Ant caste evo-devo: it's not all about size

Social administration of juvenile hormone to larvae increases body size and nutritional needs for pupation

Social insects often display extreme variation in body size and morphology within the same colony. In many species, adult morphology is socially regulated by workers during larval development. While larval nutrition may play a role in this regulation, it is often difficult to identify precisely what larvae receive from rearing workers, especially when larvae are fed through social regurgitation. Across insects, juvenile hormone is a major regulator of development. In the ant Camponotus floridanus, this hormone is present in the socially regurgitated fluid of workers. We investigated the role the social transfer of juvenile hormone in the social regulation of development. To do this, we administered an artificial regurgitate to larvae through a newly developed handfeeding method that was or was not supplemented with juvenile hormone. Orally administered juvenile hormone increased the nutritional needs of larvae, allowing them to reach a larger size at pupation. Instead of causing them to grow faster, the juvenile hormone treatment extended larval developmental time, allowing them to accumulate resources over a longer period. Handfeeding ant larvae with juvenile hormone resulted in larger adult workers after metamorphosis, suggesting a role for socially transferred juvenile hormone in the colony-level regulation of worker size over colony maturation.

A caste differentiation mutant elucidates the evolution of socially parasitic ants

Most ant species have two distinct female castes-queens and workers-yet the developmental and genetic mechanisms that produce these alternative phenotypes remain poorly understood. Working with a clonal ant, we discovered a variant strain that expresses queen-like traits in individuals that would normally become workers. The variants show changes in morphology, behavior, and fitness that cause them to rely on workers in wild-type (WT) colonies for survival. Overall, they resemble the queens of many obligately parasitic ants that have evolutionarily lost the worker caste and live inside colonies of closely related hosts. The prevailing theory for the evolution of these workerless social parasites is that they evolve from reproductively isolated populations of facultative intermediates that acquire parasitic phenotypes in a stepwise fashion. However, empirical evidence for such facultative ancestors remains weak, and it is unclear how reproductive isolation could gradually arise in sympatry. In contrast, we isolated these variants just a few generations after they arose within their WT parent colony, implying that the complex phenotype reported here was induced in a single genetic step. This suggests that a single genetic module can decouple the coordinated mechanisms of caste development, allowing an obligately parasitic variant to arise directly from a free-living ancestor. Consistent with this hypothesis, the variants have lost one of the two alleles of a putative supergene that is heterozygous in WTs. These findings provide a plausible explanation for the evolution of ant social parasites and implicate new candidate molecular mechanisms for ant caste differentiation.

The hierarchical basis of serial homology and evolutionary novelty

Given the pervasiveness of gene sharing in evolution and the extent of homology across the tree of life, why is everything not homologous with everything else? The continuity and overlapping genetic contributions to diverse traits across lineages seem to imply that no discrete determination of homology is possible. Although some argue that the widespread overlap in parts and processes should be acknowledged as "partial" homology, this threatens a broad base of presumed comparative morphological knowledge accepted by most biologists. Following a long scientific tradition, we advocate a strategy of "theoretical articulation" that introduces further distinctions to existing concepts to produce increased contrastive resolution among the labels used to represent biological phenomena. We pursue this strategy by drawing on successful patterns of reasoning from serial homology at the level of gene sequences to generate an enriched characterization of serial homology as a hierarchical, phylogenetic concept. Specifically, we propose that the concept of serial homology should be applied primarily to repeated but developmentally individualized body parts, such as cell types, differentiated body segments, or epidermal appendages. For these characters, a phylogenetic history can be reconstructed, similar to families of paralogous genes, endowing the notion of serial homology with a hierarchical, phylogenetic interpretation. On this basis, we propose a five-fold theoretical classification that permits a more fine-grained mapping of diverse trait-types. This facilitates answering the question of why everything is not homologous with everything else, as well as how novelty is possible given that any new character possesses evolutionary precursors. We illustrate the fecundity of our account by reference to debates over insect wing serial homologs and vertebrate paired appendages.

A colony-level optimization model provides a potential mechanism for the evolution of novel castes in eusocial ant colonies

Ant species often have multiple morphologically distinct ‘castes’ within a single colony. Given that most of these castes are involved in non-reproductive tasks, and since such individuals thus never reproduce, the question of how ant castes can evolve is a non-trivial one. Over the years, several models have been proposed in order to explain the evolution of castes in ant colonies. Here, we attempt to answer this question using an economics-based approach, developing an optimization model that implements adaptation and selection at the colony level. We argue that due to the nature of ant colonies, selection is shifted to the group level, and, due to this, individual ants are sheltered from negative selection. We show that our framework can explain the evolution of novel castes in ant colonies, and discuss the novelty of our model with regard to previous models that have been proposed. We also show that our model is consistent with several empirical observations of ant colonies.

My road to the ants: A model clade for eco-evo-devo

This chapter is the story of how I pioneered ants as a system for studying eco-evo-devo, a field that integrates developmental biology with ecology and evolutionary biology. One aim of eco-evo-devo is to understand how the interactions between genes and their environments during development facilitates the origin and evolution of novel phenotypes. In a series of six parts, I review some of the key discoveries from my lab on how novel worker caste systems in ants--soldiers and supersoldiers--originated and evolved. I also discuss some of the ideas that emerged from these discoveries, including the role that polyphenisms, hidden developmental potentials, and rudimentary organs play in facilitating developmental and evolutionary change. As superorganisms, I argue that ants are uniquely positioned to reveal types of variation that are often difficult to observe in nature. In doing so, they have the potential to transform our view of biology and provide new perspectives in medicine, agriculture, and biodiversity conservation. With my story I hope to inspire the next generation of biologists to continue exploring the unknown regions of phenotypic space to solve some of our most pressing societal challenges.

Warm and arid regions of the world are hotspots of superorganism complexity

Biologists have long been fascinated by the processes that give rise to phenotypic complexity of organisms, yet whether there exist geographical hotspots of phenotypic complexity remains poorly explored. Phenotypic complexity can be readily observed in ant colonies, which are superorganisms with morphologically differentiated queen and worker castes analogous to the germline and soma of multicellular organisms. Several ant species have evolved 'worker polymorphism', where workers in a single colony show quantifiable differences in size and head-to-body scaling. Here, we use 256 754 occurrence points from 8990 ant species to investigate the geography of worker polymorphism. We show that arid regions of the world are the hotspots of superorganism complexity. Tropical savannahs and deserts, which are typically species-poor relative to tropical or even temperate forests, harbour the highest densities of polymorphic ants. We discuss the possible adaptive advantages that worker polymorphism provides in arid environments. Our work may provide a window into the environmental conditions that promote the emergence of highly complex phenotypes.