Tigriopus Californicus Research Articles

Absence of heterosis for hypoxia tolerance in F1 hybrids of Tigriopus californicus.

Hybridization produces a range of outcomes from advantageous to disadvantageous, and a goal of genetic research is to understand the gene interactions that generate these outcomes. Interactions between cytoplasmic elements, such as mitochondria, and the nucleus may be particularly vulnerable to accruing disadvantageous combinations as a result of their different rates of evolution. However, mitonuclear incompatibilities often do not have an observable effect until the F2 and later generations. We used Tigriopus californicus, a model system for mitonuclear incompatibilities that is also known for exhibiting heterosis in the F1 generation, to test whether hypoxia was more stressful for mitonuclear interactions than other environmental stressors. We generated 284 parental and 436 F1 hybrids from four population crosses (720 total) and compared parental and F1 populations for hypoxia tolerance. We observed that, on average, F1 hybrids were less likely to survive a hypoxia stress test than parental populations (Parental:F1 coefficients ranged from -0.04 to 0.14 with none significantly different from 0). This suggests that hypoxia may be a particularly intense stressor for mitonuclear coordination, and that hybridization outcomes vary by trait.

Mitonuclear effects on sex ratio persist across generations in interpopulation hybrids.

Eukaryotic energy production requires tight coordination between nuclear and mitochondrial gene products. Because males and females often have different energetic strategies, optimal mitonuclear coordination may be sex-specific. Previous work found evidence for sex-specific mitonuclear effects in the copepod Tigriopus californicus by comparing two parental lines and their reciprocal F1 crosses. However, an alternative hypothesis is that the patterns were driven by the parental source of nuclear alleles. Here we test this alternative hypothesis by extending the same cross to F2 hybrids, who receive both maternal and paternal nuclear alleles from F1 hybrids. Results confirm mitonuclear effects on sex ratio, with distorted ratios persisting from the F1 to F2 generations, despite reduced fitness in F2 hybrids. No sex by cross interactions were found for other phenotypic traits measured. Mitochondrial DNA content was higher in females. Both routine metabolic rate and oxidative DNA damage were lower in F2 hybrids than in parentals. The persistence of sex-specific mitonuclear effects, even in the face of F2 hybrid breakdown, attests to the magnitude of these effects, which contribute to the maintenance of within-population mitochondrial DNA polymorphisms.

Interactive effects of temperature and salinity on metabolism and activity of the copepod Tigriopus californicus.

In natural environments, two or more abiotic parameters often vary simultaneously, and interactions between co-varying parameters frequently result in unpredictable, non-additive biological responses. To better understand the mechanisms and consequences of interactions between multiple stressors, it is important to study their effects on not only fitness (survival and reproduction) but also performance and intermediary physiological processes. The splash-pool copepod Tigriopus californicus tolerates extremely variable abiotic conditions and exhibits a non-additive, antagonistic interaction resulting in higher survival when simultaneously exposed to high salinity and acute heat stress. Here, we investigated the response of T. californicus in activity and oxygen consumption under simultaneous manipulation of salinity and temperature to identify whether this interaction also arises in these sublethal measures of performance. Oxygen consumption and activity rates decreased with increasing assay salinity. Oxygen consumption also sharply increased in response to acute transfer to lower salinities, an effect that was absent upon transfer to higher salinities. Elevated temperature led to reduced rates of activity overall, resulting in no discernible impact of increased temperature on routine metabolic rates. This suggests that swimming activity has a non-negligible effect on the metabolic rates of copepods and must be accounted for in metabolic studies. Temperature also interacted with assay salinity to affect activity, and with acclimation salinity to affect routine metabolic rates upon acute salinity transfer, implying that the sublethal impacts of these co-varying factors are also not predictable from experiments that study them in isolation.

Optimal outbreeding is shaped during larval life-history in the splash pool copepod Tigriopus californicus.

