Human bedbugs, Cimex lectularius, are “true bugs,” that is, insects in the order Hemiptera. They are an infernal pest, sucking the blood out of people and leaving a nasty, itchy rash. (I was bitten only once, but it was in a fleabag hotel in Peru, and there were many bites all over me, with the rash persisting for about three weeks.) Fortunately, bedbugs aren’t known to carry any diseases.

Still, they’re annoying, as you’ll know if you’ve followed the news over the past couple of years. Having been nearly eradicated by 1940 following applications of DDT, bedbugs started making a comeback when we declared a DDT moratorium, and the bugs are now common in American cities and a devil to eliminate.

Here’s a human bedbug sucking blood from the arm of a volunteer (photo from Wikipedia)

But where did bedbugs come from? Well, it’s long been known that their closest relative seems to be the bat bug, a similar insect that lives on bats, sucking their blood in the caves. The batbug also happens to be classified as the same species as the human bedbug, Cimex lectularius. The morphological differences between the two forms are trivial, but you can still tell them apart with a microscope. Below is a diagram and some text from Bad Bed Bugs highlighting the diagnostic differences:

The trick to identifying a bat bug is by looking at the length of hairs on the upper covering of the thorax. The picture above is the joining of one half bat bug (left side) and one half bed bug (right side). You’ll notice that the length of the bat bugs hairs is longer than the width of its eye. The bed bug however, has hairs that are smaller than the width of its eye.

There are other differences, too: as Carl Zimmer notes in a new piece in the New York Times, the human variety has longer and thinner legs than the bat variety, perhaps because the bat variety needs a firm grip on their cave-hanging hosts.

There also appear to be physiological differences. As a new paper in Molecular Ecology by Warren Booth and colleagues (reference and free link below) notes, each type does better in terms of longevity and reproduction when it feeds on its own host. A batbug forced to ingest human blood does okay, but not as well as on a bat, and vice versa. Finally, the daily rhythm (“diurnal cycle”) differs between the two forms: batbugs feed during the day, when bats are asleep in their caves, while human bedbugs feed at night, when humans are asleep in their beds.

One problem with these data, which are used by both Zimmer and Booth et al. to imply genetic differentiation, is that we don’t know whether these differences are evolved genetic differences between the forms, or are only developmental/physiological responses to feeding on different species. It’s possible that if you transferred a batbug to humans, it will develop longer legs, change its feeding cycle, and get physiologically acclimated to human blood in a generation or so, and that this is not due to evolutionary (genetic) change, but could be a purely developmental (“plastic”) response.

That’s not a far-fetched interpretation. Human head and body lice, which are not different species, also transform their physiology and morphology as a result of acclimation and not genetics, and even Anolis lizards change the shape of their legs if they’re forced to climb on thin branches rather than clamber on tree trunks or the ground. The only way to determine if the morphological differences between bedbugs and batbugs are due to genetic/evolutionary change is to rear them over several generations on a common diet, and see if the differences persist. If they do, they’re genetic.

The reason Zimmer and Booth et al. dwell on this is because bats have been suggested to be the vector that gave us human bedbugs. Bats, so the theory goes, were originally afflicted with batbugs, and early humans lived in caves alongside the infested bats. Batbugs then found a juicy new source of food nearby, a few individuals colonized humans, and the rest is history: the human bedbug.

Booth et al. wanted to see how much genetic differentiation there really is between human bedbugs and batbugs, and so their paper reports an extensive genetic analysis of several hundred of individuals from both forms of the bug. The researchers looked at mitochondrial DNA, nuclear DNA (in the form of microsatellites), and at genes that had evolved in human bedbugs to resist DDT.

What they found was that batbugs and human bedbugs do indeed show significant genetic differentiation—in all three types of genes investigated. Bedbugs and batbugs clearly form two distinct genetic lineages. This is shown by statistical analysis of bugs taken from the two hosts; the figure below shows the genetic differentiation for nuclear DNA among samples of both forms taken in Europe. Brown dots are individual human bedbugs, blue are batbugs, and you can see how well separated they are (see the caption below the figure).

There is, however, still some evidence of gene flow between the two forms, perhaps occurring when a batbug finds itself on a human and mates with bedbugs, or vice versa. Although most human bedbugs show the DDT-resistant form of “pesticide genes”, a few don’t, and those “susceptible” genes may have come from the batbugs, which never experienced DDT. Still, what we have here are two closely-related but genetically distinct lineages, and that is the big lesson from the paper of Booth et al. But they want to say more, and that is what Carl Zimmer highlighted in his NYT piece (see question #2 below).

