Luis Santos-del-Blanco

Plasticity in reproduction and growth among 52 range-wide populations of a Mediterranean conifer: adaptive responses to environmental stress

A plastic response towards enhanced reproduction is expected in stressful environments, but it is assumed to trade off against vegetative growth and efficiency in the use of available resources deployed in reproduction [reproductive efficiency (RE)]. Evidence supporting this expectation is scarce for plants, particularly for long‐lived species. Forest trees such as Mediterranean pines provide ideal models to study the adaptive value of allocation to reproduction vs. vegetative growth given their among‐population differentiation for adaptive traits and their remarkable capacity to cope with dry and low‐fertility environments. We studied 52 range‐wide Pinus halepensis populations planted into two environmentally contrasting sites during their initial reproductive stage. We investigated the effect of site, population and their interaction on vegetative growth, threshold size for female reproduction, reproductive–vegetative size relationships and RE. We quantified correlations among traits and environmental variables to identify allocation trade‐offs and ecotypic trends. Genetic variation for plasticity was high for vegetative growth, whereas it was nonsignificant for reproduction. Size‐corrected reproduction was enhanced in the more stressful site supporting the expectation for adverse conditions to elicit plastic responses in reproductive allometry. However, RE was unrelated with early reproductive investment. Our results followed theoretical predictions and support that phenotypic plasticity for reproduction is adaptive under stressful environments. Considering expectations of increased drought in the Mediterranean, we hypothesize that phenotypic plasticity together with natural selection on reproductive traits will play a relevant role in the future adaptation of forest tree species.

Genetic differentiation for size at first reproduction through male versus female functions in the widespread Mediterranean tree Pinus pinaster

BACKGROUND AND AIMS The study of local adaptation in plant reproductive traits has received substantial attention in short-lived species, but studies conducted on forest trees are scarce. This lack of research on long-lived species represents an important gap in our knowledge, because inferences about selection on the reproduction and life history of short-lived species cannot necessarily be extrapolated to trees. This study considers whether the size for first reproduction is locally adapted across a broad geographical range of the Mediterranean conifer species Pinus pinaster. In particular, the study investigates whether this monoecious species varies genetically among populations in terms of whether individuals start to reproduce through their male function, their female function or both sexual functions simultaneously. Whether differences among populations could be attributed to local adaptation across a climatic gradient is then considered. METHODS Male and female reproduction and growth were measured during early stages of sexual maturity of a P. pinaster common garden comprising 23 populations sampled across the species range. Generalized linear mixed models were used to assess genetic variability of early reproductive life-history traits. Environmental correlations with reproductive life-history traits were tested after controlling for neutral genetic structure provided by 12 nuclear simple sequence repeat markers. KEY RESULTS Trees tended to reproduce first through their male function, at a size (height) that varied little among source populations. The transition to female reproduction was slower, showed higher levels of variability and was negatively correlated with vegetative growth traits. Several female reproductive traits were correlated with a gradient of growth conditions, even after accounting for neutral genetic structure, with populations from more unfavourable sites tending to commence female reproduction at a lower individual size. CONCLUSIONS The study represents the first report of genetic variability among populations for differences in the threshold size for first reproduction between male and female sexual functions in a tree species. The relatively uniform size at which individuals begin reproducing through their male function probably represents the fact that pollen dispersal is also relatively invariant among sites. However, the genetic variability in the timing of female reproduction probably reflects environment-dependent costs of cone production. The results also suggest that early sex allocation in this species might evolve under constraints that do not apply to other conifers.

Extensive Clonal Assemblies in Populus alba and Populus x canescens from the Iberian Peninsula

Riparian vegetation in temperate zones holds great environmental importance and thus its conservation should be a priority. Among riverine tree species, genus Populus stands out, being also a model organism in research. In this work, we present a genetic analysis of the hybrid Populus x canescens and its parent species Populus alba and Populus tremula, with special emphasis in P. alba with which backcrosses frequently occur. This study focuses on the river Douro basin (Spain) where the presence of hybrids has been previously reported. Nuclear microsatellite markers and Bayesian statistical analysis have been used for the detection of hybrids and purebred stands. This methodology has also made possible the study of clonality in the taxonomic continuum P. alba–P. x canescens. Our results highlight the existence of a small number of genotypes accounting for most of the individual trees and stands. Possible causes leading to this situation are discussed. Also, the presence of individuals genetically close to ornamental cultivars is reported for the first time. Finally, concern about the present situation of P. alba genetic resources is brought up.

