Steep Ecological Gradients Research Articles

Phylogeography of Ara militaris (Military Macaw): Implications for Conservation

The Military Macaw (Ara militaris) is an endangered bird species with disjunct geographic distribution across the Neotropics, consisting of three recognized subspecies: One in Mexico (A. m. mexicanus) and two in South America (A. m. militaris and A. m. bolivianus). However, due to the limited phenotypic differentiation between these allopatric taxa, their taxonomic status has been the subject of debate. In this study, we explored mitochondrial DNA (mtDNA) variability to determine the phylogeographical pattern through phylogenetic and ecological modelling analyses. We also aimed to describe the evolutionary relationships of twelve A. militaris populations. We identified 41 haplotypes in the 300 bp region of the Cytochrome b (Cyt-b) gene of the mtDNA and low nucleotide diversity. The observed phylogeographic structure suggests the existence of two clades: One composed of A. m. militaris and A. m. bolivianus and another consisting solely of A. m. mexicanus. The A. m. mexicanus clade further divides into two recognized subclades: Sierra Madre Oriental and northeastern portion of the Sierra Madre Occidental. Ecological analyses revealed that the niche similarity between these lineages was lower than expected by chance. Additionally, results from low cross-prediction tests indicated that the two lineages have inhabited different environmental spaces since the Late Pleistocene. This divergence may be associated with a steep ecological gradient and contemporary geographical barrier. Based on our results, we suggest that at least the A. m. mexicanus has a divergent evolutionary history; therefore, it should be considered as a different evolutionarily significant and management unit. We recommend that future conservation strategies in Mexico incorporate effective protection measures, including habitat preservation and the reduction of illegal trade, to ensure the preservation of viable populations.

Liverworts show a globally consistent mid-elevation richness peak.

The study of elevational gradients allows to draw conclusions on the factors and mechanisms determining patterns in species richness distribution. Several earlier studies investigated liverwort diversity on single or few elevational transects. However, a comprehensive survey of the elevational distribution patterns of liverwort richness and their underlying factors is lacking so far. This study's purpose was to fill this gap by compiling an extensive data set of liverwort elevational patterns encompassing a broad diversity of mountains and mountain ranges around the world. Using polynomial regression analyses, we found a prevalence of hump-shaped richness patterns (19 of 25 gradients), where liverwort species richness peaked at mid-elevation and decreased towards both ends of the gradient. Against our expectation and unlike in other plant groups, in liverworts, this pattern also applies to elevational gradients at mid-latitudes in temperate climates. Indeed, relative elevation, calculated as the percentage of the elevational range potentially inhabited by liverworts, was the most powerful predictor for the distribution of liverwort species richness. We conclude from these results that the admixture of low- and high-elevation liverwort floras, in combination with steep ecological gradients, leads to a mid-elevation floristic turnover shaping elevational patterns of liverwort diversity. Our analyses further detected significant effects of climatic variables (temperature of the warmest month, potential evapotranspiration, and precipitation of the warmest month) in explaining elevational liverwort richness patterns. This indicates that montane liverwort diversity is restricted by high temperatures and subsequent low water availability especially towards lower elevations, which presumably will lead to serious effects by temperature shifts associated with global warming.

Common microgeographical selection patterns revealed in four European conifers.

Microgeographical adaptation occurs when the effects of directional selection persist despite gene flow. Traits and genetic loci under selection can then show adaptive divergence, against the backdrop of little differentiation at other traits or loci. How common such events are and how strong the selection is that underlies them remain open questions. Here, we discovered and analysed microgeographical patterns of genomic divergence in four European and Mediterranean conifers with widely differing life-history traits and ecological requirements (Abies alba MIll., Cedrus atlantica [Endl.] Manetti, Pinus halepensis Mill. and Pinus pinaster Aiton) by screening pairs from geographically close forest stands sampled along steep ecological gradients. We inferred patterns of genomic divergence by applying a combination of divergence outlier detection methods, demographic modelling, Approximate Bayesian Computation inferences and genomic annotation to genomic data. Surprisingly for such small geographical scales, we showed that selection is strong in all species but generally affects different loci in each. A clear signature of selection was systematically detected on a fraction of the genome, of the order of 0.1%-1% of the loci depending on the species. The novel modelling method we designed for estimating selection coefficients showed that the microgeographical selection coefficient scaled by population size (Ns) was 2-30. Our results convincingly suggest that selection maintains within-population diversity at microgeographical scales in spatially heterogeneous environments. Such genetic diversity is likely to be a major reservoir of adaptive potential, helping populations to adapt under fluctuating environmental conditions.

