Correlation of cytochrome titer and respiration in Apis mellifera: Adaptive response to caste determination defines workers, intercastes and queens

Differential expression of antioxidant system genes in honey bee (Apis mellifera L.) caste development mitigates ROS-mediated oxidative damage in queen larvae

The expression of morphological differences between the castes of social bees is triggered by dietary regimes that differentially activate nutrient-sensing pathways and the endocrine system, resulting in differential gene expression during larval development. In the honey bee, Apis mellifera, mitochondrial activity in the larval fat body has been postulated as a link that integrates nutrient-sensing via hypoxia signaling. To understand regulatory mechanisms in this link, we measured reactive oxygen species (ROS) levels, oxidative damage to proteins, the cellular redox environment, and the expression of genes encoding antioxidant factors in the fat body of queen and worker larvae. Despite higher mean H₂O₂ levels in queens, there were no differences in ROS-mediated protein carboxylation levels between the two castes. This can be explained by their higher expression of antioxidant genes (MnSOD, CuZnSOD, catalase, and Gst1) and the lower ratio between reduced and oxidized glutathione (GSH/GSSG). In worker larvae, the GSG/GSSH ratio is elevated and antioxidant gene expression is delayed. Hence, the higher ROS production resulting from the higher respiratory metabolism in queen larvae is effectively counterbalanced by the up-regulation of antioxidant genes, avoiding oxidative damage. In contrast, the delay in antioxidant gene expression in worker larvae may explain their endogenous hypoxia response.

Effects of larval Age at Grafting and Juvenile Hormone on Morphometry and Reproductive Quality Parameters of in Vitro Reared Honey Bees (Hymenoptera: Apidae)

The honey bee queen plays a central role in the Apis mellifera L. (Hymenoptera: Apidae) colony, and her high reproductive capacity is fundamental for building up the workforce of a colony. Caste development in honey bee females involves elaborate physiological pathways unleashed at the beginning of the first larval instars, with juvenile hormone (JH) playing a crucial role. Here we took advantage of established in vitro rearing techniques to conduct a 2 × 2 experimental design and test initial rearing age (young vs old) and JH treatment (JH III vs solvent control) to enlighten the role of nutrient quality and JH in shaping honey bee female fertility, morphological features related to queenliness, and key physiological parameters (hemolymph vitellogenin/Vg, sugar levels, and Vg transcript levels). Our results show that while the age at initial larval rearing had major impacts on external morphology development, where younger larvae exhibited a higher probability to develop into queen-like adults morphotypes, the JH application during the larval stage improved physiological pathways related to ovary development and metabolism during the ontogenic development. We detected that the supplementation of queen larvae with JH promoted important benefits regarding queen fertility as the increase of ovariole number and vg levels at hemolymph, both crucial factors at eggs production. The data presented here provide guidance in efforts to improve honey bee queen quality, especially in light of frequent episodes of queen failures in the beekeeping industry.

Mitochondrial structure and dynamics as critical factors in honey bee (Apis mellifera L.) caste development

The relationship between nutrition and phenotype is an especially challenging question in cases of facultative polyphenism, like the castes of social insects. In the honey bee, Apis mellifera, unexpected modifications in conserved signaling pathways revealed the hypoxia response as a possible mechanism underlying the regulation of body size and organ growth. Hence, the current study was designed to investigate possible causes of why the three hypoxia core genes are overexpressed in worker larvae. Parting from the hypothesis that this has an endogenous cause and is not due to differences in external oxygen levels we investigated mitochondrial numbers and distribution, as well as mitochondrial oxygen consumption rates in fat body cells of queen and worker larvae during the caste fate-critical larval stages. By immunofluorescence and electron microscopy we found higher densities of mitochondria in queen larval fat body, a finding further confirmed by a citrate synthase assay quantifying mitochondrial functional units. Oxygen consumption measurements by high-resolution respirometry revealed that queen larvae have higher maximum capacities of ATP production at lower physiological demand. Finally, the expression analysis of mitogenesis-related factors showed that the honey bee TFB1 and TFB2 homologs, and a nutritional regulator, ERR, are overexpressed in queen larvae. These results are strong evidence that the differential nutrition of queen and worker larvae by nurse bees affects mitochondrial dynamics and functionality in the fat body of these larvae, hence explaining their differential hypoxia response.

Development and evolution of caste dimorphism in honeybees - a modeling approach

The difference in phenotypes of queens and workers is a hallmark of the highly eusocial insects. The caste dimorphism is often described as a switch-controlled polyphenism, in which environmental conditions decide an individual's caste. Using theoretical modeling and empirical data from honeybees, we show that there is no discrete larval developmental switch. Instead, a combination of larval developmental plasticity and nurse worker feeding behavior make up a colony-level social and physiological system that regulates development and produces the caste dimorphism. Discrete queen and worker phenotypes are the result of discrete feeding regimes imposed by nurses, whereas a range of experimental feeding regimes produces a continuous range of phenotypes. Worker ovariole numbers are reduced through feeding-regime-mediated reduction in juvenile hormone titers, involving reduced sugar in the larval food. Based on the mechanisms identified in our analysis, we propose a scenario of the evolutionary history of honeybee development and feeding regimes.

Genetic caste determination in harvester ants: possible origin and maintenance by cyto-nuclear epistasis

While reproductive caste in eusocial insects is usually determined by environmental factors, in some populations of the harvester ants, Pogonomyrmex barbatus and P. rugosus, caste has been shown to have a strong genetic component. This system of genetic caste determination (GCD) is characterized by between-caste nuclear variation and high levels of mitochondrial haplotype variation between alternative maternal lineages. Two previous genetic models, involving a single nuclear caste-determining locus or interactions between two nuclear loci, respectively, have been proposed to explain the GCD system. We propose a new model based on interactions between nuclear and mitochondrial genes that can better explain the co-maintenance of distinct nuclear and mitochondrial lineages. In our model, females with coevolved cyto-nuclear gene complexes, derived from intra-lineage mating, develop into gynes, while females with disrupted cyto-nuclear complexes, derived from inter-lineage mating, develop into workers. Both haplodiploidy and inbreeding facilitate the buildup of such coevolved cyto-nuclear complexes within lineages. In addition, the opportunity for both intra-lineage and inter-lineage mating in polyandrous populations facilitates the accumulation of gyne-biasing genes. This model may also help to explain the evolution of workerless social, parasites. We discuss similarities of GCD and cytoplasmic male sterility in plants and how worker production of males would affect the stability of GCD. Finally, we propose experiments and observations that might help resolve the origin and maintenance of this unusual system of caste determination.

Caste development and reproduction: a genome-wide analysis of hallmarks of insect eusociality

The honey bee queen and worker castes are a model system for developmental plasticity. We used established expressed sequence tag information for a Gene Ontology based annotation of genes that are differentially expressed during caste development. Metabolic regulation emerged as a major theme, with a caste-specific difference in the expression of oxidoreductases vs. hydrolases. Motif searches in upstream regions revealed group-specific motifs, providing an entry point to cis-regulatory network studies on caste genes. For genes putatively involved in reproduction, meiosis-associated factors came out as highly conserved, whereas some determinants of embryonic axes either do not have clear orthologs (bag of marbles, gurken, torso), or appear to be lacking (trunk) in the bee genome. Our results are the outcome of a first genome-based initiative to provide an annotated framework for trends in gene regulation during female caste differentiation (representing developmental plasticity) and reproduction.