New climatically specialized lineages of Batrachochytrium dendrobatidis and their sub-lethal effects on amphibians establish the Asiatic origins of the pathogen

Abstract

Chytridiomycosis, a highly significant global wildlife disease, has caused unprecedented amphibian population declines and species extinctions worldwide. In contrast, mass die-offs due to chytridiomycosis have not been observed in Asia, which is thought to be the ancestral region of origin and a hyper-diversity hotspot of the known causal pathogens, Batrachochytrium dendrobtidis (Bd) and B. salamndrivorans (Bsal). It has been hypothesized that Asian amphibians may have evolved immunity to clinical Batrachochytrium infection. However, limited knowledge of endemic lineages, evolutionary history, and climate-related infection patterns limits our ability to explore this hypothesis. Here, we investigated the genetic diversity and infection patterns of the frog-infecting species, Bd, in Chinas poorly-explored Guangxi region. We used the internal transcribed spacer (ITS) marker and the nested PCR method to survey prevalence and haplotype diversity of Bd across 17 forest sites. A generalized linear model was used to evaluate associations between numerous variables and Bd prevalence within native amphibians. Our results identified seven new haplotypes, four of which are closely related to the early-emerging BdASIA-1 lineage recovered from South Korea. We also identified a unique Asian haplotype, close to the BdASIA-3 lineage, as the most prevalent (64.6% of Bd-infected adult individuals) in 11 out of 15 infected species. This haplotype was also detected in a salamander individual, which exhibited non-lethal skin lesions on the abdomen. The infection of Bd within amphibians was found to be positively associated with temperature and elevation. Our findings suggest that there is significant undiscovered genetic diversity of Asian Bd lineages in this region. Longer-term studies are required to further investigate Bd diversity, prevalence, seasonality and impact on native species and populations in Southern China and across the region of origin in Asia. Author SummaryChytridiomycosis is a disease which is responsible for the sharp decline of amphibian populations and species extinctions around the world. Surprisingly, it has not yet been well-studied in Asia, the region where the two causal pathogens of the disease, Batrachochytrium dendrobatidis (Bd) and B. salamandrivorans (Bsal) originated. In order to better understand the lack of mass die-offs in Asia, we recently conducted a study in south Chinas Guangxi region to investigate the genetic diversity and infection patterns of Bd. Through the use of internal transcribed spacer (ITS) markers and nested PCR, we discovered seven new types of Bd, four of which were closely related to the early-emerging BdASIA-1 lineage from South Korea. The highest prevalence of Bd infection was observed in 11 species of amphibians, including a salamander which had non-lethal skin lesions. It was also noted that infection of Bd in amphibians was associated with temperature and elevation. This study has provided important information on Bd diversity and prevalence in the region, and further research is needed to explore Asia as the putative region of origin for this disease.