Parallel diversification of the African tree toad genus Nectophryne (Bufonidae)

African amphibian diversity remains underestimated with many cryptic lineages awaiting formal description. An important hotspot of amphibian diversification is the Guineo-Congolian rainforest in Central Africa, its richness attributable to present day and ancestral range fragmentation through geological barriers, habitat expansion and contraction, and the presence of steep ecological gradients. The charismatic Nectophryne tree toads present an interesting case study for diversification in this region. The two formally described species comprising this genus show nearly identical geographic distributions extending across most of the Guineo-Congolian rainforest, but show little morphological disparity. Both species harbour extensive genetic diversity warranting taxonomic revisions, and interestingly, when comparing the subclades within each, the two species show remarkably parallel diversification histories, both in terms of timing of phylogenetic splits and their geographic distributions. This indicates that common processes may have shaped the evolutionary history of these lineages.

Little genetic structure in a Bornean endemic small mammal across a steep ecological gradient.

Janzen's influential "mountain passes are higher in the tropics" hypothesis predicts restricted gene flow and genetic isolation among populations spanning elevational gradients in the tropics. Few studies have tested this prediction, and studies that focus on population genetic structure in Southeast Asia are particularly underrepresented in the literature. Here, we test the hypothesis that mountain treeshrews (Tupaia montana) exhibit limited dispersal across their broad elevational range which spans ~2,300m on two peaks in Kinabalu National Park (KNP) in Borneo: Mt Tambuyukon (MT) and Mt Kinabalu (MK). We sampled 83 individuals across elevations on both peaks and performed population genomics analyses on mitogenomes and single nucleotide polymorphisms from 4,106 ultraconserved element loci. We detected weak genetic structure and infer gene flow both across elevations and between peaks. We found higher genetic differentiation on MT than MK despite its lower elevation and associated environmental variation. This implies that, contrary to our hypothesis, genetic structure in this system is not primarily shaped by elevation. We propose that this pattern may instead be the result of historical processes and limited upslope gene flow on MT. Importantly, our results serve as a foundational estimate of genetic diversity and population structure from which to track potential future effects of climate change on mountain treeshrews in KNP, an important conservation stronghold for the mountain treeshrew and other montane species.

Differential divergence in autosomes and sex chromosomes is associated with intra-island diversification at a very small spatial scale in a songbird lineage.

Recently diverged taxa showing marked phenotypic and ecological diversity provide optimal systems to understand the genetic processes underlying speciation. We used genome-wide markers to investigate the diversification of the Reunion grey white-eye (Zosterops borbonicus) on the small volcanic island of Reunion (Mascarene archipelago), where this species complex exhibits four geographical forms that are parapatrically distributed across the island and differ strikingly in plumage colour. One form restricted to the highlands is separated by a steep ecological gradient from three distinct lowland forms which meet at narrow hybrid zones that are not associated with environmental variables. Analyses of genomic variation based on single nucleotide polymorphism data from genotyping-by-sequencing and pooled RAD-seq approaches show that signatures of selection associated with elevation can be found at multiple regions across the genome, whereas most loci associated with the lowland forms are located on the Z sex chromosome. We identified TYRP1, a Z-linked colour gene, as a likely candidate locus underlying colour variation among lowland forms. Tests of demographic models revealed that highland and lowland forms diverged in the presence of gene flow, and divergence has progressed as gene flow was restricted by selection at loci across the genome. This system holds promise for investigating how adaptation and reproductive isolation shape the genomic landscape of divergence at multiple stages of the speciation process.

