Caste development and evolution in ants: it's all about size

Abstract

Female ants display a wide variety of morphological castes, including workers, soldiers, ergatoid (worker-like) queens and queens. Alternative caste development within a species arises from a variable array of genetic and environmental factors. Castes themselves are also variable across species and have been repeatedly gained and lost throughout the evolutionary history of ants. Here, we propose a simple theory of caste development and evolution. We propose that female morphology varies as a function of size, such that larger individuals possess more queen-like traits. Thus, the diverse mechanisms that influence caste development are simply mechanisms that affect size in ants. Each caste-associated trait has a unique relationship with size, producing a phenotypic space that permits some combinations of worker-and queen-like traits, but not others. We propose that castes are gained and lost by modifying the regions of this phenotypic space that are realized within a species. These modifications can result from changing the sizefrequency distribution of individuals within a species, or by changing the association of tissue growth and size. We hope this synthesis will help unify the literature on caste in ants, and facilitate the discovery of molecular mechanisms underlying caste development and evolution.

Juvenile hormone interacts with multiple factors to modulate aggression and dominance in groups of orphan bumble bee (Bombus terrestris) workers

“…These findings are consistent with earlier studies showing that body size affects dominance in untreated B. terrestris (P. Van Doorn, 1989) and B. atratus worker bees (Matos and Garofalo, 1995). Body size is also correlated with dominance rank in other social bees (e.g., 2 6 carpenter bees, Withee and Rehan, (2016), Lawson et al, (2017); halictine bee, Smith and Weller, (1989)) as well as other social insects such as paper wasps (Cervo et al, 2008;Chandrashekara and Gadagkar, 1991;Zanette and Field, 2009) and queenless ants (Heinze and Oberstadt, 1999;Nowbahari et al, 1999;Trible and Kronauer, 2017). The common correlation between body size and dominance rank can be explained by the importance of body size in determining the outcome of the agonistic interactions that are used for the establishment of dominance hierarchies.…”

Convergent eusocial evolution is based on a shared reproductive groundplan plus lineage-specific plastic genes

“…Future studies including more species will be necessary to determine the generality of the patterns (e.g., the precise numbers of shared and lineage-specific genes) we found. It is likely that a relatively small number of core conserved genes exist as upstream hubs in regulatory networks, and layered downstream of this core is a myriad of taxonomically-restricted genes as well as conserved genes with lineage-specific expression patterns 6,32,33,60 . This is consistent with models for the evolution of hierarchical developmental gene regulatory networks, whereby a relatively small number of highly conserved genes act upstream to initiate gene cascades (e.g., to set up body-patterning), while batteries of downstream genes are evolutionarily labile and largely responsible for lineage-specific features 61 .…”

Effects of nutritional deprivation on development and behavior in the subsocial beeCeratina calcarata(Hymenoptera: Xylocopinae)

“…In forced association studies, largely solitary species will form social dominance hierarchies, indicating these species possess behavioral precursors to higher levels of social organization (Sakagami and Maeta, 1977;Michener, 1985;Arneson and Wcislo, 2003). Dominance hierarchies are often determined by body size, where the smaller individual assumes a subordinate role (Smith et al, 2009;Trible and Kronauer, 2017); thus, poor larval nutrition would reduce competitive ability (Huntingford and Turner, 1987;Withee and Rehan, 2016). For example, hierarchies in subsocial and primitively eusocial Hymenoptera, such as C. calcarata, Lasioglossum zephyrum and Mischocyttarus mastigophorus, are determined by dominance interactions, where larger females often become the reproductive individual and smaller females serve as workers (Kumar, 1975;Buckle, 1982;Molina and O'Donnell, 2008;Withee and Rehan, 2016).…”