Phenotypic integration and life history strategies among populations of Pinus halepensis: an insight through structural equation modelling

Maternal risk-management elucidates the evolution of reproductive adaptations in sharks by means of natural selection

Maternal investment theory is the study of how breeding females allocate resources between offspring size and brood size to achieve reproductive success. In classical trade-off models, r/K-selection and bet-hedging selection, the primary predictors of maternal investments in offspring are population density and resource stability. In crowded, stable environments, K-selected females invest in large offspring at an equivalent cost in brood size. In uncrowded, unstable environments, r-selected females invest in large broods at an equivalent cost in offspring size. In unpredictable resource environments, bet-hedging females invest moderately in brood size and offspring size. The maternal risk-management model represents a profound departure from classical trade-off models. Maternal investments in offspring size, brood size, and brood number are shaped independently by autonomous risk factors: the duration of gaps in resources during seasonal cycles, rates of predation, and unpredictable catastrophic events. To date, no single model has risen to a position of preeminence. Here in sharks, we show that maternal investments within and across species do not agree with the predictions of trade-off models and instead agree with the predictions of the maternal risk-management model. Within and across shark species, offspring size and brood size were independent maternal investment strategies. The risk of starvation favored investments in larger offspring. The risk of predation favored investments in larger broods. If empirical studies continue to confirm its predictions, maternal-risk management may yet emerge as a unifying model of diverse reproductive adaptations by means of natural selection.

Adaptive potential of maritime pine under contrasting environments

BACKGROUND

Predicting the adaptability of forest tree populations under future climates requires a better knowledge of both the adaptive significance and evolvability of measurable key traits. Phenotypic plasticity, standing genetic variation and degree of phenotypic integration shape the actual and future population genetic structure, but empirical estimations in forest tree species are still extremely scarce. We analysed 11 maritime pine populations covering the distribution range of the species (119 families and 8 trees/family, ca. 1300 trees) in a common garden experiment planted at two sites with contrasting productivity. We used plant height as a surrogate of fitness and measured five traits (mean and plasticity of carbon isotope discrimination, specific leaf area, needle biomass, Phenology growth index) related to four different strategies (acquisitive economics, photosynthetic organ size, growth allocation and avoidance of water stress).

RESULTS

Estimated values of additive genetic variation would allow adaptation of the populations to future environmental conditions. Overall phenotypic integration and selection gradients were higher at the high productivity site, while phenotypic integration within populations was higher at the low productivity site. Response to selection was related mainly to photosynthetic organ size and drought-avoidance mechanisms rather than to water use efficiency. Phenotypic plasticity of water use efficiency could be maladaptive, resulting from selection for height growth.

CONCLUSIONS

Contrary to the expectations in a drought tolerant species, our study suggests that variation in traits related to photosynthetic organ size and acquisitive investment of resources drive phenotypic selection across and within maritime pine populations. Both genetic variation and evolvability of key adaptive traits were considerably high, including plasticity of water use efficiency. These characteristics would enable a relatively fast micro-evolution of populations in response to the ongoing climate changes. Moreover, differentiation among populations in the studied traits would increase under the expected more productive future Atlantic conditions.

Leaf phenotypic plasticity coupled with integration facilitates the adaptation of plants to enhanced N deposition

The response of leaf functional traits can provide vital insight into the adaptive strategies of plants under global change. However, empirical knowledge on the acclimation of functional coordination between phenotypic plasticity and integration to increased nitrogen (N) deposition is still scarce. The variation of leaf functional traits of two dominant seedling species, Machilus gamblei and Neolitsea polycarpa, across four N deposition rates (0, 3, 6, and 12 kg N ha^-1yr^-1), along with the relationship between leaf phenotypic plasticity and integration were investigated in a subtropical montane forest. We found that enhanced N deposition promoted the development of seedling traits toward the direction of resource acquisition, including improved leaf N content, specific leaf area and photosynthetic performance. Appropriate N deposition (≤6 kg N ha^-1 yr^-1) might induce the optimization of leaf functional traits to promote the capability and efficiency of nutrient use and photosynthesis in seedlings. However, excessive N deposition (12 kg N ha^-1 yr^-1) would result in detrimental effects on leaf morphological and physiological traits, thus inhibiting the efficiency in resource acquisition. A positive relationship occurred between leaf phenotypic plasticity and integration in both seedling species, implied that higher plasticity of leaf functional traits likely led to better integration with other traits under N deposition. Overall, our study emphasized that leaf functional traits could rapidly respond to changes in N resource, while the coordination between leaf phenotypic plasticity and integration can facilitate the adaptation of tree seedlings in coping with enhanced N deposition. Further studies are still needed on the role of leaf phenotypic plasticity and integration in plant fitness for predicting ecosystem functioning and forest dynamics, especially in the context of future high N deposition.

Harnessing tree-ring phenotypes to disentangle gene by environment interactions and their climate dependencies in a circum-Mediterranean pine

BACKGROUND AND AIMS

Understanding the genetic basis of adaptation and plasticity in trees constitutes a knowledge gap. We linked dendrochronology and genomics [single nucleotide polymorphisms (SNPs)] for a widespread conifer (Pinus halepensis Mill.) to characterize intraspecific growth differences elicited by climate.

