Cryptic infection of a broad taxonomic and geographic diversity of tadpoles by Perkinsea protists

Abstract

The decline of amphibian populations, particularly frogs, is often cited as an example in support of the claim that Earth is undergoing its sixth mass extinction event. Amphibians appear to be particularly sensitive to emerging diseases (e.g. fungal and viral pathogens), yet the diversity and geographic distribution of infectious agents are only starting to be investigated. Recent work has linked a previously undescribed protist with mass-mortality events in the USA, in which infected frog tadpoles have an abnormally enlarged yellowish liver filled with protist cells of a presumed parasite. Phylogenetic analyses revealed that this infectious agent was affiliated with the Perkinsea; a parasitic group within the alveolates exemplified by Perkinsus sp. a ‘marine’ protist responsible for mass-mortality events in commercial shellfish populations. Using small subunit rDNA sequencing, we developed a targeted PCR protocol for preferentially sampling a clade of the Perkinsea. We test this protocol on freshwater environmental DNA revealing a wide diversity of Perkinsea lineages in these environments. Then we used the same protocol to test for Perkinsea-like lineages in livers of 182 tadpoles from multiple families of frogs. We identified a distinct Perkinsea clade, encompassing a low level of SSU rDNA variation different from the lineage previously associated with tadpole mass-mortality events. Members of this clade were present in 38 tadpoles sampled from 14 distinct genera/phylogroups, from five countries across three continents. This provides the first evidence that Perkinsea-like protists infect tadpoles across a wide taxonomic range of frogs in tropical and temperate environments, including oceanic islands.