Human transport of species across oceans disrupts natural dispersal barriers and facilitates hybridisation between previously allopatric species. The recent introduction of the North Pacific sea squirt, Ciona robusta, into the native range of the North Atlantic sea squirt, C. intestinalis, is a good example of this outcome. Recent studies have revealed an adaptive introgression, in a single chromosomal region, from the introduced into the native species. Here, we monitored this adaptive introgression over time, examining both the frequency of adaptive alien genes at the core and the hitchhiking footprint in the shoulders of the introgression island, by studying a thousand Ciona spp. individuals collected in 22 ports of the contact zone, 14 of which were sampled 20 generations apart. For that purpose, we developed a KASP multiplex genotyping approach, which proved effective in identifying native, non-indigenous and hybrid individuals and in detecting introgressed haplotypes. Adaptive alien genes were detected where they had been found 20 generations ago, as well as in some newly sampled ports. The core region of the introgression, where the frequency of C. robusta alleles is the highest and locally adaptive genes must be, remains stable in both space and time. In contrast, we observed an erosion of C. robusta ancestry tracts in flanking chromosomal shoulders on the edges of the core, consistent with the second phase of a local sweep and a purge of incompatible introgressed alleles. Our study reveals how the shoulders of an adaptive introgression island fall over time after the initial sweep.