Pleistocene glacial cycles drove lineage diversification and fusion in the Yosemite toad (Anaxyrus canorus)

Species endemic to alpine environments can evolve via steep ecological selection gradients between lowland and upland environments. Additionally, many alpine environments have faced repeated glacial episodes over the past two million years, fracturing these endemics into isolated populations. In this "glacial pulse" model of alpine diversification, cycles of allopatry and ecologically divergent glacial refugia play a role in generating biodiversity, including novel admixed ("fused") lineages. We tested for patterns of glacial pulse lineage diversification in the Yosemite toad (Anaxyrus [Bufo] canorus), an alpine endemic tied to glacially influenced meadow environments. Using double-digest RADseq on populations densely sampled from a portion of the species range, we identified nine distinct lineages with divergence times ranging from 18 to 724 thousand years ago (ka), coinciding with multiple Sierra Nevada glacial events. Three lineages have admixed origins, and demographic models suggest these fused lineages have persisted throughout past glacial cycles. Directionality indices supported the hypothesis that some lineages recolonized Yosemite from east of the ice sheet, whereas other lineages remained in western refugia. Finally, refugial niche reconstructions suggest that low- and high-elevation lineages have convergently adapted to similar climatic niches. Our results suggest glacial cycles and refugia may be important crucibles of adaptive diversity across deep evolutionary time.

No combination of morphological, ecological or chemical characters can reliably diagnose species in theParmelia saxatilisaggregate in Scotland

AbstractTheParmelia saxatilisaggregate is comprised of three species in Europe, proposed to differ in morphological, distributional or chemical characters. In this study, we sampled nearly 200 thalli from five sites across a steep ecological gradient in Scotland to investigate the distribution of the species in the aggregate, and we characterized all specimens by morphological, chemical and ITS sequence variation. In our sample, 191 specimens were identified to species using ITS. We confirm that a PCR length assay can be used for separation ofP. saxatiliss. str. fromP. ernstiaeandP. serranabecause across our sample,P. saxatiliss. str. consistently includes a group I intronc. 200 bp. Using sequences for specimen identification, we test previously proposed characters to diagnose specimens and use multivariate analysis to identify the most consistent features which may be used for identification among species. First, we test lobe morphology, presence and amount of pruina, distribution of isidia, lobe tip colour, and chemistry. Second, we use classification trees that quantify the contributions of 1) morphological and chemical factors, and 2) morphological and ecological factors, toa prioriITS-barcoded specimens.Parmelia saxatiliss. str.,P. ernstiaeandP. serranaall occur across the sampled gradient but differ in the frequency of occurrence, withP. saxatiliss. str. more frequent in the relatively drier east, andP. ernstiaemore frequent in the wetter west.Parmelia serranawas collected around a third as often as the other two species, but more frequently on tree branches than expected. For all the morphological characters examined, all the species show some overlap and no morphological features are diagnostic, though trends are apparent by species. The classification tree approach holds promise for discovering the most meaningful variation for field workers to approach correct identifications. Chemical variation using TLC is perhaps the best way to distinguish most specimens but, even here, overlap in chemosyndromes exists among the species.

Strong host specificity of a root hemi-parasite (Santalum acuminatum) limits its local distribution: beggars can be choosers

Santalum acuminatum (quandong) is a root hemi-parasite with a very wide distribution across southern Australia. Despite its very wide distribution, along the Jurien Bay chronosequence, it only occurs on the young Quindalup dunes, and it is absent on older dunes. The soils and local vegetation community change across the 10 km chronosequence, with higher species diversity correlated with lower soil phosphorus (P) levels. Here, we aimed to test whether the distribution of quandong on the dune systems can be explained by different neighbours (potential hosts) or different soil P concentrations across the chronosequence. Quandongs were grown in pots with 18 potential hosts for a year at three P levels, reflecting conditions across the chronosequence. Hemi-parasite growth and neighbour response were measured through the assessment of biomass, root mass ratios, haustorial size and frequency, δ15N and δ13C isotope signatures, as well as amino acid composition of xylem sap. Effects of neighbour species on the growth of quandongs were stronger when they were paired with Acacia saligna than when grown with other legumes and non-legumes, indicating strong host specificity. Quandong growth with all other species was significantly less than when grown with A. saligna or without a host, indicating strong competition with a conspecific neighbour. Soil P concentration had little effect on quandong growth. Host specificity and competition from non-hosts comprise main drivers for the distribution of quandong across the Jurien Bay chronosequence, rather than soil P availability. Our results show the importance of host specificity and how it may restrict the distribution of hemi-parasitic plants in different plant communities along a steep ecological gradient.

