Graduate Thesis Or Dissertation
 

Phenotypic and Transcriptomic Evolution of the Circadian Clock Network in an Intertidal Marine Crustacean

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  • Daily cyclical changes of light are brought on by the axial rotation of the Earth while seasonal changes in light are caused by Earth’s tilt as it revolves around the sun. Major life history traits, such as development, which are tied to metabolic processes, have a strong link to the timing and length of daylight as it is advantageous to be synchronous with a changing environment. The circadian clock is a network of feedback loops of rhythmic genes and proteins that coordinates and entrains cellular processes to the presence of light within our 24-hour day. The molecular components and functioning of the circadian clock responses to photoperiod have been well-studied in many systems, but population-level evolutionary patterns in nature are poorly understood. Moreover, there have been limited studies in marine organisms compared to terrestrial systems, and those that do often focus solely on characterizing the molecular basis of the circadian clock. Further, most studies examine only one species without testing how diverging populations that inhabit unique light regimes may have evolved distinctive strategies within their photoperiod range. To address the gaps in our knowledge we sought to examine divergent, latitudinally separated populations of the intertidal copepod, Tigriopus californicus, across multiple daylengths to uncover differences in response to changing photoperiod, including how a population may respond under a novel photoperiod. We first investigated how allopatric populations of T. californicus exhibit plastic and divergent responses in growth due to changes in daylength. We observed that shorter days promoted increased growth rates but produced smaller sized adults. However, the northern population displayed faster growth on average, while a subset of known circadian genes displayed both plastic and divergent responses to changing photoperiods. To further explore how differences in growth manifested, we examined how total gene expression oscillates under a short-day photoperiod that only one of two allopatric populations of T. californicus naturally inhabits. We uncovered that populations have largely distinct sets of genes that have cyclical rhythms. We found, however, that many of the canonical circadian clock genes oscillated in similar patterns between the populations, suggesting that the core regulatory machinery that drives circadian rhythms is conserved, and therefore that phenotypic differences most likely evolved by divergence in molecules located more peripherally in the gene network. Finally, by examining patterns of alternative splicing (AS) across two photoperiods, we found unique usage of isoforms for both core circadian genes and numerous clock-controlled genes, suggesting that AS may play a large role in modulating responses to photoperiods in coordination with the circadian clock. Together these studies provide novel insights to adaption to photoperiods between divergent populations of a marine invertebrate. We conclude that photoperiods can exert a potentially important selective pressure, driving differences in life histories that have been underexplored in many systems. This body of work will be useful in informing future research that aims to quantify the adaptive potential of diverse organisms to photoperiods in “the light of evolution.”
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  • This work was supported by Oregon State University faculty startup funds to Felipe S. Barreto.
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