Résumé
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.