Juvenile hormone , but not nutrition or social cues ,

18 Eusocial insect colonies are defined by extreme variation in reproductive activity among 19 castes, but the ancestral conditions from which this variation arose are unknown. 20 Investigating the factors that contribute to variation in reproductive physiology among 21 solitary insects that are closely related to social species can help to fill this gap. We 22 experimentally tested the role of nutrition, juvenile hormone, and social cues on 23 reproductive maturation in solitary alkali bees (Halictidae: Nomia melanderi). We find that 24 alkali bee females emerge from overwintering with small Dufour’s glands and small ovaries, 25 containing oocytes in the early stages of development. Oocyte maturation occurs rapidly, 26 and is staggered between the two ovaries. Lab-reared females reached reproductive 27 maturity without access to mates or nesting opportunities, and many had resorbed oocytes. 28 Initial activation of these reproductive structures does not depend on pollen consumption, 29 though dietary protein or lipids may be necessary for long-term reproductive activity. JH is 30 likely to be a limiting factor in alkali bee reproductive activation, as females treated with JH 31 were more likely to develop mature oocytes and Dufour’s glands. Unlike for related social 32 bees, the effects of JH were not suppressed by the presence of older, reproductive females. 33 These results provide important insight into the factors that influence reproductive activity 34 in an important native pollinator, and those that may have been particularly important in 35 the evolution of reproductive castes. 36 37 Introduction 38 Complex social organization, such as that observed among honey bees (Apis mellifera), ants 39 (Formicidae), and vespid wasps (Vespidae), is marked by a high degree of variance in 40 reproductive activity among individuals within a colony. This variation is demarcated among 41 reproductive castes, whereby workers do not reproduce, despite engaging in maternal 42 behaviors (e.g., brood feeding or nest defense), and queens reproduce while largely 43 refraining from brood care (Michener, 1974). Workers of many social insect species have 44 similar reproductive anatomy to queens (e.g., ovaries, spermathecae, glands, ovipositor), 45 yet remain functionally sterile. This suggests the factors that influence the function of these 46 structures differ between queens and workers, and that understanding this variation may 47 provide insights into the physiological basis for the origin of social insect castes. 48 49 The factors that differentially influence reproductive activation among social insect castes 50 include nutritional, endocrine, and social cues (Kapheim, 2017). For example, both queens 51 and workers acting as nurses within a honey bee colony consume a protein-rich diet, but 52 . CC-BY-NC-ND 4.0 International license peer-reviewed) is the author/funder. It is made available under a The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/134387 doi: bioRxiv preprint first posted online May. 4, 2017;