Researchers with Baylor College of Medicine and Texas A&M AgriLife Research collaborated with the U.S. Department of Agriculture Agricultural Research Service, SCINet project and Ag100Pest Initiative to assemble the first genome for a soft tick, Ornithodoros turicata. Their findings were published in G3: Genes | Genomes | Genetics.
Populations of this tick are found in Florida, and in regions from Texas, Oklahoma, Kansas and into the southwestern states and Mexico. It is the vector of a pathogen causing human relapsing fever and has agricultural significance as a potential vector of African swine fever virus.
These ticks are cavity dwellers, meaning that they live in animal burrows, caves and root hollows, as well as under pier and beam buildings. They are secretive and blood-feed on animals that visit those cavities/environments. They can live longer than 5 years without a bloodmeal and female ticks can transfer pathogens to their offspring. This enables pathogens to be passed across tick generations, which not only makes the ticks important vectors of disease but also as pathogen reservoirs.
"The genome opens up a whole new avenue of research to study the tick, the life cycle, its biology and tick-pathogen interactions," said Dr. Job Lopez, senior author and associate professor in the National School of Tropical Medicine at Baylor. "For example, we know very little about how these ticks disperse from cavity to cavity, and how closely related they are over their wide geography."
The researchers started a colony of ticks from a cave in Travis County, Texas, due to a case of relapsing fever: "We have maintained that colony over 3 decades and it has been the source for numerous research projects, thus it made sense to use them for the first genome of this family of tick," said Dr. Pete Teel, co-author and regents professor in the Department of Entomology at Texas A&M AgriLife Research.
The biology of soft ticks is complicated and quite different from hard ticks, which are more extensively studied. Gender of hard ticks is easily determined from a nymphal stage, but in soft ticks, gender determination is difficult. The egg hatches into a larva, then a larva will feed and molt into a nymph. Nymphs have upwards of five to six instar stages. Each instar stage takes a blood meal to molt into the subsequent stage, and it can take about a year to go from a larva to an adult. Finally, when they molt into adults, sex can be determined. One of the main goals of this genome project is to figure out the sex at a genetic level and to identify the chromosome that plays a role with sex determination.
"It's one thing to generate a genome, but there's a lot of post-processing once you're done with sequencing. Our goal was to generate a high-quality, chromosome-level genome instead of a highly fragmented genome that is challenging to work with," Lopez said.
Teel said the completed genome will allow researchers to investigate chromosomes linked to specific biological functions. This creates opportunities for researchers to dvelop novel tools to control the pest.
"We have a basis now to really look at population genetics, and we are looking at the vast geography that we're discovering for this species from Florida all the way through the High Plains and back to southwest and down to Mexico," Teel said. "This is a huge step toward the connection of surveillance and development of control."
About a decade ago, African swine fever moved globally due to trade of infected domestic swine, which infect each other by direct transmission of the virus. The disease gets its name from the natural cycle involving ticks, virus and warthogs, a cycle in which warthogs do not get sick, but maintain the virus. When virus gets into the domestic swine population, mortality is nearly 100%. Today, African warthog are now ranging in Texas.
"Texas has all the puzzle pieces for the emergence of a natural cycle for the African swine fever virus. We have the tick and African warthogs (and one of the largest populations of feral swine). That's where the significance of this tick vector comes into play," Lopez said.
The team aims to look at genetic diversity and adaptation to diverse ecological settings. They can also now delineate the molecular mechanisms of this tick's reproductive cycle and vertical transmission of pathogens.
"Physiology, development and reproduction of these ticks is all based on their genetics, and this allows us to open the doors to those discoveries," Teel said.
Other contributors to this work include Mackenzie Tietjen, Amanda R. Stahlke, David Luecke, Perot Saelao, Sheina B. Sim, Scott M. Geib, Brian E. Scheffler, Anna K. Childers and Alexander R. Kneubehl.
This work was supported by the U.S. Department of Agriculture Agricultural Research Service (USDA-ARS). This research used resources provided by the SCINet project of the USDA-ARS project number 0500-00093-001-00-D. The genome assembly was generated as part of the USDA-ARS Ag100Pest Initiative.
