Humoral and Cellular Defense Mechanisms in Rebel Workers of Apis mellifera.

Aneta Strachecka,Patrycja Skowronek,Jerzy Paleolog,Karolina Kuszewska,Paweł Migdał,Michał Woyciechowski,Marcin Grabowski

doi:10.3390/biology10111146

Abstract

Simple SummaryImmune 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.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.

Highlights

Fat body cells and hemocytes play a crucial role in insect immunity
The phagocytic index decreased with the age in both sub-castes, quicker in normal workers, and was higher in the rebels compared to normal workers in four tested age groups (Figure 2; two-way ANOVA with multiple comparison testing using the post hoc Tukey HSD test; age F(3,232) = 248.67; p < 0.0001; castes/sub-caste F(1,232) = 1265.7; p < 0.0001, post-hoc for each group max. p = 0.941535; castes/sub-caste*age F(3,232) = 124,54, confirmed that the workers tested belonged to two different sub-castes, the rebels, and normal workers
The phagocytic index decreased with the age in both sub-castes, quicker in normal workers, and was higher in the rebels compared to normal workers in four tested age groups (Figure 2; two-way ANOVA with multiple comparison testing using the post hoc Tukey HSD test; age F(3,232)= 248.67; p < 0.0001; castes/sub-caste F(1,232)=

Summary

Introduction

Fat body cells and hemocytes play a crucial role in insect immunity. Most proteins from hemolymph are produced in fat body cells, which secrete antimicrobial peptides and constitute an important component of the humoral defense. The fat body has a segmental character and functions as a multifunctional system made up of many organs/segments where each segment works separately and they all contribute to a common metabolism [4] This segmental nature of the two cooperating tissues/cell types (hemocytes in hemolymph and fat body cells) is crucial in the cascading processes related to immunity and aging of honeybee workers. Vg is synthesized in the trophocytes [10] (most likely in the third tergite of the fat body) and is immediately directed to the appropriate tissues by the circulatory system [4] The changes of these hormones as a result of aging induce hemocyte death [6]. High JH titers of nurses influence the hypopharyngeal glands (HG) development [13] and high levels of abdominal lipids [14]

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