We are Borg: How Ticks Assimilate Genetic Adaptation

To most people parasites are gruesome creatures, likely the feature of nightmares. Yet, they are fascinating organisms from an evolutionary perspective (read more about Tick Origins and the Fossil Record). They are prime examples of the evolutionary arms-race, where parasite and host try to outwit one another through biological adaptation. Unlike plant ectoparasites, animal parasites are blood feeding and have to overcome far more complex defense systems and immune responses from their animal hosts.

During my honours and masters degrees (REF 1), I worked on cattle ticks. Specifically, the blue cattle ticks, Rhipicephalus decoloratus (a native South African) and R. microplus (an invasive species). Out native cattle, such as the Bonsmara and Nguni (read more about Cattle History in South Africa) are tolerant to R. decoloratus (RD), initiating strong immune responses to these. Thus, cattle and tick populations remain at an equilibrium where cattle do not suffer large infestations nor do they contract the associated tick borne diseases too often. R. microplus (RM) is a different tick all together. Since, our native cattle have not had prolonged exposure to this cattle tick, thus they have ineffective immune detection and responses to them. Additionally, RM carries a more virulent version of Redwater (Babesia), a haemolytic disease, which causes the bursting of red blood cells, much like malaria. The internal blood parasite responsible for this belongs to the Babesia genus, B. bigemina for RD and B. bovis for RM. Native cattle easily suffer large infestations of RM and readily contract B. bovis when farms become overrun by RM.

It is common to find both species of blue tick (along with several other and larger tick species) on one animal. RM may have more sophisticated evolutionary tricks with which to bypass cattle immune systems, but RD has an ecological advantage. RD is more at home with our semi-arid climate, whereas RM hails from tropical regions. This is where RM deploys its nefarious plans of evolutionary domination: it is assimilating RD into its genetic pool!

Sheep tick, Ixodus ricinus, 5x magnification, Richard Bartz
Sheep tick, Ixodus ricinus, 5x magnification, Richard Bartz, Wikipedia

It seems that RM has the ability to hybridize with RD (REF 2), but with a silver bullet added to the mix. RM back-crosses far more efficiently with its hybrids than RD. This, along with RM’s general increased fitness (such as having a larger number of offspring, quicker generational turnover, shorter life-cycles and greater pesticide resistance, REF 3) means that the RD population shrink and eventually become replaced by RM and its hybrids. All the while RM is gathering genetic contributions from RD, which seem to result in RM adapting to South Africa’s semi-arid conditions. This means, RM becomes the proverbial boogey-tick as well as the architect of RD’s destruction.

An extinct tick may not be a great loss to society, but it is the means and the cause of which that should be alarming. Ticks are an understudied field in South Africa and more research is required to develop methods of effective control and eventual eradication efforts. South Africa should follow the examples of other countries, such as the USA and AUS who had setup quarantine zones for RM and monitor these closely (REF 4-8). It may not lead to the swift eradication of RM, but it will limit its spread and the devastation that RM brings. This will bring relief to our cattle farmers whilst buying time for researcher to develop a defense plan. I will discuss more about the Introduction and Spread of Rhipicephalus microplus in South Africa with a future article.

Beware of Ticks XI International Jena Symposium on Tick Borne DIseases 2011 Tanzelle Oberholster andDNAsci.wordpress

References

  1. T. Oberholster (2014) Characterisation of the genetic diversity of the southern cattle tick, Rhipicepahlus microplus populations. pp 11. Masters Dissertation, University of Pretoria.
  2. Nyangiwe, N., A. Harrison and I. G. Horak, 2013a Displacement of Rhipicephalus decoloratus by Rhipicephalus microplus (Acari: Ixodidae) in the Eastern Cape Province, South Africa. Exp Appl Acarol: 10.1007/s10493-10013-19705-10497.
  3. Walker, A. R., A. Bouattour, J. L. Camicas, A. Estrada-Peña, I. G. Horak et al., 2003 Ticks of Domestic Animals in Africa: a Guide to Identification of Species. Bioscience Reports, Edinburgh
  4. Lohmeyer, K. H., J. M. Pound, M. A. May, D. M. Kammlah and R. B. Davey, 2011 Distribution of Rhipicephalus (Boophilus) microplus and Rhipicephalus (Boophilus) annulatus (Acari: Ixodidae) Infestations Detected in the United States Along the Texas/Mexico Border. J Med Entomol 48: 770-774.
  5. Pound, J. M., J. E. George, D. M. Kammlah, K. H. Lohmeyer and R. B. Davey, 2010 Evidence for Role of White-Tailed Deer (Artiodactyla: Cervidae) in Epizootiology of Cattle Ticks and Southern Cattle Ticks (Acari: Ixodidae) in Reinfestations Along the Texas/Mexico Border in South Texas: A Review and Update. J Econ Entomol 103: 211-218.
  6. Texas cattle fever ticks are back with a vengeance, 2 February 2017, Steve Byrns, Texas A&M AgriLife Communications
  7. Cattle Fever Ticks: Frequently Asked Questions, January 2017, Texas Animal Health Commission
  8. Fever Tick Situation Report: January 30, 2017, Texas Animal Health Commission

 

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