Determining the native region of the putatively invasive ascidian Didemnum vexillum Kott, 2002

Over the past 40 years, an increasing number of previously unrecorded populations of a colonial ascidian, recently identified as Didemnum vexillum, have been documented in most temperate coastal regions of the world, impacting aquaculture operations, natural rocky habitats, cobble/gravel substrates, and eelgrass beds. The earliest sample thought to be D. vexillum was collected in Mutsu Bay, Japan in 1926, but was not identified to species at the time. Lack of a published description led to widespread mis-identification of this species. Because of incomplete historical records and the numerous mis-identifications of this species, the native range of D. vexillum has not been conclusively known. To determine which portion of the current known range of D. vexillum is within its native region, we obtained DNA sequences of two genes, cytochrome c oxidase subunit 1(co1; mitochondrial) and THO complex subunit (tho2; nuclear), from 365 and 75 samples of D. vexillum from around the world, respectively. Both population genetics (co1 only) and phylogenetics (co1 and tho2) were used to measure and compare the amount of genetic variation contained in each region where D. vexillum is currently found (Eastern North America, Japan, New Zealand, Northwestern Europe, and Western North America). We found that genetic diversity in Japan was greater than in any other region. Northwestern Europe. western North America, and New Zealand all showed evidence of having recently undergone a genetic bottleneck. The gene tree for co1 in D. vexillum was divided into three clades: one which is found globally; the other two found only in Japan. Analysis of a partial genomic sequence of the nuclear gene tho2 confirmed that the co1 clades belong to the same species. In agreement with the sparse historical data, the molecular evidence suggests that Japan lies within the native range of D. vexillum. (C) 2012 Elsevier B.V. All rights reserved.