Population genetics of the frog-killing fungus Batrachochytrium dendrobatidis

Abstract

Global amphibian decline by chytridiomycosis is a major environmental disaster that has been attributed to either recent fungal spread or environmental change that promotes disease. Here, we present a population genetic comparison of Batrachochytrium dendrobatidis isolates from an intensively studied region of frog decline, the Sierra Nevada of California. In support of a novel pathogen, we find low diversity, no amphibian-host specificity, little correlation between fungal genotype and geography, local frog extirpation by a single fungal genotype, and evidence of human-assisted fungus migration. In support of endemism, at a local scale, we find some diverse, recombining populations. Therefore neither epidemic spread nor endemism alone explains this particular amphibian decline. Recombination raises the possibility of resistant sporangia and a mechanism for rapid spread as well as persistence that could greatly complicate global control of the pathogen.

Fig. 1.

Multilocus genotype network of B. dendrobatidis isolates. Boxes signify more than one isolate (number given) from the same location with the same genotype. Circled in blue and green are groups 1 and 2 as predicted by Structure for the Sierra Nevada B. dendrobatidis population samples. The key indicates frog host and number of samples with site abbreviations following those in Table 1. Each step in the network represents a change in genotype at a single locus.

Fig. 2.

Map of California showing B. dendrobatidis collection sites and the inferred genetic groupings in the Sierra Nevada (blue, group 1; green, group 2). Gray shading corresponds to the distribution of the mountain yellow-legged frog species complex. The black bar marks the approximate boundary between R. sierrae in the north (LIV, SM, MP) and R. muscosa in the south (WL, LC, HL). Other Californian B. dendrobatidis isolates were from Trinity River (TR-Bul, R. catesbeiana) and Point Reyes (PR-Bul, R. catesbeiana and PR-Red, R. draytonii).