Résumé
Twenty-years of genetic studies of marine invaders have shown that
successful invaders are often characterized by native and introduced
populations displaying similar levels of genetic diversity. This pattern
is presumably due to high propagule pressure and repeated introductions.
The opposite pattern is reported in this study of the brown seaweed,
Sargassum muticum, an emblematic species for circumglobal invasions.
Albeit demonstrating polymorphism in the native range, microsatellites
failed to detect any genetic variation over 1269 individuals sampled from
46 locations over the Pacific-Atlantic introduction range.
Single-Nucleotide Polymorphisms (SNPs) obtained from ddRAD-sequencing
revealed some genetic variation, but confirmed severe founder events in
both the Pacific and Atlantic introduction ranges. Our study thus
exemplifies the need for extreme caution in interpreting neutral genetic
diversity as a proxy for invasive potential. Our results confirm a
previously hypothesized trans-oceanic secondary introduction from NE
Pacific to Europe. However, the SNPs panel unexpectedly revealed two
additional distinct genetic origins of introductions. Also, conversely to
scenarios based on historical records, southern rather than northern NE
Pacific populations could have seeded most of the European populations.
Finally, the most recently introduced populations showed the lowest
selfing rates, suggesting higher levels of recombination might be
beneficial at the early stage of the introduction process (i.e.,
facilitating evolutionary novelties), whereas uniparental reproduction
might be favored later in sustainably established populations (i.e.,
sustaining local adaptation).