list of valid genus and species names. Parasitology 129: S15-S36.

Figures:

1. Pleistocene landscape fauna: Sedwick C (2008) What Killed the Woolly Mammoth? PLoS Biol 6(4): e99.
2. Tick in amber: George Poinar, Jr., courtesy of Oregon State University.
3. Tick fossil record: several Wikipedia images
   • Sea spider by Bernard Picton
   • Horseshoe Crab by Didier Descouens
   • Velvet mite by André Karwath
   • Hooded-tick spider by M. Hedin
   • Namaqua tick by Mans et. al. 2011
   • Spinose ear tick by Mat Pound-USDA Agricultural Research Service Country-United States 
   • Hedgehog tick by André Karwath
4. Namaqua tick by Mans et. al. 2011
5. Therapsid by [email protected]