Do Differences in Latitudinal Distributions of Species and Organelle Haplotypes Reflect Thermal Reaction Norms Within the Emiliania/Gephyrocapsa Complex?

The cosmopolitan phytoplankter Emiliania huxleyi contrasts with its closest relatives that are restricted to narrower latitudinal bands, making it interesting for exploring how alternative outcomes in phytoplankton range distributions arise. Mitochondrial and chloroplast haplogroups within E. huxleyi are shared with their closest relatives: Some E. huxleyi share organelle haplogroups with Gephyrocapsa parvula and G. ericsonii which inhabit lower latitudes, while other E. huxleyi share organelle haplogroups with G. muellerae, which inhabit high latitudes. We investigated whether the phylogeny of E. huxleyi organelles reflects environmental gradients, focusing on the Southeast Pacific where the different haplogroups and species co-occur. There was a high congruence between mitochondrial and chloroplast haplogroups within E. huxleyi. Haplogroup II of E. huxleyi is negatively associated with cooler less saline waters, compared to haplogroup I, both when analyzed globally and across temporal variability at the small special scale of a center of coastal upwelling at 30° S. A new mitochondrial haplogroup Ib detected in coastal Chile was associated with warmer waters. In an experiment focused on inter-species comparisons, laboratory-determined thermal reaction norms were consistent with latitudinal/thermal distributions of species, with G. oceanica exhibiting warm thermal optima and tolerance and G. muellerae exhibiting cooler thermal optima and tolerances. Emiliania huxleyi haplogroups I and II tended to exhibit a wider thermal niche compared to the other Gephyrocapsa, but no differences among haplogroups within E. huxleyi were found. A second experiment, controlling for local adaptation and time in culture, found a significant difference between E. huxleyi haplogroups. The difference between I and II was of the expected sign, but not the difference between I and Ib. The differences were small (≤1°C) compared to differences reported previously within E. huxleyi by local adaptation and even in-culture evolution. Haplogroup Ib showed a narrower thermal niche. The cosmopolitanism of E. huxleyi might result from both wide-spread generalist phenotypes and specialist phenotypes, as well as a capacity for local adaptation. Thermal reaction norm differences can well explain the species distributions but poorly explain distributions among mitochondrial haplogroups within E. huxleyi. Perhaps organelle haplogroup distributions reflect historical rather than selective processes.

Sperm of more colorful males are better adapted to ovarian fluids in lake char (Salmonidae)

Fish often spawn eggs with ovarian fluids that have been hypothesized to support sperm of some males over others (cryptic female choice). Alternatively, sperm reactions to ovarian fluids could reveal male strategies linked to their likely roles during spawning. Sperm of males who would usually be close to females during spawning are then expected to be better adapted to the presence of ovarian fluids than to water only, while the reverse would be expected for males that typically spawn at larger distance to the females. We tested these predictions with gametes and ovarian fluids from wild-caught lake char (Salvelinus umbla). We found that sperm of more colorful males showed increased sperm velocity in diluted ovarian fluids while sperm of paler males were fastest in water only. We then let equal numbers of sperm compete for fertilizations in the presence or absence of ovarian fluids and used microsatellite markers to assign in total 1,464 embryos (from 70 experimental trials) to their fathers. Overall, sperm of more colorful males reached higher fertilization success than sperm of pale males. This difference was enhanced by the presence of ovarian fluids and best explained by the increased sperm velocity. Sperm competitiveness was not enhanced with decreasing male inbreeding coefficients or decreasing genetic distance to a given female, although parallel stress tests on embryos revealed that females would profit more from mating with least related males rather than most colored ones. We conclude that sperm of more colorful males are best adapted to ovarian fluids, and that the observed reaction norms reveal male strategies rather than cryptic female choice.

