Female caste development in the honeybee (Apis mellifera)

Abstract

Honeybees are incredible organisms. They produce two morphologically, and behaviourally distinct adult forms from a single genome; a remarkable example of developmental plasticity. The diet a female honeybee larva receives dictates whether it develops into a short-lived infertile worker or a large long-lived fertile queen. By studying gene expression in queen and worker larvae throughout larval development we can gain insight into how the genome is able to respond to signals from the environment and begin to understand how such regulatory mechanisms have evolved.

Previous studies of larval gene expression have been hampered by a lack of appropriate technologies. In the current study, custom honeybee microarrays and high-throughput sequencing technologies were used to study queen and worker gene expression at nine time-points during larval development. This study represents the most comprehensive analysis, of larval caste-specific gene expression in the honeybee, to date.

A large number of genes were found to be differentially expressed between queens and workers throughout development. There are changes in gene expression between queens and workers as early as six hours after differential feeding. It was previously believed that queen and worker larvae followed a similar developmental trajectory up until the third instar. It is clear from the current study that queen and worker larvae follow different developmental trajectories from very early in larval development.

Several key processes and pathways that contribute to these caste-specific developmental trajectories were identified in gene expression studies. Workers up-regulate genes with orthologs in Drosophila involved in muscle development and drug metabolism during early to mid larval development. During late larval development workers up-regulate genes that encode ribosomal proteins and genes involved in programmed cell death. Queens up-regulate physiometabolic genes early in development, putting their energy into cellular growth and maintenance. During mid-larval development they switch to an up-regulation of genes involved in energy generation and then during late development, genes with Drosophila orthologs involved in proteolysis.

In situ hybridisation and larval RNAi, were used to further investigate the role of several candidate genes in caste development. From larval RNAi, the hexamerins were identified as key regulators of caste development. In addition to these studies, a non-coding RNA was found in royal jelly; studies of this transcript identified a possible role in larval survival.

Overall, this study has provided a valuable reservoir of information on caste-specific gene expression during larval development. The data generated in this analysis will facilitate future functional studies to solidify the roles of candidate genes in caste development. This will result in a comprehensive understanding of caste development in the honeybee. This information can then be applied to other organisms to provide insight into the evolution of eusociality, and, more importantly, how the regulation of the genome has evolved to respond to signals from the environment.