Introduction to the Thorne Lab's research on the evolution of eusociality, especially reproductive division of labor and caste polyphenism, in termites: All 2500+ termite species live in highly social colonies in which few insects ever reproduce and hundreds, thousands, or over a million other individuals are helpers or defenders. How would such a system evolve? The origin and maintenance of eusociality, cooperative societies composed mainly of subfertile or sterile members, is an evolutionary paradox because it seems to conflict with Darwin's concept of reproductive self-interest.

Previous, non-mutually exclusive theories regarding the evolution of eusociality in termites (reviewed in Shellman-Reeve 1997, Thorne 1997, Roisin 1999, Higashi et al. 2000) have been constructive but indecisive; the current consensus is that termite eusocial evolution was driven by a suite of selective forces (Shellman-Reeve 1997, Thorne 1997, Rosengaus & Traniello 1998, Roisin 1999). By studying primitive termites (Zootermopsis; Termopsidae), the Thorne lab has demonstrated (Thorne et al. 2003, Johns et al. 2009) that inevitable meetings between termite families living within the same piece of wood result in assassination of some reproductives, merging of remaining populations of the two colonies, and differentiation of some helpers into new reproductives. This competitive circumstance provides a missing link in explaining how offspring helpers in ancestral termite families might have had opportunities to inherit their parents' nest resources. The same context of battles between neighboring colonies frequently produces reproductive soldiers, a highly unusual termite caste that can be aggressive during intercolony fights and therefore appears to be a precursor to modern, sterile soldier termites.

Higashi, M. et al. (2000) in Termites: Evolution, Sociality, Symbioses, Ecology, eds. Abe, T., Bignell, D.E. & Higashi, M. (Kluwer AcademicPublishers,Dordrecht, The Netherlands), pp. 169-187.

Johns, P.M., K.J. Howard, N.L. Breisch, A. Riviera, & B.L. Thorne. (2009) Proc. National Acad. Sci. USA 106: 17452-17456.

Roisin, Y. (1999) Insectes Soc. 46, 297-305.

Rosengaus, R.B., Maxmen, A.B., Coates L.E., & Traniello, J.F.A. (1998) Behav. Ecol. Sociobiol. 44, 125-134.

Shellman-Reeve, J.S. (1997) in The Evolution of Social Behavior in Insects and Arachnids, eds. Choe, J.C. & Crespi, B.J. (Cambridge University Press, Cambridge,) pp. 52-93.

Thorne, B.L. (1997) Ann. Rev. Ecol. Syst. 28, 27-54.

Thorne, B.L., Breisch, N.L. & Muscedere, M.L. (2003) Proc. National Acad. Sci. USA 22, 12808-12813.

Thorne, B.L., N.L. Breisch and M.I. Haverty. 2002. Longevity of kings and queens and first time of production of fertile progeny in dampwood termite (Isoptera; Termopsidae; Zootermopsis) colonies with different reproductive structures. Journal of Animal Ecology 71: 1030-1041.

Thorne, B.L. and J.F.A. Traniello. 2003. Comparative social biology of basal taxa of ants and termites. Annual Review of Entomology, 48: 283-306.

Eusocial insects such as termites, ants and bees live in colonies that contain overlapping generations. Unlike other insects their colonies include specialised sterile castes, whose members help raise new broods. But it is not clear how this cooperative division of labour evolved.

To investigate, Barbara Thorne, Nancy Breisch and Mario Muscedere of the University of Maryland arranged scores of interactions between colonies of the termite Zootermopsis nevadensis in the lab (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.2133530100).

Reproductive soldiers are thought to be a relatively rare and docile termite caste. But reproductive soldiers of Z. nevadensis from one colony quickly killed the kings and queens of the other, and the two colonies merged. In subsequent turf wars between colonies, the larger, merged colonies typically prevailed.

In the course of her 25 years of research on these insects, Barbara Thorne, Professor of Entomology, has uncovered these and other unexpected findings about them. They initially intrigued her because their lives seemed to contradict Darwin's theory of evolution by natural selection, according to which a huge incentive is to pass genes on to the next generation. In addition, for many social animals a driving force towards social living includes the prospect of inheriting the parents' nest, territory, resources, food or mate.

At first glance, termites who are not kings or queens have none of these incentives. The king and queen mate and found a colony, producing successive broods of offspring in a highly organized social colony. All the rest of the colony's members are sterile. They are either helpers or soldiers, acting as babysitters to rear and defend the young of the royal couple. What could possibly be the evolutionary incentive for them to do so, wondered Dr. Thorne, especially as their genetic system is akin to that of humans: both parents pass on their genes to their offspring in equal numbers?

True, if one of the parents dies, other termites can inherit the nest and become reproductive. But the Catch 22 is that the royal couple generally lives a great deal longer than do the helpers and soldiers, so there is virtually no chance of the latter being given the opportunity to blossom with reproductive ability.

