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Tick Genome Provides Resource for Understanding Bloodsucker's Biology, Disease Transmission

NEW YORK (GenomeWeb) – An international team reported online today in Nature Communications that is has produced and started analyzing a genome assembly for the deer tick, also known as the blacklegged tick, Ixodes scapularis, a species known for transmitting Lyme disease-causing bacteria and other infectious culprits.

Using DNA from the I. scapularis Wikel strain, researchers from Purdue University, the Broad Institute, the J. Craig Venter Institute, the National Institutes of Allergy and Infectious Diseases, and elsewhere put together a tick genome assembly dubbed IscaW1. To that, they added polymorphism patterns for I. scapularis populations in various parts of the US, along with proteomic and metagenomic information for deer ticks interacting with human granulocytic anaplasmosis-causing bacteria.

"Genomic resources for the tick were desperately needed," corresponding author Catherine Hill, a medical entomology researcher at Purdue University, said in a statement. "These enable us to look at tick biology in a systems way."

Their analysis of the tick genome — the largest arthropod genome analyzed so far — has already revealed a raft of repetitive DNA sequence, for example, as well as a large collection of tick-specific genes and genes coding for detoxification enzymes that may be relevant to tick control efforts.

"We've got our eye on this because these [detoxification] enzymes are also involved in detoxifying insecticides," Hill noted. "As we develop new chemicals to control ticks, we'll be going up against this massive arsenal of detoxification enzymes, far more than insects have."

Deer ticks are infamous for transmitting Lyme disease — a condition that's diagnosed tens of thousands of times each year in the US, but believed to affect far more individuals who do not report their symptoms or are erroneously diagnosed with another condition.

The blood-sucking pests can also pass on other pathogens and parasites as they ingest host blood and regurgitate saliva into host skin wounds. For example, deer ticks are believed to vectors for bacteria that cause human granulocytic anaplasmosis or tick-borne relapsing fever; the parasite behind babesiosis; and the Powassan virus, which can produce fevers, flu-like symptoms, and, in some cases, encephalitis.

Members of the team used Sanger sequence data to put together a 1.8-billion-base IscaW1 genome assembly for the deer tick, which is believed to have a 2.1-billion-base total genome size. The genome was covered to an average depth of nearly fourfold, while fluorescence in situ hybridization data made it possible for the group to produce a physical genome map.

Roughly 70 percent of the deer tick genome was made up of repetitive DNA, the researchers reported, with many tandem repeat sequences turning up in and around chromosome centromeres.

The team's analyses also uncovered more than 4,400 non-coding RNA genes and 20,486 predicted protein-coding genes within the annotated deer tick sequences, which spanned some 57 percent of the genome.

While the researchers identified orthologs for some 60 percent of tick genes in other arthropods, the genome contained a significant proportion of tick-specific genes and paralogous sequences produced through gene duplication.

For example, the team identified neuropeptide-related genes suspected of helping ticks balloon in size as they gorge on blood — a process that requires rapid production of new cuticle material, among other things — and to digest blood meals that would be too iron-rich for most organisms to handle.

Along with genes coding for anticoagulants and compounds with anti-inflammatory activity, the researchers noted that the tick genome was rife with sequences encoding detoxification-related enzymes and components of pathways that prevent ticks from falling prey to pathogens they carry.

Finally, the researchers used restriction site-associated DNA sequencing on samples from 74 female ticks from eight I. scapularis populations in Maine, Massachusetts, New Hampshire, Virginia, Indiana, Wisconsin, North Carolina, and Florida to characterize deer tick patterns across the US.

Their results argued against the notion that distinct deer tick species exist in southern and northern parts of the country. Even so, it suggested a north-south population structure, which may eventually offer clues to understanding why Lyme disease cases are more common in the Northeast and upper Midwest. 

A collection of related companion papers expected out shortly will reportedly tackle more detailed studies on everything from potential duplicated gene functions in the tick genome to mechanisms of infection for specific pathogens carried by ticks.

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