Hybridization and introgression between species is remarkably common, even between distantly-related taxa. This suggests that the frequency of hybridization between species has been greatly constrained, not by evolutionary divergence in isolation, but by lack of opportunity. This constraint is being relaxed by human-mediated dispersal. Here we document a case where such dispersal of a widespread species of freshwater turtle (Emydura macquarii) into the highly restricted range of a critically endangered endemic turtle (Myuchelys georgesi) has provided opportunity for the two to hybridize. This has raised concerns about the potential impact of hybridization on the endemic species, and its continued persistence in the face of challenges brought about by habitat alteration, increased competition, disease, and genetic pollution. This study serves to highlight the risks associated with human-mediated dispersal, which can bring into contact species that would otherwise have never or rarely met, and so provide opportunity for hybridization and introgression between even distantly-related species with uncertain consequences for already threatened species.

DFwt18-3294.Rdata SNP data file in adegenet compressed format.DFwt18-3294.Rdata is a genlight object (adegenet] that contains the SNP data and associated metadata. It can be accessed and interrogated by loading it in R.DFwt18-3294.RdataDFwt18-3294_read.rA script to read in the data to a binary genlight object (Jombart 2008, Jombart & Ahmed 2011) to facilitate analysis. Draws on R package dartR (Gruber et al. 2018). Shows the code to create DFwt18-23294.Rdata.DFwt18-3294_analysisA script to replicate the analysis. Also draws on R package dartR (Gruber et al. 2018)Table_S1Details of individuals assigned to likelihood classes (parental Emydura macquarii and Myuchelys georgesi); F1 hybrids (F1); backcrosses of F1 hybrids to one or the other parentals, F1xEM or F1xMY) by New Hybrids (Anderson & Thompson 2002).