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1. Nitrogen stable isotope turnover and discrimination in lizards

   https://doi.org/10.1002/rcm.9030  

   Warne, Robin W. ; Wolf, Blair O. ( January 2021 , Rapid Communications in Mass Spectrometry)

   Nitrogen stable isotope ratio (δ¹⁵N) processes are not well described in reptiles, which limits reliable inference of trophic and nutrient dynamics. In this study we detailed δ¹⁵N turnover and discrimination (Δ¹⁵N) in diverse tissues of two lizard species, and compared these results with previously published carbon data (δ¹³C) to inform estimates of reptilian foraging ecology and nutrient physiology.

   We quantified¹⁵N incorporation and discrimination dynamics over 360 days in blood fractions, skin, muscle, and liver ofSceloporus undulatusandCrotaphytus collaristhat differed in body mass. Tissue samples were analyzed on a continuous flow isotope ratio mass spectrometer.

   Δ¹⁵N for plasma and red blood cells (RBCs) ranged between +2.7 and +3.5‰; however, skin, muscle, and liver did not equilibrate, hindering estimates for these somatic tissues.¹⁵N turnover in plasma and RBCs ranged from 20.7 ± 4 to 303 ± 166 days among both species. Comparison with previously published δ¹³C results for these same samples showed that¹⁵N and¹³C incorporation patterns were uncoupled, especially during winter when hibernation physiology could have played a role.

   Our results provide estimates of¹⁵N turnover rates and discrimination values that are essential to using and interpreting isotopes in studies of diet reconstruction, nutrient allocation, and trophic characterization in reptiles. These results also suggest that somatic tissues can be unreliable, while life history shifts in nutrient routing and metabolism potentially cause¹⁵N and¹³C dynamics to be decoupled.

    

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2. Juvenile Albacore tuna ( Thunnus alalunga ) foraging ecology varies with environmental conditions in the California Current Large Marine Ecosystem

   https://doi.org/10.1111/fog.12638  

   Nickels, Catherine F. ; Portner, Elan J. ; Snodgrass, Owyn ; Muhling, Barbara ; Dewar, Heidi ( September 2023 , Fisheries Oceanography)

   Juvenile North Pacific Albacore tuna (Thunnus alalunga) support commercial and recreational fisheries in the California Current Large Marine Ecosystem (CCLME), where they forage during summer and fall. The distributions of the commercial and recreational fisheries and estimates of forage availability have varied substantially over the past century. Time‐series quantifying Albacore diet can help link forage composition to variability in Albacore abundance and distribution and, consequently, their availability to fishers. Previous diet studies in the CCLME are of relatively short duration, and long‐term variability in Albacore diet remains poorly understood. We describe the diets of juvenile Albacore from three regions in the CCLME from 2007 to 2019 and use classification and regression tree analysis to explore environmental drivers of variability. Important prey include Northern Anchovy (Engraulis mordax), rockfishes (Sebastesspp.), Boreal Clubhook Squid (Onychoteuthis borealijaponica), euphausiids (Order: Euphausiidae), and amphipods (Order: Amphipoda), each contributing >5% mean proportional abundance. Most prey items were short lived species or young‐of‐the‐year smaller than 10 cm. Diet variability was related to environmental conditions over the first 6 months of the year (PDO, sea surface temperature, and NPGO) and conditions concurrent with Albacore capture (region and surface nitrate flux). We describe foraging flexibility over regional and annual scales associated with these environmental influences. Continuous, long‐term studies offer the opportunity to identify flexibility in Albacore foraging behavior and begin to make a predictive link between environmental conditions early in the year and Albacore foraging during summer and fall.

    

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3. Genetic basis of octanoic acid resistance in Drosophila sechellia : functional analysis of a fine‐mapped region

   https://doi.org/10.1111/mec.14001  

   Andrade López, J. M. ; Lanno, S. M. ; Auerbach, J. M. ; Moskowitz, E. C. ; Sligar, L. A. ; Wittkopp, P. J. ; Coolon, J. D. ( February 2017 , Molecular Ecology)

   Drosophila sechelliais a species of fruit fly endemic to the Seychelles islands. Unlike its generalist sister species,D. sechelliahas evolved to be a specialist on the host plantMorinda citrifolia. This specialization is interesting because the plant's fruit contains secondary defence compounds, primarily octanoic acid (OA), that are lethal to most other Drosophilids. Although ecological and behavioural adaptations to this toxic fruit are known, the genetic basis for evolutionary changes inOAresistance is not. Prior work showed that a genomic region on chromosome 3R containing 18 genes has the greatest contribution to differences inOAresistance betweenD. sechelliaandD. simulans. To determine which gene(s) in this region might be involved in the evolutionary change inOAresistance, we knocked down expression of each gene in this region inD. melanogasterwithRNAinterference (RNAi) (i) ubiquitously throughout development, (ii) during only the adult stage and (iii) within specific tissues. We identified three neighbouring genes in theOsirisfamily,Osiris 6(Osi6),Osi7andOsi8, that led to decreasedOAresistance when ubiquitously knocked down. Tissue‐specificRNAi, however, showed that decreasing expression ofOsi6andOsi7specifically in the fat body and/or salivary glands increasedOAresistance. Gene expression analyses ofOsi6andOsi7revealed that while standing levels of expression are higher inD. sechellia,Osi6expression is significantly downregulated in salivary glands in response toOAexposure, suggesting that evolved tissue‐specific environmental plasticity ofOsi6expression may be responsible forOAresistance inD. sechellia.

