Transcriptional regulation of caste development in the honey bee, Apis mellifera L.

Abstract

The castes of social insects, the queens and workers, were not only a challenge to the Darwinian theory, only being understood in evolutionary terms once the concept of kin selection was developed by Hamilton, but equally defied the understanding of processes underlying ontogenetic questions of developmental biology. The problem herein is that a single genotype can give rise to two different phenotypes as a consequence of differential feeding of the larvae. The present project aims at (i) elucidating signaling pathways and their interaction in caste development of the honey bee, and (ii) analyzing libraries of genes differentially expressed in tissues and organs that show major caste differences in their morphologies.As to the first aspect, we intend to investigate the expression pattern of the gene methoprene-tolerant (met) in queen and worker larvae. Based on results obtained in Drosophila, Tribolium and Pyrrhocoris, Met is the most promising candidate for a juvenile hormone (JH) receptor. JH has for decades been known as a prime factor regulating caste development, but its mode of action was unknown due to the elusive nature of its receptor. Being a member of the HLH-PAS transcription factor family, Met would also have great potential for interacting with other signaling pathways which were either experimentally confirmed as playing a role in honey bee caste development (IIS/TOR and EGFR), or those evidenced as of interest by their expression patterns (the hypoxia pathway core genes). Besides performing correlative analyses of the temporal expression of core genes in these pathways, we also plan to interrupt gene functions by means of RNA interference experiments. With respect to the second aspect, the cellular and molecular basis of caste-specific differentiation of specific organs and tissues we will concentrate, in the case of the ovaries, on a gene (lncov-1) recently sequenced from subtractive libraries and identified as a putative long non-coding RNA. It maps within a QTL for variation in ovariole number of honey bees workers and was localized within an intron of a protein-coding gene of unknown function. Besides analyzing the temporal expression pattern of lncov-1, as well as the gene to which it is intronic, during post-embryonic development of the ovaries, we will also further investigate its intracellular function(s) by means of fluorescent in situ hybridization and colocalization with organelles, with the centrosome being a prime candidate. In the case of the hind legs, which, in the worker caste, carry structures specialized for the collection and transport of pollen, we will as a first step validate the differential expression of genes sequenced from subtractive libraries and subsequently run more profound investigations on the expression patterns of genes that emerged as of special interest in the development of hindleg imaginal discs.Through the combination of hypothesis-driven approaches (candidate genes in signaling pathways) with those not driven by hypotheses (differentially expressed genes in ovaries and hindlegs) we intend to generate an ample base of knowledge on mechanisms of transcriptional regulation underlying the caste development in bees. (AU)