Abstract
Caste differentiation in social insects is a phenotypic plasticity for the high efficiency of individual tasks within a group. Morphological differences between the castes are caused by effects of neuro-endocrine systems and different gene expressions during larval stages. Recently, a lot of metabolic gene expressions between the castes have been determined and molecular mechanisms underlying caste differentiation of internal and external morphology have been gradually clarified. Sterile adult workers have a potential to change the reproductive states including reproductive organs and behaviors without any changes of external morphology under queenless conditions. Behavioral transition to reproductive individuals from sterile workers is caused by physiological changes in the brain and may eventually reconstruct the brain specialized to the caste specific behaviors. The basic physiological and molecular mechanisms of caste differentiation and caste transition seem common among species in eusocial Hymenoptera. Some of the hormonal functions for the caste transition, however, has been lost in highly eusocial species. In the present paper, I introduce common mechanisms underlying reproductive regulations among eusocial insects and discuss the evolution of endocrine functions for reproduction.
