Humoral and Cellular Defense Mechanisms in Rebel Workers of Apis mellifera

SIMPLE SUMMARY: Immune mechanisms in insects include cellular and humoral defenses. One of these cellular defense mechanisms is phagocytosis. The humoral defense components are produced in the fat body cells, the development of which depends on the juvenile hormone (JH) titers and vitellogenin (Vg) concentration. These mechanisms also regulate caste formation. In a colony, a queen, a worker, or a rebel can develop from eggs of the same genome. Rebels are reproductive workers that can lay eggs while maintaining tasks inside and outside the colony the same as sterile workers. The aim of our studies was to determine the phagocytic index, JH titer, Vg concentration, the number of oenocytes (fat body cells), and their size in the rebels and to compare them to those in normal workers. The rebels were characterized by high phagocytic indices, JH and Vg levels, and increased numbers and sizes of oenocytes in the fat body cells in comparison to the normal workers. These characteristics can be viewed as an evolutionary adaptation of these reproductive workers to life in a eusocial society. ABSTRACT: The physiological state of an insect depends on efficiently functioning immune mechanisms such as cellular and humoral defenses. However, compounds participating in these mechanisms also regulate reproductive caste formation and are responsible for reproductive division of labor as well as for labor division in sterile workers. Divergent reaction of the same genotype yielding reproductive queens and worker castes led to shaping of the physiological and behavioral plasticity of sterile or reproductive workers. Rebels that can lay eggs while maintaining tasks inside and outside the colony exhibit both queen and worker traits. So, we expected that the phagocytic index, JH3 titer, and Vg concentration would be higher in rebels than in normal workers and would increase with their age. We also assumed that the numbers of oenocytes and their sizes would be greater in rebels than in normal workers. The rebels and the normal workers were collected at the age of 1, 7, 14, and 21 days, respectively. Hemolymph and fat bodies were collected for biochemical and morphological analyses. The high levels of JH, Vg, and the phagocytic index, as well as increased numbers and sizes of oenocytes in the fat body cells demonstrate the physiological and phenotypic adaptation of rebels to the eusocial life of honeybees.