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Genetic analysis of different tick genomes reveals that genetic diversity between species arises from ecogeographic factors and that understanding tick-pathogen interactions is essential to combating tick-borne diseases
Ticks are parasitic arachnids that transmit a wide variety of pathogens to humans and animals. Research within this field has increased as the incidence of tick-borne diseases increases worldwide in both humans and animals. In the United States, ticks are the most common agents of vector-borne diseases, therefore understanding their genetic makeup and pathogen interaction is essential for the control of ticks and tick-borne diseases (Merino et al. 2013). The authors of this study aim to explore the genetic diversity, geographic distribution, and ecological adaptation of ticks by analyzing six different tick genomes. Their results demonstrate that genetic diversity and pathogen composition in different tick species is mainly shaped by ecological and geographic factors, which further emphasizes the need for continued research within this field to develop appropriate control strategies and mitigate the spread of disease (Jia et al. 2020).
The life cycle of a tick is a bit more complicated than one might expect (Figure 1), so knowing this is essential to understanding why tick-borne diseases are so versatile and are able to be transmitted at various stages of a tick’s life. Most ticks follow a three-host cycle, which means that they attach to three different host animals, however certain species such as R. microplus follow a one-host life cycle. Ticks progress through three stages, larva, nymph, and adult with each stage requiring a blood-meal to develop to the next stage. This lifestyle ensures that a tick may become infectious at any stage and that infection lasts through each developmental stage, which allows for the opportunity to transmit the infection back to another host (Mansfield et al. 2017). The distribution of ticks and their specie-pathogen interactions are largely dependent on ecological and geographic factors, which serve to give a framework of where a given species may exist. A given tick species may be distributed in the same ecological fauna while also being geographically separated, this gives rise to local selective pressures playing an important role on genetic diversity and tick-pathogen interactions. These species-specific characteristics can be seen, for instance, in H. asiaticum (the other five species studied also demonstrate similar characteristic) which prefers to live in desert environments but are geographically separated between two regions of china—Xinjiang and Xizang.
Genetic composition facilities diversity as well via protein family expansion and loss of genes. Hematophagous ticks have an expanded family of proteins relevant to the blood-sucking process and iron-metabolism related genes. Ticks also have loss several genes involved in the immune system’s ability to recognize and respond to certain bacteria, indicating a unique strategy of immunological defense against microbes in ticks (Palmer and Jiggins 2015). Diversity can arise from many different aspects within or between species and to investigate these different avenues, the authors comparatively analyzed six tick species dominant throughout China: Ixodes persulcatus, Haemaphysalis longicornis, Dermacentor silvarum, Hyalomma asiaticum, Rhipicephalus sanguineus, and Rhipicephalus microplus. They did this by collecting samples of 678 tick specimen’s representative of the six species abovementioned across China and sequencing them to determine and compare chromosome size, abundance of repetitive elements, GC content, and gene content. To further elucidate genetic diversity on the population level, researchers created phylogenetic trees of the tick populations and overlapped it with the ecological fauna where the population was found. Lastly, pathogen distribution was explored by mapping the tick genome and analyzing the microbial composition which led to pathogen identification. There is now a framework for which species contained certain pathogen(s) and the location where the tick-pathogen sample was collected, together this data sheds immense insight on genetic diversity and tick-pathogen interactions.
Their results found that genetic diversity was evident on the genetic and population level. Chromosome size and gene content varied greatly across the six genomes. Different species consisted of various numbers of genes dedicated to specialized function, such as iron metabolism and hemoglobin digestion, indicating genetic diversity between species. In terms of population, this was due to a common dispersal strategy that the different tick species had evolved. This strategy was a distribution pattern in which populations located themselves based on ecological setting. Local adaptation to different ecological niches coupled with geographic distance and the availability of different animal hosts leads to differences in subpopulations of the same species. the relative abundance of certain pathogens was quite different across the six tick species and that driving factors are geographical fauna and ecological features of the region, indicating that the different forms of animal life play a key role in tick-pathogen interaction and distribution.
Although this analysis of tick genomes has provided invaluable information regarding genetic diversity and the complexity of tick’s interactions with host and pathogens, there is still a need for more genomic data on ticks to further expand our growing knowledge. Given the information known about tick’s dispersal pattern, future research should include data of climate change as a tool for analyzing potential trends in the dispersion of tick species. The expansion of ticks worldwide further emphasizes the need for continued research on tick genomics and tick-pathogen identification as the risk for spreading tick-borne diseases spread.
Dreylan Hines is an undergraduate student at Davidson College pursing a Bachelor’s degree in Biology. Contact him at drhines@davidson.edu.
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References
Jia, N., Wang, J., Shi, W., & Du, L. (2020). Large-Scale Comparative Analyses of Tick Genomes Elucidate Their Genetic Diversity and Vector Capacities. Cell, 182, 1328-1340.e13. https://doi.org/10.1016/j.cell.2020.07.023
Mansfield, K. L., Jizhou, L., Phipps, L. P., & Johnson, N. (2017). Emerging Tick-Borne Viruses in the Twenty-First Century. Frontiers in cellular and infection microbiology, 7, 298. https://doi.org/10.3389/fcimb.2017.00298
Merino, O., Alberdi, P., Pérez de la Lastra, J. M., & de la Fuente, J. (2013). Tick vaccines and the control of tick-borne pathogens. Frontiers in cellular and infection microbiology, 3, 30. https://doi.org/10.3389/fcimb.2013.00030
Palmer, W., Jiggins, F. (2015). Comparative Genomics Reveals the Origins and Diversity of Arthropod Immune Systems, Molecular Biology and Evolution, 32, 8, 2111-2129. https://doi.org/10.1093/molbev/msv093