METHODS

The analysis comprised 20-year tree-ring series of 130 trees structured in 23 populations evaluated in a common garden. We tested for genotype by environment interactions (G × E) of indexed ring width (RWI) and early- to latewood ratios (ELI) using factorial regression, which describes G × E as differential gene sensitivity to climate.

KEY RESULTS

The species' annual growth was positively influenced by winter temperature and spring moisture and negatively influenced by previous autumn precipitation and warm springs. Four and five climate factors explained 10 % (RWI) and 16 % (ELI) of population-specific interannual variability, respectively, with populations from drought-prone areas and with uneven precipitation experiencing larger growth reductions during dry vegetative periods. Furthermore, four and two SNPs explained 14 % (RWI) and 10 % (ELI) of interannual variability among trees, respectively. Two SNPs played a putative role in adaptation to climate: one identified from transcriptome sequencing of P. halepensis and another involved in response regulation to environmental stressors.

CONCLUSIONS

We highlight how tree-ring phenotypes, obtained from a common garden experiment, combined with a candidate-gene approach allow the quantification of genetic and environmental effects determining adaptation for a conifer with a large and complex genome.

Straightening the crooked: intraspecific divergence of stem posture control and associated trade-offs in a model conifer

Although the straightening capacity of the stem is key for light capture and mechanical stability in forest trees, little is known about its adaptive implications. Assuming that stem straightening is costly, trade-offs are expected with competing processes such as growth, maintenance, and defence. We established a manipulative experiment in a common garden of Pinus pinaster including provenances typically showing either straight-stemmed or crooked-stemmed phenotypes. We imposed a bending up to 35º on plants aged 9 years of both provenance groups and followed the straightening kinetics and shoot elongation after releasing. Eight months later, we destructively assessed biomass partitioning, reaction wood, wood microdensity, xylem reserve carbohydrates, and phloem secondary metabolites. The experimental bending and release caused significant, complex changes with a marked difference between straight- and crooked-type plants. The straight-type recovered verticality faster and to a higher degree and developed more compression wood, while displaying a transitory delay in shoot elongation, reducing resource allocation to defence and maintaining the levels of non-structural carbohydrates compared with the crooked type. This combination of responses indicates the existence of intraspecific divergence in the reaction to mechanical stresses that may be related to different adaptive phenotypic plasticity.

Intraspecific responses to climate reveal nonintuitive warming impacts on a widespread thermophilic conifer

Many ecologically important forest trees from dry areas have been insufficiently investigated for their ability to adapt to the challenges posed by climate change, which hampers the implementation of mitigation policies. We analyzed 14 common-garden experiments across the Mediterranean which studied the widespread thermophilic conifer Pinus halepensis and involved 157 populations categorized into five ecotypes. Ecotype-specific tree height responses to climate were applied to projected climate change (2071-2100 ad), to project potential growth patterns both locally and across the species' range. We found contrasting ecotypic sensitivities to annual precipitation but comparatively uniform responses to mean temperature, while evidence of local adaptation for tree height was limited to mesic ecotypes. We projected intriguing patterns of response range-wide, implying either height inhibition or stimulation of up to 75%, and deduced that the ecotype currently experiencing more favorable (wetter) conditions will show the largest inhibition. Extensive height reductions can be expected for coastal areas of France, Greece, Spain and northern Africa. Our findings underline the fact that intraspecific variations in sensitivity to precipitation must be considered when projecting tree height responses of dry forests to future climate. The ecotype-specific projected performances call for management activities to ensure forest resilience in the Mediterranean through, for example, tailored deployment strategies.

Physiological Stress Integrates Resistance to Rattlesnake Venom and the Onset of Risky Foraging in California Ground Squirrels

Using venom for predation often leads to the evolution of resistance in prey. Understanding individual variation in venom resistance is key to unlocking basic mechanisms by which antagonistic coevolution can sustain variation in traits under selection. For prey, the opposing challenges of predator avoidance and resource acquisition often lead to correlated levels of risk and reward, which in turn can favor suites of integrated morphological, physiological and behavioral traits. We investigate the relationship between risk-sensitive behaviors, physiological resistance to rattlesnake venom, and stress in a population of California ground squirrels. For the same individuals, we quantified foraging decisions in the presence of snake predators, fecal corticosterone metabolites (a measure of “stress”), and blood serum inhibition of venom enzymatic activity (a measure of venom resistance). Individual responses to snakes were repeatable for three measures of risk-sensitive behavior, indicating that some individuals were consistently risk-averse whereas others were risk tolerant. Venom resistance was lower in squirrels with higher glucocorticoid levels and poorer body condition. Whereas resistance failed to predict proximity to and interactions with snake predators, individuals with higher glucocorticoid levels and in lower body condition waited the longest to feed when near a snake. We compared alternative structural equation models to evaluate alternative hypotheses for the relationships among stress, venom resistance, and behavior. We found support for stress as a shared physiological correlate that independently lowers venom resistance and leads to squirrels that wait longer to feed in the presence of a snake, whereas we did not find evidence that resistance directly facilitates latency to forage. Our findings suggest that stress may help less-resistant squirrels avoid a deadly snakebite, but also reduces feeding opportunities. The combined lethal and non-lethal effects of stressors in predator–prey interactions simultaneously impact multiple key traits in this system, making environmental stress a potential contributor to geographic variation in trait expression of toxic predators and resistant prey.