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Evolutionary Transitions Towards Eusociality in Snapping Shrimps ARTICLES PUBLISHED: 20 MARCH 2017 | VOLUME: 1 | ARTICLE NUMBER: 0096 Evolutionary transitions towards eusociality in snapping shrimps Solomon Tin Chi Chak1, 2*, J. Emmett Duffy1 †, Kristin M. Hultgren3 and Dustin R. Rubenstein2 Animal social organization varies from complex societies where reproduction is dominated by a single individual (eusociality) to those where reproduction is more evenly distributed among group members (communal breeding). Yet, how simple groups transition evolutionarily to more complex societies remains unclear. Competing hypotheses suggest that eusociality and com- munal breeding are alternative evolutionary endpoints, or that communal breeding is an intermediate stage in the transition towards eusociality. We tested these alternative hypotheses in sponge-dwelling shrimps, Synalpheus spp. Although species varied continuously in reproductive skew, they clustered into pair-forming, communal and eusocial categories based on several demographic traits. Evolutionary transition models suggested that eusocial and communal species are discrete evolutionary endpoints that evolved independently from pair-forming ancestors along alternative paths. This ‘family-centred’ origin of euso- ciality parallels observations in insects and vertebrates, reinforcing the role of kin selection in the evolution of eusociality and suggesting a general model of animal social evolution. lthough sociality in the form of cooperative group living1 is retention of offspring16. These ideas are further elaborated in the widespread among animals, the structure of animal societ- ‘lifetime monogamy hypothesis’15,17, which suggests that high relat- Aies varies widely2–6. While many species associate tempo- edness among parents and offspring is a necessary precondition for rarily or permanently in the minimal social group consisting of a eusociality to evolve. An alternative to these family-centred models mated pair, many species form larger groups in which the partition- of social evolution is the ‘parasocial model’14, which suggests that ing of reproduction (that is, reproductive skew7) ranges from low eusocial insects evolved from an intermediate ancestor where unre- to highly skewed8. Since reproductive skew is a trait of the social lated breeders of the same generation aggregated to form ‘communal group that varies continuously among species8, it has been used to groups’ characterized by low skew. Recent synthesis suggests distinguish among and define different forms of social organization that communal breeding societies with low skew and low genetic in animals as taxonomically diverse as insects and vertebrates8–11. relatedness among group members are more common than once At one extreme are the eusocial species, which are defined by over- thought1 in both insects (where females share a domicile but do not lapping generations, cooperative care of young, and a reproductive cooperatively care for young18) and vertebrates (where females share division of labour where many group members are temporarily or a domicile but do cooperatively care for young1,3,4). In essence, these permanently sterile. In these species, skew is typically high because alternative hypotheses (the subsocial and parasocial models) for the one or a few individuals monopolize the groups’ reproduction12. evolution of extreme reproductive altruism disagree on whether or In contrast, skew is generally lower in more egalitarian societies not eusocial societies passed through a low-skew, intermediate form where reproduction is shared among many, or even all, group mem- of social organization (such as communal breeding) during their bers3,4,13. Given that reproductive skew is only one of many inter- evolution. Moreover, these models also disagree about whether high connected traits that characterize social syndromes, determining relatedness among the initial group members was a key prerequisite whether these different forms of social organization—high- versus for advanced sociality or a subsequent consequence of group-living, low-skew societies—are variations on a common theme9 or repre- a question that has generated intense debate19,20. Therefore, testing sent independent and alternative social syndromes10,13 remains a whether communally breeding species represent an intermediate fundamental problem in biology that requires testing alternative stage in the evolution of eusociality remains crucial to understand- models of social evolution. ing the evolution of animal sociality. Two alternative models of social evolution have been proposed Clarifying the evolutionary transition to eusociality may also help to explain the evolutionary transition to eusociality from a non- resolve a long-standing debate in social