Résumé
Introduced species can impose profound impacts on the evolution of
receiving communities with which they interact. If native and introduced
taxa remain reproductively semi-isolated, human-mediated secondary contact
may promote genetic exchange across newly created hybrid zones,
potentially impacting native genetic diversity and invasive species
spread. Here, we investigate the contributions of recent divergence
histories and ongoing (post-introduction) gene flow between the invasive
marine mussel, Mytilus galloprovincialis and a morphologically
indistinguishable and taxonomically contentious native Australian taxon,
Mytilus planulatus. Using transcriptome-wide markers, we demonstrate that
two contemporary M. galloprovincialis introductions into southeastern
Australia originate from genetically divergent lineages from its native
range in the Mediterranean Sea and Atlantic Europe, where both
introductions have led to repeated instances of admixture between
introduced and endemic populations. Through increased genome-wide
resolution of species relationships, combined with demographic modelling,
we validate that mussels sampled in Tasmania are representative of the
endemic Australian taxon (M. planulatus), but share strong genetic
affinities to M. galloprovincialis. Demographic inferences indicate
late-Pleistocene divergence times and historical gene flow between the
Tasmanian endemic lineage and northern M. galloprovincialis, suggesting
that native and introduced taxa have experienced a period of historical
isolation of at least 100,000 years. Our results demonstrate that many
genomic loci and sufficient sampling of closely related lineages in both
sympatric (e.g., Australian populations) and allopatric (e.g.,
northern-hemisphere Mytilus taxa) ranges are necessary to accurately (i)
interpret patterns of intraspecific differentiation and to (ii)
distinguish contemporary invasive introgression from signatures left by
recent divergence histories in high dispersal marine species. More
broadly, our study fills a significant gap in systematic knowledge of
native Australian biodiversity and sheds light on the intrinsic challenges
for invasive species research when native and introduced species
boundaries are not well-defined.