Metabolic division of labor in social insects

Social insects are known for reproductive and behavioral division of labor, but little attention 20 has been paid to metabolic forms of division of labor. Metabolic division of labor is the


Introduction 34
Division of labor is a central aspect of living organisms and can be observed across levels of 35 biological organization: between organelles within a cell, between cells within a multicellular 36 organism, between microbes in a community and between multicellular organisms within a 37 social group [1]. Social insects display many types of division of labor including the dissociation 38 of reproduction between workers and reproductives, task specialization within the worker 39 caste, and also metabolic division of labor. While most research has focused on behavioral 40 and reproductive division of labor, less attention has been paid to metabolic division of labor, 41 a potentially defining feature of advanced eusociality. 42 We define metabolic division of labor as the partitioning of a given metabolic process into 43 several elementary metabolic tasks performed by separate units within a cooperative entity. 44 Broadly, metabolic division of labor allows the decoupling of breakdown of incoming food 45 (catabolism) and synthesis of endogenous molecules (anabolism), although there can be much 46 finer subdivisions of subsections of metabolic pathways across individuals. Separating 47 metabolic tasks across individual units requires the transfer of metabolites between cells, 48 tissues or individuals. Thus, for a system to have metabolic division of labor, the system needs 49 a processor, a user and a direct or indirect transfer of metabolized material from the processor 50 to the user. The transfer of metabolized material can be synchronous (direct passage) or 51 asynchronous (externally stored) [2]. Dissociation of producer and user allows signature 52 asymmetries to come about between units (e.g. germline and soma, [3]). 53 Metabolic division of labor occurs between the cells of multicellular organisms [4,5], within 54 microbial communities [6][7][8] and between colony-members in social insect colonies. In 55 multicellular organisms, certain cells produce and secrete molecules into the extracellular 56 space or bloodstream, these molecules travel through the body and are taken up by other cells 57 where they act or are used. In microbial communities, related or unrelated microbes secrete 58 signals or waste-products into the medium and other microbes take these up and use them. 59 An important difference is that the cells of a multicellular organism are clonal and produce such 60 molecules at a clear metabolic cost to themselves, while in microbial communities, cross-61 feeding is more often a form of opportunism, recycling or tit-for-tat cooperation. Social transfers 62 of material are frequent in social insect colonies [9,10], for example through trophallaxis or 63 trophic eggs [11,2]. Across social insects, there are likely a range of degrees of metabolic 64 division of labor depending on within-colony relatedness and reproductive opportunity, from 65 simple recycling to full altruistic cooperation. At the eusocial extreme, fully altruistic metabolic 66 division of labor may be a signature of superorganismality, where it creates an integrated metabolism across the colony in a major evolutionary transition in individuality [1]. To better 68 understand social insects and their major evolutionary transitions to superorganismality, we 69 need a better understanding of metabolic division of labor. 70

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We highlight three forms of transfer that often result in metabolic division of labor: 72 Trophallaxis, trophic eggs, cannibalism (Figure 1). In most cases, social transfer behaviors 73 that came to mediate metabolic division of labor likely evolved originally for another purpose 74 and Non-destructive cannibalism). For division of metabolic labor to occur, the system requires 79 a processor, a user and a direct or indirect transfer of metabolized material from the processor 80 to the user. Here, we highlight ten examples of metabolic division of labor in social insects 81 cited throughout the text and here classified according to their method of transfer. Source 82 refers to the source material that is transformed or metabolized. 83