Inbreeding and outbreeding depression are dynamic forms of selection critical to mating system evolution and the efficacy of conservation biology. Most evidence on how the relative severity and timing of these forces are shaped is confined to self-fertilization, distant outcrossing, and intermediate 'optimal outcrossing' in hermaphrodites. We tested the notion that closed population demographics may reduce and delay the costs of inbreeding relative to distant outbreeding in an intertidal copepod with separate sexes and a biphasic larval / post-metamorphic life-history (Tigriopus californicus). At three lifecycle stages (fecundity, metamorphosis, and post-metamorphosis), we quantified the effects of inbreeding and outbreeding in crosses with varying degrees of recent common ancestry. Although inbreeding and outbreeding depression have distinct genetic mechanisms, both manifested the same stage-specific consequences for fitness. Inbreeding and outbreeding depression were not apparent for fecundity, post-metamorphic survival, sex ratio, or the ability to acquire mates, but inbreeding between full siblings and outbreeding between interpopulation hybrids reduced the fraction of offspring that completed metamorphosis by 32% and 47%, respectively. On average, the effects of inbreeding on metamorphic rate were weaker and nearly twice as variable among families than those of outbreeding, suggesting genetic load was less pervasive than the incompatibilities accrued between divergent populations. Overall, our results indicate the transition from larval to juvenile life stages is markedly susceptible to both inbreeding and outbreeding depression in T. californicus. We suggest stage-specific selection acting concurrently with the timing of metamorphosis may be an instrumental factor shaping reproductive optima in species with complex life-histories.

The role of mitochondria in sex- and age-specific gene expression in a species without sex chromosomes

Mitochondria perform an array of functions, many of which involve interactions with gene products encoded by the nucleus. These mitochondrial functions, particularly those involving energy production, can be expected to differ between sexes and across ages. Here, we measured mitochondrial effects on sex- and age-specific gene expression in parental and reciprocal F1 hybrids between allopatric populations of Tigriopus californicus with over 20% mitochondrial DNA divergence. Because the species lacks sex chromosomes, sex-biased mitochondrial effects are not confounded by the effects of sex chromosomes. Results revealed pervasive sex differences in mitochondrial effects, including effects on energetics and aging involving nuclear interactions throughout the genome. Using single-individual RNA sequencing, sex differences were found to explain more than 80% of the variance in gene expression. Males had higher expression of mitochondrial genes and mitochondrially targeted proteins (MTPs) involved in oxidative phosphorylation (OXPHOS), while females had elevated expression of non-OXPHOS MTPs, indicating strongly sex-dimorphic energy metabolism at the whole organism level. Comparison of reciprocal F1 hybrids allowed insights into the nature of mito-nuclear interactions, showing both mitochondrial effects on nuclear expression, and nuclear effects on mitochondrial expression. While based on a small set of crosses, sex-specific increases in mitochondrial expression with age were associated with longer life. Network analyses identified nuclear components of strong mito-nuclear interactions and found them to be sexually dimorphic. These results highlight the profound impact of mitochondria and mito-nuclear interactions on sex- and age-specific gene expression.

Thermal plasticity has higher fitness costs among thermally tolerant genotypes of Tigriopus californicus

Abstract Under climate change, ectotherms will likely face pressure to adapt to novel thermal environments by increasing their upper thermal tolerance and its plasticity, a measure of thermal acclimation. Ectotherm populations with high thermal tolerance are often less thermally plastic, a trade‐off hypothesized to result from (i) a phenotypic limit on thermal tolerance above which plasticity cannot further increase the trait, (ii) negative genetic correlation or (iii) fitness trade‐offs between the two traits. Whether each hypothesis causes negative associations between thermal tolerance and plasticity has implications for the evolution of each trait. We empirically tested the limit and trade‐off hypotheses by leveraging the experimental tractability and thermal biology of the intertidal copepod Tigriopus californicus. Using populations from four latitudinally distributed sites in coastal California, six lines per population were reared under a laboratory common garden for two generations. Ninety‐six full sibling replicates (n = 4–5 per line) from a third generation were developmentally conditioned to 21.5 and 16.5°C until adulthood. We then measured the upper thermal tolerance and fecundity of sibships at each temperature. We detected a significant trade‐off in fecundity, a fitness corollary, between baseline thermal tolerance and its plasticity. Tigriopus californicus populations and genotypes with higher thermal tolerance were less thermally plastic. We detected negative directional selection on thermal plasticity under ambient temperature evidenced by reduced fecundity. These fitness costs of plasticity were significantly higher among thermally tolerant genotypes, consistent with the trade‐off hypothesis. This trade‐off was evident under ambient conditions, but not high temperature. Observed thermal plasticity and fecundity were best explained by a model incorporating both the limit and trade‐off hypotheses rather than models with parameters associated with one hypothesis. Effects of population and family on tolerance and plasticity negatively covaried, suggesting that a negative genetic correlation could not be ruled as contributing to negative associations between the traits. Our study provides a novel empirical test of the fitness trade‐off hypothesis that leverages a strong inference approach. We discuss our results' insights into how thermal adaptation may be constrained by physiological limits, genetic correlations, and fitness trade‐offs between thermal tolerance and its plasticity. Read the free Plain Language Summary for this article on the Journal blog.