Capturing neutral and adaptive genetic diversity for conservation in a highly structured tree species

Preserving intraspecific genetic diversity is essential for long-term forest sustainability in a climate change scenario. Despite that, genetic information is largely neglected in conservation planning, and how conservation units should be defined is still heatedly debated. Here, we use maritime pine (Pinus pinaster Ait.), an outcrossing long-lived tree with a highly fragmented distribution in the Mediterranean biodiversity hotspot, to prove the importance of accounting for genetic variation, of both neutral molecular markers and quantitative traits, to define useful conservation units. Six gene pools associated to distinct evolutionary histories were identified within the species using 12 microsatellites and 266 single nucleotide polymorphisms (SNPs). In addition, height and survival standing variation, their genetic control, and plasticity were assessed in a multisite clonal common garden experiment (16 544 trees). We found high levels of quantitative genetic differentiation within previously defined neutral gene pools. Subsequent cluster analysis and post hoc trait distribution comparisons allowed us to define 10 genetically homogeneous population groups with high evolutionary potential. They constitute the minimum number of units to be represented in a maritime pine dynamic conservation program. Our results uphold that the identification of conservation units below the species level should account for key neutral and adaptive components of genetic diversity, especially in species with strong population structure and complex evolutionary histories. The environmental zonation approach currently used by the pan-European genetic conservation strategy for forest trees would be largely improved by gradually integrating molecular and quantitative trait information, as data become available.

Disentangling plasticity of serotiny, a key adaptive trait in a Mediterranean conifer

PREMISE OF THE STUDY Serotiny, the maintenance of ripe seeds in closed fruits or cones until fire causes dehiscence, is a key adaptive trait of plants in fire-prone ecosystems, but knowledge of phenotypic plasticity for cone retention in woody plants is extremely scarce. On the basis of published literature and our field observations, we hypothesized that increased aridity might decrease the aerial seed bank as a plastic response, not necessarily adaptive. METHODS We used a Pinus halepensis common garden replicated in three contrasted sites (mild, cold, and dry) to separate population differentiation from phenotypic plasticity of cone serotiny and canopy cone bank (CCB). Differences in growth among trees of the same provenance allowed us to include size effect as a proxy of ontogenetic age for the same chronological age of the trees. KEY RESULTS Tree size had a strong negative effect on serotiny, but serotiny degree differed among trial sites even after accounting for size effects. As hypothesized, serotiny was lower at the harsh (dry and cold) sites compared with the mild site. Genetic variation for size-dependent cone serotiny and significant population × site interaction were confirmed, the latter implying different plasticity of serotiny among populations. Population differentiation for CCB showed an ecotypic trend, with positive correlation with temperature oscillation (continentality) and negative correlation with summer rainfall. CONCLUSIONS Growth-limiting environments exacerbated the precocious release of seeds, contrary to the ecotypic trend found for the aerial cone bank, suggesting a counter-gradient plasticity. This plastic response is potentially maladaptive under a scenario of frequent wildfires.

Correlated genetic effects on reproduction define a domestication syndrome in a forest tree

Compared to natural selection, domestication implies a dramatic change in traits linked to fitness. A number of traits conferring fitness in the wild might be detrimental under domestication, and domesticated species typically differ from their ancestors in a set of traits known as the domestication syndrome. Specifically, trade‐offs between growth and reproduction are well established across the tree of life. According to allocation theory, selection for growth rate is expected to indirectly alter life‐history reproductive traits, diverting resources from reproduction to growth. Here we tested this hypothesis by examining the genetic change and correlated responses of reproductive traits as a result of selection for timber yield in the tree Pinus pinaster. Phenotypic selection was carried out in a natural population, and progenies from selected trees were compared with those of control trees in a common garden experiment. According to expectations, we detected a genetic change in important life‐history traits due to selection. Specifically, threshold sizes for reproduction were much higher and reproductive investment relative to size significantly lower in the selected progenies just after a single artificial selection event. Our study helps to define the domestication syndrome in exploited forest trees and shows that changes affecting developmental pathways are relevant in domestication processes of long‐lived plants.

