Abstract
Human-mediated dispersal interplays with natural processes and complicates understanding of the biogeographical history of species. This is exemplified by two invasive tunicates, Ciona robusta (formerly Ciona intestinalis type A) and C. intestinalis (formerly Ciona intestinalis type B), globally distributed and sympatric in Europe. By gathering new mitochondrial sequences that were merged with published datasets, we analysed genetic patterns in different regions, with a focus on 1) their sympatric range and 2) allopatric populations in N and S America and southern Europe. In the sympatric range, the two species display contrasting genetic diversity patterns, with low polymorphism in C. robusta supporting the prevalent view of its recent introduction. In the E Pacific, several genetic traits support the non-native status of C. robusta. However, in the NE Pacific, this appraisal requires a complex scenario of introduction and should be further examined supported by extensive sampling efforts in the NW Pacific (putative native range). For C. intestinalis, Bayesian analysis suggested a natural amphi-North Atlantic distribution, casting doubt on its non-native status in the NW Atlantic. This study shows that both natural and human-mediated dispersal have influenced genetic patterns at broad scales; this interaction lessens our ability to confidently ascertain native vs. non-native status of populations, particularly of those species that are globally distributed.
Highlights
The two species are distributed along temperate and warm-temperate coasts[20,21] (Fig. 1)
By merging our dataset with the published dataset of Zhan et al.[25] on Pacific and Atlantic coasts of North America, totals of 714 C. robusta and 1140 C. intestinalis individuals were examined on COX3-ND1 sequences of 580 and 529 bp respectively
A subset of 501 individuals of C. robusta and 683 individuals of C. intestinalis sampled for this study were sequenced on an additional mtDNA fragment (COI)
Summary
The two species are distributed along temperate and warm-temperate coasts[20,21] (Fig. 1). Several studies have considered phylogenetic relationships within the genus Ciona or the C. intestinalis species complex, (e.g.12,18,21), or investigated genetic diversity and connectivity of local populations (e.g. in North America[25], in South Africa[30], and in Mediterranean Sea[31]) None of these examined in detail the global genetic patterns of the two species comparing different regions of introduction and notably the single sympatric area described so far. They are both sessile as adults, with a short life cycle involving broadcast spawning for external fertilization, and with a non-feeding larva providing a short planktonic phase[20,22,32] It has been argued[33,34,35,36,37] that species’ basic biology will profoundly affect patterns of natural dispersal and population-genetic structuring and the extent of natural geographical ranges and rates of speciation. Close comparison of the species is possible in their contact zone in the EC, where the two species live side by side at many sites[22]
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