Variación estacional del contenido de lípidos en Tigriopus californicus (Backer) (Copépoda, harpacticoida)

Seasonal variations of the body weight, water content and lipid amount in adult individuals of Tigriopus californicus (Backer) from peninsula Foca, Puerto Deseado (Sta. Cruz) were studied. Correlations among them were found.
 Experiences were realized during 1978 and 1979, using 100 individuals as optimun number for each test, taking them at random.
 Maximun values for body weight and for water content are observed during the spring and minimun values at the end of the summer. In relation to the lipid percentage (based in the dry weight) it reaches its maximum values during the spring, existing an inverse correlation with the former parameters.
 It was also proved the influence of the ovigerous female on the lipid content of the adult population during 1978.
 Maximun values were obtained in summer and minimum ones in spring, according to the population movement of the species (Lombardo y Pallares, 1981).
 Complementary data of nitrogen and protein percentages (based in the dry weight) during 1979 were given. In addition to the lipid and protein percentage, the calorific contenta owing to them are expressed in Kcal/g of dry weight.

Disproportionate role of nuclear-encoded proteins in organismal and mitochondrial thermal performance in a copepod.

Determining the mechanisms by which organisms evolve thermal tolerance is crucial to predicting how populations may respond to changes in local temperature regimes. Although evidence of relationships between mitochondrial background and thermal adaptation have been found, the presence of both nuclear-encoded and mitochondrial DNA (mtDNA)-encoded proteins warrants experiments aimed at parsing out the relative role of each genome in thermal adaptation. We investigated the relative role of mtDNA-encoded products in thermal tolerance between two divergent populations of Tigriopus californicus using first-generation (F1) hybrids that vary in maternally inherited mtDNA but are heterozygous for population-specific alleles across nuclear loci. We tested two measures of thermal tolerance, (1) survivorship to acute thermal stress and (2) thermal stability of mitochondrial performance in Complex I-fueled ATP synthesis, both across a range of increasing temperatures. We found that the southern population (San Diego, CA, USA) outperformed the northern population (Strawberry Hill, OR, USA) in survivorship, and that both reciprocal F1 hybrid crosses had intermediate survival. Mitochondria from the San Diego population displayed greater stability in ATP synthesis with increasing temperatures compared with those from Strawberry Hill. Interestingly, hybrids from both cross directions had synthesis profiles that were very similar to that of Strawberry Hill. Taken together, these results suggest that the relative role of the mtDNA in these phenotypes is negligible compared with that of elements encoded by nuclear DNA in this system.

Genetic incompatibilities in reciprocal hybrids between populations of Tigriopus californicus with low to moderate mitochondrial sequence divergence.

All mitochondrial-encoded proteins and RNAs function through interactions with nuclear-encoded proteins, which are critical for mitochondrial performance and eukaryotic fitness. Coevolution maintains inter-genomic (i.e., mitonuclear) compatibility within a taxon, but hybridization can disrupt coevolved interactions, resulting in hybrid breakdown. Thus, mitonuclear incompatibilities may be important mechanisms underlying reproductive isolation and, potentially, speciation. Here we utilize Pool-seq to assess the effects of mitochondrial genotype on nuclear allele frequencies in fast- and slow-developing reciprocal inter-population F2 hybrids between relatively low-divergence populations of the intertidal copepod Tigriopus californicus. We show that mitonuclear interactions lead to elevated frequencies of coevolved (i.e., maternal) nuclear alleles on two chromosomes in crosses between populations with 1.5% or 9.6% fixed differences in mitochondrial DNA nucleotide sequence. However, we also find evidence of excess mismatched (i.e., noncoevolved) alleles on three or four chromosomes per cross, respectively, and of allele frequency differences consistent with effects involving only nuclear loci (i.e., unaffected by mitochondrial genotype). Thus, our results for low-divergence crosses suggest an underlying role for mitonuclear interactions in variation in hybrid developmental rate, but despite substantial effects of mitonuclear coevolution on individual chromosomes, no clear bias favoring coevolved interactions overall.

