Globalized infectious diseases are causing species declines worldwide, but their source often remains elusive. We used whole-genome sequencing to solve the spatiotemporal origins of the most devastating panzootic to date, caused by the fungus Batrachochytrium dendrobatidis, a proximate driver of global amphibian declines. We traced the source of B. dendrobatidis to the Korean peninsula, where one lineage, BdASIA-1, exhibits the genetic hallmarks of an ancestral population that seeded the panzootic. We date the emergence of this pathogen to the early 20th century, coinciding with the global expansion of commercial trade in amphibians, and we show that intercontinental transmission is ongoing. Our findings point to East Asia as a geographic hotspot for B. dendrobatidis biodiversity and the original source of these lineages that now parasitize amphibians worldwide.

Dispersal is central in ecology and evolution because it influences population regulation, adaptation, and speciation. In many species, dispersal is different between genders, leading to sex-biased dispersal. Several theoretical hypotheses have been proposed to explain the evolution of this bias: the resource competition hypothesis proposed by Greenwood, the local mate competition hypothesis, and the inbreeding avoidance hypothesis. Those hypotheses argued that the mating system should be the major factor explaining the direction of such bias. Sociality and the presence of handicap in genders (exaggerated sexual characters or parental care) have recently been proposed to be linked with the direction of this bias. We tested these expected coevolutions using a database of 257 species. Based on phylogenetic approaches, our findings marginally corroborated Greenwood’s hypothesis by showing relationships between the direction of sex-biased dispersal, mating systems, and territoriality. More importantly, our results highlighted that the evolution of this bias was more linked to parental care and sexual dimorphism. These traits were also found to be associated with mating systems, suggesting that sexual asymmetry in morphology and parental care might be the main determinant of the evolution of sex-biased dispersal across species and not mating systems per se, as proposed in Greenwood’s hypothesis.

Summary1. The complex structure of tree bark reflects its many functions, which include structural support as well as defence against fire, pests and pathogens. Thick bark, however, might limit respiration by the living tissues of the trunk. Nevertheless, little research has addressed community-level variation in bark thickness, and to the best of our knowledge, no one has tested multiple hypotheses to explain variation in bark thickness. 2. We conducted an extensive survey of bark thickness within and among species of trees in the tropical rain forests of French Guiana. Trunk bark thickness increased by 1AE2 mm per 10 cm increase in stem diameter, and varied widely at all taxonomic levels. Mean trunk bark thickness was 4AE5 mm (range: 0AE5-29 mm), which was less that found in two Amazonian rain forests in previous studies. This survey of bark thickness should be of use for forest management since tree survival through fire is strongly predicted by bark thickness. 3. We combined the survey data with multiple datasets to test several functional hypotheses proposed to explain variation in bark thickness. We found bark to provide an average of 10% of the flexural rigidity of tree stems, which was substantially less than that found in the only other study of bark stiffness. Bark thickness was uncorrelated with species' association with fire-prone habitats, suggesting that the influence of fire on bark thickness does not extend into moist Forests. There was also little evidence that bark thickness is affected by its function as a defence against herbivory. Nor was there evidence that thick bark limits trunk respiration. 4. A re-analysis of previously collected anatomical data indicated that variation in rhytidome (non-conducting outer bark) thickness explains much of the variation in overall bark thickness. As rhytidome is primarily involved in protecting the living tissues of the trunk, we suggest that bark thickness is driven mostly by its defensive function. 5. Functional explanations for the variation in bark thickness were not clear-cut. Nevertheless, this study provides a foundation for further investigation of the functional bases of bark in tropical trees.