Comparative analysis of the molecular starvation response of Southern Ocean copepods.

Large lipid-storing copepods dominate mesozooplankton biomass in the polar oceans and form a critical link between primary production and higher trophic levels. The ecological success of these species depends on their ability to survive periods of food deprivation in a highly seasonal environment, but the molecular changes that mediate starvation tolerance in these taxa are unknown. We conducted starvation experiments for two dominant Southern Ocean copepods, Calanoides acutus and Calanus propinquus, allowing us to compare the molecular starvation response between species. These species differ in life history, diet and metabolic traits, and expressed overlapping but distinct transcriptomic responses to starvation. Most starvation-response genes were species-specific, but we identified a conserved core set of starvation-response genes related to RNA and protein metabolism. We used phylotranscriptomics to place these results in the context of copepod evolution and found that starvation-response genes are under strong purifying selection at the sequence level and stabilizing selection at the expression level, consistent with their role in mediating essential biological functions. Selection on starvation-response genes was especially strong in our focal lipid-storing lineage relative to other copepod taxa, underscoring the significance of starvation tolerance for these species. We also found that certain key lipid enzymes (elongases and desaturases) have experienced diversification and positive selection in lipid-storing lineages, reflecting the unique lipid storage needs of these animals. Our results shed light on the molecular adaptations of high-latitude zooplankton to variable food conditions and suggest that starvation-response genes are under particularly strong sequence and expression constraints.

Sensory conflict disrupts circadian rhythms in the sea anemone Nematostella vectensis.

Circadian clocks infer time of day by integrating information from cyclic environmental factors called zeitgebers, including light and temperature. Single zeitgebers entrain circadian rhythms, but few studies have addressed how multiple, simultaneous zeitgeber cycles interact to affect clock behavior. Misalignment between zeitgebers ('sensory conflict') can disrupt circadian rhythms, or alternatively clocks may privilege information from one zeitgeber over another. Here, we show that temperature cycles modulate circadian locomotor rhythms in Nematostella vectensis, a model system for cnidarian circadian biology. We conduct behavioral experiments across a comprehensive range of light and temperature cycles and find that Nematostella's circadian behavior is disrupted by chronic misalignment between light and temperature, which involves disruption of the endogenous clock itself rather than a simple masking effect. Sensory conflict also disrupts the rhythmic transcriptome, with numerous genes losing rhythmic expression. However, many metabolic genes remained rhythmic and in-phase with temperature, and other genes even gained rhythmicity, implying that some rhythmic metabolic processes persist even when behavior is disrupted. Our results show that a cnidarian clock relies on information from light and temperature, rather than prioritizing one signal over the other. Although we identify limits to the clock's ability to integrate conflicting sensory information, there is also a surprising robustness of behavioral and transcriptional rhythmicity.

Almost all living things exhibit circadian rhythms ­ internally driven biological processes ­ which regulate important bodily functions, including sleep and wake cycles, over a roughly 24-hour period. Circadian clocks govern these rhythms by receiving information from the environment that allows them to tell what time of day it is. Two of the most important environmental signals, known as 'zeitgebers' ­ meaning 'time giver' ­ are light and temperature. In nature, circadian clocks must integrate information from multiple zeitgebers simultaneously. Typically, over a 24-hour period, temperature increases and decreases with the light cycle, getting warmer during the day and colder at night. However, artificial light pollution and circadian disruption ­ such as shift work ­ can impact the natural relationship between light and temperature. This 'sensory conflict', where two zeitgebers provide conflicting information about the time of day, can impact ecosystems such as coral reefs; and is also linked to poor health in humans. How circadian clocks behave in complex multi-zeitgeber environments and specifically, whether they prioritize one zeitgeber over another is not fully understood. To investigate how cnidarians ­ a group of marine animals including corals and jellyfish ­ respond to sensory conflict, Berger and Tarrant varied the relationship between light and temperature cycles using the sea anemone Nematostella vectensis as a model system. Nematostella is a nocturnal cnidarian, meaning it moves most at night. First, Berger and Tarrant kept Nematostella in dark conditions with 24-hour temperature cycles ­ starting cold, increasing to a peak in the middle of the day before decreasing towards the end of the day. Monitoring Nematostella movement revealed that they moved most during the cold phase, showing that temperature cycles alone can maintain rhythmic behavior. Similarly, when temperature and light cycles were aligned such that both rose and fell together, nocturnal behavior was preserved. However, when large misalignments between light and temperature cycles were introduced ­ such that temperature decreased during light periods and increased in the dark ­ nocturnal behavior was almost completely lost. This suggests that both light and temperature interact to produce complex patterns of circadian behavior, with neither signal being prioritized over the other. Additionally, Berger and Tarrant investigated how sensory conflict impacts the activity of Nematostella genes. While many genes remained rhythmic, suggesting some gene expression persists when behavior is disturbed, others that were rhythmic became arrhythmic. In contrast, a selection of genes that do not normally display rhythmic behavior gained rhythmic expression. Genes related to protein metabolism and other energy-intensive processes were particularly disrupted. In an increasingly 24/7 society, it is important to understand how complex multi-sensory environments impact circadian rhythms and as a result, health and fitness. The findings show that certain light and temperature regimes severely disrupt Nematostella behavior and could be useful in predicting how other organisms might respond to disruptions such as light pollution. In the future, such information could be used to design optimal light regimes for ecosystems in which the relationship between light cycles and other environmental signals is disrupted by human behavior.