Genomic Architecture of Hybrid Male Sterility in a Species Without Sex Chromosomes (Tigriopus californicus, Copepoda: Harpacticoida).

Sterility among hybrids is one of the most prevalent forms of reproductive isolation delineating species boundaries and is expressed disproportionately in heterogametic XY males. While hybrid male sterility (HMS) due to the "large X effect" is a well-recognized mechanism of reproductive isolation, it is less clear how HMS manifests in species that lack heteromorphic sex chromosomes. We evaluated differences in allele frequencies at approximately 460,000 SNPs between fertile and sterile F2 interpopulation male hybrids to characterize the genomic architecture of HMS in a species without sex chromosomes (Tigriopus californicus). We tested associations between HMS and mitochondrial-nuclear and/or nuclear-nuclear signatures of incompatibility. Genomic regions associated with HMS were concentrated on a single chromosome with the same primary 2-Mbp regions identified in one pair of reciprocal crosses. Gene Ontology analysis revealed that annotations associated with spermatogenesis were the most overrepresented within the implicated region, with nine protein-coding genes connected with this process found in the quantitative trait locus of chromosome 2. Our results indicate that a narrow genomic region was associated with the sterility of male hybrids in T. californicus and suggest that incompatibilities among select nuclear loci may replace the large X effect when sex chromosomes are absent.

Loss of mitochondrial performance at high temperatures is correlated with upper thermal tolerance among populations of an intertidal copepod.

Environmental temperatures have pervasive effects on the performance and tolerance of ectothermic organisms, and thermal tolerance limits likely play key roles underlying biogeographic ranges and responses to environmental change. Mitochondria are central to metabolic processes in eukaryotic cells, and these metabolic functions are thermally sensitive; however, potential relationships between mitochondrial function, thermal tolerance limits and local thermal adaptation in general remain unresolved. Loss of ATP synthesis capacity at high temperatures has recently been suggested as a mechanistic link between mitochondrial function and upper thermal tolerance limits. Here we use a common-garden experiment with seven locally adapted populations of intertidal copepods (Tigriopus californicus), spanning approximately 21.5° latitude, to assess genetically based variation in the thermal performance curves of maximal ATP synthesis rates in isolated mitochondria. These thermal performance curves displayed substantial variation among populations with higher ATP synthesis rates at lower temperatures (20-25°C) in northern populations than in southern populations. In contrast, mitochondria from southern populations maintained ATP synthesis rates at higher temperatures than the temperatures that caused loss of ATP synthesis capacity in mitochondria from northern populations. Additionally, there was a tight correlation between the thermal limits of ATP synthesis and previously determined variation in upper thermal tolerance limits among populations. This suggests that mitochondria may play an important role in latitudinal thermal adaptation in T. californicus, and supports the hypothesis that loss of mitochondrial performance at high temperatures is linked to whole-organism thermal tolerance limits in this ectotherm.

Developmental rate displays effects of inheritance but not of sex in interpopulation hybrids of Tigriopus californicus.

Coevolved genetic interactions within populations can be disrupted by hybridization resulting in loss of fitness in hybrid individuals (i.e., hybrid breakdown). However, the extent to which variation in fitness-related traits among hybrids is inherited across generations remains unclear, and variation in these traits may be sex-specific in hybrids due to differential effects of genetic incompatibilities in females and males. Here we present two experiments investigating variation in developmental rate among reciprocal interpopulation hybrids of the intertidal copepod Tigriopus californicus. Developmental rate is a fitness-related trait in this species that is affected by interactions between mitochondrial-encoded and nuclear-encoded genes in hybrids that result in variation in mitochondrial ATP synthesis capacities. First, we show that F2 -hybrid developmental rate is equivalent in two reciprocal crosses and is unaffected by sex, suggesting that breakdown of developmental rate is likely experienced equally by females and males. Second, we demonstrate that variation in developmental rate among F3 hybrids is heritable; times to copepodid metamorphosis of F4 offspring of fast-developing F3 parents (12.25 ± 0.05 days, μ ± SEM)were significantly faster than those of F4 offspring of slow-developing parents (14.58 ± 0.05 days). Third, we find that ATP synthesis rates in these F4 hybrids are unaffected by the developmental rates of their parents, but that mitochondria from females synthesize ATP at faster rates than mitochondria from males. Taken together, these results suggest that sex-specific effects vary among fitness-related traits in these hybrids, and that effects likely associated with hybrid breakdown display substantial inheritance across hybrid generations.

