Frequent colony fusions provide opportunities for helpers to become reproductives in the termite Zootermopsis nevadensis

In many termite taxa, colonies occupying the same nesting resource can meet, interact, and merge with unrelated conspecific neighbors. Because proto-termite ancestors likely also co-inhabited resources and experienced interactions with neighboring conspecific families, extant species that form fused colony units may offer fundamental clues to explaining the origins of eusociality in Isoptera, particularly if both original families retain the potential for reproduction. We allowed entire colonies of Zootermopsis nevadensis (Archotermopsidae) to interact, merge, and develop in the lab, then used genetic markers to determine the family of origin of reproductives, soldiers, and helpers. Persisting and new members of all castes arose from both original colonies and in some cases were hybrids of the two original lineages. We also measured the frequency of mixed-family colonies in natural settings. Ten out of 30 field sampled colonies contained mixed families, demonstrating that interactions and fusions are common in nature. We discuss the implications of our findings as a model system for understanding the evolution of eusociality in termites, highlighting the importance of ecological circumstances impacting direct, indirect, and colony-level fitness.