Behavioral plasticity and the regulation of caste
Ants represent an outstanding system from which to study mechanisms governing behavioral adaptation, due to the production of multiple alternative behavioral states (castes) which are amenable to experimental manipulation. Previous work by us and others have established a strong experimental basis for inducing and understanding the molecular and neuronal bases for these behavioral states and shifts between them in both H. saltator and C. floridanus, however these are just starting points and what we don’t know far outweighs what we do know.
Previously, on ants
Neurohormonal basis for reproductive caste in H. saltator: Previously, in collaboration with Janko Gospocic (UTSW) we demonstrated that two hormones determine the worker-gamergate axis: Juvenile Hormone (JH3) signaling is associated with the worker state while Ecdysone (20E) is associated with an active reproductive status. We further found that the repressor Kr-h1 is involved in maintaining both of these, via repression of genes normally active only in the alternative caste. Thus a single repressor effects repression of caste-inappropriate genes in the alternative caste via currently-unknown mechanisms. |
Origin, convergence, and regulation of the blood-brain barrier as a regulatory hub for behavioral plasticity: Recently, we demonstrated that one of the defining behavioral difference between Minor and Major workers in C. floridanus, foraging behavior, is controlled by differential degradation of the hormone Juvenile Hormone (JH3) by the workers’ blood-brain barrier (BBB). In Major workers which rarely forage, the BBB expresses high levels of a copy of Juvenile Hormone Esterase (Jhe, CfJhe hereafter), which unlike the copy in D. melanogaster, is retained in the cytoplasm and exclusively expressed in surface glia (cell of the insect BBB). In Minor workers, CfJhe is much more lowly expressed, leading to increased JH3 entry into the brain, and subsequently, foraging. How this mechanism evolved, if such mechanisms exist elsewhere among ants, and how development primes caste-specific BBB activity are all open questions we are actively investigating. |
Projects
The unexpected and exciting finding that the blood-brain barrier regulates worker behavior opens the door to several projects related to understanding 1. Where did this mechanism first originated in ants? 2. Have such mechanisms evolved repeatedly (convergent evolution) in ants with independent origins of multiple worker types and if such hormone-gating by the BBB occurs in other taxa outside of Formicidae (ants)? 3. What other hormones or neuropeptides are gated differentially between ant castes? 4. How does development prime CfJhe (JH3 esterase) for expression immediately preceding adulthood, and how does this fit in the wider context of pupal caste determination? |
We seek to understand the mechanistic basis of context-specific repression via Kr-h1 in both workers and gamergates of H. saltator. This work will be performed in collaboration with long-time colleague and Karl’s science-spouse Janko Gospocic. 1. Is Kr-h1 binding to alternative sets of genes depending on caste mediated by reciprocal nuclear hormone receptors? We have found that Juvenile Hormone (JH3) signaling dominates the worker transcriptome and Ecdysone (20E) dominates that of the gamergate, but how this is translated into differences in Kr-h1 binding is currently not known. 2. Have other such mechanisms evolved independently? We have previously found that a repressive axis involving CoREST and tramtrack acts in C. floridanus to repress hormone-degrading genes (among others) however the involvement of Kr-h1 or another such ‘universal repressor’ is currently unknown. 3. Do these hormonal signaling pathways have a homologous equivalents in mammals? Mammals lack the core hormones we have connected to caste determination (JH3/20E) however functionally homologous hormonal signaling likely exists in mammals, mediated via their associated nuclear hormone receptors. This project will attempt to identify conserved and novel pathways associated with reproductive activation, social dominance, and appetitive responses in mammals, and in collaboration with the Gorbunova lab, evaluate if these act in one of the only eusocial mammals: The Naked Mole Rat. |