"Unfortunately there's no time-machine to allow us to go back 150 million years to the time when termite societies first began," says Dr. Thorne. So there's no way to know whether the first king and queen were equally long-lived, or what the evolutionary incentive was for those first offspring to remain in the first colony and rear the royal offspring, rather than leave the colony, find a mate and found a colony of their own.

She reasoned that the best approach was to study modern species that have retained many of the primitive characteristics of their distant ancestors. Whatever the evolutionary dynamic was, she decided, it would probably have happened in small, young colonies, because there must have been some evolutionary scenario favoring even the first offspring to stay as helpers in their parents' nest.

The primitive termites that Dr. Thorne studies typically establish their colonies in huge trees, usually pines, but they are highly selective in their choice of tree. First, tens of thousands of bark beetles must attack a tree en masse, drilling holes in the bark, which makes the sap leak out and the tree die. Then the beetles lay eggs, the adults die, and the young beetles emerge from the tree, drilling exit holes. At that point the termites march in and take possession of the beetles' empty entrance and exit holes. A royal couple enter a hole, pass right through it and establish their royal chamber directly under the soft, most nutritious part of the tree's bark. Now thousands of termite families are burgeoning in a single dead tree.

The life cycle of these termites is quite different from the subterranean termites (genus Reticulitermes) that invade and ravage people's homes in Maryland and elsewhere. The latter live underground and forage for wooden gate-posts, trees that have been felled by hurricanes and winter storms, or the wooden structure of people's houses. Dr. Thorne's primitive termites (Zootermopsis nevadensis, commonly known as the dark Pacific Coast dampwood termite), by contrast, never leave the wood of the pine tree in which they start their colony. "What happens when these termite colonies start to grow and expand their territory under the bark?" mused Dr. Thorne. Neighbor colonies must inevitably meet.

She had already done a great deal of field work when she made up her mind to simulate in the laboratory the situation of the colonies in the tree bark. She and her group collected the termites in California and bred a whole nursery of small termite families. Each colony was kept separate, complete with their entire social situation: king, queen and first few groups of offspring, a total of about 50-100 individuals.

Dr. Thorne later connected the colonies with Tygon tubing. The colony members were under no obligation to meet others through the tubing. They could have foregone the opportunity. But they did not. Instead they entered into an interaction, (i.e. a battle royal). In the carnage that ensued, usually only the reproductive pair in one colony was killed. Very occasionally the bloodshed included some of the helpers. Then the two once-independent colonies fused into a mega-colony.

A few months after the interaction, some helpers within the fused colony began to develop reproductive abilities. This shows that the colonies in the tree will also meet one another and the reproductive royals will be killed, thus providing an opportunity for new reproductive termites to develop. Dr. Thorne had at last identified the circumstance where there was an opportunity for early inheritance of the colony resources by termite offspring.

"A further, very frequent result of the interaction is the development of a totally bizarre caste in termites, called reproductive soldiers," says Dr. Thorne. Normal soldier termites have a dramatically different head shape and big strong mandibles. Interestingly, the reproductive soldiers are a blend of a soldier head and mandibles, plus a reproductive body. "They had previously been found, very incidentally, in some primitive termites," says Dr. Thorne, "but people knew very little about them. We were finding them virtually every time we initiated an interaction. A reproductive soldier, or more than one, would develop. They can be male or female. And they're fertile. We saw them mate with other reproductives."

She and her group began to focus on the role of these reproductive soldiers, especially the males, which are more common. In all the early experiments involving single termite families, the reproductive soldiers had been docile and passive, staying with the brood in the nursery. "We couldn't get them to snap or bite at anybody," says Dr. Thorne. "They didn't act like soldiers at all, just like reproductives. Then the group arranged interactions between two colonies, one or both of which contained a male reproductive soldier. That's when the soldier's true personality emerged," she said. "Male reproductive soldiers would march over to the other side and attack the other male reproductive - whether it was a king or another reproductive soldier and kill him, often by cutting off his head."

From that she and her group inferred that the sterile soldier form of termites typical of modern termites evolved from a reproductive soldier, rather than vice versa. This matches similar inferences drawn for ants and aphids.

The paper by Dr. Thorne and her group on the subject was published in the Proceedings of the National Academy of Sciences. She was assisted in the work by Dr. Nancy Breisch, a senior research associate in the Entomology Department, and Mario Muscedere while he was an undergraduate student here. He was supported by a Howard Hughes Fellowship and is now a graduate student at Boston University. The National Geographic Foundation and the U.S. Department of Agriculture Forest Service provided funding for the research.

In developing his theory, one head-scratcher for Darwin was how some species such as termites, ants and bees, which thrive in large social families where only a few members reproduce, fit into the scheme of evolution by natural selection.

