The complex social lifestyle associated with phenotypic plasticity make the honey bee, Apis mellifera, an interesting model for Developmental Biology studies. Within the female sex, the queen is the primary reproductive female in a colony, while the workers are subfertile, responsible for colony maintenance. Males (drones) contribute little to the social structure, except for mating with a single queen during a mating flight. Accordingly, most studies on the reproductive system of bees have their focus on the divergent ovarian morphology between queens and workers. They differ in the number of ovarioles that compose their ovaries. This is a result of hormonally regulated programmed cell death in the larval ovary of workers that leads to the destruction of over 90% of the ovariolar primordial, leaving only 2-20 ovarioles in the ovaries of adult workers, while in queens practically all of the larval 150-200 ovarioles persist. Compared to all other bees, honey bee queens have exceptionally large gonads and interestingly, similar to queens, the testes of drones are also composed of a high number of testiolar tubules, which also sets them apart from the males of other bees. This led us to propose the hypothesis that the two sexes of A. mellifera may share gene regulatory modules during the development of their reproductive systems that differ from those of other bees. The genus Apis differs from other bees not only with respect to gonad morphology, but also in their mating system. While in most bees the females are monandric, i.e., they mate only with a single male, honey bee queens are highly polyandric. Although this character combination sets the genus Apis apart from all other bees, this cannot be considered a prerequisite for their advanced state of sociality because in the equally highly eusocial stingless bees the queens are monandric and have ovaries composed of only 4-8 ovarioles per ovary. Thus, in this respect, the stingless bees are more similar to the primitively eusocial bumble bees than to the honey bees. With the aim of understanding the developmental processes underlying the exaggerated gonad morphology in honey bees from an evolutionary perspective, we intend to: (1) identify differentially expressed genes (DEGs) by comparing RNA-Seq libraries from male and female gonads of honey bee larvae and (2) compare these with the gene expression patterns in gonads of stingless bees and bumble bees. In our prior work we already identified DEGs in honey bee ovaries regulated by juvenile hormone, and within the framework of the current project we expect to generate a database that will permit us to build an evolutionary framework that connects gonad development with mating strategies in social bees.
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