Automixis in Artemia: solving a century‐old controversy

Parthenogenesis (reproduction through unfertilized eggs) encompasses a variety of reproduction modes with (automixis) or without (apomixis) meiosis. Different modes of automixis have very different genetic and evolutionary consequences but can be particularly difficult to tease apart. In this study, we propose a new method to discriminate different types of automixis from population‐level genetic data. We apply this method to diploid Artemia parthenogenetica, a crustacean whose reproductive mode remains controversial despite a century of intensive cytogenetic observations. We focus on A. parthenogenetica from two western Mediterranean populations. We show that they are diploid and that markers remain heterozygous in cultures maintained up to ~36 generations in the laboratory. Moreover, parallel patterns of population‐wide heterozygosity levels between the two natural populations strongly support the conclusion that diploid A. parthenogenetica reproduce by automictic parthenogenesis with central fusion and low, but nonzero recombination. This settles a century‐old controversy on Artemia, and, more generally, suggests that many automictic organisms harbour steep within‐chromosome gradients of heterozygosity due to a transition from clonal transmission in centromere‐proximal regions to a form of inbreeding similar to self‐fertilization in centromere‐distal regions. Such systems therefore offer a new avenue for contrasting the genomic consequences of asexuality and inbreeding.     

Vegetation in clear‐cuts depends on previous land use: a century‐old grassland legacy

Plant species richness in central and northern European seminatural grasslands is often more closely linked to past than present habitat configuration, which is indicative of an extinction debt. In this study, we investigate whether signs of historical grassland management can be found in clear‐cuts after at least 80 years as coniferous production forest by comparing floras between clear‐cuts with a history as meadow and as forest in the 1870s in Sweden. Study sites were selected using old land‐use maps and data on present‐day clear‐cuts. Species traits reflecting high capacities for dispersal and persistence were used to explain any possible links between the plants and the historical land use. Clear‐cuts that were formerly meadow had, on average, 36% higher species richness and 35% higher richness of grassland indicator species, as well as a larger overall seed mass and lower anemochory, compared to clear‐cuts with history as forest. We suggest that the plants in former meadows never disappeared after afforestation but survived as remnant populations. Many contemporary forests in Sweden were managed as grasslands in the 1800s. As conservation of remaining grassland fragments will not be enough to reduce the existing extinction debts of the flora, these young forests offer opportunities for grassland restoration at large scales. Our study supports the concept of remnant populations and highlights the importance of considering historical land use for understanding the distribution of grassland plant species in fragmented landscapes, as well as for policy‐making and conservation.     

New‐age ideas about age‐old sex: separating meiosis from mating could solve a century‐old conundrum

Ever since Darwin first addressed it, sexual reproduction reigns as the ‘queen’ of evolutionary questions. Multiple theories tried to explain how this apparently costly and cumbersome method has become the universal mode of eukaryote reproduction. Most theories stress the adaptive advantages of sex by generating variation, they fail however to explain the ubiquitous persistence of sexual reproduction also where adaptation is not an issue. I argue that the obstacle for comprehending the role of sex stems from the conceptual entanglement of two distinct processes – gamete production by meiosis and gamete fusion by mating (mixis). Meiosis is an ancient, highly rigid and evolutionary conserved process identical and ubiquitous in all eukaryotes. Mating, by contrast, shows tremendous evolutionary variability even in closely related clades and exhibits wonderful ecological adaptability. To appreciate the respective roles of these two processes, which are normally linked and alternating, we require cases where one takes place without the other. Such cases are rather common. The heteromorphic sex chromosomes Y and W, that do not undergo meiotic recombination are an evolutionary test case for demonstrating the role of meiosis. Substantial recent genomic evidence highlights the accelerated rates of change and attrition these chromosomes undergo in comparison to those of recombining autosomes. I thus propose that the most basic role of meiosis is conserving integrity of the genome. A reciprocal case of meiosis without bi‐parental mating, is presented by self‐fertilization, which is fairly common in flowering plants, as well as most types of apomixis. I argue that deconstructing sex into these two distinct processes – meiosis and mating – will greatly facilitate their analysis and promote our understanding of sexual reproduction.     