Variance components of sex determination in the copepod Tigriopus californicus estimated from a pedigree analysis.

Extensive theory exists regarding population sex ratio evolution that predicts equal sex ratio (when parental investment is equal). In most animals, sex chromosomes determine the sex of offspring, and this fixed genotype for sex has made theory difficult to test since genotypic variance for the trait (sex) is lacking. It has long been argued that the genotype has become fixed in most animals due to the strong selection for equal sex ratios. The marine copepod Tigriopus californicus has no sex chromosomes, multiple genes affecting female brood sex ratio, and a brood sex ratio that responds to selection. The species thus provides an opportune system in which to test established sex ratio theory. In this paper, we further our exploration of polygenic sex determination in T. californicus using an incomplete diallel crossing design for analysis of the variance components of sex determination in the species. Our data confirm the presence of extra-binomial variance for sex, further confirming that sex is not determined through simple Mendelian trait inheritance. In addition, our crosses and backcrosses of isofemale lines selected for biased brood sex ratios show intermediate phenotypic means, as expected if sex is a threshold trait determined by an underlying "liability" trait controlled by many genes of small effects. Furthermore, crosses between families from the same selection line had similar increases in phenotypic variance as crosses between families from different selection lines, suggesting families from artificial selection lines responded to selection pressure through different underlying genetic bases. Finally, we estimate heritability of an individual to be male or female on the observed binary scale as 0.09 (95% CI: 0.034-0.14). This work furthers our accumulating evidence for polygenic sex determination in T. californicus laying the foundation for this as a model species in future studies of sex ratio evolution theory.

Northern copepod mitochondria can't beat the heat

How hot is too hot and why is it too hot? Unravelling the physiology of how animals cope with warming environments can tell us a lot about their evolutionary history as well as their current (and future) habitats. One possible mechanism by which an animal's physiology could limit its heat tolerance is if high temperatures interfered with routine but important metabolic processes such as making energy, ATP, by the mitochondria. Copepods, which often live in splash pools above the high tide line, adapt to the peculiarities of their particular pools. The copepod Tigriopus californicus has made its home in rock pools from balmy Mexico to chilly northern California, and has adapted its mitochondria to the vastly differing climes. With beakers of these copepods, descended from various populations, available in the lab, Tim Healy and Ron Burton from the Scripps Institution of Oceanography, USA, compared how quickly the mitochondria from different populations of copepods made ATP when in warm water and how that related to each populations’ natural habitat to find out whether their mitochondria influenced what temperatures they tolerated and, in doing so, their natural habitat ranges.For each population, the researchers isolated mitochondria from several individuals and provided them with all the materials they needed to make ATP. The team then compared how quickly the mitochondria did this at a range of temperatures from 20°C to a scorching 36°C, and found that the mitochondria from copepods from cold climates generally made ATP faster than those from warm climates, especially at temperatures below 25°C; although that was not surprising, because some animals such as copepods can counteract the natural slowing effect of cold temperatures on their metabolism. However, the northern population's mitochondria were also much more sensitive to high temperatures, as their ability to make ATP declined sharply when the temperature was above the mid-30s. Meanwhile, the mitochondria from southern copepods resisted the heat and happily made ATP at temperatures hot enough to hinder making ATP in the northern populations. Taken together, this suggested that the mitochondria of each population are best adjusted to make ATP at the temperatures that they would naturally encounter in the wild. Healy and Burton then turned their attention to figuring out whether the differences in mitochondrial physiology uncovered in the lab contributed to differences in heat tolerance in the wild.The duo compared the temperature at which each population of copepods made ATP half as fast as normal with the temperature at which they stopped swimming, a sign that it was too hot for them, and discovered that the southern populations, whose mitochondria could churn out decent amounts of ATP at relatively high temperatures, also tended to keep swimming at hotter temperatures, supporting the idea that copepods hit their upper temperature limit when their mitochondria stop performing well. This suggested that having ‘southern style’ or ‘northern style’ mitochondria not only impacted which temperatures the copepods could make the most ATP, but also which temperatures they could survive in the wild.Taken together, it is clear that northern populations of copepods like the cold, while southern populations like the heat, and how fast mitochondria can make ATP at these temperatures underlies at least some of this variation. But how copepod mitochondria react to heat does more than distinguish different populations – it also seems to set the maximum temperature where different populations can survive. As animals scramble to respond to the looming threat of climate change, understanding what sets the temperatures an animal can tolerate will be key to understanding why animals live where they do in the present and where they might live in the future.

