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Abstract

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The global emergence of the amphibian chytrid pathogen Batrachochytrium dendrobatidis (Bd) is one of the most compelling, and troubling, examples of a panzootic. Only discovered in 1998, Bd is now recognized as a proximate driver of global declines in amphibian diversity and is now widely acknowledged as a key threatening process for this ancient class of vertebrates. Moreover, Bd has become a member of a small group of highly virulent multihost pathogens that are known to have had effects on entire vertebrate communities and the ecosystem-level effects of Bd-driven amphibian declines are starting to emerge as a consequence of regional decreases in amphibian diversity. Despite the speed at which this species of aquatic chytrid has become a focus of research efforts, major questions still exist about where Bd originated, how it spreads, where it occurs and what are Bd’s effects on populations and species inhabiting different regions and biomes. In this issue, Goka et al. (2009) make an important contribution by publishing the first nationwide surveillance for Bd in Asia. Although previous data had suggested that amphibians in Asia are largely uninfected by Bd, these surveys were limited in their extent and few firm conclusions could be drawn about the true extent of infection. Goka et al. herein describe a systematic surveillance of Japan for both native and exotic species in the wild, as well as amphibians housed in captivity, using a Bd-specific nested PCR reaction on a sample of over 2600 amphibians. Their results show that Bd is widely prevalent in native species across Japan in at least three of the islands that make up the archipelago, proving for the first time that Asia harbours Bd.

Following the discovery that Bd was the driver of declines in amphibian species in Australia, the Americas and Europe (Berger et al. 1998), much attention has been focused on finding out how Bd was being spread, and from where. Answers to this latter question have been sought by attempting to identify geographic regions where Bd has had a long and stable association with host species, indicative of co-evolution, as well as substantially increased levels of genetic diversity when compared against the various regional epizootics. One such study by Weldon et al. (2004) has identified Africa as a potential source of the panzootic. Histology on historical museum specimens showed that Bd has infected amphibians in Southern Africa since at least 1938, and the ‘Bd Out of Africa’ hypothesis was coined to suggest that Bd was spread around the world via the extensive trade in the African clawed frog Xenopus laevis from the 1930s onwards. However, the recently published molecular analysis by James et al. (2009) on global strains of the pathogen failed to find evidence that Africa contains more diversity than occurs in other regions, and in fact found that north American isolates of Bd were more highly diverse than elsewhere. Therefore, this overarching question on the origin of Bd remains unanswered to date. What is clear, however, is that the global trade in amphibians is a potent force in spreading Bd into naïve populations and species. This statement is especially true for the so-called ‘Typhoid Mary’ species such as X. laevis and the North American bullfrog Rana catesbeiana (Fig. 1a); these species carry Bd infections, however, rarely exhibit the disease, chytridiomycosis. They are also widely traded and are often highly invasive when introduced by accident or purpose into new environments (Fisher & Garner 2007). Therefore, these two species constitute ideal vectors for introducing Bd into uninfected regions of the globe (Garner et al. 2006).

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Figure 1.  (a) The North American bullfrog Rana catesbeiana (photo credit: Matthieu Berroneau). (b) The Chinese giant salamander Andrius davidianus, a close relative of A. japonicus (photo credit: International Cooperation Network for Giant Salamander Conservation).

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With this in mind, it is perhaps not surprising to find that Japan harbours Bd. Both R. catesbeiana and X. laevis are captive in Japan, and it is likely that the aquatic zoospores of Bd have long been contaminating water supplies in discard from pet shops and research/breeding facilities. Further, both species of amphibian have formed naturalized alien populations in at least five islands, and Goka et al. show that the prevalence of Bd is higher in these two invasive species than occurs in native species. What Goka et al. also demonstrate through their sequencing of ribosomal haplotypes is that the majority of the genetic diversity is found residing in samples of Bd taken from natural and captive R. catesbeiana, where some populations of bullfrogs are infected by up to seven haplotypes of the pathogen. Also, two of the Bd haplotypes found infecting R. catesbeiana are also found infecting native species. These patterns are supportive of the hypothesis that R. catasbeiana are introducing Bd into Japan, that this has occurred multiple times and that trans-specific transmission of Bd has occurred between these alien bullfrog ‘vectors’ into endemic Japanese species; similar results demonstrating trans-specific transmission and introduction of Bd have been observed within the UK (Cunningham et al. 2005) and Mallorca (Walker et al. 2008).

However, this interpretation of the data becomes less clear-cut when the actual distribution of haplotypes is examined. First, the small southerly island of Okinawa contains populations of alien North American bullfrogs. However, these appear to be uninfected (although low levels of infection could have been missed due to the study’s small sample sizes). In this region, populations of the endemic and threatened sword-tailed newt Cynops ensicauda carry a high prevalence of infection (50%) as well as four haplotypes of Bd. These haplotypes are also found infecting bullfrog populations elsewhere in Japan; perhaps in Okinawa, the chytrid has jumped into the C. ensicauda populations, while becoming extinct in the alien bullfrog populations through stochastic processes? At this stage, it is not possible to say. Second, Goka et al. show that individuals of the giant Japanese salamander, Andrius japonicus (Fig. 1b) are infected by Bd both in captivity and in their natural habitat. This aspect of the study becomes controversial when Goka et al. report the results of a phylogenetic analysis of the sequences of the Bd haplotypes that are infecting these salamanders. They show that the salamander-associated haplotypes (called B, J and K) are dramatically different to those found infecting other Japanese species as well as the introduced alien species. So what does this mean?

No evidence of chytridiomycosis has been observed in these salamanders, which suggests that either they have a relatively high tolerance to infection or that these lineages of Bd are avirulent. The authors also cite unpublished evidence that they have found evidence of infection in museum specimens dating back to 1902. If this is the case, then these data suggest that Bd may have been endemic within Japan for a period longer than has been documented elsewhere in the globe, and that co-evolution with the pathogen may have occurred. Does this mean that Japan, or other southeast Asian countries (such as Indonesia, Kusrini et al. 2008), may have been a source of the original lineage of Bd and that we are actually observing a panzootic that stems from ‘out of Asia’ rather than ‘out of Africa’? It is of course too early to make any conclusions, and there are some aspects of this study that deserve to be worked on further. In particular, the nested PCR-assay developed for this study may be biased to detecting some ribosomal haplotypes over others; we need to remember that the multicopy ribosomal DNA array can be heterogeneous within fungal individuals due to a breakdown in concerted evolution. If this is the case, then sequencing more genes from isolates of Bd that have been recovered from native Japanese amphibians will be required to ascertain the true phylogeny. Comparing these genotypes against those recovered by James et al. would then indicate whether the ‘endemic’ Japanese strains of Bd represent a basal lineage, or are in fact a closely related but separate species of chytrid. It is also necessary to establish whether these A. japonicus-associated Bd lineages are virulent when compared against more well-studied lineages of Bd; recently, it has been shown that different strains of the pathogen are differentially virulent against the common toad Bufo bufo (Fisher et al. 2009) and such an approach would shed light on the potential co-evolutionary dynamics that this Asian lineage of the pathogen may have experienced.

Goka et al. have made an important contribution to our understanding of where Bd occurs, and where it may have come from. Their study will no doubt inject renewed vigour into surveying thus-far untouched amphibian communities, as well as raising new, and testable, hypotheses about the global spread and evolution of Bd. The findings that stem from this research will likely prove to be essential weapons in the fight against what has become one of the most devastating infections to impact upon a class of vertebrates.

References

  1. Top of page
  2. Abstract
  3. References