Ever since a few enterprising bed bugs hopped off a bat and attached themselves to a Neanderthal walking out of a cave 60,000 years ago, bed bugs have enjoyed a thriving relationship with their human hosts.
Not so for the unadventurous bed bugs that stayed with the bats - their populations have continued to decline since the Last Glacial Maximum, also known as the ice age, which was about 20,000 years ago.
A team led by two Virginia Tech researchers recently compared the whole genome sequence of these two genetically distinct lineages of bed bugs. Published in Biology Letters on Tuesday, May 28, their findings indicate the human-associated lineage followed a similar demographic pattern as humans and may well be the first true urban pest.
We wanted to look at changes in effective population size, which is the number of breeding individuals that are contributing to the next generation, because that can tell you what's been happening in their past."
Lindsay Miles, lead author and postdoctoral fellow in the Department of Entomology
According to the researchers, the historical and evolutionary symbiotic relationship between humans and bed bugs will inform models that predict the spread of pests and diseases under urban population expansion.
By directly tying human global expansion to the emergence and evolution of urban pests like bed bugs, researchers may identify the traits that co-evolved in both humans and pests during urban expansion.
A stairway graph (at left) shows that the genome-wide patterns of bed bug demography mirrors global human expansion, courtesy of Biology Letters 21: 20250061. The image of bed bugs is courtesy of Warren Booth.
"Initially with both populations, we saw a general decline that is consistent with the Last Glacial Maximum; the bat-associated lineage never bounced back, and it is still decreasing in size," said Miles, an affiliate with the Fralin Life Sciences Institute. "The really exciting part is that the human-associated lineage did recover and their effective population increased."
Miles points to the early establishment of large human settlements that expanded into cities such as Mesopotamia about 12,000 years ago.
"That makes sense because modern humans moved out of caves about 60,000 years ago," said Warren Booth, the Joseph R. and Mary W. Wilson Urban Entomology Associate Professor. "There were bed bugs living in the caves with these humans, and when they moved out they took a subset of the population with them so there's less genetic diversity in that human-associated lineage."
As humans increased their population size and continued living in communities and cities expanded, the human-associated lineage of the bed bugs saw an exponential growth in their effective population size.
By using the whole genome data, the researchers now have a foundation for further study of this 245,000 year old lineage split. Since the two lineages have genetic differences yet not enough to have evolved into two distinct species, the researchers are interested in focusing on the evolutionary alterations of the human-associated lineage compared with the bat-associated lineage that have taken place more recently.
"What will be interesting is to look at what's happening in the last 100 to 120 years," said Booth. "Bed bugs were pretty common in the old world, but once DDT [dichloro-diphenyl-trichloroethane] was introduced for pest control, populations crashed. They were thought to have been essentially eradicated, but within five years they started reappearing and were resisting the pesticide."
Booth, Miles, and graduate student Camille Block have already discovered a gene mutation that could contribute to that insecticide resistance in a previous study, and they are looking further into the genomic evolution of the bed bugs and relevance to the pest's insecticide resistance.
Booth said the project is a good example of what happens when researchers "follow the science," which he is afforded the space to do thanks in part to the Joseph R. and Mary W. Wilson endowment that supports his faculty position.
"It's a great resource to have," said Booth. "We are using it for work investigating the evolution of insecticide resistance and species spread using museum specimens collected from 120 years ago to our present-day samples. "I'm very lucky to have that freedom to explore."
Genetic discovery sheds light on infection-triggered neuropathy