The epizootic episodes caused by chytridiomycosis are recognized as one of the main causes in the decline of amphibians worldwide. Genetic studies of Batrachochytrium dendrobatidis, the fungus that causes chytridiomycosis, showed important data on the existence of different strains of endemic Bd lineages in different regions, but the introduction of the global hypervirulent strain Bd-GPL and its possible recombination with local strains brought catastrophic epizootic events throughout the world. The observed declines in Mexico, central and south America were associated with the introduction of the Bd-GPL lineage, and the presence of Bd since the 1970s in Mexico suggests that the epizootic wave that caused the disease began in this country. However, there are no genetic studies of the wild strains of Bd present in Mexico that associate the Bd genotype of Mexico with the epizootic waves in Central and South America. Currently, 24% of amphibian species in Mexico are critically endangered, and most of these species belong to the Plethodontidae and Hylidae families, with chytridiomycosis being one of the factors that increases their risk of extinction. Based on the evaluation of Bd’s historical and current impact on these species, prevention, mitigation and conservation measures could be established. In order to generate this evaluation, the objectives of this project are to characterize the genetic variation and Bd lineages of Mexico, and analyze their relationship with the incidence of historical and current Bd in critically endangered amphibians of the families Plethodontidae and Hylidae. Finally, this information will analyze the risk of these species due to Bd infection, and a hypothesis of Bd space-time dispersion from Mexico to Central and South America will be proposed.

Results: Chytridiomycosis caused by Batrachochytrium dendrobatidis (Bd) is considered as the worst infectious disease in vertebrates. Prior research in reconstructing the history of the disease has revealed a great evolutive history involving possible origin areas, routes of spread and several genetic lineages. Retrospective studies of Bd presence in museum vouchers and genetic population studies have supported the hypothesis of Bd arriving and spreading into Mexico coming from North America and spreading into Central and South America. The pathogen has been found in Mexico since the 1970s with current infections in 74 amphibian species. However, the information about the historical presence and genetic lineages across the country is limited. In this study, we aim to determine the historical and contemporary patterns of prevalence and genetic variation of Bd in Mexico. We combined swabbing of historical vouchers and Bd isolation techniques from wild amphibians to analyze Bd presence, prevalence and to determine its genetic diversity in the country using a genotyping method. The results of this study significantly increase our understanding of the known time frame of Bd presence and genetic diversity of Bd in Mexico and America. Our study corroborates that Bd had an early presence in Mexico with a contemporary dominance of the global panzootic lineage. The prevalence pattern of Bd detected by qPCR suggests it has been present since the 1890s, with a high prevalence in the 1930s. This suggests that Bd has been present in Mexico for a very long time, interacting with amphibians for longer than previously suspected and long before the declines of the 1970-1980 ́s.

The Bd genetic diversity does not show a geographic pattern, and the different genetic clusters and mixes admixture patterns suggest spread and recombination in several zones, such as Chiapas. Our study demonstrates that the evolutionary history of Bd in Mexico is substantially more complex than previously realized, providing a framework to test new questions about historical Bd genotypes and historical invasions related to the amphibian declines observed in Mexico and to help develop conservation strategies from highly virulent Bd genotypes. On the search for conservation management of endangered species, it has been shown that symbiotic microbiome plays a relevant role in host defense against pathogens across a variety of organisms. In amphibians, the host-associated bacterial communities are able to protect amphibians from pathogens and may mediate diseases such as the chytridiomycosis. While it is known that symbiotic skin bacteria can inhibit Bd growth, it is not known about the skin bacterial community structure in the light of Bd genetics and the environmental conditions in which the host interacts with the pathogen. In this study, we sampled skin bacteria and Bd to examine the interaction between infection load and Bd genetic diversity with skin bacterial community structure on five populations of Hyliola regilla (hypochondriaca) that differ in environments in Baja California, Mexico. We found differences in Bd genetics, prevalence and infection load among sites and environments, and the skin bacterial composition was correlated with Bd infection load and Bd genetics. Our results provide evidence that Bd genetics and infection load may explain the variation of bacterial composition in H. regilla (hypochondriaca), suggesting that they are significant factors in shaping the amphibian skin bacterial communities.