Increased Inter-Colony Fusion Rates Are Associated with Reduced COI Haplotype Diversity in an Invasive Colonial Ascidian Didemnum vexillum

Considerable progress in our understanding of the population genetic changes associated with biological invasions has been made over the past decade. Using selectively neutral loci, it has been established that reductions in genetic diversity, reflecting founder effects, have occurred during the establishment of some invasive populations. However, some colonial organisms may actually gain an ecological advantage from reduced genetic diversity because of the associated reduction in inter-colony conflict. Here we report population genetic analyses, along with colony fusion experiments, for a highly invasive colonial ascidian, Didemnum vexillum. Analyses based on mitochondrial cytochrome oxidase I (COI) partial coding sequences revealed two distinct D. vexillum clades. One COI clade appears to be restricted to the probable native region (i.e., north-west Pacific Ocean), while the other clade is present in widely dispersed temperate coastal waters around the world. This clade structure was supported by 18S ribosomal DNA (rDNA) sequence data, which revealed a one base-pair difference between the two clades. Recently established populations of D. vexillum in New Zealand displayed greatly reduced COI genetic diversity when compared with D. vexillum in Japan. In association with this reduction in genetic diversity was a significantly higher inter-colony fusion rate between randomly paired New Zealand D. vexillum colonies (80%, standard deviation ±18%) when compared with colonies found in Japan (27%, standard deviation ±15%). The results of this study add to growing evidence that for colonial organisms reductions in population level genetic diversity may alter colony interaction dynamics and enhance the invasive potential of newly colonizing species

Pritchardia flynnii (Arecaceae), a New Endemic Species from Kaua\u27i, Hawaiian Islands

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<i>Didemnum vexillum</i> cytochrome oxidase I (COI) sequence diversity measures from five geographic regions.

<p>Sequence dataset is combined from this study and Stefaniak et al. (2009) <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0030473#pone.0030473-Stefaniak1" target="_blank">[52]</a>.</p><p>*Dates from Lambert (2009) <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0030473#pone.0030473-Lambert1" target="_blank">[49]</a>. Abbreviations: Japan (A+B), Japan Clade A and B data; Japan (A), Japan Clade A data only; NZ, New Zealand; WCNA, West Coast North America; ECNA, East Coast North America.</p

Statistical parsimony network of 16 <i>Didemnum vexillum</i> cytochrome oxidase I (COI) haplotypes.

<p>Areas of circles are proportional to the frequency of each haplotype in the dataset and differing shading indicates five different geographic regions as indicated. Small black circles on the branches indicate hypothetical intermediate haplotypes that were not observed.</p

Bayesian phylogenetic analysis of 16 <i>Didemnum vexillum</i> cytochrome oxidase I (COI) haplotypes.

<p>Numbers on nodes denote posterior probability values. GenBank accession numbers of the COI haplotypes are shown. Asterisks indicate seven COI haplotypes (H11–H17) not previously reported by Stefaniak et al. (2009) <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0030473#pone.0030473-Stefaniak1" target="_blank">[52]</a>. COI sequences from <i>Didemnum psammatode</i> (EU742661, JN624758), <i>D. albidum</i> (EU419432, EU419456) and <i>Didemnum</i> sp. B (EU419407, EU419408) were used as outgroups.</p