Globally invasive genotypes of the amphibian chytrid outcompete an enzootic lineage in coinfections

Abstract

Competition between genotypes is likely to be a key driver of pathogen evolution, particularly following a geographical invasion by distant strains. Theory predicts that competition between disease strains will result in the most virulent strain persisting. Despite its evolutionary implications, the role of strain competition in shaping populations remains untested for most pathogens. We experimentally investigated the in vivo competitive differences between two divergent lineages of the amphibian-killing chytrid fungus ( Batrachochytrium dendrobatidis , Bd ). These Bd lineages are hypothesized to have diverged in allopatry but been recently brought back into secondary contact by human introduction. Prior studies indicate that a panzootically-distributed, global lineage of Bd was recently introduced into southern Brazil, and is competitively excluding enzootic lineages in the southern Atlantic Forest. To test for differences in competitive ability between invasive and enzootic Brazilian Bd isolates, we coinfected a model host frog system which we developed for this study ( Hymenochirus curtipes ). We tracked isolate-specific zoospore production over the course of the coinfection experiment with chip-based digital PCR (dPCR). The globally invasive panzootic lineage had a competitive advantage in spore production especially during the first one to four weeks of infection, and on frogs that eventually succumbed to Bd infection. Our study provides new evidence that competitive pressure resulting from the human movement of pathogen strains can rapidly alter the genetics, community dynamics and spatial epidemiology of pathogens in the wild.

Presence of low virulence chytrid fungi could protect European amphibians from more deadly strains

“…Their protective effect against disease is associated with several traits that correlate with their colonisation ability and invasiveness: expression and function of Bd crinkler genes, metalloproteases and other effector proteins. However, the hypervirulent isolate's high fecundity, marked invasiveness in amphibian cells and more pronounced colonization capacity in vivo suggest a distinct competitive advantage over the local isolates 39 . Moreover, co-infections may result in superinfection and/or hybridisation 3,40,41 , with highly unpredictable outcomes.…”

Incapacitating effects of fungal coinfection in a novel pathogen system

“…Batrachochytrium, or with various combinations of lineages of Bd that differ in genotype (Jenkinson et al, 2018;Longo, Fleischer, & Lips, 2019;Ossiboff et al, 2019). Bd is widespread, and one or more lineages can be found on all continents inhabited by amphibians, including North America (O'Hanlon et al, 2018).…”

Genetic analysis of post‐epizootic amphibian chytrid strains in Bolivia: Adding a piece to the puzzle

“…If Bd ‐ GPL introduced to Bolivia via the cloud forests in the 1990s has indeed been transported to the altiplano by waterfowl (Burrowes & De la Riva, 2017b; Garmyn et al., 2012; Johnson & Speare, 2005) or other potential vectors, it would explain the recent declines of certain altiplano species such as Rhinella spinulosa and Telmatobius marmoratus among others (De la Riva & Reichle, 2014), as well as its presence in extant T. culeus from Lago Titicaca. This hypervirulent strain ( Bd‐GPL ) with its aggressive infection advantage (Jenkinson et al., 2018) could potentially outcompete older endemic strains and put at risk populations that survived what may have been an old, historically unassessed, Bd epidemic (‘the ghost of Bd past’) likely caused by a Bd genotype that has been either extirpated, or not detected in this preliminary study.…”

“…Few places are known to contain multiple lineages (Byrne et al., 2019; O’hanlon et al., 2018; Schloegel et al., 2012), and this may speak to the ability of Bd‐GP L to outcompete enzootic lineages. Recent findings have shown that when Bd‐GPL and the older Brazilian strain (now considered Bd ‐ASIA‐2/ Bd ‐BRAZIL; see O’hanlon et al., 2018) are co‐inoculated onto a single host, Bd‐GPL grows much faster, suggesting that it has the potential to outcompete endemic strains (Jenkinson et al., 2018). If this is the case in the wild, the global invasion of the hypervirulent Bd‐GPL may hinder our possibility to detect ancient Bd strains and, thus, truly understand the evolutionary dynamics and spatial epidemiology of this pathogen.…”