Direct Effects of an Invasive European Buckthorn Metabolite on Embryo Survival and Development in Xenopus laevis and Pseudacris triseriata

We demonstrate novel direct effects of an invasive plant metabolite on embryo development in the native Western Chorus Frog (Pseudacris triseriata) and a model organism, African Clawed Frog (Xenopus laevis). European buckthorn (Rhamnus cathartica) exhibits aggressive growth in amphibian breeding sites and releases the secondary metabolite, emodin, into soil and water. Emodin is known to have several deleterious, bioactive properties in mammals and birds, but its effects on amphibians have not been assessed. We used the FETAX (Frog Embryo Teratogenesis Assay-Xenopus) protocol to assess the effect of emodin on amphibian development in X. laevis, and modified the assay with P. triseriata to determine whether effects were consistent in a native species occurring within the range of the invasive R. cathartica. We detected and quantified emodin at amphibian breeding ponds that were infested heavily with buckthorn and have experienced recent declines in amphibian diversity and abundance. The X. laevis assay demonstrated significant embryo mortality and malformation in the presence of a gradient of concentrations of emodin including those detected in the environment. Teratogenicity indices (TI) >2 indicate the strong development-disrupting potential of emodin in amphibians. The P. triseriata assay produced similar patterns of embryo mortality and malformation as observed in the X. laevis assay. However, P. triseriata were more sensitive to emodin than X. laevis with TIs >40. Such effects may contribute to amphibian declines through depressed hatching success and poor larval survival and may represent an unrecognized impact of invasive plants more generally.