Beyond Allopatric Speciation: Testing for Genetic Homogeneity in Duttaphrynus melanostictus in Relation to Human- induced Dispersal

Human- induced dispersal of species is accelerating along with the increase in human movements. This unnatural dispersal contributes to range expansions, such as for the Asian black -spined toad, Duttaphrynus melanostictus . The species became invasive in numerous ecosystems, such as Madagascar and the Komodo islands area, where it has a strong ecological impact. Here, we investigated the origin of D . melanostictus in Taiwan using statistical parsimony networks together with joint Bayesian inference of phylogeny and population clustering approaches. We tested four major dispersal hypotheses to explain the relation between the Taiwanese and other D. melanostictus populations: 1) the species originates from South East Asia (SEA) and it is invasive in T aiwan as a result of human- induced dispersal, 2) the species originates from South East Asia, and dispersed ov er land bridges, 3) the species comes from the Chinese mainland through human- induced dispersal, 4) the species originates from the Chinese mainland, and dispersed over land bridges during glacial maxima. Our unrooted haplotype network based on a fragment of contiguous tRNA Gly -ND3 mitochondrial DNA from 22 individuals, together with homologous sequences extracted from GenBank for China and SEA, confirmed the non- clustering of haplotypes from SEA and Taiwan. Likewise, Bayesian phylogenetic inferences further clarified the absence of genetic segregation between Taiwanese and mainland Chinese populations. The haplotypes from Taiwan were segregated with in a monophyletic clade, shared with mainland Chinese populations. The origin of the Taiwanese D. melanostictus is consequently linked to the Chinese clade, and rejects our first and second hypothesis. The clustering with haplotypes from geographically close localities in mainland China, as well as with haplotypes present in geographically distant but active trading areas, here Hong- Kong, suggests both past dispersion over land- bridges, and recent human- induced dispersal. Interestingly, we also found four haplotypes from the SE Asian clade clustering with the mainland Chinese clade, and two mainland Chinese haplotypes clustered within the SE Asian clade. This is another potential indicator of additional human- induced dispersal events. Past dispersal over land bridges reflects the highly vagrant behaviour of the species, followed by inadvertent translocations, favoured by the resilience of the species in dry environments. Our study emphasises the risk of future hybridisation between Taiwanese, SE Asian and mai nland Chinese clades as a result of human activities.