Metabolic and physiological responses of Mediterranean high-mountain and alpine plants to combined abiotic stresses.

High-mountain areas provide excellent opportunities to study the effects of combined abiotic stresses on plant physiology given their variety of steep ecological gradients, low anthropogenic disturbance and remarkable levels of taxonomic diversity. Efficient photoprotective and antioxidant scavenging mechanisms are vital for survival in high-mountain plants, having its altitudinal and seasonal variations determined by environmental or ontogenetic factors such as the decrease in mean temperatures and water availability. A number of stress indicators have been described in order to rapidly assess plant fitness in high-mountain environments. For instance, carbon isotope (δ13 C) and proline content as drought and temperature stress indicators, because of their link to water-use efficiency and osmotic adjustment; photosynthetic pigments, related to phenology, nutrient status, light and temperature stress; and non-structural carbohydrate accumulation in response to mild or brief drought conditions. The present review unveils the wide research opportunities available for the study of adaptive responses in high-mountain plants via stress indicators, and calls attention to the substantial knowledge gap existing between alpine zones and other mountainous regions, such as Mediterranean high-mountains. The aim is to grant a more holistic understanding of the physiological mechanisms driving plant life in high altitudes and improve the predictions of the effects of changing environments in these species and across ecological scales.

Independent homoploid hybrid speciation events in the Macaronesian endemic genus Argyranthemum.

Well-characterized examples of homoploid hybrid speciation (HHS) are rare in nature, yet they offer the potential to study a number of evolutionary processes. In this study, we investigate putative homoploid hybrid species in the genus Argyranthemum (Asteraceae), a group of plants endemic to the Macaronesian archipelagos of the North Atlantic Ocean. We specifically address a number of knowledge gaps surrounding the origin(s) of A.sundingii and A.lemsii, which are thought to be derived from the same parental cross. Comparisons of leaf morphology suggest that A.sundingii and A.lemsii are distinct from their parental progenitors and distinguishable from each other based on leaf area. Ecological niche modelling (ENM) demonstrated that the homoploid hybrid species occupy novel habitats that are intermediate relative to the parental species. Nuclear simple sequence repeat markers (SSRs) and single nucleotide polymorphism (SNP) data indicate that the homoploid hybrid species are distinct from the parental taxa, while population-level sampling of chloroplast SSRs and approximate Bayesian computation show that A.sundingii and A.lemsii are independently derived from the same parental cross. As such, Argyranthemum represents an example of independent homoploid hybrid speciation events with evidence of divergence in leaf morphology and adaptation to novel intermediate habitats. On oceanic islands, which are often typified by steep ecological gradients and inhabited by recently derived species with weak reproductive barriers, multiple HHS events from the same parental cross are not only possible but also likely to have played a more important role in oceanic island radiations than we currently think.

Trade-offs between sampling effort and data quality in habitat monitoring

The transect method has been widely used to monitor habitat conservation status and has been recently recommended as the best tool to monitor steep ecological gradients, such as those in coastal systems. Despite that, the effectiveness of the transect approach can be limited when considering the sampling effort in terms of time needed for sampling. Our work aimed at evaluating the efficacy of the transect approach in a Mediterranean coastal system. Specifically we aimed at evaluating the sampling effort versus the completeness of datasets obtained by performing belt transects in different ways specifically designed to progressively reduce the sampling effort: (i) sampling plots adjacently (“adjacent-plot transect”); (ii) sampling plots alternately (“alternate-plot transect”); (iii) sampling one plot at each plant community along the vegetation zonation (“zonation-plot transect”). We evaluated method efficiency in terms of number and type of habitats identified, spatial extent, species richness and composition, through multivariate analyses, null models and rarefaction curves. The sampling effort was measured in terms of time needed for sampling. The zonation-plot transect had the lowest sampling effort, but provided only an approximation of the state of the dunal communities. The alternate-plot transect showed the best trade-off between the sampling effort and the completeness of information obtained, and may be considered as a efficient option in very wide coastal systems. Our research provides guidelines that can be used in other coastal systems to choose the most cost-effective monitoring method thereby maximising the efficient use of monitoring resources.

