Résumé
Biological introductions bring into contact species that can still
hybridize. The evolutionary outcomes of such secondary contacts may be
diverse (e.g. adaptive introgression from or into the introduced species)
but are not yet well examined in the wild. The recent secondary contact
between the non-native sea squirt Ciona robusta (formerly known as C.
intestinalis type A) and its native congener C. intestinalis (formerly
known as C. intestinalis type B), in the Western English Channel, provides
an excellent case study to examine. To examine contemporary hybridization
between the two species, we developed a panel of 310 ancestry-informative
SNPs from a population transcriptomic study. Hybridization rates were
examined on 449 individuals sampled in eight sites from the sympatric
range and five sites from allopatric ranges. The results clearly showed an
almost complete absence of contemporary hybridization between the two
species in syntopic localities, with only one-first-generation hybrid and
no other genotype compatible with recent backcrosses. Despite the almost
lack of contemporary hybridization, shared polymorphisms were observed in
sympatric and allopatric populations of both species. Furthermore, one
allopatric population from SE Pacific exhibited a higher rate of shared
polymorphisms compared to all other C. robusta populations. Altogether,
these results indicate that the observed level of shared polymorphism is
more probably the outcome of ancient gene flow spread afterwards at a
worldwide scale. They also emphasize efficient reproductive barriers
preventing hybridization between introduced and native species, which
suggests hybridization should not impede too much the expansion and the
establishment of the non-native species in its introduction range.