The effects of fire severity on ectomycorrhizal colonization and morphometric features in Pinus pinaster Ait. seedlings

Aim of the study: Mycorrhizal fungi in Mediterranean forests play a key role in the complex process of recovery after wildfires. A broader understanding of an important pyrophytic species as Pinus pinaster and its fungal symbionts is thus necessary for forest restoration purposes. This study aims to assess the effects of ectomycorrhizal symbiosis on maritime pine seedlings and how fire severity affects fungal colonization ability. Area of study: Central Spain, in a Mediterranean region typically affected by wildfires dominated by Pinus pinaster , a species adapted to fire disturbance. Material and Methods : We studied P. pinaster root apexes from seedlings grown in soils collected one year after fire in undisturbed sites, sites moderately affected by fire and sites highly affected by fire. Natural ectomycorrhization was observed at the whole root system level as well as at two root vertical sections (0-10 cm and 10-20 cm). We also measured several morphometric traits (tap root length, shoot length, dry biomass of shoots and root/shoot ratio), which were used to test the influence of fire severity and soil chemistry upon them. Main results : Ectomycorrhizal colonization in undisturbed soils for total and separated root vertical sections was higher than in soils that had been affected by fire to some degree. Inversely, seedling vegetative size increased according to fire severity. Research highlights: Fire severity affected soil properties and mycorrhizal colonization one year after occurrence, thus affecting plant development. These findings can contribute to a better knowledge of the factors mediating successful establishment of P. pinaster in Mediterranean forests after wildfires.

Local effects drive heterozygosity-fitness correlations in an outcrossing long-lived tree.

Heterozygosity–fitness correlations (HFCs) have been used to understand the complex interactions between inbreeding, genetic diversity and evolution. Although frequently reported for decades, evidence for HFCs was often based on underpowered studies or inappropriate methods, and hence their underlying mechanisms are still under debate. Here, we used 6100 genome-wide single nucleotide polymorphisms (SNPs) to test for general and local effect HFCs in maritime pine (Pinus pinaster Ait.), an iconic Mediterranean forest tree. Survival was used as a fitness proxy, and HFCs were assessed at a four-site common garden under contrasting environmental conditions (total of 16 288 trees). We found no significant correlations between genome-wide heterozygosity and fitness at any location, despite variation in inbreeding explaining a substantial proportion of the total variance for survival. However, four SNPs (including two non-synonymous mutations) were involved in significant associations with survival, in particular in the common gardens with higher environmental stress, as shown by a novel heterozygosity–fitness association test at the species-wide level. Fitness effects of SNPs involved in significant HFCs were stable across maritime pine gene pools naturally growing in distinct environments. These results led us to dismiss the general effect hypothesis and suggested a significant role of heterozygosity in specific candidate genes for increasing fitness in maritime pine. Our study highlights the importance of considering the species evolutionary and demographic history and different spatial scales and testing environments when assessing and interpreting HFCs.

A ghost of dioecy past and the legacy of sexual dimorphism: low siring success of hermaphrodites after the breakdown of dioecy

Evolutionary transitions from dioecy to hermaphroditism must overcome the inertia of sexual dimorphism because modified males or females will express the opposite sexual function for which their phenotypes have been optimized. We tested this prediction by comparing the siring success of female-derived hermaphrodites of the plant Mercurialis annua with males and hermaphrodites that present a male-like inflorescence. We found that pollen dispersed by female-derived hermaphrodites was about a third poorer at siring outcross offspring than that from hermaphrodites with male-like inflorescences, illustrating the notion that a ‘ghost of dioecy past’ compromises the fitness of derived hermaphrodites in outcrossing populations. We conclude that whereas dioecy might evolve from hermaphroditism by conferring upon individuals certain benefits of sexual specialization, reversals from dioecy to hermaphroditism must often be limited to situations in which outcrossing cannot be maintained and inbreeding is favored. Our study provides novel empirical support for evolutionary models for the breakdown of dioecy.