Phylogeography on the rocks: The contribution of current and historical factors in shaping the genetic structure of Chthamalus montagui (Crustacea, Cirripedia)

The model marine broadcast-spawner barnacle Chthamalus montagui was investigated to understand its genetic structure and quantify levels of population divergence, and to make inference on historical demography in terms of time of divergence and changes in population size. We collected specimens from rocky shores of the north-east Atlantic Ocean (4 locations), Mediterranean Sea (8) and Black Sea (1). The 312 sequences 537 bp) of the mitochondrial cytochrome c oxidase I allowed to detect 130 haplotypes. High within-location genetic variability was recorded, with haplotype diversity ranging between h = 0.750 and 0.967. Parameters of genetic divergence, haplotype network and Bayesian assignment analysis were consistent in rejecting the hypothesis of panmixia. C. montagui is genetically structured in three geographically discrete populations, which corresponded to north-eastern Atlantic Ocean, western-central Mediterranean Sea, and Aegean Sea-Black Sea. These populations are separated by two main effective barriers to gene flow located at the Almeria-Oran Front and in correspondence of the Cyclades Islands. According to the 'isolation with migration' model, adjacent population pairs diverged during the early to middle Pleistocene transition, a period in which geological events provoked significant changes in the structure and composition of palaeocommunities. Mismatch distributions, neutrality tests and Bayesian skyline plots showed past population expansions, which started approximately in the Mindel-Riss interglacial, in which ecological conditions were favourable for temperate species and calcium-uptaking marine organisms

The genetic structure of the exotic ascidian Styela plicata (Tunicata) from Italian ports, with a re-appraisal of its worldwide genetic pattern

The pleated ascidian Styela plicata (Lesueur, 1823) is a solitary species commonly found in ports and marinas around the world. It has been recorded in the Mediterranean region since the mid-19th century. In the present work, the species' genetic diversity was analysed, employing a 613-bp portion of the mitochondrial cytochrome c oxidase subunit I (COI) gene from 149 individuals collected in 14 ports along Italian coasts at spatial scales ranging from 1 to approximately 2200 km. Haplotype and nucleotide diversity values were h = 0-0.933 (total h = 0.789) and π = 0-0.145 (total π = 0.0094), respectively. A general southward trend of increasing within-population genetic diversity was observed. Analysis of molecular variance revealed significant genetic structuring but no significant differences were detected among basins, and no isolation by distance was found. Our data were integrated with the COI sequences available from previous studies and re-analysed in order to investigate the possible routes of introduction of this ascidian into the Mediterranean Sea. The presence of the two COI haplogroups detected in previous molecular investigations on S. plicata at intercontinental spatial scale was confirmed in the Mediterranean Sea. The results revealed multiple introductions of S. plicata, although some locations appear to have experienced rapid expansion from few founding individuals with reduced genetic diversity. However, continuous introductions would confound the pattern deriving from single founder events and make it difficult to estimate the time needed for gene diffusion into established populations. This mixing of effects creates difficulties in understanding the past and current dynamics of this introduction, and managing this alien invasive ascidian whose genetic structure is continuously shuffled by vessel-mediated transport

Phylogeography on the rocks: The contribution of current and historical factors in shaping the genetic structure of <i>Chthamalus montagui</i> (Crustacea, Cirripedia) - Fig 2

<p>A. Median-joining network of <i>COI</i> haplotypes obtained in <i>Chthamalus montagui</i> from the 13 sampled locations. Each line in the network represents one mutational step; each small bar on the branches represents an additional mutational step. The area of each circle is proportional to the number of individuals showing that haplotype. B. Bayesian assignment analysis of <i>COI</i> sequences. Each vertical bar represents an individual and its associated probability of belonging to one of the five genetic clusters detected. Asterisks on the bar graph indicate individuals with uncertain assignment (<i>P</i> < 0.05). Pie charts indicate the percentage of genetic clusters contributing to each location. Pie charts were constructed with individual net assignments and after removing individuals with uncertain assignment.</p

<i>Chthamalus montagui</i>.

<p>Sum of squared deviations (SSD) between the expected and observed mismatch distributions of pairwise differences, Harpending’s [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0178287#pone.0178287.ref038" target="_blank">38</a>] raggedness index (<i>r</i>) of the observed mismatch distribution and Fu’s [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0178287#pone.0178287.ref039" target="_blank">39</a>] <i>F</i>_S neutrality test on <i>COI</i> sequences pooled according to the three biogeographical areas; <i>P</i> values were obtained by coalescent simulations with 10000 replicates.</p

Frequency distribution of the number of pairwise nucleotide differences (mismatch) between <i>COI</i> haplotypes in the three populations of <i>Chthamalus montagui</i>.

<p>The solid line is the theoretical distribution under the model of demographic expansion.</p

Location of <i>Chthamalus montagui</i> sampling sites.

<p>Location abbreviations are as reported in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0178287#pone.0178287.t001" target="_blank">Table 1</a>.</p

Bayesian skyline plots of effective population size through time in <i>Chthamalus montagui</i> from the three biogeographical areas, based on the 537bp sequences of <i>COI</i> and a nucleotide substitution rate of 3.1%/my.

<p>The bold black curve is the median of the parameter <i>N</i>_e<i>T</i>, which is proportional to the effective population size; the dotted lines delimit the 95% highest posterior density. For comparison, all <i>x</i>-axes have the same scale. The plots are truncated to the median estimate of each area’s TMRCA.</p