Project description:Populations that tolerate extreme environmental conditions with frequent fluctuations can give valuable insights into physiological limits and adaptation. In some estuarine and marine ecosystems, organisms must adapt to extreme and fluctuating salinities, but not much is known how varying salinities impact local adaptation across a wide geographic range. We used eight geographically and genetically divergent populations of the intertidal copepod Tigriopus californicus to test if northern populations have greater tolerance to low salinity stresses, as they experience greater precipitation and less evaporation. We used a common garden experiment approach and exposed all populations to acute low (1, 3ppt) and high (110, 130ppt) salinities for 24 hours, and a fluctuation between baseline salinity and moderate low (7ppt) and high (80ppt) salinities over 49 hours. We also performed RNA-sequencing at several time points during the fluctuation between baseline and 7ppt to understand the molecular basis of divergence between two populations with differing physiological responses. We present these novel findings: 1) acute low salinity conditions caused more deaths than high salinity, 2) molecular processes that elevate proline levels increased in 7ppt, which contrasts with other T. californicus studies that mainly associated accumulation of proline with hyperosmotic stress. We also find that 3) tolerance to a salinity fluctuation did not follow a latitudinal trend, but was instead governed by a complex interplay of factors including population and the duration of salinity stress. This highlights the importance of including a wider variety of environmental conditions in empirical studies to understand local adaptation.
Project description:Background: Geographic variation in the thermal environment impacts a broad range of biochemical and physiological processes and can be a major selective force leading to local population adaptation. In the intertidal copepod Tigriopus californicus, populations along the coast of California show differences in thermal tolerance that are consistent with adaptation, i.e., southern populations withstand thermal stresses that are lethal to northern populations. To understand the genetic basis of these physiological differences, we use an RNA-seq approach to compare genome-wide patterns of gene expression in two populations known to differ in thermal tolerance. Results: Observed differences in gene expression between the southern (San Diego) and the northern (Santa Cruz) populations included both the number of affected loci as well as the identity of these loci. However, the most pronounced differences concerned the amplitude of up-regulation of genes producing heat shock proteins (Hsps) and genes involved in ubiquitination and proteolysis. Cuticle genes were up-regulated in SD but down-regulated in SC, and mitochondrial genes were downregulated in both populations. Among the hsp genes, orthologous pairs show markedly different thermal responses as the amplitude of hsp response was greatly elevated in the San Diego population, most notably in members of the hsp70 gene family. There was no evidence of accelerated evolution at the sequence level for hsp genes. Conclusions: Marked changes in gene expression were observed in response to acute sublethal thermal stress in the copepod T. californicus. Although some qualitative differences were observed between populations (e.g., cuticle gene regulation), the most pronounced differences involved the magnitude of induction of numerous hsp and ubiquitin genes. These differences in gene expression suggest that evolutionary divergence in the regulatory pathway(s) involved in acute temperature stress may offer at least a partial explanation of latitudinal trends in thermal tolerance observed in Tigriopus.
Project description:Background: Geographic variation in the thermal environment impacts a broad range of biochemical and physiological processes and can be a major selective force leading to local population adaptation. In the intertidal copepod Tigriopus californicus, populations along the coast of California show differences in thermal tolerance that are consistent with adaptation, i.e., southern populations withstand thermal stresses that are lethal to northern populations. To understand the genetic basis of these physiological differences, we use an RNA-seq approach to compare genome-wide patterns of gene expression in two populations known to differ in thermal tolerance. Results: Observed differences in gene expression between the southern (San Diego) and the northern (Santa Cruz) populations included both the number of affected loci as well as the identity of these loci. However, the most pronounced differences concerned the amplitude of up-regulation of genes producing heat shock proteins (Hsps) and genes involved in ubiquitination and proteolysis. Cuticle genes were up-regulated in SD but down-regulated in SC, and mitochondrial genes were downregulated in both populations. Among the hsp genes, orthologous pairs show markedly different thermal responses as the amplitude of hsp response was greatly elevated in the San Diego population, most notably in members of the hsp70 gene family. There was no evidence of accelerated evolution at the sequence level for hsp genes. Conclusions: Marked changes in gene expression were observed in response to acute sublethal thermal stress in the copepod T. californicus. Although some qualitative differences were observed between populations (e.g., cuticle gene regulation), the most pronounced differences involved the magnitude of induction of numerous hsp and ubiquitin genes. These differences in gene expression suggest that evolutionary divergence in the regulatory pathway(s) involved in acute temperature stress may offer at least a partial explanation of latitudinal trends in thermal tolerance observed in Tigriopus. For each population, ~600 copepods were split into two equal samples, one for control and one for treatment. Each sample was placed in a 50 mL Falcon tube containing 30 mL filtered seawater. After equilibrating samples to 20 degrees C, each tube was immersed in water bath at its target temperature (control: 20 C; treatment: 35 C) for one hour, and then immersed at 20 C for one hour for recovery. Copepods were then collected in a net mesh and quickly transferred to a tube containing 5 mL Tri-reagent for standard RNA extraction.