Population structure, adaptation and divergence of the meadow spittlebug, Philaenus spumarius (Hemiptera, Aphrophoridae), revealed by genomic and morphological data

Understanding patterns of population differentiation and gene flow in insect vectors of plant diseases is crucial for the implementation of management programs of disease. We investigated morphological and genome-wide variation across the distribution range of the spittlebug Philaenus spumarius (Linnaeus, 1758) (Hemiptera, Auchenorrhyncha, Aphrophoridae), presently the most important vector of the plant pathogenic bacterium Xylella fastidiosa Wells et al., 1987 in Europe. We found genome-wide divergence between P. spumarius and a very closely related species, P. tesselatus Melichar, 1899, at RAD sequencing markers. The two species may be identified by the morphology of male genitalia but are not differentiated at mitochondrial COI, making DNA barcoding with this gene ineffective. This highlights the importance of using integrative approaches in taxonomy. We detected admixture between P. tesselatus from Morocco and P. spumarius from the Iberian Peninsula, suggesting gene-flow between them. Within P. spumarius, we found a pattern of isolation-by-distance in European populations, likely acting alongside other factors restricting gene flow. Varying levels of co-occurrence of different lineages, showing heterogeneous levels of admixture, suggest other isolation mechanisms. The transatlantic populations of North America and Azores were genetically closer to the British population analyzed here, suggesting an origin from North-Western Europe, as already detected with mitochondrial DNA. Nevertheless, these may have been produced through different colonization events. We detected SNPs with signatures of positive selection associated with environmental variables, especially related to extremes and range variation in temperature and precipitation. The population genomics approach provided new insights into the patterns of divergence, gene flow and adaptation in these spittlebugs and led to several hypotheses that require further local investigation.Peer reviewe

New mitochondrial and nuclear evidences support recent demographic expansion and an atypical phylogeographic pattern in the spittlebug Philaenus spumarius (Hemiptera, Aphrophoridae)

Peer reviewe

Climate change in the Baltic Sea region : a summary

Based on the Baltic Earth Assessment Reports of this thematic issue in Earth System Dynamics and recent peer-reviewed literature, current knowledge of the effects of global warming on past and future changes in climate of the Baltic Sea region is summarised and assessed. The study is an update of the Second Assessment of Climate Change (BACC II) published in 2015 and focuses on the atmosphere, land, cryosphere, ocean, sediments, and the terrestrial and marine biosphere. Based on the summaries of the recent knowledge gained in palaeo-, historical, and future regional climate research, we find that the main conclusions from earlier assessments still remain valid. However, new long-term, homogenous observational records, for example, for Scandinavian glacier inventories, sea-level-driven saltwater inflows, so-called Major Baltic Inflows, and phytoplankton species distribution, and new scenario simulations with improved models, for example, for glaciers, lake ice, and marine food web, have become available. In many cases, uncertainties can now be better estimated than before because more models were included in the ensembles, especially for the Baltic Sea. With the help of coupled models, feedbacks between several components of the Earth system have been studied, and multiple driver studies were performed, e.g. projections of the food web that include fisheries, eutrophication, and climate change. New datasets and projections have led to a revised understanding of changes in some variables such as salinity. Furthermore, it has become evident that natural variability, in particular for the ocean on multidecadal timescales, is greater than previously estimated, challenging our ability to detect observed and projected changes in climate. In this context, the first palaeoclimate simulations regionalised for the Baltic Sea region are instructive. Hence, estimated uncertainties for the projections of many variables increased. In addition to the well-known influence of the North Atlantic Oscillation, it was found that also other low-frequency modes of internal variability, such as the Atlantic Multidecadal Variability, have profound effects on the climate of the Baltic Sea region. Challenges were also identified, such as the systematic discrepancy between future cloudiness trends in global and regional models and the difficulty of confidently attributing large observed changes in marine ecosystems to climate change. Finally, we compare our results with other coastal sea assessments, such as the North Sea Region Climate Change Assessment (NOSCCA), and find that the effects of climate change on the Baltic Sea differ from those on the North Sea, since Baltic Sea oceanography and ecosystems are very different from other coastal seas such as the North Sea. While the North Sea dynamics are dominated by tides, the Baltic Sea is characterised by brackish water, a perennial vertical stratification in the southern subbasins, and a seasonal sea ice cover in the northern subbasins.Peer reviewe

Maximum Likelihood tree based on the 3 concatenated mtDNA genes (COI, COII and cyt <i>b</i>) (1527bp).

<p>Values above branches correspond to MP and ML bootstrap values (only values > 50% are shown) and values below branches correspond to Bayesian posterior probability. PT – Portugal; SP – Spain; UK – United Kingdom; BL – Belgium; FN – Finland; SC – Sicily; TK – European Turkey; AT – Anatolia; USA – United States of America; CA – Canada.</p

Maximum Parsimony tree based on nuclear gene elongation factor-1α.

<p>Values above branches correspond to MP bootstrap (only values > 50% are shown) and Bayesian posterior probability values. Black: GenBank sequences (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0098375#pone.0098375-MaryanskaNadachowska1" target="_blank">[26]</a>); blue individuals correspond to the eastern mtDNA group; red individuals correspond to the eastern-Mediterranean mtDNA group and green individuals to the western-Mediterranean mtDNA group.</p

Sampling locations of <i>Philaenus spumarius</i> in (a) Europe and Anatolia (b) North America and (c) New Zealand in each geographic region.

<p>1 – Azores; 2 – Iberian Peninsula; 3 – Morocco; 4 – France; 5 – United Kingdom; 6 – Belgium; 7 – Italian Peninsula; 8 – Sicily; 9 – Slovenia; 10 – Balkans (Bulgaria; Greece; European Turkey); 11 – Anatolian Peninsula; 12 – Finland; 13 – North America (Canada and United States of America); 14 – New Zealand. Circle sizes are proportional to the number of individuals. Circles: green – “western-Mediterranean” mtDNA group; red – “eastern-Mediterranean” mtDNA group; blue – “eastern” mtDNA group. Circle sizes are proportional to the number of samples.</p

Median-joining haplotype network of <i>Philaenus spumarius</i> sampled geographic regions for mitochondrial gene COI (539bp).

<p>Size of the circles is in proportion to the number of haplotypes. Branches begin in the centre of the circles and their sizes are in proportion to the number of mutations.</p

Tajima's <i>D</i> and [44] Fu's <i>F</i>s test values and their statistical significance for <i>Philaenus spumarius</i> Cytochrome <i>c</i> oxidase I mtDNA groups.

<p>*: indicates significant values at P<0.05; **: indicates significant values at P<0.01 and ***: indicates significant values at P<0.001.</p

Parameters from the mismatch distribution for <i>Philaenus spumarius</i> COI groups.

<p>Numbers in parenthesis are the upper and lower bound of 95% CI (1000 bootstrap replicates).</p><p>θ₀ and θ₁: pre-expansion and post-expansion populations size; τ: time in number of generations elapsed since the sudden/demographic expansion and spatial expansion episodes; SSD: sum of squared deviations; Raggedness: raggedness index following <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0098375#pone.0098375-Harpending2" target="_blank">[49]</a>; P_SSD and P_RAG.: probability that expected mismatch distributions (1000 bootstrap replicates) be larger than observed mismatch distributions.</p

Number of individuals, number of haplotypes and genetic diversity indices calculated for geographic regions of <i>Philaenus spumarius</i> and for mitochondrial gene Cytochrome <i>c</i> oxidase I (COI).

<p>Western Europe: Belgium and France; Italy: Italian peninsula and Sicily.</p