Abstract
BackgroundThe cane toad, Bufo (Chaunus) marinus, is one of the most notorious vertebrate pests introduced into Australia over the last 200 years and, so far, efforts to identify a naturally occurring B. marinus-specific pathogen for use as a biological control agent have been unsuccessful. We explored an alternative approach that entailed genetically modifying a pathogen with broad host specificity so that it no longer caused disease, but carried a gene to disrupt the cane toad life cycle in a species specific manner.Methodology/Principal FindingsThe adult beta globin gene was selected as the model gene for proof of concept of autoimmunity as a biocontrol method for cane toads. A previous report showed injection of bullfrog tadpoles with adult beta globin resulted in an alteration in the form of beta globin expressed in metamorphs as well as reduced survival. In B. marinus we established for the first time that the switch from tadpole to adult globin exists. The effect of injecting B. marinus tadpoles with purified recombinant adult globin protein was then assessed using behavioural (swim speed in tadpoles and jump length in metamorphs), developmental (time to metamorphosis, weight and length at various developmental stages, protein profile of adult globin) and genetic (adult globin mRNA levels) measures. However, we were unable to detect any differences between treated and control animals. Further, globin delivery using Bohle iridovirus, an Australian ranavirus isolate belonging to the Iridovirus family, did not reduce the survival of metamorphs or alter the form of beta globin expressed in metamorphs.Conclusions/SignificanceWhile we were able to show for the first time that the switch from tadpole to adult globin does occur in B. marinus, we were not able to induce autoimmunity and disrupt metamorphosis. The short development time of B. marinus tadpoles may preclude this approach.
Highlights
The spread of the cane toad, Bufo (Chaunus) marinus, into the Australian environment following the initial introduction at Gordonvale near Cairns, Queensland in 1935 has been spectacularly successful
The positive control used for the experiments was recombinant adult globin that migrates as a larger protein than the native globin on polyacrylamide gel electrophoresis (PAGE) gels due to the addition of a 6XHis-tag and plasmid linker sequence
Our proof of concept approach was largely influenced by previous observations that in bullfrog tadpoles immunised with purified adult globin, the adult globin protein profile was altered in surviving metamorphs [18]
Summary
The spread of the cane toad, Bufo (Chaunus) marinus, into the Australian environment following the initial introduction at Gordonvale near Cairns, Queensland in 1935 has been spectacularly successful. The cane toad is a highly toxic and hardy introduced species and presents wide ranging ecological and social impacts within the Australian landscape. Since other vaccines developed against diseases of wildlife include a rabies virus based vector used to immunise wildlife against SARS [6] Extension of this concept has seen recombinant viruses developed to control a host’s biological processes. Injecting tadpoles with adult globin interfered with expression of this protein in Rana catesbeiana, and induced changes in gene expression profiles of metamorphs [18]. We hypothesise that it may be possible to alter metamorphosis by immunologically sensitising larval stages (tadpoles) to proteins expressed only in later post-metamorphic stages. Our results indicate that the altered adult globin protein profile seen in Rana catesbeiana metamorphs after exposure of tadpoles to adult globin does not occur in B.marinus. The short larval stage in B. marinus compared with R. catesbeiana may preclude this approach to cane toad biocontrol
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