We examined the emergence of replacement reproductives in the primitive dampwood termite, Zootermopsis nevadensis (Termopsidae) to gain insight into the evolution of eusociality. Termite ancestors likely had life histories similar to extant Termopsids; nuclear families (colonies) within a single decaying tree compete for limited resources, leading to frequent encounters between neighbors and the deaths of founding kings and queens (primary reproductives). In Termopsids, non-soldier helpers can differentiate into fertile replacement reproductives, which often develop after colonies interact.

We staged laboratory interactions between pairs of similarly sized colonies generated from controlled crosses between unrelated alates. We used microsatellite markers to determine the family of origin of replacement reproductives, alates, and a subsample of workers that developed after the interactions.

In one treatment we removed replacement reproductives as soon as they were detected. We found replacement reproductives and alates could develop from both interacting colonies. A small fraction of workers in the merged group had genotypes consistent with crossbreeding between members of both interacting colonies or inbreeding between a surviving primary reproductive and its own offspring.

In the second treatment, we left colonies undisturbed for one and two years following an interaction. Most replacement reproductives and alates within the merged social group were offspring of the original primary reproductives, but some were the product of inbreeding or crossbreeding within or among families.

The frequency of these outcomes depended in part on which primaries survived the interaction. We discuss the implications of these results to the evolution of eusociality in termites.