Abstract
Stingless bees of the genus Melipona, have long been considered an enigmatic case among social insects for their mode of caste determination, where in addition to larval food type and quantity, the genotype also has a saying, as proposed over 50 years ago by Warwick E. Kerr. Several attempts have since tried to test his Mendelian two-loci/two-alleles segregation hypothesis, but only recently a single gene crucial for sex determination in bees was evidenced to be sex-specifically spliced and also caste-specifically expressed in a Melipona species. Since alternative splicing is frequently associated with epigenetic marks, and the epigenetic status plays a major role in setting the caste phenotype in the honey bee, we investigated here epigenetic chromatin modification in the stingless bee Melipona scutellaris. We used an ELISA-based methodology to quantify global methylation status and western blot assays to reveal histone modifications. The results evidenced DNA methylation/demethylation events in larvae and pupae, and significant differences in histone methylation and phosphorylation between newly emerged adult queens and workers. The epigenetic dynamics seen in this stingless bee species represent a new facet in the caste determination process in Melipona bees and suggest a possible mechanism that is likely to link a genotype component to the larval diet and adult social behavior of these bees.
Highlights
Studies on caste determination in bees have largely benefited from the availability of a well annotated genome sequence from Apis mellifera (The Honey Bee Genome Sequencing Consortium, 2006)
The differential DNA methylation content may play a role in the expression of the caste and sex phenotypes of M. scutellaris, similar to what has been denoted in honey bees
Analyses of the methylome of honey bee larvae revealed differentially methylated genes related to the regulation of juvenile hormone (JH) biosynthesis and showed that splicing is regulated by DNA methylation (Foret et al, 2012; Li-Byarlay et al, 2013)
Summary
Studies on caste determination in bees have largely benefited from the availability of a well annotated genome sequence from Apis mellifera (The Honey Bee Genome Sequencing Consortium, 2006) This directed the focus to a key issue, the role and function of nutrient sensing pathways as connectors between the differential nutrition of the queen and worker larvae and the endocrine signals related to differential gene expression (Hartfelder et al, 2015). The still most accepted mechanistic hypothesis of caste determination in Melipona is the interaction of genetic and environmental factors that jointly influence juvenile hormone (JH) biosynthesis (Velthuis, 1976; Bonetti et al, 1995) According to this hypothesis, double heterozygosity at two not yet identified loci, coupled with an adequate food supply would result in high levels of JH production, and under optimal colony conditions, this predicted mechanism could explain the observed 3:1 worker to queen ratio in newly emerged female brood (Kerr and Nielsen, 1966)
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