Insights into the life strategy of the common marine diatom Chaetoceros peruvianus Brightwell.

Chaetoceros peruvianus is a marine diatom species with circumglobal distribution. While frequently observed, it appears never to dominate the marine phytoplankton community hence it can be characterized as a rather opportunistic, generalistic species. Here we present ecological interpretations from a long-term data set on marine microphytoplankton in the northern Adriatic Sea, where the abundancies and relative contributions of C. peruvianus were observed along a set of steep ecological gradients. Limited supply of dissolved inorganic phosphate was identified as the driving ecological factor for this ecosystem. In parallel C. peruvianus was cultivated in monoclonal cultures and its morphological and physiological reaction to replete and phosphorus depleted medium was analysed. C. peruvianus reacted to phosphorus depletion by an increase in cell height and length as well as thickness and length of setae. This morphological reaction included an increase in cellular volume and calculated carbon content. Additionally, it represents the transition between two described morphological varieties, C. peruvianus and C. peruvianus var. robusta. C. peruvianus showed a significant induction of extracellular alkaline phosphatase activity if grown in phosphate depleted medium. Microscopical analysis demonstrated this activity to be located exclusively on the setae of the cells.

Forest Adaptation to Climate Change along Steep Ecological Gradients: The Case of the Mediterranean-Temperate Transition in South-Western Europe

Impacts of climate change are likely to be marked in areas with steep climatic transitions. Species turnover, spread of invasive species, altered productivity, and modified processes such as fire regimes can all spread rapidly along ecotones, which challenge the current paradigms of ecosystem management. We conducted a literature review at a continental-wide scale of South-Western European forests, where the drier and warmer conditions of the Mediterranean have been widely used as examples of what is expected in more temperate areas. Results from the literature point to: (a) an expansion of slow-growing evergreen hardwood trees; (b) increased dieback and mortality episodes in forests (both natural and planted) mostly related to competition and droughts, and mainly affecting conifers; and (c) an increase in emergent diseases and pests of keystone-trees used in agroforestry zones. There is no consensus in the literature that fire regimes are directly increasing due to climate change, but available satellite data of fire intensity in the last 17 years has been lower in zones where agroforestry practices are dominant compared to unmanaged forests. In contrast, there is agreement in the literature that the current spread of fire events is probably related to land abandonment patterns. The practice of agroforestry, common in all Mediterranean countries, emerges as a frequent recommendation in the literature to cope with drought, reduce fire risk, and maintain biodiverse landscapes and rural jobs. However, it is unknown the extent to which the open vegetation resulting from agroforestry is of interest to forest managers in temperate areas used to exploiting closed forest vegetation. Hence, many transitional areas surrounding the Mediterranean Basin may be left unmanaged with potentially higher climate-change risks, which require active monitoring in order to understand and help ongoing natural adaptation processes.

Positive selection and climatic effects on MHC class II gene diversity in hares (Lepus capensis) from a steep ecological gradient

In natural populations, allelic diversity of the major histocompatibility complex (MHC) is commonly interpreted as resulting from positive selection in varying spatiotemporal pathogenic landscapes. Composite pathogenic landscape data are, however, rarely available. We studied the spatial distribution of allelic diversity at two MHC class II loci (DQA, DQB) in hares, Lepus capensis, along a steep ecological gradient in North Africa and tested the role of climatic parameters for the spatial distribution of DQA and DQB proteins. Climatic parameters were considered to reflect to some extent pathogenic landscape variation. We investigated historical and contemporary forces that have shaped the variability at both genes, and tested for differential selective pressure across the ecological gradient by comparing allelic variation at MHC and neutral loci. We found positive selection on both MHC loci and significantly decreasing diversity from North to South Tunisia. Our multinomial linear models revealed significant effects of geographical positions that were correlated with mean annual temperature and precipitation on the occurrence of protein variants, but no effects of co-occurring DQA or DQB proteins, respectively. Diversifying selection, recombination, adaptation to local pathogenic landscapes (supposedly reflected by climate parameters) and neutral demographic processes have shaped the observed MHC diversity and differentiation patterns.