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Trophic eggs are a high-quality processed food source, where the producer has transformed 85 exogenous food into endogenous, readily usable and storable materials. Trophic eggs are 86 frequently observed in ants and stingless bees [13,14]. Production and consumption of trophic 87 eggs can be considered a form of metabolic division of labor (Figure 1); in some ponerine 88 ants, non-reproductive workers produce trophic eggs and offer them to the reproductives. In 89 invasive yellow crazy ants, larvae are exclusively fed with trophic eggs produced by a special 90 caste of physogastric workers, themselves receiving food from other workers via trophallaxis 91 [15,16]. 92 Trophic eggs as a form of metabolic division of labor could have come about through the 93 combination of worker reproduction and worker policing [13,14]. In species with totipotent 94 workers or where workers have ovaries, it can be in a worker's interest to perform the metabolic 95 labor of transforming exogenous food into eggs. Dominant individuals eat worker-produced 96 eggs through policing and this provides a metabolic 'shortcut' relative to feeding on solely 97 exogenous food. Thus, with these two behaviors in place, all elements required for metabolic 98 division of labor are fulfilled (Figure 1). 99 Social insect larvae have been suggested and shown to provide a metabolic service to adult 112 nestmates, acting as a digestive caste [23,24]. Larva-to-adult and adult-to-larva trophallaxis 113 occurs in ants, bees and wasps. In social wasps, larval regurgitate is a major source of adult 114 nutrition [24,25]. 115 Anal-oral trophallaxis transmits microbes, but it has also been shown to transmit endogenously 116 produced proteins and metabolites [26]. Termites are best-studied for this form of trophallaxis 117 [27], but it has also been documented in ants and bees (Figure 1) [11,28]. A new form of 118 trophallaxis has recently been discovered wherein ant pupae secrete pupal moulting fluid that 119 adults and larvae drink [26]. This fluid is quite similar to that passed during oral-oral trophallaxis 120 [17,26] indicating that more recently evolved forms of trophallaxis, e.g. oral-oral, may have co-121 opted similar currencies from a more ancient form of social transfer, e.g. pupal moulting fluid. 122

Cannibalism: Why and why not metabolic division of labor 123
Cannibalism is observed in response to starvation across social insects [29][30][31][32] Under 124 starvation, it is hard to argue that cannibalized larvae were initially produced for this purpose. 125 But beyond response to suboptimal conditions, there is evidence that larvae are a form of live 126 food storage or act as metabolic processors (Figure 1 hemolymph through small cuts, using larval "secretions" as primary food source. In these 144 "Dracula ants" [41], the queens engage in non-destructive cannibalism of larvae. In other ant 145 species, mostly of the genus Platythyrea, adults drink from evolved taps, glands or tubercles 146 found on their larvae (Figure 1) [42]. 147 Why would a mother drink the blood of her young? For insects that undergo complete 148 metamorphosis, as larvae pass through the stages of growth they must accumulate and store 149 nutrients to provide resources for metamorphosis. If larvae perform the metabolic labor of 150 transforming exogenous nutrients into the ideal composition to build new ant biomass, drinking 151 their hemolymph would allow reproductives to avoid this metabolic labor. 152 Larval storage proteins as currency for Hymenoptera

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The Hymenopteran evolutionary history (parasitic wasps), the habit of feeding on larval and 154 pupal fluids (both rich in larval storage proteins), and the presence of larval storage proteins in 155 adult oral secretions collectively suggest that social Hymenoptera may rely on the currency of 156 larval storage proteins ancestrally sourced from prey [17,19,21,22,43]. 157 Consequences for superorganismality across forms As with other forms of division of labor, the benefits of metabolic division of labor within a 170 cooperative group include the increase in task performance due to task specialization, the 171 reduction in costs related to switching between tasks, and the ability to collectively circumvent 172 trade-offs each entity is facing individually [46]. 173

Complexification of metabolic processes 174
In microbial communities, metabolic division of labor increases the yield of a reaction by 175 reducing metabolic burden [47], and allows for an elongation of metabolic pathways [48]. If 176 these principles apply in social insects, we would expect that the need for complex metabolic 177 processes should favor the emergence of metabolic division of labor. Therefore, in 178 species feeding on a low-quality diet requiring ample processing to create social insect 179 biomass, metabolic division of labor should be necessary for colony productivity (Figure 2). 180 For example, to build insect biomass, nectar and honeydew are likely much lower quality than 181 are insect prey, and require more molecular processing to satisfy larval needs for growth. 182 Colonies of species feeding on low-quality diets tend to be much larger [12], possibly indicating 183 a need for more metabolic laborers. In contrast to nectar and pollen, the royal jelly synthesized 184 by honeybee nurses is a highly elaborated cocktail of proteins and macromolecules [49,50]. 185 Thus, metabolic division of labor may be an adaptive mechanism to manipulate larval diet 186 quality and thus build adaptive asymmetries within the collective (e.g body size, longevity; 187