evolution—whether social social ancestor. In insects, it is generally accepted that eusociality organization should be viewed as a continuous or discrete trait. The arose through what has become known as the ‘subsocial model’. concept of a eusocial continuum treats eusocial species as an end- This model suggests that eusocial species evolved from an interme- point along a continuum of reproductive options, which implicitly diate ancestor whose immature offspring remained in the paren- assumes that eusocial species with high reproductive skew evolved tal nest under the mother’s care, forming an aggregation of related via low-skew intermediates (that is, via the parasocial hypoth- individuals that did not interbreed14,15. The subsocial model pro- esis). In contrast, the alternative idea that eusociality is a qualita- posed for insects is similar to the ‘extended-family model’ proposed tively and evolutionarily distinct form of social organization8,9,21 for vertebrates, in which high-skew societies evolved through the marked by unique adaptations such as the lack of reproductive 1Virginia Institute of Marine Science, The College of William and Mary, Gloucester Point, Virginia 23062, USA. 2Department of Ecology, Evolution and Environmental Biology, Columbia University, New York, New York 10027, USA. 3Biology Department, Seattle University, Seattle, Washington 98122, USA. †Present address: Tennenbaum Marine Observatories Network, Smithsonian Institution, PO Box 37012 NHB MRC 106, Washington DC 20013, USA. *e-mail: [email protected] NATURE ECOLOGY & EVOLUTION 1, 0096 (2017) | DOI: 10.1038/s41559-017-0096 | www.nature.com/natecolevol 1 © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. ARTICLES NATURE ECOLOGY & EVOLUTION ab 0.55 CS ≥ 8 CS < 8 0.50 0.45 NOF < 3 NOF ≥ 3 Pair-forming 0.40 Avg. silhouette distance 0.35 0.30 Eusocial Communal 23456 Number of clusters c d 8 Social organizations 2 Communal Eusocial Intermediate 1 Intermediate 2 4 Intermediate 3 1 Pair-forming 2 0 Skewness of colony size Number of ovigerous females –1 1 124816 32 64 128 0.00 0.25 0.50 0.75 Colony size Eusociality index Figure 1 | Demographic classification of social states in Synalpheus shrimp. a, Optimum number of clusters (social categories) as determined from average silhouette distances (si) from PAM analyses with different k (number of clusters) and different input variables. The high si for k =​ 3 indicates that Synalpheus species are best separated into three clusters. Error bars indicate standard errors (n =​ 7). b, Classification tree used to characterize the three clusters, namely pair-forming, communal and eusocial, by colony size (CS) and the number of ovigerous females (NOF). c,d, Bivariate plots of two sets of variables (number of ovigerous females and colony size, and skewness of colony size and eusociality index) that resulted in the highest average si (n =​ 31). Pair-forming, communal and eusocial species were unequivocally identified in all PAM analyses regardless of input variables. Intermediate 1 were clustered with either pair-forming or communal species. Intermediate 2 was clustered with either communal or eusocial species. Intermediate 3 was clustered with either pair-forming, communal or eusocial species. In c, the number of ovigerous females and colony size are shown on geometric scales. totipotency15 or behaviourally distinct castes22 is consistent with the The snapping shrimp genus Synalpheus provides an opportunity subsocial hypothesis in which low-skew societies evolved along their to independently study the evolutionary transition towards eusoci- own evolutionary path, and are not intermediate social forms on ality because these marine animals not only exhibit nearly the full the path towards eusociality. However, whether societies character- range of forms of animal social organization, but have also undergone ized by different forms of social organization evolved continuously at least four relatively recent independent origins of eusociality11. along an axis of reproductive skew or along divergent evolutionary The ~45 West Atlantic species in the gambarelloides species group paths towards different states of social organizations (for example, of Synalpheus31 live obligatorily within the canals of sponges32 and models 1 versus 2 in Results) has yet to be tested broadly outside vary in social organization from pair-forming to communal breed- of the Hymenoptera23,24. ing to eusociality31. Pair-forming is the ancestral state shared by most Examining the evolutionary transition to eusociality is diffi- other alpheid snapping shrimps33. Pair-forming Synalpheus species cult in most insects because eusociality appears to be ancient and live in sponges that typically contain only a single breeding pair31. ancestral to many lineages
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