The Developmental Basis of Worker Caste Polymorphism in Ants

Abstract

Among social Hymenoptera, the evolution of the worker caste has reached its apex in the ants, in which some taxa have evolved complex physical worker caste systems. Diverse worker caste systems can be generated through regulation of three aspects of larval growth: critical size, growth parameters, and reprogramming of these factors. Even the most complex caste systems could have evolved simply by the addition of revised programs to the end of an ancestral developmental pathway for workers. Worker castes in ants provide a system in which to study the evolution of reaction norms and developmental switches. In ants, the simplest developmental switch, revision of critical size alone, does not lead to discontinuous phenotypes. Only when changes in growth rules are tied to the decision to revise critical size are distinct phenotypes produced from the alternative developmental programs. The addition of ever more physical castes may be limited by both developmental and ecological factors.