Developmental and physiological determinants of caste in social Hymenoptera: evolutionary implications.

The eusociality threshold, marked by cooperative brood care by mother and daughters, is crossed by both halictine bees and polistine wasps. Beyond the eusociality threshold lies the continued evolution and elaboration of social systems. Two particularly important thresholds of advanced colony organization are marked by morphological commitment to caste: 1) a commitment to queen-worker dimorphism, and 2) a commitment to morphological diversity within the worker form. Queen-worker dimorphism is a defining character of highly eusocial caste systems. If morphological differences exist between queens and workers, the pattern of queen and worker development must diverge no later than the larval stage. Larvae must be able to translate their own developmental, especially nutritional, history into a development decision. The linking and coordinate expression of gyne characters are advantageous when intermediates are less fit than full queens or full workers. Once such a developmental system evolves, individuals become extremely vulnerable to control by other colony members that have an interest in their developmental fate. In highly eusocial Hymenoptera, queens use 2 principal mechanisms to control offspring development: direct control of nutrition (eg honeybees Apis) and interference with larval response to nutrition (eg Bombus terrestris, many ants). A 2nd major threshold in the evolution of morphologically complex caste systems is the addition of multiple physical worker castes. -from Author