Sex‐biased transcriptome divergence along a latitudinal gradient

Sex‐dependent gene expression is likely an important genomic mechanism that allows sex‐specific adaptation to environmental changes. Among Drosophila species, sex‐biased genes display remarkably consistent evolutionary patterns; male‐biased genes evolve faster than unbiased genes in both coding sequence and expression level, suggesting sex differences in selection through time. However, comparatively little is known of the evolutionary process shaping sex‐biased expression within species. Latitudinal clines offer an opportunity to examine how changes in key ecological parameters also influence sex‐specific selection and the evolution of sex‐biased gene expression. We assayed male and female gene expression in Drosophila serrata along a latitudinal gradient in eastern Australia spanning most of its endemic distribution. Analysis of 11 631 genes across eight populations revealed strong sex differences in the frequency, mode and strength of divergence. Divergence was far stronger in males than females and while latitudinal clines were evident in both sexes, male divergence was often population specific, suggesting responses to localized selection pressures that do not covary predictably with latitude. While divergence was enriched for male‐biased genes, there was no overrepresentation of X‐linked genes in males. By contrast, X‐linked divergence was elevated in females, especially for female‐biased genes. Many genes that diverged in D. serrata have homologs also showing latitudinal divergence in Drosophila simulans and Drosophila melanogaster on other continents, likely indicating parallel adaptation in these distantly related species. Our results suggest that sex differences in selection play an important role in shaping the evolution of gene expression over macro‐ and micro‐ecological spatial scales.     

Post‐glacial colonization routes coincide with a life‐history breakpoint along a latitudinal gradient

Although adaptive divergence along environmental gradients has repeatedly been demonstrated, the role of post‐glacial colonization routes in determining phenotypic variation along gradients has received little attention. Here, we used a hierarchical Q_ST–F_ST approach to separate the roles of adaptive and neutral processes in shaping phenotypic variation in moor frog (Rana arvalis) larval life histories along a 1,700 km latitudinal gradient across northern Europe. This species has colonized Scandinavia via two routes with a contact zone in northern Sweden. By using neutral SNP and common garden phenotypic data from 13 populations at two temperatures, we showed that most of the variation along the gradient occurred between the two colonizing lineages. We found little phenotypic divergence within the lineages; however, all phenotypic traits were strongly diverged between the southern and northern colonization routes, with higher growth and development rates and larger body size in the north. The Q_ST estimates between the colonization routes were four times higher than F_ST, indicating a prominent role for natural selection. Q_ST within the colonization routes did not generally differ from F_ST, but we found temperature‐dependent adaptive divergence close to the contact zone. These results indicate that lineage‐specific variation can account for much of the adaptive divergence along a latitudinal gradient.     

Within-species correlations in leaf traits of three boreal plant species along a latitudinal gradient

Evergreen boreal plant species express high variability in their leaf traits. It remains controversial whether this within-species variability is constrained to the same leaf trait relationships as has been observed across species. We sampled leaves of three boreal evergreen woody species along a latitudinal gradient (from 57o56′N to 69o55′N). Leaf longevity (LL) of Pinus sylvestris L. and Vaccinium vitis-idaea L. correlated negatively with mean annual air temperature (MAT), whereas the LL of Ledum palustre L. was not affected by MAT. V. vitis-idaea and L. palustre had a negative relationship between leaf mass per area (LMA) and MAT. In P. sylvestris, the LMA–MAT relationship was positive. A negative correlation between LL and LMA was significant only for P. sylvestris. Leaf nitrogen concentration was positively related to leaf phosphorus concentration in all three species. Leaf potassium concentration was related to nitrogen concentration only in L. palustre, and to phosphorus concentration in P. sylvestris and L. palustre. Our results demonstrate that although within the studied species the variation in some of the leaf traits may have the same degree as interspecific variation, there is no such intercorrelation of leaf traits within the studied species as has been observed across species.     

Assessing the frequency and drivers of early‐greening in broad‐leaved woodlands along a latitudinal gradient in southern Africa

Savannas are the only deciduous system where new leaf flush pre‐empts the onset of suitable conditions for growth, a phenological phenomenon known as early‐greening. Limited understanding of the frequency and drivers of the occurrence of early‐greening in southern African savanna trees exists. We aimed to estimate the frequency of early‐greening events across southern Africa and investigated potential environmental drivers of green‐up. We selected and compared seven broad‐leaved woodland sites where Burkea africana was a dominant species using remotely sensed data along a latitudinal gradient from South Africa to Zambia. Normalized difference vegetation index (NDVI) values were extracted from the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery at each site from January 2002 to June 2014. Using an austral year (July 1st–June 30th), early‐greening was recorded if the green‐up start date occurred prior to the onset date of seasonal rainfall. A latitudinal gradient of early‐green‐up was detected across southern Africa (R² = 0.74) with the two most northerly (Zambian) sites showing the earliest and most consistent green‐up start dates (3 October ± 5.34 days). A strong latitudinal gradient was observed between the variability in the amount of rainfall in the first 6 months of green‐up and the green‐up start dates across southern Africa (R² = 0.92). Photoperiod appeared to play a role in areas where the onset of rainfall commenced late into the austral year. Mean maximum temperatures recorded 10 days prior to green‐up start dates suggested a potential threshold of about 35°C, which could drive early‐greening in the absence of rainfall. Correlations between the proportion of early‐greening years and the above mentioned environmental factors indicated that rainfall variability had the strongest influence over the observed phenological gradient (R² = 0.96). Understanding early‐greening in complex savanna systems is a vital step in furthering predictive phenological models under changing climatic conditions.     