Modeling phenological reaction norms over an elevational gradient reveals contrasting strategies of Dusky Flycatchers and Mountain Chickadees in response to early-season temperatures

We developed an approach to distinguish among 3 alternative strategies that birds may employ relating the timing of egg laying across elevations to annual variation in spring temperature (phenological reaction norms), which we applied to analyze the breeding phenology of 2 species over an elevational gradient in the Sierra Nevada, California. In a “simple-offset” strategy, birds at different elevations initiate breeding relative to environmental temperature in a consistent manner, in that breeding onset is triggered by a critical temperature regardless of when it occurs in the spring. Elevation-specific reaction norms based on multiple years are offset (high-elevation birds start breeding later) but parallel. In a “delay” strategy, in cooler springs, populations at higher elevations that are sensitive to early-season weather-related risks delay laying onset relative to those at lower elevations, yielding a high-elevation reaction norm that diverges from a low-elevation one at cooler temperatures. Conversely, high-elevation populations in cooler springs that are sensitive to the risk of having insufficient time to complete a breeding cycle advance laying onset relative to lower populations (“advance” strategy), yielding a high-elevation reaction norm than converges with a low-elevation one. Both delay and advance strategies imply an elevation-dependent interaction between temperature and date (photoperiod) in influencing laying onset. Examined across 3 elevation groupings, phenological reaction norms of Mountain Chickadees (Poecile gambeli) were essentially parallel, consistent with simple offset, whereas Dusky Flycatchers (Empidonax oberholseri) relationships were more complex. In cooler springs, mid-elevation flycatchers bred comparatively late relative to lowest-elevation birds (delay), implying greater sensitivity to early-season risks, but still with sufficient time to complete a breeding cycle. However, high-elevation flycatchers bred comparatively early relative to mid-elevation populations (advance); delaying at these highest elevations may not be an option. Our approach revealed differences in risk sensitivity that were consistent with other ecological differences between the 2 species.

Impact of Developmental Temperatures On The Repeatability of Thermal Plasticity in Metabolic Rate

Phenotypic plasticity is an important mechanism that allows populations to adjust to changing environments. Plastic responses induced by early life experiences can have lasting impacts on how individuals respond to environmental variation later in life (i.e., reversible plasticity). Developmental environments can also influence repeatability of plastic responses thereby altering the capacity for reaction norms to respond to selection. Here, we compared metabolic thermal reaction norms in lizards (Lampropholis delicata) that were incubated at two developmental temperatures (ncold = 26, nhot = 25). We repeatedly measured individual reaction norms across six acute temperatures 10 times over ~3.5 months (nobs = 3,818) to estimate the repeatability of average metabolic rate (intercept) and thermal plasticity (slope). The intercept and the slope of the population-level thermal reaction norm did not change with developmental temperatures. Repeatability of average metabolic rate was, on average, 10% lower in hot incubated lizards and was stable across acute temperatures. The slope of the reaction norm was moderately repeatable (R = 0.44, 95% CI = 0.035 – 0.93) suggesting that individual metabolic rate changed consistently with acute temperature, although credible intervals were quite broad. Importantly, reaction norm repeatability did not depend on early developmental temperature. Our work implies that thermal plasticity has the capacity to evolve, despite there being less consistent variation in metabolic rate under hot environments. This capacity for thermal plasticity to evolve will be increasingly more important for terrestrial ectotherms living in changing climate.

Comparative analysis of phenotypic plasticity sheds light on the evolution and molecular underpinnings of locust phase polyphenism

Locusts exhibit one of nature’s most spectacular examples of complex phenotypic plasticity, in which changes in density cause solitary and cryptic individuals to transform into gregarious and conspicuous locusts forming large migrating swarms. We investigated how these coordinated alternative phenotypes might have evolved by studying the Central American locust and three closely related non-swarming grasshoppers in a comparative framework. By experimentally isolating and crowding during nymphal development, we induced density-dependent phenotypic plasticity and quantified the resulting behavioural, morphological, and molecular reaction norms. All four species exhibited clear plasticity, but the individual reaction norms varied among species and showed different magnitudes. Transcriptomic responses were species-specific, but density-responsive genes were functionally similar across species. There were modules of co-expressed genes that were highly correlated with plastic reaction norms, revealing a potential molecular basis of density-dependent phenotypic plasticity. These findings collectively highlight the importance of studying multiple reaction norms from a comparative perspective.