Differential gene expression and mitonuclear incompatibilities in fast- and slow-developing interpopulation Tigriopus californicus hybrids.

Mitochondrial functions are intimately reliant on proteins and RNAs encoded in both the nuclear and mitochondrial genomes, leading to inter-genomic coevolution within taxa. Hybridization can break apart coevolved mitonuclear genotypes, resulting in decreased mitochondrial performance and reduced fitness. This hybrid breakdown is an important component of outbreeding depression and early-stage reproductive isolation. However, the mechanisms contributing to mitonuclear interactions remain poorly resolved. Here, we scored variation in developmental rate (a proxy for fitness) among reciprocal F2 interpopulation hybrids of the intertidal copepod Tigriopus californicus and used RNA sequencing to assess differences in gene expression between fast- and slow-developing hybrids. In total, differences in expression associated with developmental rate were detected for 2925 genes, whereas only 135 genes were differentially expressed as a result of differences in mitochondrial genotype. Upregulated expression in fast developers was enriched for genes involved in chitin-based cuticle development, oxidation-reduction processes, hydrogen peroxide catabolic processes and mitochondrial respiratory chain complex I. In contrast, upregulation in slow developers was enriched for DNA replication, cell division, DNA damage and DNA repair. Eighty-four nuclear-encoded mitochondrial genes were differentially expressed between fast- and slow-developing copepods, including 12 subunits of the electron transport system (ETS) which all had higher expression in fast developers than in slow developers. Nine of these genes were subunits of ETS complex I. Our results emphasize the major roles that mitonuclear interactions within the ETS, particularly in complex I, play in hybrid breakdown, and resolve strong candidate genes for involvement in mitonuclear interactions.

Mitochondrial effects on fertility and longevity in Tigriopus californicus contradict predictions of the mother's curse hypothesis.

Strict maternal inheritance of mitochondria favours the evolutionary accumulation of sex-biased fitness effects, as mitochondrial evolution occurs exclusively in female lineages. The 'mother's curse' hypothesis proposes that male-harming mutations should accumulate in mitochondrial genomes when they have neutral or beneficial effects on female fitness. Rigorous empirical tests have largely focused on Drosophila, where support for the predictions of mother's curse has been mixed. We investigated the impact of mother's curse mutations in Tigriopus californicus, a minute crustacean. Using non-recombinant backcrosses, we introgressed four divergent mitochondrial haplotypes into two nuclear backgrounds and recorded measures of fertility and longevity. We found that the phenotypic effects of mitochondrial mutations were context dependent, being influenced by the nuclear background in which they were expressed, as well as the sex of the individual and rearing temperature. Mitochondrial haplotype effects were greater for fertility than longevity, and temperature effects were greater for longevity. However, in opposition to mother's curse expectations, females had higher mitochondrial genetic variance than males for fertility and longevity, little evidence of sexual antagonism favouring females was found, and the impacts of mitonuclear mismatch harmed females but not males. Together, this indicates that selection on mitochondrial variation has not resulted in the accumulation of male mutation load in Tigriopus californicus.

Hypoxia tolerance, but not low pH tolerance, is associated with a latitudinal cline across populations of Tigriopus californicus.

Intertidal organisms must tolerate daily fluctuations in environmental parameters, and repeated exposure to co-occurring conditions may result in tolerance to multiple stressors correlating. The intertidal copepod Tigriopus californicus experiences diurnal variation in dissolved oxygen levels and pH as the opposing processes of photosynthesis and cellular respiration lead to coordinated highs during the day and lows at night. While environmental parameters with overlapping spatial gradients frequently result in correlated traits, less attention has been given to exploring temporally correlated stressors. We investigated whether hypoxia tolerance correlates with low pH tolerance by separately testing the hypoxia and low pH stress tolerance separately of 6 genetically differentiated populations of T. californicus. We independently checked for similarities in tolerance for each of the two stressors by latitude, sex, size, and time since collection as predictors. We found that although hypoxia tolerance correlated with latitude, low pH tolerance did not, and no predictor was significant for both stressors. We concluded that temporally coordinated exposure to low pH and low oxygen did not result in populations developing equivalent tolerance for both. Although climate change alters several environmental variables simultaneously, organisms' abilities to tolerate these changes may not be similarly coupled.