Niche width of above‐ and below‐ground organisms varied in predicting biodiversity profiling along a latitudinal gradient

Biodiversity is the foundation of all ecosystems across the planet, and having a better understanding of its global distribution mechanism could be important for biodiversity conservation under global change. A niche width model, combined with metabolic theory, has successfully predicted the increase of α‐diversity and decrease of β‐diversity in the below‐ground microbial community along an altitudinal mountain gradient. In this study, we evaluated this niche width model of above‐ground plants (mainly trees and shrubs) and below‐ground bulk soil microbial communities (i.e., bacteria and archaea) along a latitudinal gradient of forests in China. The niche widths of both plants and microbes increased with increasing temperature and precipitation, and with proximity to circumneutral pH. However, the α‐ and β‐diversities (observed richness and Bray–Curtis dissimilarity, respectively) could not be accurately predicted by a single niche width model alone, either temperature, precipitation or pH. Considering the interactions among different niche width models, all three niche width models were combined to predict biodiversity at the community level using structural equation modelling. The results showed that the niche width model of circumneutral pH was most important in predicting diversity profiling (i.e., α‐ and β‐diversity) for both plants and microbes, while niche width of precipitation and temperature showed both direct and indirect importance for microbe and plant biodiversity, respectively. Because the current niche width model neglects several scenarios related to taxon and environmental attributes, it still needs to be treated with caution in predicting biodiversity trends.     

A 450 million years long latitudinal gradient in age‐dependent extinction

Leigh Van Valen famously stated that under constant conditions extinction probability is independent of species age. To test this 'law of constant extinction', we developed a new method using deep learning to infer age‐dependent extinction and analysed 450 myr of marine life across 21 invertebrate clades. We show that extinction rate significantly decreases with age in > 90% of the cases, indicating that most species died out soon after their appearance while those which survived experienced ever decreasing extinction risk. This age‐dependent extinction pattern is stronger towards the Equator and holds true when the potential effects of mass extinctions and taxonomic inflation are accounted for. These results suggest that the effect of biological interactions on age‐dependent extinction rate is more intense towards the tropics. We propose that the latitudinal diversity gradient and selection at the species level account for this exceptional, yet little recognised, macroevolutionary and macroecological pattern.     

What’s in a frog stomach? Solving a 150‐year‐old mystery (Diptera: Calliphoridae)

Abstract.  The nominal taxon Acanthosoma chrysalis Mayer, 1844 is revised, and a lectotype is designated. The species, which was described from Germany from a number of alleged parasites encysted in the peritoneal wall of the stomach of edible frogs, is shown to be based on first instar larvae of blow flies (Calliphoridae). Argued from the shape and configuration of mouthhooks and abdominal cuticular spines, Acanthosoma Mayer, 1844 is shown to be a junior synonym of Onesia Robineau-Desvoidy, 1830 , syn.n. , and A. chrysalis is shown to be a junior synonym of O. floralis Robineau-Desvoidy, 1830 , syn.n. This species is an obligate parasitoid of earthworms, and it is hypothesized that first instar larvae enter the frogs through infected earthworms.     

Temporal stability of European rocky shore assemblages: variation across a latitudinal gradient and the role of habitat‐formers

Compensatory dynamics, overyielding and statistical averaging are mechanisms promoting the temporal stability of natural communities. Using the model of European intertidal rocky shore assemblages and collating 17 datasets, we investigated how the strength of these stability‐enhancing mechanisms varies with latitude and how it can be altered by the loss of habitat‐formers (e.g. canopy‐forming macroalgae). Community stability decreased with increasing latitude, mostly as a consequence of a greater synchronization of species fluctuations. Statistical averaging and overyielding (i.e. richness effect) promoted stability, but their strength did not vary with latitude. An experimental removal of macroalgal canopies caused a strengthening of the statistical averaging effect that was consistent across the latitudinal gradient investigated. Nonetheless, the loss of canopies depressed stability by enhancing the synchronization of species fluctuations on southernmost shores, while it had weak effects on shores at higher latitudes. Variation in life‐history traits among canopy‐forming species and/or in prevailing environmental conditions across a gradient of latitude could underlie variable effects of habitat‐formers on species fluctuations. Our study shows 1) that the stability of intertidal assemblages and strength of compensatory dynamics varies with latitude, 2) that canopy‐forming macroalgae, exerting a strong control on understorey species, can influence the strength of compensatory dynamics and 3) that biological forcing (i.e. facilitation) can be as important as environmental forcing in enhancing the synchronization of species fluctuations.     

