Multiple lineages, same molecular basis: task specialization is commonly regulated across all eusocial bee groups

Abstract

14A striking feature of advanced insect societies is the existence of workers that forgo 15 reproduction. Two broad types of workers exist in eusocial bees: nurses which care 16 for their young siblings and the queen, and foragers who guard the nest and forage for 17 food. Comparisons between this two worker subcastes have been performed in 18 honeybees, but data from other bees are scarce. To understand whether similar 19 molecular mechanisms are involved in nurse-forager differences across distinct 20 species, we compared gene expression and DNA methylation profiles between nurses 21 and foragers of the buff-tailed bumblebee Bombus terrestris and of the stingless bee 22Tetragonisca angustula. These datasets were then discussed comparatively to 23 previous findings on honeybees. Our analyses revealed that although the expression 24 pattern of genes is often species-specific, many of the biological processes and 25 molecular pathways involved are common. Moreover, DNA methylation and gene 26 expression correlation were dependent on the nucleotide context. 27 subcastes evolved it is necessary to differentiate such more recent changes -that 53 could be species-specific -from those shared across species, and thus likely ancestral. 54The highly eusocial stingless bees have age-based division of labour 18 , similarly to 55 that of honeybees despite their most common ancestor being 50 to 80 million years 56 ago 19,20 . To date, no global expression or epigenetic studies have been performed in 57 stingless bees to understand worker task specialization. Similarly, while primitively 58 eusocial bumblebees are largely studied ecological biological models and important 59 wild and managed pollinators, we know comparatively little about the molecular 60 underpinnings of differences between its worker subcastes. Indeed, studies have been 61 restricted to few genes, leaving many open questions [21][22][23] . A major limiting element 62 for these studies is that this species display a somewhat fluctuating division of labour 63 with indistinctive separation between subcastes 10,21,23 . 64We aim to fill in this knowledge gap through the analyses of the global gene 65 expression differences between nurses and foragers, and the characterization of nurses 66 DNA methylation profile in two eusocial bee species, the primitively eusocial buff-67 tailed bumblebee, Bombus terrestris, and the highly eusocial stingless bee, 68Tetragonisca angustula. Combined, these two bee species and the honeybee represent 69 the three evolutionary branches of eusocial corbiculates sharing a common social 70 origin 24 . Hence, in addition to using the generated datasets to uncover unique and 71 more recent molecular traits linked to task division in B. terrestris and T. angustula, 72 we also verified whether common genes and pathways could be involved in task 73 specialization across all the eusocial bee groups. 74 75Results 76 task-dependent foraging gene expression in the bumblebee Bombus terrestris. Arch. Insect