Using functional genomics to provide insights into social parasitism by Cape honey bee clonal workers based on mandibular gland pheromones (Apis mellifera capensis Eschscholtz)

Abstract

The queen mandibular gland pheromone (QMP) enables the queen to maintain reproductive dominance by inhibiting worker ovary activation and production of queen-like multi-glandular secretions. While the queen is typically able to maintain the reproductive role in most colonies, some rogue workers evade this strict control to become reproductively active. An extreme form of this evasion is seen in Apis mellifera capensis (Eschscholtz 1822) where, through a short-sighted evolutionary process, a specific invasive lineage of the A. m. capensis workers developed into facultative social parasites. These parasitic workers (A. m. capensis clones) infest susceptible honey bee colonies and develop into pseudo-queens, taking over the role of reproduction. While relatively more has been described regarding the behavioural basis accompanying reproductive parasitism, the genetic basis describing how these morphologically-worker honey bees become pseudo-queens still requires more investigation. Here, the composition of the mandibular gland secretions from young, old and field-collected (age-unspecified) A. m. capensis clones from queenright and queenless social environments was investigated and compared to the mandibular secretions of A. m. scutellata workers (a subspecies with a comparatively lower reproductive potential). Techniques in gas chromatography and functional genomics were used to investigate the chemical composition of the mandibular gland secretions and the molecular pathways involved in the biosynthesis of mandibular gland fatty acid components, respectively. In the first part of this work, it is shown that even at less than 24 hours old, queenless A. m. capensis clones show queen-like signatures in their pheromone profiles through the production of 9-HDA, the precursor to the ‘queen-substance’ 9-ODA. Using high-throughput RNA sequencing technology, about 48 differentially expressed transcripts (DEGs) directly associated with the mandibular gland pheromone biosynthetic pathway in Apis mellifera, were identified. Of these DEGs, 25 were orthologues to Cytochrome P450s, enzymes involved in the caste-specific hydroxylation of acylated stearic acid, a major regulatory point in the biosynthesis of mandibular gland fatty acids. The in-situ biosynthesis and activation of stearic acid, uncompleted β- oxidation and the oxidative conversion of 9-HDA to 9-ODA were also identified as putative points of queen-associated regulation in the multi-step biosynthetic pathway. Finally, using field-collected A. m. capensis clones, it is shown that some host queens can suppress reproductive parasitism in clones by regulating the synthesis of multiple enzymes key in the production of mandibular gland components, such as Cytochrome P450 enzymes and alcohol dehydrogenases, leading to a multi-step regulation of worker reproduction. This work contributes to our understanding of the molecular-level mechanisms related to regulation of reproductive dominance and deepens our understanding of the evolution of reproductive division of labour.