A genome‐wide investigation of the worldwide invader Sargassum muticum shows high success albeit (almost) no genetic diversity

Abstract

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 1,269 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 transoceanic secondary introduction from NE Pacific to Europe. However, the SNP 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).

Genomic signatures of clonality in the deep water kelp Laminaria rodriguezii

“…The development of reduced representation sequencing methods (see for review Davey et al, 2011) now allows such investigations for non-model species. Among these methods, Restriction Site Associated DNA sequencing (RAD sequencing) seems to be promising to more precisely delineate Multi-locus lineages (MLLs), where microsatellite loci have failed to detect any genetic variation, as in the case of the selfing species Sargassum muticum in the introduction range (Le Cam et al, 2019). This study illustrates the fact that genotypic richness (R) in partially clonal populations could be underestimated if clonal assessment is performed with a reduced number of molecular markers (Arnaud-Haond et al, 2005).…”

“…These studies have enabled the detection of multilocus lineages (MLLs) for determining the spatial architecture of clonal lineages. However, while RAD-sequencing data provide a better resolution than other genotyping methods to assess multilocus lineages (MLLs) (see Le Cam et al, 2019), genotyping errors and missing data introduce additional noise than microsatellite data. It follows that investigating clonality with these data cannot rely on the detection of strict Multilocus Genotypes (MLGs).…”

An evolutionary perspective on marine invasions

“…These patterns reflect the emergence of new adaptive genotypes early during the invasion that were then followed by selfing at a later stage, which favored the establishment of advantageous genotypes. Overall, Le Cam et al (2019) pinpoint a rare case of a successful marine invader using nonclonal reproduction, but with limited genomic diversity. where host and parasite are coevolved: The host is native, but the parasite is invasive (short-term coevolution); and the parasite is absent (hosts are naive).…”

Linking Ecology to Genetics to Better Understand Adaptation and Evolution: A Review in Marine Macrophytes

“…For example, epigenetic effects have been identified to contribute to phenotypic variation in genetically depauperate invasive populations (Richards et al, 2012). One could examine whether the worldwide invader Sargassum muticum (Yendo) Fensholt with almost no intraspecific genetic diversity (Le Cam et al, 2020) was driven by ecological adaptation or by epigenetic variation (no changes in DNA sequences but heritable structural modification such as DNA methylation).…”