DOI: 10.25549/usctheses-c40-341026
title: Tolerance to thermal, osmotic, and copper stress in the intertidal copepod Tigriopus californicus

description: Intertidal environments fluctuate due to many environmental stressors, including temperature, UV exposure, wind, and tidal influences. Organisms that inhabit the intertidal must be capable of surviving stressful conditions, and insight into these response mechanisms may help explain how organisms succeed in challenging environments. ❧ Tigriopus californicus is an intertidal harpacticoid copepod that inhabits high tidepools on rocky outcrops along the western shore of North America, with strong genetic differentiation between populations. As a result of local adaptation, populations have different upper thermal tolerance limits: southern populations can survive higher temperatures than northern populations. ❧ Sixty to seventy population-specific single nucleotide polymorphisms (SNPs) were typed on F₂ hybrid individuals after exposure to thermal, salinity, and copper stress. Quantitative trait locus (QTL) analysis was used to identify loci associated with survivorship. F₂ hybrid individuals sharing combinations of genes from stress-tolerant and stress-sensitive populations varied in their responses to stress challenges. For thermal stress, only one locus, located on chromosome 1 was associated with survivorship, but was too large to identify candidate genes in the region. Other analyses failed to identify QTL that could explain the difference in copper and salinity tolerance between populations. ❧ While thermotolerance has a strong relationship to environmental temperature, levels of copper sufficient to elicit a selective force are orders of magnitude greater than found in marine waters. Both temperature and copper exert oxidative stress creating reactive oxygen species that attack DNA and cellular processes. High levels of copper tolerance may be supported by a robust response to thermal stress because of shared mechanisms. When applied in combination, stressors affecting the same mechanisms may be subject to interactive effects, causing more harm than expected from single-stress assessments. ❧ Three combinations of temperature, copper and salinity give different perspectives on combined tolerance. 1.) High temperature and high salinity commonly co-occur in tidepools because of evaporation. 2.) High temperature and low salinity could co-occur in tidepools if a rain event was followed by high air temperatures, but may be rare. 3.) High temperature and high copper content only occur under laboratory conditions. ❧ Levels of survivorship and fitness under each of the stressors individually and in combination were tested and synergistic effects were observed for some populations and stress combinations. No antagonistic effects were observed, indicating that none of these stressors protects against another of stressors tested. ❧ RNA-sequencing the same individuals in the single and combination testing groups showed few transcripts were shared between the disparate stressors applied individually. Overlapping transcriptomic responses were observed between the high temperature only and combined temperature and copper or salinity conditions, showing that some of the response was driven by high temperatures. Profiles consistent with synergistic interactions were also observed: a large proportion transcripts of transcripts were unique to the combined temperature-copper stress condition, instead of predicted by the stressors assessed individually (47%, including both up- and downregulated transcripts). High and low salinity stress showed a less robust response, in both individual and combined stress conditions, but also had many unique significantly up- and downregulated transcripts (100%). ❧ Stress responses vary between populations, sexes, and stressors, and cannot be easily predicted from individual stressors. In light of climate change, at least additive and synergistic effects of multiple stressors suggest that negative consequences follow increased stress loads. While it is difficult to predict how populations will respond, continued studies of robust stress responses, such as those in T. californicus will help explain how tolerant organisms will continue to success in challenging conditions.