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Abstract
Caste polymorphism in eusocial insects is based on morphological plasticity and linked to physiological and behavioral characteristics. To test the possibility that dopamine production in the brain is associated with the caste-specific morphology and behavior in female honey bees, an intermediate caste was produced via artificial rearing using different amounts of diet, before quantifying the dopamine levels and conducting behavioral tests. In field colonies, individual traits such as mandibular shape, number of ovarioles, diameter of spermatheca, and dopamine levels in the brain differed significantly between workers and queens. Females given 1.5 times the amount of artificial diet that control worker receives during the larval stage in the laboratory had characteristics intermediate between castes. The dopamine levels in the brain were positively correlated with the mandibular shape indexes, number of ovarioles, and spermatheca diameter among artificially reared females. The dopamine levels were significantly higher in females with mandibular notches than those without. In fighting experiments with the intermediate caste females, the winners had significantly higher dopamine levels in the brain than the losers. Brain levels of tyrosine were positively correlated with those of catecholamines but not phenolamines, thereby suggesting a strong metabolic relationship between tyrosine and dopamine. Thus, the caste-specific characteristics of the honey bee are potentially continuous in the same manner as those in primitively eusocial species. Dopamine production in the brain is associated with the continuous caste-specific morphology, as well as being linked to the amount of tyrosine taken from food, and it supports the aggressive behavior of queen-type females.
Highlights
Caste polymorphism in eusocial insects is based on phenotypic plasticity to allow specialization in particular tasks by different colony members
To evaluate the degree of queen-like morphology, we focused on the mandible as an external morphological characteristic and the reproductive organs as internal morphological characteristics, especially the ovaries and spermatheca, as done previously [3,45]
Caste is exhibited in terms of discontinuous characteristics that are specialized for each task under natural conditions
Summary
Caste polymorphism in eusocial insects is based on phenotypic plasticity to allow specialization in particular tasks by different colony members. Dopamine and aggression in intermediate honey bee caste discontinuously into different morphological and behavioral types in advanced eusocial species, whereas those in primitively eusocial species are continuous with overlapping morphology between workers and queens, including body size [1,2,3,4]. In advanced eusocial Hymenoptera, including honey bees, the female caste is determined by nutritional states in the early larval stage. In female honey bee larvae, feeding on royal jelly during the first three larval instars leads to a queen phenotype [3,5]. Nutrition of female larvae can be manipulated by nurse workers in the nest, such that discontinuous phenotypes of the castes in advanced eusocial Hymenoptera might be caused by the fine manipulation of feeding by workers [10]
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