    

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4. Dietary protein content and digestibility influences discrimination of amino acid nitrogen isotope values in a terrestrial omnivorous mammal

   https://doi.org/10.1002/rcm.9073  

   Whiteman, John P. ; Rodriguez Curras, Mauriel ; Feeser, Kelli L. ; Newsome, Seth D. ( April 2021 , Rapid Communications in Mass Spectrometry)

   Ecologists increasingly determine the δ¹⁵N values of amino acids (AA) in animal tissue; “source” AA typically exhibit minor variation between diet and consumer, while “trophic” AA have increased δ¹⁵N values in consumers. Thus, trophic‐source δ¹⁵N offsets (i.e., Δ¹⁵N_T‐S) reflect trophic position in a food web. However, even minor variations in δ¹⁵N_{source AA}values may influence the magnitude of offset that represents a trophic step, known as the trophic discrimination factor (i.e., TDF_T‐S). Diet digestibility and protein content can influence the δ¹⁵N values of bulk animal tissue, but the effects of these factors on AA Δ¹⁵N_T‐Sand TDF_T‐Sin mammals are unknown.

   We fed captive mice (Mus musculus) either (A) a low‐fat, high‐fiber diet with low, intermediate, or high protein; or (B) a high‐fat, low‐fiber diet with low or intermediate protein. Mouse muscle and dietary protein were analyzed for bulk tissue δ¹⁵N using elemental analyzer‐isotope ratio mass spectrometry (EA‐IRMS), and were also hydrolyzed into free AA that were analyzed for δ¹⁵N using gas chromatography‐combustion‐IRMS.

   As dietary protein increased, Δ¹⁵N_{Consumer‐Diet}slightly declined for bulk muscle tissue in both experiments; increased for AA in the low‐fat, high‐fiber diet (A); and remained the same or decreased for AA in the high‐fat, low‐fiber diet (B). The effects of dietary protein on Δ¹⁵N_T‐Sand on TDF_T‐Svaried by AA but were consistent between variables.

   Diets were less digestible and included more protein in Experiment A than in Experiment B. As a result, the mice in Experiment A probably oxidized more AA, resulting in greater Δ¹⁵N_{Consumer‐Diet}values. However, the similar responses of Δ¹⁵N_T‐Sand of TDF_T‐Sto diet variation suggest that if diet samples are available, Δ¹⁵N_T‐Saccurately tracks trophic position. If diet samples are not available, the patterns presented here provide a basis to interpret Δ¹⁵N_T‐Svalues. The trophic‐source offset of Pro‐Lys did not vary across diets, and therefore may be more reliable for omnivores than other offsets (e.g., Glu‐Phe).

    

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5. Effects of chemical preservation on bulk and amino acid isotope ratios of zooplankton, fish, and squid tissues

   https://doi.org/10.1002/rcm.8408  

   Hetherington, Elizabeth D. ; Kurle, Carolyn M. ; Ohman, Mark D. ; Popp, Brian N. ( April 2019 , Rapid Communications in Mass Spectrometry)

   It is imperative to understand how chemical preservation alters tissue isotopic compositions before using historical samples in ecological studies. Specifically, although compound‐specific isotope analysis of amino acids (CSIA‐AA) is becoming a widely used tool, there is little information on how preservation techniques affect amino acidδ¹⁵N values.

   We evaluated the effects of chemical preservatives on bulk tissueδ¹³C andδ¹⁵N and amino acidδ¹⁵N values, measured by gas chromatography/isotope ratio mass spectrometry (GC/IRMS), of (a) tuna (Thunnus albacares) and squid (Dosidicus gigas) muscle tissues that were fixed in formaldehyde and stored in ethanol for 2 years and (b) two copepod species,Calanus pacificusandEucalanus californicus, which were preserved in formaldehyde for 24–25 years.

   Tissues in formaldehyde‐ethanol had higher bulkδ¹⁵N values (+1.4,D. gigas; +1.6‰,T. albacares), higherδ¹³C values forD. gigas(+0.5‰), and lowerδ¹³C values forT. albacares(−0.8‰) than frozen samples. The bulkδ¹⁵N values from copepods were not different those from frozen samples, although theδ¹³C values from both species were lower (−1.0‰ forE. californicusand −2.2‰ forC. pacificus) than those from frozen samples. The mean amino acidδ¹⁵N values from chemically preserved tissues were largely within 1‰ of those of frozen tissues, but the phenylalanineδ¹⁵N values were altered to a larger extent (range: 0.5–4.5‰).

   The effects of preservation on bulkδ¹³C values were variable, where the direction and magnitude of change varied among taxa. The changes in bulkδ¹⁵N values associated with chemical preservation were mostly minimal, suggesting that storage in formaldehyde or ethanol will not affect the interpretation ofδ¹⁵N values used in ecological studies. The preservation effects on amino acidδ¹⁵N values were also mostly minimal, mirroring bulkδ¹⁵N trends, which is promising for future CSIA‐AA studies of archived specimens. However, there were substantial differences in phenylalanine and valineδ¹⁵N values, which we speculate resulted from interference in the chromatographic resolution of unknown compounds rather than alteration of tissue isotopic composition due to chemical preservation.

    

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