Nematostella vectensis exhibits an enhanced molecular stress response upon co-exposure to highly weathered oil and surface UV radiation.

Crude oil released into the environment undergoes weathering processes that gradually change its composition and toxicity. Co-exposure to petroleum mixtures and other stressors, including ultraviolet (UV) radiation, may lead to synergistic effects and increased toxicity. Laboratory studies should consider these factors when testing the effects of oil exposure on aquatic organisms. Here, we study transcriptomic responses of the estuarine sea anemone Nematostella vectensis to naturally weathered oil, with or without co-exposure to environmental levels of UV radiation. We find that co-exposure greatly enhances the response. We use bioinformatic analyses to identify molecular pathways implicated in this response, which suggest phototoxicity and oxidative damage as mechanisms for the enhanced stress response. Nematostella's stress response shares similarities with the vertebrate oxidative stress response, implying deep conservation of certain stress pathways in animals. We show that exposure to weathered oil along with surface-level UV exposure has substantial physiological consequences in a model cnidarian.

Shifts in morphology, gene expression, and selection underlie web loss in Hawaiian Tetragnatha spiders.

BACKGROUND: A striking aspect of evolution is that it often converges on similar trajectories. Evolutionary convergence can occur in deep time or over short time scales, and is associated with the imposition of similar selective pressures. Repeated convergent events provide a framework to infer the genetic basis of adaptive traits. The current study examines the genetic basis of secondary web loss within web-building spiders (Araneoidea). Specifically, we use a lineage of spiders in the genus Tetragnatha (Tetragnathidae) that has diverged into two clades associated with the relatively recent (5 mya) colonization of, and subsequent adaptive radiation within, the Hawaiian Islands. One clade has adopted a cursorial lifestyle, and the other has retained the ancestral behavior of capturing prey with sticky orb webs. We explore how these behavioral phenotypes are reflected in the morphology of the spinning apparatus and internal silk glands, and the expression of silk genes. Several sister families to the Tetragnathidae have undergone similar web loss, so we also ask whether convergent patterns of selection can be detected in these lineages. RESULTS: The cursorial clade has lost spigots associated with the sticky spiral of the orb web. This appears to have been accompanied by loss of silk glands themselves. We generated phylogenies of silk proteins (spidroins), which showed that the transcriptomes of cursorial Tetragnatha contain all major spidroins except for flagelliform. We also found an uncharacterized spidroin that has higher expression in cursorial species. We found evidence for convergent selection acting on this spidroin, as well as genes involved in protein metabolism, in the cursorial Tetragnatha and divergent cursorial lineages in the families Malkaridae and Mimetidae. CONCLUSIONS: Our results provide strong evidence that independent web loss events and the associated adoption of a cursorial lifestyle are based on similar genetic mechanisms. Many genes we identified as having evolved convergently are associated with protein synthesis, degradation, and processing, which are processes that play important roles in silk production. This study demonstrates, in the case of independent evolution of web loss, that similar selective pressures act on many of the same genes to produce the same phenotypes and behaviors.

De novo transcriptome assembly of the Southern Ocean copepod Rhincalanus gigas sheds light on developmental changes in gene expression.

Copepods are small crustaceans that dominate most zooplankton communities in terms of both abundance and biomass. In the polar oceans, a subset of large lipid-storing copepods occupy central positions in the food web because of their important role in linking phytoplankton and microzooplankton with higher trophic levels. In this paper, we generated a high-quality de novo transcriptome for Rhincalanus gigas, the largest-and among the most abundant-of the Southern Ocean copepods. We then conducted transcriptional profiling to characterize the developmental transition between late-stage juveniles and adult females. We found that juvenile R. gigas substantially upregulate lipid synthesis and glycolysis pathways relative to females, as part of a developmental gene expression program that also implicates processes such as muscle growth, chitin formation, and ion transport. This study provides the first transcriptional profile of a developmental transition within Rhincalanus gigas or any endemic Southern Ocean copepod, thereby extending our understanding of copepod molecular physiology.