Abstract
Genetic tools have greatly aided in tracing the sources and colonization history of introduced species. However, recurrent introductions and repeated shuffling of populations may have blurred some of the genetic signals left by ancient introductions. Styela plicata is a solitary ascidian distributed worldwide. Although its origin remains unclear, this species is believed to have spread worldwide by travelling on ship's hulls. The goals of this study were to infer the genetic structure and global phylogeography of S. plicata and to look for present-day and historical genetic patterns. Two genetic markers were used: a fragment of the mitochondrial gene Cytochrome Oxidase subunit I (COI) and a fragment of the nuclear gene Adenine Nucleotide Transporter/ADP-ATP Translocase (ANT). A total of 368 individuals for COI and 315 for ANT were sequenced from 17 locations worldwide. The levels of gene diversity were moderate for COI to high for ANT. The Mediterranean populations showed the least diversity and allelic richness for both markers, while the Indian, Atlantic and Pacific Oceans had the highest gene and nucleotide diversities. Network and phylogenetic analyses with COI and ANT revealed two groups of alleles separated by 15 and 4 mutational steps, respectively. The existence of different lineages suggested an ancient population split. However, the geographic distributions of these groups did not show any consistent pattern, indicating different phylogeographic histories for each gene. Genetic divergence was significant for many population-pairs irrespective of the geographic distance among them. Stochastic introduction events are reflected in the uneven distribution of COI and ANT allele frequencies and groups among many populations. Our results confirmed that S. plicata has been present in all studied oceans for a long time, and that recurrent colonization events and occasional shuffling among populations have determined the actual genetic structure of this species.
Highlights
Biological introductions have notably increased during the last century, posing a major threat to global biodiversity and altering the structure and function of many communities [1,2,3,4,5,6,7]
Several remarkable features emerged from the recovered distribution of the genetic variability
There seems to be an effect of selection on the genetic makeup of this species, as illustrated by the highly divergent population of North Carolina and the intra-individual distribution of both groups of Adenine Nucleotide Transporter (ANT) sequences
Summary
Biological introductions have notably increased during the last century, posing a major threat to global biodiversity and altering the structure and function of many communities [1,2,3,4,5,6,7]. The establishment of new genetic variants and spread of exotic species has been facilitated by a proliferation of harbours and other artificial structures along the coast [20,21,22,23,24,25]. Genetic diversity plays a crucial role on the successful establishment of an introduced species or variant in a new area [26,27,28,29,30]. A throughout assessment of the genetic structure of an introduced species, including its history of subdivision and gene flow, allows the identification of range expansions, colonization events, and an understanding of the invasive potential and the relative contributions of artificial and natural dispersal [e.g. 31–34]
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