Are individuals consistent? Endocrine reaction norms under different ecological challenges

Quantifying organismal capacity for compensatory mechanisms is essential to forecast response to environmental change. Despite accumulating evidence for individual variation in physiological plasticity, the causes and consequences of this variation remain unclear. An outstanding question is whether individual reaction norms are consistent across different environmental challenges, i.e., whether an individual that is responsive to one environmental variable will be equally responsive to a different environmental variable. Additionally, are these reaction norms themselves consistent over time, i.e., repeatable? Here, we quantified individual baseline glucocorticoid responses in house sparrows, Passer domesticus, to sequential manipulations of temperature, wind speed, and food unpredictability that were repeated in discrete blocks of sampling under both control and stressor-exposed conditions. Individuals significantly decreased their baseline corticosterone levels and increased their mass during treatment exposure. This response was consistent across environmental challenge types. There was high repeatability in the intercept and slope of the baseline corticosterone reaction norm between environmental challenges but broad credible intervals in the repeatability of the reaction norm slope, suggesting that although glucocorticoid levels during baseline conditions are repeatable, among-individual variation in the shape of the glucocorticoid response may be higher than within-individual variation. Within-subject variation in baseline corticosterone levels was mainly explained by within-individual variation in body mass during stressor exposure. Despite the high lability in physiological traits, endocrine plasticity is repeatable across environmental challenges and may be able to evolve due to genetic accommodation, in which selection acts on genetic variation of reaction norms.

Temperature Induced Physiological Reaction Norms of the Coccolithophore Gephyrocapsa oceanica and Resulting Coccolith Sr/Ca and Mg/Ca Ratios

Coccolithophores are one of the major contributors to the pelagic production of calcium carbonate and their fossilized remains are a key component of the biogeochemical cycles of calcium (Ca), magnesium (Mg), and other divalent cations present in the intracellular precipitated calcitic structures (coccoliths). The geochemical signature of coccoliths (e.g., Sr/Ca and Mg/Ca ratios) is used as paleoproxy to reconstruct past environmental conditions and to understand the underlying physiological precipitation kinetics. Here, we present the elemental fractionation of Sr and Mg in calcite of the coccolithophore Gephyrocapsa oceanica from controlled laboratory experiments applying an extended temperature gradient (12 to 27°C). The physiological reaction norm of G. oceanica, in terms of growth rate, exhibited optimum behavior while the partition coefficient of Sr (DSr) was linearly correlated with temperature and DMg indicated no specific trend. Our results indicate: (1) a presumably secondary physiological control of DSr, and (2) the importance of calibrating coccolithophore-based proxies using experiments that include the full physiological reaction norms (i.e., a possible non-linear response) to environmental drivers (e.g., temperature, salinity, and pH, etc.). The presented results contribute to an improved understanding of the underlying physiological kinetics involved in regulating coccolith elemental fractionation and give additional implications for designing future laboratory experiments to calibrate and apply coccolithophore based paleoproxies on the fossil sediment record.

A reaction norm perspective on reproducibility

Reproducibility in biomedical research, and more specifically in preclinical animal research, has been seriously questioned. Several cases of spectacular failures to replicate findings published in the primary scientific literature have led to a perceived reproducibility crisis. Diverse threats to reproducibility have been proposed, including lack of scientific rigour, low statistical power, publication bias, analytical flexibility and fraud. An important aspect that is generally overlooked is the lack of external validity caused by rigorous standardization of both the animals and the environment. Here, we argue that a reaction norm approach to phenotypic variation, acknowledging gene-by-environment interactions, can help us seeing reproducibility of animal experiments in a new light. We illustrate how dominating environmental effects can affect inference and effect size estimates of studies and how elimination of dominant factors through standardization affects the nature of the expected phenotype variation through the reaction norms of small effect. Finally, we discuss the consequences of reaction norms of small effect for statistical analysis, specifically for random effect latent variable models and the random lab model.