Dynamic Formation of Asexual Diploid and Polyploid Lineages: Multilocus Analysis of <em>Cobitis</em> Reveals the Mechanisms Maintaining the Diversity of Clones

<div><p>Given the hybrid genomic constitutions and increased ploidy of many asexual animals, the identification of processes governing the origin and maintenance of clonal diversity provides useful information about the evolutionary consequences of interspecific hybridization, asexuality and polyploidy. In order to understand the processes driving observed diversity of biotypes and clones in the <em>Cobitis taenia</em> hybrid complex, we performed fine-scale genetic analysis of Central European hybrid zone between two sexual species using microsatellite genotyping and mtDNA sequencing. We found that the hybrid zone is populated by an assemblage of clonally (gynogenetically) reproducing di-, tri- and tetraploid hybrid lineages and that successful clones, which are able of spatial expansion, recruit from two ploidy levels, i.e. diploid and triploid. We further compared the distribution of observed estimates of clonal ages to theoretical distributions simulated under various assumptions and showed that new clones are most likely continuously recruited from ancestral populations. This suggests that the clonal diversity is maintained by dynamic equilibrium between origination and extinction of clonal lineages. On the other hand, an interclonal selection is implied by nonrandom spatial distribution of individual clones with respect to the coexisting sexual species. Importantly, there was no evidence for sexually reproducing hybrids or clonally reproducing non-hybrid forms. Together with previous successful laboratory synthesis of clonal <em>Cobitis</em> hybrids, our data thus provide the most compelling evidence that 1) the origin of asexuality is causally linked to interspecific hybridization; 2) successful establishment of clones is not restricted to one specific ploidy level and 3) the initiation of clonality and polyploidy may be dynamic and continuous in asexual complexes.</p> </div

Unrooted statistical parsimony networks of haplotypes belonging to <i>C. taenia</i>-like (T) (upper panel) and <i>C. elongatoides</i>-like (E) (lower panel) clades (sensu [<b>13</b>]).

<p>White circles denote haplotypes found in sexual individuals only, dark grey circles denote those found in hybrids only, and light grey circles denote haplotypes shared by both hybrid and sexual individuals. The sizes of haplotypes are proportional to their frequency. Small blank circles represent missing (unobserved) haplotypes. Newly sequenced haplotypes are in bold. Rectangles delimit the hybrid clades I and II.</p

Observed and simulated distribution of clonal age estimators.

<p>(a) Correlation between the observed values of <i>dist.bp</i> and Tomiuk and Loeschcke’s <i>I</i>; (b, d) Distributions of Hartigan’s <i>D</i> values calculated from simulated <i>dist.bp</i> histograms at every 200^th generation of the simulation. Each value was calculated from a 50∶50 mixture of sympatric and allopatric clones. Arrows indicate the observed value; frequency distributions of the observed values of <i>dist.bp,</i> and <i>I</i>, respectively (c, e).</p

Europe-wide distribution, reproductive pathways and contact zone in Odra R. basin of studied species.

<p>(a) Distribution area of <i>C. tanaitica</i> (blue circles), <i>C. taenia</i> (red area), and <i>C. elongatoides</i> (yellow area), with the directions of postglacial colonization of Europe by <i>C. taenia</i> (red arrows) and <i>C. elongatoides</i> (yellow arrow). Secondary contact zones are indicated by zigzag symbols, and the dispersal of clonal lineages is indicated by dotted arrows (modified from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0045384#pone.0045384-Janko2" target="_blank">[13]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0045384#pone.0045384-Culling1" target="_blank">[46]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0045384#pone.0045384-Janko7" target="_blank">[64]</a>); (b) inferred reproductive pathways in <i>Cobitis</i> complex. Sperm at straight arrow indicates true fertilization, whereas sperm at round arrow indicates a triggering the egǵs development without paternal genetic contribution to the offspring; (c) Sampling sites in the Odra R. hybrid zone. For each locality, the presence of <i>C. taenia</i> and <i>C. elongatoides</i> is indicated by red and yellow dots, respectively and we indicate co-occurring hybrid biotypes, and their number in parentheses, if more than one. For each biotype on a given sample site, we list the presence of clones (MLL) and their absolute frequencies in parentheses, if more than one.</p

Histograms of the frequency distribution of pairwise distances in bp among genotyped individuals.

<p>(a–b) Data from <i>C. elongatoides</i> and <i>C. taenia</i>; (c–d) Distributions from observed (natural) and simulated diploid hybrids; (e–f) distributions from the observed (natural) and simulated triploid hybrids.</p

The plots of <i>dist.mut</i> and <i>dist.bp</i> values as a function of true age.

<p>Each gray line tracks the evolution of one clonal lineage over the time. Simulations are shown for the 2 values of mutation rate (µ) under SSM mutation model. At selected times, we represent the boxplots showing the median of simulated values of <i>dist.mut</i> and <i>dist.bp</i>. as well as first and third quartile; whiskers represent 1.5 times the interquartile range.</p