Date Available
5-7-2024
Year of Publication
2024
Degree Name
Doctor of Philosophy (PhD)
Document Type
Doctoral Dissertation
College
Agriculture, Food and Environment
Department/School/Program
Entomology
First Advisor
Dr. Xuguo Zhou
Abstract
Developmental plasticity, manifested as a caste system, is integral to the success of eusocial insects because of the way in which it decouples the various tasks that a single, solitary organism would normally be responsible for, allowing individuals to specialize towards their given roles while keeping vulnerable members of the colony, such as those responsible for reproduction, safe from harm. Termites exhibit particularly complex caste systems that can be divided into three types of castes: workers perform manual labor, such as foraging and brood care; soldiers defend the nest; and reproductives lay eggs. Each of these castes, as well as the interactions between them, is vital to the growth of the colony, yet knowledge concerning differences and interactions among termite castes is lacking. In my dissertation research, using the eastern subterranean termite, Reticulitermes flavipes, the most widely distributed termite species in North America, as a model, I broadly examined the various castes, what differentiates them, and how their plasticity leads to the extreme resiliency observed in termite colonies.
In Chapter 1, I provide a broad overview of the state of termite behavioral research, with a focus on recent advances in the last decade. Topics such as hygienic behaviors, foraging, and vibroacoustic communication are particularly important to the success of termites and have received significant recent attention, although there is still much that remains unknown.
In Chapter 2, I performed a morphometric comparison of brain structure between termite castes, predicting that I would observe polymorphism in brain structure potentially correlated with the different roles and sensory capabilities of each caste. I observed significant differences in brain structure, measured as regional expansion, between castes, which were mapped to distinct brain neuropils. For example, alates, a reproductive caste that disperses from the nest to found new colonies, showed expansion in brain regions that process visual cues. These results provide the groundwork for future study of the termite brain by identifying potential regions of interest for functional study.
In Chapter 3, I manipulated caste ratios within artificially constructed termite colonies to observe the influence of excessive soldier populations on colony fitness, predicting that this “soldier overloading” would negatively impact colony growth. Soldiers can neither feed nor groom themselves and must be maintained by workers. Therefore, excessive amounts of soldiers could potentially place additional stress on the worker caste, negatively impacting worker health. However, I observed no effect of soldier overloading on colony fitness, contrasting with previous studies and suggesting that termite colonies are capable of enduring and restoring significant imbalances in caste composition.
In Chapter 4, I examined the phenomenon of “competitor intimidation”, in which workers indirectly exposed to competitor cues show significant mortality within several days. Previous research indicated that this mortality could be counteracted by the presence of a nestmate soldier, suggesting a vital role in worker-soldier interactions alleviating competitor-induced stress. However, my investigation of this phenomenon indicated that both the timing and severity of competitor cue-induced mortality is influenced by multiple factors, such as the species used and colony origin, obfuscating potential study of worker-soldier interactions and reflecting the complexity of termite behavior.
Overall, my research provides a deeper understanding of various aspects of termite biology, particularly in relation to their castes and the interactions between them.
Digital Object Identifier (DOI)
https://doi.org/10.13023/etd.2024.91
Funding Information
This research was supported by the following funds:
1) A United States Department of Agriculture Predoctoral Fellowship (2021-67034-34975) in 2021.
2) A United States Department of Agriculture Hatch Grant (KY008090) in 2021.
3) A United States Department of Agriculture Hatch Grant (KY008071) in 2019.
Recommended Citation
Merchant, Austin, "Polymorphism and Plasticity in the Termite Caste System" (2024). Theses and Dissertations--Entomology. 82.
https://uknowledge.uky.edu/entomology_etds/82
Data used for statistical analysis
BrainStats.zip (42441 kB)
Stats files used in brain visualization