Two questions remain:

1. Were batbugs the ancestors of the human bedbug? It seems likely, although neither Zimmer nor Booth et al. explicitly give the information that is be crucial for ansering this question: Are the batbug and bedbug more genetically similar to each other than either is to any other species in the genus? If the batbug is the ancestor of the bedbug, then the two forms have to be “sister taxa,” that is, each other’s closest relatives. Now this may indeed be the case, and may be cited in one of Booth et al.’s references, but I didn’t look them all up. I’ll take it for granted that both Zimmer and Booth et al. know that these are in fact sister taxa.

But one problem remains: do they only look like sister taxa because there has been gene flow between batbugs and human bedbugs, making them look as if they evolved recently when in fact they didn’t? This is a problem with trying to suss out the evolutionary history for any pair of species that live in the same place and occasionally hybridize. Fortunately, it can be taken care of. For example, if bedbugs and batbugs had distinct forms of genes (as they do), but those forms are still more similar to each other than to the gene forms of other species or populations in the genus, then that would imply that they are indeed sister taxa. Neither the authors nor Zimmer discuss this, but it may be such a well-known result that neither thought it necessary to mention it explicitly.

Also, the human bedbug is genetically depauperate compared to the batbug, and that’s what one would expect if only a few individual batbugs originally colonized humans, going through what we call a “population bottleneck.” The genetically depauperate nature of the human bedbug compared to the batbug also implies that if there was a colonization from bats to humans, it happened only once or a very, very few times. If colonization was frequent, human bedbugs would be much more genetically variable among populations than we see. If the bat transfer theory is correct, the colonization of humans by batbugs must have occurred in the distant past when humans lived in caves along with bats, and that would probably be about 50,000 years ago in Eurasia. (No molecular dating of the divergence was reported.)

But what the authors and Zimmer find most exciting about the study is encapsulated in the second question:

2. Are these forms on the road to becoming different species? Are we seeing, in the form of batbugs and human bedbugs, two groups that descended from a common ancestor (on bats), and are now in the process of becoming different biological species? Indeed, Zimmer calls his piece, “In bedbugs, scientists see a model of evolution.” What he means by that is “a model of how new species form.”

We evolutionists, by and large, conceive of species as being different groups that cannot exchange genes because of biologically-produced “isolating barriers” that prevent the formation of fertile hybrids. Bedbugs and batbugs do appear to have such barriers: they don’t do well on each other’s hosts, they are active at different times of day, they seem to maintain differences in appearance, and, of course, the DNA data show a lack of genetic exchange. Now, as I said, we don’t know whether the differences in activity period, ability to thrive on the host, or morphology are based on differences in genes (we can’t assume blithely that they are), but the DNA data clearly show that these lineages don’t exchange genes very often. Could it be that we have a case of speciation in action due to host shift by the batbugs?

The answer is that we don’t know for sure. What we see are two diverged lineages, but we can’t know whether they will continue their evolutionary divergence and go on to form two “full species”, totally incapable of exchanging genes, that deserve different Latin names. It’s possible that they will maintain their status as somewhat distinct lineages, but that gene flow will be enough to keep them from achieving full reproductive isolation.

There are in fact many known cases of groups that are similar to these bedbugs in having achieved partial but not full reproductive isolation, so to imply that these bugs are unique, or that we have here a rare model of speciation in statu nascendi, is incorrect. In the book Speciation that I co-wrote with Allen Orr, we discuss many cases of “host races” in insects that show significant genetic divergence of forms living on different plants (aphids and the”true” fruit flies [tephritids] are two examples), but in which there is still gene flow between the forms. They are not considered “full species” since reproductive isolation is incomplete. In all of these cases we simply have no idea about whether they’ll go on to evolve into fully isolated species. We’d have to wait around for a couple of hundred thousand years to find that out.

The fact is that most populations of a species showing some reproductive incompatibility probably do not go on to form full species. Either they fuse back into one species, or one form goes extinct, or they maintain their status as incompletely isolated forms. To ask the question, “Are these going to become full species?” is to ask a question that can’t be answered.

Nevertheless, there is of course a lot of interesting information about batbugs and bedbugs in the paper of Booth et al., regardless of their unknown evolutionary fate. At least we know (probably) where an annoying human parasite came from, and something about the evolutionary differences between them. That might not help us eradicate bedbugs, but isn’t it fascinating to contemplate that our affliction with that creature is a remnant of our evolutionary history as cave-dwellers?

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Booth, W. et al. 2015. Host association drives genetic divergence in the bed bug, Cimex lectuarius. Molecular Ecology, online.