12.4.2. The origins of eusociality in Isoptera
In contrast to the haplodiploidy of Hymenoptera, termite sex is determined universally by an XX—XY chromosome system and thus there is no genetic predisposition toward kinship-based eusociality. Furthermore, and in contrast to the widespread subsociality of hymenopterans, the lack of any intermediate stages on the route to termite eusociality has obscured its origin. Subsocial behaviors in some mantids and cockroaches (the nearest relatives of the termites) have been proposed to be an evolutionary precursor to the eusociality in Isoptera. Notably, behavior in the family Cryptocercidae, which is sister branch to the termite lineage (Fig. 7.4), demonstrates how reliance on a nutrient-poor food source and adult longevity might predispose to social living.
The internal symbiotic organisms needed to aid the digestion of a cellulose-rich, but nutrient-poor, diet of wood is central to this argument. The need to transfer symbionts to replenish supplies lost at each molt encourages unusual levels of intracolony interaction through trophallaxis. Furthermore, transfer of symbionts between members of successive generations requires overlapping generations. Trophallaxis, slow growth induced by the poor diet, and parental longevity, act together to encourage group cohesion. These factors, together with patchiness of adequate food resources such as rotting logs, can lead to colonial life, but do not readily explain altruistic caste origins. When an individual gains substantial benefits from successful foundation of a colony, and where there is a high degree of intracolony relatedness (as is found in some termites), eusociality may arise. However, the origin of eusociality in termites remains much less clear-cut than in eusocial hymenopterans.
The broken line indicates a paraphyletic taxon. (Data from several sources)