Building asymmetry through dissociation of metabolic costs and benefits 189
Metabolic division of labor can minimize metabolic costs for reproductives, relegate those costs 190 to more disposable workers or larvae and thus maximize the lifespan and egg production of 191 the reproductive(s), beneficial to colony fitness (Figure 2, [51]). For instance the process of 192 digestion is metabolically costly but essential for nutrient intake. Thus allocating metabolic cost 193 related to digestion of exogenous macronutrients to the worker caste and the intake of 194 processed nutrients to the queen castes would optimize egg production while minimizing 195 metabolic costs to queens [21,52,53].

Queens and workers divergence in body size, lifespan and reproduction 219
Metabolic division of labor leads to an unequal allocation of metabolic costs and benefits 220 between individuals, and as such, it can accentuate asymmetries between colony members. 221 These asymmetries can have developmental and life history consequences [51][52][53] including 222 the adult size dimorphism or differences in longevity and fecundity between reproductives and 223 workers (Figure 2). Termites workers are lacking of uric-acid oxidase which prevent them from 224 using stocks of uric acid as a source of nitrogen; instead this enzyme is highly abundant in 225 reproductives which can thus metabolize uric acid that they receive from non-reproductives 226 and use this nitrogen source for their reproduction [20]. 227 228

Reinforcement of superorganismality 229
Colony level-metabolic division of labor leads to increasing interdependence between colony 230 members, and thus reinforces the higher-level individual, the colony or superorganism. In 231 larvae of species that are fed by trophallaxis, larval development is dependent on trophallactic 232 fluid metabolites and proteins provided by workers, as in the case of the honeybee [50]. 233

Development of anatomical structures 235
Metabolic division of labor can favor the specialization of anatomical structures that enable the 236 transfer (Figure 2). For trophallaxis, the proventriculus separating the crop from the midgut is 237 highly elaborated in ants that engage in trophallaxis [11]. Glands can also be highly elaborated, Given the novelty of this view of social insects, we need to explore the importance of metabolic 255 division of labor to colony fitness in order to ultimately establish its role in social insect life-256 history evolution. 257 The fact that metabolic division of labor occurs so broadly in social insects will also inform our 258 understanding of superorganismality, and the major evolutionary transitions in individuality that 259 have occurred multiple times across social insect lineages. As opposed to multicellular 260 systems where metabolic division of labor occurs between cells, in social insects it occurs 261 between distinct and accessible individuals, making social insects a good study system to 262 explore networked metabolism.

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Instead, they found that division of labor or asymmetries induce the evolution of differential aging 288 between constituent parts, typically the separation between germline and soma.  ** Ant species engaging in mouth-to-mouth trophallaxis are ecologically successful and tend to have 317 larger colonies than species that do not display this behavior. In this article authors have investigated 318 how and why this behavior evolved. They found that trophallaxis evolved primarily in two major events 319 in lineages that include over one third of all ant species. They show that evolution of trophallaxis was 320 associated with having a liquid diet but also with reduced intra-colonial conflict, making trophallaxis a 321 signature of superorganismality.

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13. subcaste specialized in the production of trophic eggs. The authors have monitored the dissemination 335 of dyed food in the colony through mouth-to-mouth trophallaxis. They show that most of the food is 336 received by physogastric workers, and confirm that the exclusive food source of larvae are trophic eggs.

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They thus highlight the critical role of the physogastric workers as trophic-eggs center specialized in the 338 transformation of exogenous food endogenously produced diet for larvae.

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They show that individuals exchange insect storage proteins, antioxidants and digestive enzymes, and