An intraspecific application of the leaf‐height‐seed ecology strategy scheme to forest herbs along a latitudinal gradient

We applied the leaf‐height‐seed (LHS) ecology strategy scheme (a combination of three ecologically important traits: specific leaf area (SLA), seed mass and plant height) intraspecifically to two widespread European forest herbs along a latitudinal gradient. The aims of this study were to quantify LHS trait variation, disentangle the environmental factors affecting these traits and compare the within‐species LHS trait relationships with latitude to previously established cross‐species comparisons. We measured LHS traits in 41 Anemone nemorosa and 44 Milium effusum populations along a 1900–2300 km latitudinal gradient from N France to N Sweden. We then applied multilevel models to identify the effects of regional (temperature, latitude) and local (soil fertility and acidity, overstorey canopy cover) environmental factors on LHS traits. Both species displayed a significant 4% increase in plant height with every degree northward shift (almost a two‐fold plant height difference between the southernmost and northernmost populations). Neither seed mass nor SLA showed a significant latitudinal cline. Temperature had a large effect on the three LHS traits of Anemone. Latitude, canopy cover and soil nutrients were related to the SLA and plant height of Milium. None of the investigated variables appeared to be related to the seed mass of Milium. The variation in LHS traits indicates that the ecological strategy determined by the position of each population in this three‐factor triangle is not constant along the latitudinal gradient. The significant increase in plant height suggests greater competitive abilities for both species in the northernmost populations. We also found that the studied environmental factors affected the LHS traits of the two species on various scales: spring‐flowering Anemone was affected more by temperature, whereas early‐summer flowering Milium was affected more by local and other latitude‐related factors. Finally, previously reported cross‐species correlations between LHS traits and latitude were generally unsupported by our within‐species approach.     

Explaining global variation in the latitudinal diversity gradient: Meta‐analysis confirms known patterns and uncovers new ones

Aim

The pattern of increasing biological diversity from high latitudes to the equator [latitudinal diversity gradient (LDG)] has been recognized for > 200 years. Empirical studies have documented this pattern across many different organisms and locations. Our goal was to quantify the evidence for the global LDG and the associated spatial, taxonomic and environmental factors. We performed a meta‐analysis on a large number of individual LDGs that have been published in the 14 years since Hillebrand's ground‐breaking meta‐analysis of the LDG, using meta‐analysis and meta‐regression approaches largely new to the fields of ecology and biogeography.

Location

Global.

Time period

January 2003–September 2015.

Major taxa studied

Bacteria, protists, plants, fungi and animals.

Methods

We synthesized the outcomes of 389 individual cases of LDGs from 199 papers published since 2003, using hierarchical mixed‐effects meta‐analysis and multiple meta‐regression. Additionally, we re‐analysed Hillebrand's original dataset using modern methods.

Results

We confirmed the generality of the LDG, but found the pattern to be weaker than was found in Hillebrand's study. We identified previously unreported variation in LDG strength and slope across longitude, with evidence that the LDG is strongest in the Western Hemisphere. Locational characteristics, such as habitat and latitude range, contributed significantly to LDG strength, whereas organismal characteristics, including taxonomic group and trophic level, did not. Modern meta‐analytical models that incorporate hierarchical structure led to more conservative and sometimes contrasting effect size estimates relative to Hillebrand's initial analysis, whereas meta‐regression revealed underlying patterns in Hillebrand's dataset that were not apparent with a traditional analysis.

Main conclusions

We present evidence of global latitudinal, longitudinal and habitat‐based patterns in the LDG, which are apparent across both marine and terrestrial realms and over a broad taxonomic range of organisms, from bacteria to plants and vertebrates.     

Southern hospitality: a latitudinal gradient in gene flow in the marine environment

In recent years population genetics and phylogeographic studies have become increasingly valuable tools for inferring both historical and present-day genetic patterns within marine species. Here, we take a comparative approach to population-level study, analyzing original mitochondrial DNA data from 969 individuals representing 28 chiton (Mollusca: Polyplacophora) species to uncover large-scale genetic patterns along the Pacific coast of North America. The data reveal a distinct latitudinal connectivity gradient among chitons: species that exist at lower latitudes tend to have more isolated populations. This trend appears to be a product of between-species differences; within species, no significant gradient in connectivity is observed. Lower average annual sea surface temperatures are hypothesized to contribute to longer larval duration (and by extension, greater connectivity) among lecithotrophic species, providing a mechanism for the observed positive correlation between gene flow and latitude. Because increased isolation among populations may lead to speciation, a latitudinal trend in gene flow may contribute to the increased species diversity observed at lower latitudes.