Carl Zimmer reported this week in the New York Times on research into the natural history of bedbugs in Europe: “In Bedbugs, Scientists See a Model of Evolution”. Zimmer describes a new paper by Warren Booth and colleagues that examined European bedbugs, focusing on the differences between the bedbugs that infest human hosts and those that infest bats living across the same region, often sharing human structures like attics and eaves. Both the bat-infesting and human-infesting bedbug populations are classified within the same species (Cimex lenticularis) but their evolution has taken them in different directions. The case is interesting from the point of view of evolutionary theory because the differentiation may have taken place across a single geographic region—it is a case of sympatric speciation in action, where the mode of isolation is adaptation to different host populations.
The bat-infesting bedbugs exhibit greater genetic diversity than the ones that infest humans. Greater genetic diversity can result from an older origin for the bat-infesting population, or greater gene flow across the population’s range (so that founder events are less important to dispersal). Or a low genetic diversity in the human-infesting population may reflect the long history of humans attempting to eradicate the pests, with today’s local populations having been recolonized at long distance from a small number of source populations.
Most of the human-associated bedbug populations are highly inbred, which reflects their dispersal pattern in which homes may be initially colonized by a single male-female pair or egg-laying female. This contrasts with the pattern in the bat-associated populations. From Booth and colleagues’ paper:
The genetically depauperate nature of the human-associated populations may also provide insight into the stability of bed bug populations. Population establishment, high population turnover, and extinction events are expected to be common with little opportunity for population admixture, likely due to human-mediated movement and frequent interventions through pest control. Thus, human-associated bed bugs appear to live in highly structured metapopulations. In contrast, C. lectularius populations within bat roosts may be expected to be more stable, albeit with temporal fluctuations due to weather events, bug mortality and host dispersal, with multiple introduction events resulting from the latter.
We still have a lot to learn about the origins of the bedbugs that are obligate parasites of humans. The great extent of divergence does tend to suggest that bedbugs started their association with ancient humans sometime long before people began living in sedentary villages. However, such great divergence between the human and bat-infesting forms in Europe might not be reflected in other parts of the world. Broader comparisons would tell us more about the early phase of host-parasite association.
My intuition is that Pleistocene humans did not inhabit any caves with the kind of regularity that would allow a stable human-infesting bedbug population. Adult bedbugs can go for several weeks without a blood meal, but young bedbugs need to feed at least every few days, which I would have thought requires a human occupation more intense than most caves show. My intuition could be wrong, and some caves in the Near East do show evidence of substantial amounts of plant material carried in for bedding over some period of time—just the kind of place where bedbugs might have sheltered. And perhaps we should consider whether the transition to humans might have been facilitated by reliance on other animal hosts, like hibernating cave bears.
Booth, William, Ondřej Balvín, Edward L. Vargo, Jitka Vilímová and Coby Schal. (2015) Host Association Drives Genetic Divergence in the Bed Bug, Cimex lectularius. Molecular Ecology (in press) doi:10.1111/mec.13086