Abstract

Evolution has created different castes of females in eusocial haplodiploids. The difference between them lies in their functions and vulnerability but above all in their reproductive potentials. Honeybee queens are highly fertile. On the other hand, the workers are facultatively sterile. However, rebel workers, i.e. workers that develop in a queenless colony, reproduce more often than normal workers. As a result, the fat body of these bees, which apart from acting as the energy reserve, is also the site of numerous metabolic processes, had to specialize in different functions perfected over millions of years of eusocial evolution. Assuming that the variety of functions manifests itself in the pleomorphic structure of the fat body cells, we predicted that also different parts of the fat body, e.g. from different segments of the abdomen, contain different sets of cells. Such differences could be expected between queens, rebels and normal workers, i.e. females with dramatically different reproductive potentials. We confirmed all these expectations. Although all bees had the same types of cells, their proportion and segmental character corresponded with the caste reproductive potential and physiological characteristics shaped in the evolutionary process. The females with an increased reproductive potential were characterized by the presence of oenocytes in the third tergite and high concentrations of compounds responsible for energy reserves, like glucose, glycogen and triglycerides. Queens had very large trophocytes, especially in the third tergite. Only in workers did we observe intercellular spaces in all the segments of the fat body, as well as high protein concentrations—especially in the sternite. As expected, the rebels combined many features of the queens and normal workers, what with other findings can help understand the ways that led to the origin of different castes in females of eusocial Hymenoptera.

Highlights

  • Evolution has created different castes of females in eusocial haplodiploids

  • We confirmed that fat body cells in the queens, rebels and normal workers contain the same components, but have original morphological and physiological traits characteristic only of the specific castes, shaped mainly by their reproductive potential

  • The segmental character of the fat body, which so far has not been considered in scientific research, in the individual insects of a given caste/sub-caste allows for a broader look at the processes taking place in each of those locations

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Summary

Introduction

Evolution has created different castes of females in eusocial haplodiploids. The difference between them lies in their functions and vulnerability but above all in their reproductive potentials. Assuming that the variety of functions manifests itself in the pleomorphic structure of the fat body cells, we predicted that different parts of the fat body, e.g. from different segments of the abdomen, contain different sets of cells Such differences could be expected between queens, rebels and normal workers, i.e. females with dramatically different reproductive potentials. In the advanced eusocial insects the reproductive role is limited to one or a few highly specialised queens supported by a mostly sterile worker caste This segregates investment in reproduction and colony growth and maintenance between different individuals, which increases the total reproductive output of the group of k­ in[10,11,12,13]. On the basis of this information, it can be expected that the fat body cells of the worker/reproductive castes/sub-caste will contain the same components (e.g. nuclei, vacuoles, and the protein and lipid stores), but will retain original morphological and physiological characteristics shaped in the evolutionary process

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