Plasticity of physiological caste in a social insect
Solis, Carlos Rodolfo
Strassmann, Joan E.
Doctor of Philosophy
In social insect colonies workers realize their reproductive potential through rearing the queen's brood. In the primitively eusocial wasp Polistes exclamans (Hymenoptera: Vespidae) castes are not morphologically distinct. This species has an annual colony cycle that begins when mated, hibernated females start colonies in the spring. One foundress becomes the queen while the rest become workers. The worker population grows throughout the year until the appearance of gynes, females that do not work but become foundresses the following spring. Males are also produced by the end of the season, but they die in winter with the workers. While morphological caste differences are absent, caste can be identified using behavioral and physiological parameters. In contrast with other social insects, female caste remains undetermined until adulthood, and even then females switch between castes under the appropriate circumstances. This can be advantageous since high nest predation rates and unpredictable environmental variation, accompanied by frequent queen supersedure are typical. I studied three aspects of the P. exclamans caste system: (i) morphology and physiology in gynes and workers; (ii) seasonal resource allocation into brood and (iii) effects of brood loss on caste determination. Physiological indicators were derived from qualitative and quantitative studies of biochemical components, and from measurements of metabolic rates. I show that differences between castes are mostly quantitative, rather than qualitative. I suggest that lack of physical and qualitative differentiation is what allows caste transitions. Seasonal resource allocation on brood shows that while young, adult females produced throughout the season increase in weight and size, energy reserves lipids peak in May and June. I suggest that this peak in reserves is related to high predation experienced by colonies during that time and that it allows females to reconstruct their nests more efficiently. Finally, I examine effects of loss of brood in late summer and early fall colonies showing that females that do not have the opportunity to care for brood develop gyne-like characters, but that females that have develop worker-like characters. These results show that caste in P. exclamans is plastic, behaviorally and physiologically, allowing individuals to respond to varying environmental and social conditions.
Physiology; Entomology; Ecology