UC San Diego

In supralittoral splash pools, the copepod Tigriopus californicus is frequently exposed to environmental extremes, and synthesis and accumulation of the red ketocarotenoid astaxanthin from dietary precursors may be an important component of survival. To search for evidence of genes involved in the synthesis pathway and for potential further-reaching effects of the pigment, we evaluated the transcriptomic response to astaxanthin synthesis. By comparing RNA sequencing of copepods fed diets with varying amounts of astaxanthin precursors (Tetraselmis algae, spirulina, nutritional yeast) across two timescales (lifelong and raised on yeast but exposed for 16 hours to Tetraselmis), we found limited or mixed responses to the shift in carotenoids and other dietary antioxidants. Although we did not observe evidence in favor of any candidate carotenoid ketolases, the lack of differential expression in response to diet is not evidence against these candidates. A carotenoid oxygenase homologous to vertebrate BCO2 did shift in expression over a short-term exposure to carotenoids, potentially to prevent oxidative stress induced by carotenoid accumulation. Other changes in gene expression may relate to nutrient sensing and assimilation pathways. Both the upregulation of myosins in spirulina-fed copepods and shifts in neural signaling transcript levels in copepods exposed briefly to Tetraselmis may be related to the highly conserved ways that organisms sense, assimilate, and respond to nutrient changes. These include the TOR and IIS pathways. Protease and protease inhibitor levels could be influenced by multiple factors, including shifts in both antioxidant and nutrient assimilation systems.