Exogenous oxidative stressors elicit differing age and sex effects in Tigriopus californicus.

As organisms age, cellular function declines in a time-dependent manner. Oxidative stress induced by reactive oxygen species damages cellular machinery and contributes to senescence which narrows the homeostatic window needed to maintain function and survive stress. Sex differences in longevity are apparent in many species and may be related to sex-specific homeostatic responses. Here we use the emerging aging model system Tigriopus californicus, the splashpool copepod, to estimate sex- and age-specific tolerances to two chemical oxidants, hydrogen peroxide and paraquat. Sex-specific tolerance was estimated for both oxidants simultaneously for 15 age-classes. As animals aged, hydrogen peroxide tolerance decreased but paraquat tolerance increased. Also, we observed no sex difference for hydrogen peroxide tolerance, while females were more tolerant of paraquat. Our results demonstrate that oxidative stressors can have dramatically different sex and age effects in Tigriopus californicus. These findings underscore the challenges ahead in understanding relationships among oxidative stressors, sex, and aging.

Elevated Salinity Rapidly Confers Cross-Tolerance to High Temperature in a Splash-Pool Copepod.

SynopsisAccurate forecasting of organismal responses to climate change requires a deep mechanistic understanding of how physiology responds to present-day variation in the physical environment. However, the road to physiological enlightenment is fraught with complications: predictable environmental fluctuations of any single factor are often accompanied by substantial stochastic variation and rare extreme events, and several factors may interact to affect physiology. Lacking sufficient knowledge of temporal patterns of co-variation in multiple environmental stressors, biologists struggle to design and implement realistic and relevant laboratory experiments. In this study, we directly address these issues, using measurements of the thermal tolerance of freshly collected animals and long-term field records of environmental conditions to explore how the splash-pool copepod Tigriopus californicus adjusts its physiology as its environment changes. Salinity and daily maximum temperature—two dominant environmental stressors experienced by T. californicus—are extraordinarily variable and unpredictable more than 2–3 days in advance. However, they substantially co-vary such that when temperature is high salinity is also likely to be high. Copepods appear to take advantage of this correlation: median lethal temperature of field-collected copepods increases by 7.5°C over a roughly 120 parts-per-thousand range of ambient salinity. Complementary laboratory experiments show that exposure to a single sublethal thermal event or to an abrupt shift in salinity also elicits rapid augmentation of heat tolerance via physiological plasticity, although the effect of salinity dwarfs that of temperature. These results suggest that T. californicus’s physiology keeps pace with the rapid, unpredictable fluctuations of its hypervariable physical environment by responding to the cues provided by recent sublethal stress and, more importantly, by leveraging the mechanistic cross-talk between responses to salinity and heat stress.

Chemical manipulation of mitochondrial function affects metabolism of red carotenoids in a marine copepod (Tigriopus californicus).

The shared-pathway hypothesis offers a cellular explanation for the connection between ketocarotenoid pigmentation and individual quality. Under this hypothesis, ketocarotenoid metabolism shares cellular pathways with mitochondrial oxidative phosphorylation such that red carotenoid-based coloration is inextricably linked mitochondrial function. To test this hypothesis, we exposed Tigriopus californicus copepods to a mitochondrially targeted protonophore, 2,4-dinitrophenol (DNP), to induce proton leak in the inner mitochondrial membranes. We then measured whole-animal metabolic rate and ketocarotenoid accumulation. As observed in prior studies of vertebrates, we observed that DNP treatment of copepods significantly increased respiration and that DNP-treated copepods accumulated more ketocarotenoid than control animals. Moreover, we observed a relationship between ketocarotenoid concentration and metabolic rate, and this association was strongest in DNP-treated copepods. These data support the hypothesis that ketocarotenoid and mitochondrial metabolism are biochemically intertwined. Moreover, these results corroborate observations in vertebrates, perhaps suggesting a fundamental connection between ketocarotenoid pigmentation and mitochondrial function that should be explored further.