Reproductive self-interest the passing of genes to offspring to perpetuate their lineage -- is one of the basics of Darwin's theory. But evolving and thriving for millions of years are creatures who, except for the queen and king of their colony, don't ever reproduce. In fact, most of them spend their lives just helping and defending the royal couple, a celibate lifestyle that seems to fly in the face of evolution's drive for organisms to pass their genes to the next generation.

Almost 150 years after Darwin published The Origin of Species, University of Maryland entomologists Barbara Thorne and Nancy Breisch have discovered a phenomenon that would have intrigued Darwin. By studying a primitive species of termite, they found how evolution of highly social living, called eusocial, may have occurred and been so successful. It seems the ancient road to social success may have all begun in battle.

Termite Society

A termite colony has a queen and king and their hundreds or even thousands of offspring, called workers and soldiers, who are unable to reproduce. If one of the parents dies, some of the offspring helpers may rise to the throne and become replacement reproductives, but in the termite world, they could wait a very long for such an inheritance. In fact, the queen and king often outlive their first generation of progeny.

"So why do helpers sacrifice their own reproduction and hang out in their parents' colony?" Barbara Thorne asks about the insects she has studied for 25 years. "Darwin thought it had something to do with family. We think he got that right, although in a way he probably never considered. We revealed a situation in which parents are quite vulnerable so offspring stand to gain an early inheritance."

Primitive Species

To study the circumstances of termite evolution, Thorne and Breisch went to Zootermopsis nevadensis, a primitive species that lives in California and other Pacific states. Descendants of the first termites of 140 million years ago, they are eusocial like modern termites, but socially primitive in the ways of their ancestors.

Like the ancients, reproductive kings and queens of this species move into trees that have been killed and softened by other invading insects, to set up housekeeping and start churning out lots of little termites. They never leave the tree because all the food and shelter they need are right there. At the same time, other couples land on the same tree to set up court and start their own royal nurseries.

"Tens or even hundreds of termite families occupy a single tree, each with their own king and queen. It's like an apartment complex," says Thorne. "We wondered what happens when colonies meet."

Not the Welcome Wagon

Thorne and Breisch replicated the primitive termite real estate in their lab at Maryland. Each colony began with a queen and king and small family of offspring. The scientists ran a tube to connect two evenly sized families, similar to pathways between neighbors in a tree full of colonies in the wild. Within hours, the termites used the tube as an HOV lane to a neighbor colony and, in the process, revealed an evolutionary secret of their eusocial society. When the two colonies met, a battle took place in which the typical result was that one family's king and queen were killed, but the helpers from both colonies survived and merged into a single megacolony. In nature, the merged colonies would eventually meet other neighbors, so the scientists also explored this situation in the lab, staging meetings between the now larger, merged colonies and smaller families. When colonies were not evenly matched in size, reproductives from the larger colony won the battles.

No More Mr. Nice Guy

In addition, within several months of a battle, new reproductives developed from among the helpers, including the surprise appearance of reproductive soldiers. "In most modern termites all soldiers are sterile," says Thorne. "Reproductive soldiers have previously been found only rarely and considered an anomaly. They seemed docile, like regular reproductives, guarding the eggs and babysitting the nursery. They didn't patrol the colony."

But in further experiments involving meetings between colonies that had a reproductive soldier and families with a king and queen, soldier reproductives quickly shed the Mr. Nice Guy image. "When they encountered the neighbor colony, the male reproductive soldier immediately killed the king," Thorne says. "In one instance that would have drawn Lady MacBeth's praise," Thorne says. "We saw the male reproductive soldier go right over to the king and cut off his head, leaving the rest of the colony alone."

A Family Affair

Thorne and Breisch think the less-than-neighborly competition in the primitive colonies is the key to how termites evolved a successful eusocial society. "The rationale Darwin used was that it must have had something to do with the family unit," says Thorne. "With termites, it appears that it is indeed a family deal. By staying with their family, offspring are protected, they have food, they help rear their siblings, and they have a chance to inherit the nest and colony if a parent is killed in battle. That's a bonanza payoff if it happens, whereas it's a big risk to leave the nest to try to start one's own colony."

"The larger colonies are the ones that win the battles," says Thorne, which means small families don't have much of a chance when the hordes rumble over from the other side of the tree looking for trouble. The bigger the colony, the better the chance the queen and king won't lose their heads in a battle, which means there will be a nest to inherit.

The entomologists also think their experiments explain the evolution of the sterile soldier caste in modern termite species. "We showed that reproductive soldiers can be highly aggressive in intercolony interactions," says Thorne. "We think modern sterile soldiers evolved from these reproductive soldiers."

"Termites are an awesome success story of social insects," says Thorne. She is continuing this line of evolutionary research, currently focusing on molecular genetic techniques to reveal the family origin of reproductives that develop following battles.

Thorne also studies termite pest management, including approaches to detection, prevention, and control that minimize use of pesticides.