Niche divergence along a fine‐scale ecological gradient in sympatric color morphs of a coral reef fish

AbstractColor polymorphisms offer unique opportunities for testing the role of ecological adaptation and natural selection in the origin of species. However, the ecological conditions that facilitate the coexistence and speciation of color morphs in nature remain poorly understood. In this study, we investigate the ecological mechanisms maintaining the color polymorphism in the arc‐eye hawkfish (Paracirrhites arcatus), which consists of two common morphs (pink white‐striped [PWS] and melanistic [MEL]) and less common intermediates that all coexist on shallow coral reefs throughout the Indo‐Pacific. Our approach was to test whether ecologically based disruptive selection on color could explain the maintenance of and possible incipient divergence between sympatric color morphs. We compared patterns of spatial distribution, abundance, habitat use and preference, and niche overlap among color morphs at nine sites along the leeward coast of Hawaii Island. We demonstrate that despite co‐occurring at all sites, sympatric color morphs show significant niche divergence along a steep ecological gradient. Morph frequencies correlate strongly with a multivariate gradient in coral community and reef topography, which explained 81% of the variability in relative abundance of the two main color morphs. Melanistic morphs were more frequent in shallow, steep surge zones dominated byPocilloporacorals. In contrast, PWS morphs were more frequent in deeper, sub‐surge zones with higher coral cover dominated byPoritescorals. Niche overlap analyses highlight how the two main morphs are partitioned on opposite ends of this continuous ecological gradient, while phenotypic intermediates are mostly restricted to intermediate habitats. The strong correlation between phenotype and environment suggests that morphs have fitness advantages in their respective habitats. We speculate that disruptive natural selection on color pattern to increase crypsis in heterogeneous visual environments has led to divergence in habitat preferences. The resulting ecological isolation provides a plausible mechanism for the persistence of multiple coexisting color phenotypes inP. arcatus. If divergence in color also affects mate choice, the evolution of reproductive isolation is likely to be promoted.

The Ecology of One Cosmopolitan, One Newly Introduced and One Occasionally Advected Species from the Genus Skeletonema in a Highly Structured Ecosystem, the Northern Adriatic.

The diatom genus Skeletonema is globally distributed and often an important constituent of the phytoplankton community. In the marine phytoplankton of the northern Adriatic Sea, we found three species of the genus Skeletonema: Skeletonema menzelii, Skeletonema marinoi and Skeletonema grevillei. Making use of the steep ecological gradients that characterise the northern Adriatic, along which we could observe those species, we report here on the ecological circumstances under which those species thrive and how their respective populations are globally connected. This is the first detailed ecological study for the species S. grevillei. This study is also the first report for S. grevillei for the Adriatic Sea and Mediterranean together with additional electron microscopic details on fresh in situ samples for this species. S. marinoi appears to clearly prefer strong freshwater influence and high nutrient concentrations delivered by low salinity waters. It can outcompete other diatom species and dominate microphytoplankton blooms. S. grevillei on the other hand appears to thrive in high nutrient concentrations triggered by water column mixing. It also appears to prefer higher salinity waters and coastal embayments. Genetic analysis of S. grevillei demonstrated a peculiar dissimilarity with isolates from coastal waters off Yemen, India, Oman and China. However, a closely related sequence was isolated from coastal waters off Japan. These results indicate that S. grevillei is an introduced species, possibly transported by ballast waters. S. menzelii is a sporadic visitor in the northern Adriatic, advected from rather oligotrophic middle Adriatic waters and never dominates the phytoplankton community in the northern Adriatic.

Selection on the mitochondrial ATP synthase 6 and the NADH dehydrogenase 2 genes in hares (Lepus capensis L., 1758) from a steep ecological gradient in North Africa

