New Method Makes for Faster Genetic ID of Eastern U.S. Termites
By Andrew Porterfield
Termites (genus Reticulitermes) are not only an expensive pest—they also serve an important ecological role, decomposing cellulose material. Twelve species of termites are known in the U.S., and the species responsible for structural damage cost Americans about $11 billion a year to control.
In the U.S., four Reticulitermes species (R. flavipes, R. tibialis, R. virginicus, and R. hesperus) and one invasive species (Coptotermes formosanus) are responsible for nearly all termite control efforts. Identification of the right species in a structure is important for control, because the insects have different colony sizes and foraging areas. Identification also is important for ecological studies on the termites (pest or non-pest), to weed through large sample sizes.
But termite identification is not an easy task. Three current methods—morphology, cuticle hydrocarbon profiles, and genetic methods—all have their drawbacks. Morphological identification requires large sample sizes of reproductive or soldier castes, which are difficult to obtain. Cuticle hydrocarbon profiles can vary between colonies, making species harder to distinguish. DNA sequencing is effective but time-consuming, and it requires expertise in molecular biology and informatics to analyze data.
To solve this problem, researchers from Texas A&M University, the University of Arkansas, and the BASF Corporation developed a simpler genetic method of identification that can distinguish five species of eastern termites in a matter of hours. Their results were published this week in the Journal of Economic Entomology.
Mark Janowiecki, Ph.D., led the research project during his doctoral work at Texas A&M and is now an entomologist with the New Orleans Mosquito, Termite, and Rodent Control Board. He and his colleagues used a genetic tool called inter-simple sequence repeats (ISSR) to identify the termites. ISSRs are fragments of DNA (about 100 to 3,000 base pairs of nucleotides), located between identical “microsatellite” regions of DNA. These can provide a unique DNA “fingerprint” of a species.
The ISSR DNA is amplified from small sample sizes using the polymerase chain reaction (PCR). The technique relies on a primer (a small strip of DNA) that is a complementary match to the microsatellite region. This makes the technique easier, because no sequence data for primer construction is needed, and more accurate because ISSRs are found randomly in the genome.
“Our goal was to update previous methods … for identifying subterranean termites using a simple tool that could be run concurrently with microsatellite genotyping analysis,” Janowiecki says. “At the time, another student in the department was working in our lab to run ISSRs for identification of fly species, so I decided to see if this method could work for termites. I found a paper that described ISSR primers for other termite species and began to test them for these species of Reticulitermes.”
The researchers screened several species of termites for 16S mitochondrial DNA (a specific segment of a ribosome found in mitochondria that is generally less variable than other DNA) and screened 10 ISSR primers. Other termites from the western U.S. and Canada were also screened to look for potential species overlap.
Of the 10 ISSRs, one was found that contained species-specific fragments with enough variability to distinguish species (R. flavipes, R. hageni, R. virginicus, R. tibialis, and R. malletei). Even though some overlap was found among the species, the researchers report that all five had “a diagnostic, species-specific fragment.”
The ISSR method can be conducted in about 11 hours, versus the 14 to 34 hours needed for other genetic methods. However, the authors warn that the method might not work for all geographic regions. Some termite species in the western U.S. and some invasive termites have ISSR fragments similar to eastern species, which could cause confusion. “We screened other species, and found that they at times had overlapping fragments to the focal species we included. To include all species, it might be possible to use multiple ISSRs, but at that point the ISSR method would lose its efficiency,” Janowiecki says.
Journal of Economic Entomology
Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He writes frequently about agriculture issues for the Genetic Literacy Project. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.