Fertility and Viability of Hybrid Offspring Imply the Absence of Major Postzygotic Isolation Between Two Reticulitermes Termite Species

(1) Reproductive isolation serves as a critical mechanism that prevents interspecific hybridization among closely related species, thereby preserving species integrity. In termites, hybridization between certain closely related species can overcome prezygotic isolation and produce offspring. However, whether these hybrids can overcome postzygotic barriers remains substantially underexplored. (2) This investigation conducted a comparative analysis of reproductive output (egg production and offspring count), physiological traits (body weight), functional characteristics (locomotor capacity), and hybrid fertility between the hybrid colony that was established by <i>Reticulitermes flaviceps</i> and <i>R. chinensis</i> and the conspecific pairing colonies of <i>R. flaviceps</i> and <i>R. chinensis</i>, respectively. (3) The results showed that hybrid colonies laid significantly more eggs and produced significantly more larvae than conspecific colonies. The hybrid offspring showed no decline in weight and locomotor capacity. Furthermore, the hybrid offspring maintained balanced sex ratios with unimpaired caste differentiation and fertility in both sexes. (4) These results demonstrate that the prezygotic reproductive isolation mechanism is incomplete between two closely related termite species, and there is a risk of forming hybrid populations. This work not only provides a theoretical basis for monitoring the risk of hybrid populations in termite management but also offers new insights into the evolution of reproductive isolation and speciation in social insects.