BackgroundRecent studies of selection on mitochondrial (mt) OXPHOS genes suggest adaptation due mainly to environmental variation. In this context, Tunisian hares that display several external phenotypes with phylogenetically rather homogenous gene pool and shallow population structure provide a good precondition to detect positive selection on mt genes related to environmental/climatic variation, specifically ambient temperature and precipitation.ResultsWe used codon-based methods along with population genetic data to test for positive selection on ATPase synthase 6 (ATP6) and NADH dehydrogenase 2 (ND2) of cape hares (Lepus capensis) collected along a steep ecological gradient in Tunisia. We found significantly higher differentiation at the ATP6 locus across Tunisia, with sub-humid Mediterranean, semi-arid, and arid Sahara climate than for fourteen unlinked supposedly neutrally evolving nuclear microsatellites and mt control region sequences. This suggested positive selection on ATP6 sequences, which was confirmed by several codon-based tests for one sequence site that together with a second site translated into four different amino acids. Positive selection on ND2 sequences was also confirmed by several codon-based tests. The corresponding frequencies of the two most prevalent variants at each locus varied significantly across climate regions, and our logistic general linear models of occurrence of those proteins indicated significant effects of mean annual temperature for ATP6 and mean minimum temperature of the coldest month of the year for ND2, independent of geographical location, annual precipitation, and the respective co-occurring protein at the second locus. Moreover, presence of the ancestral ATP6 protein, as inferred from phylogenetic networks, was positively affected by the simultaneous presence of the derived ND2 protein and vice versa, independent of temperature, precipitation, or geographic location. Finally, we obtained a significant coevolution signal for the ancestral ATP6 and derived ND2 sequences and vice versa.Conclusionspositive selection was strongly suggested by the population genetic approach and the codon-based tests in both mtDNA genes. Moreover, the two most prevalent proteins at the ATP6 locus were distributed at significantly varying frequencies across the study area with a significant effect of mean annual temperature on the occurrence of the ATP6 proteins independent of geographical coordinates and the co-occuring ND2 protein variant. For ND2, occurrence of the two most frequent protein variants was significantly influenced by the mean minimum temperature of the coldest month, independent of the co-occurring ATP6 protein variant and geographical coordinates. This strongly suggests direct involvement of ambient temperature in the adaptation of the studied mtOXPHOS genes.

Hybridization between a Euro‐Siberian (Vipera berus) and a Para‐Mediterranean viper (V. aspis) at their contact zone in western France

AbstractWestern European vipers are well‐defined species with parapatric distributions that reflect contrasting thermal niches and climatic adaptations. Contact zones are usually narrow, coincide with steep ecological gradients and are associated with clear habitat segregation. Natural hybridization has been demonstrated between several species but has not been detected in others. The cold‐adapted adder (V. berus, subgenus Pelias) is not known to hybridize with the warm‐adapted aspic viper (V. aspis subgenus Vipera). For over 12 years, we have monitored sympatric populations of V. berus and V. aspis in western France where the two species exhibit very similar life cycles. We tested for possible hybridization because individuals with intermediate morphological traits have been reported in the past and were recently detected in the study population. Our results demonstrate that hybridization actually occurs and is directional since it involves females V. aspis viper and male V. berus in all analyzed cases. We discuss our results in the frame of previous findings on contact zones to evaluate in which conditions hybridization may occur.

Strong natural selection on juveniles maintains a narrow adult hybrid zone in a broadcast spawner.

Natural selection can maintain and help form species across different habitats, even when dispersal is high. Selection against inferior migrants (immigrant inviability) acts when locally adapted populations suffer high mortality on dispersal to unsuitable habitats. Habitat-specific populations undergoing divergent selection via immigrant inviability should thus show (1) a change in the ratio of adapted to nonadapted individuals among age/size classes and (2) a cline (defined by the environmental gradient) as selection counterbalances migration. Here we examine the frequencies of two depth-segregated lineages in juveniles and adults of a Caribbean octocoral, Eunicea flexuosa. Distributions of the two lineages in both shallow and deep environments were more distinct when inferred from adults than juveniles. Despite broad larval dispersal, we also found an extremely narrow hybrid zone (<100 m), with coincident clines for molecular and morphological characters of the host coral and its algal symbiont. Effective dispersal estimates derived from the hybrid zone are remarkably small (<20 m) for a broadcast spawner. The large selection coefficient against mismatched genotypes derived from cohort data is consistent with that from field transplant experiments. Narrow hybrid zones and limited effective dispersal may be a common outcome of long periods of postsettlement, prereproductive selection across steep ecological gradients. Strong diversifying selection provides a mechanism to explain the prevalence of depth-segregated sibling species in the sea.